The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 and 2016 that were designed to address the
development of monsoon climate systems in Asia and Australia. Significant
progress was made in recovering Neogene sections spanning the region from
the Arabian Sea to the Sea of Japan and southward to western Australia. High
recovery by advanced piston corer (APC) has provided a host of
semi-continuous sections that have been used to examine monsoonal evolution. Use of the half-length APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ∼ 4 Ma, likely linked to global cooling. A transition from C3 to C4 vegetation often
accompanied the drying but may be more linked to global cooling. Western
Australia and possibly southern China diverge from the general trend in
becoming wetter during the late Miocene, with the Australian monsoon being
more affected by the Indonesian Throughflow, while the Asian monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of northern hemispheric
Glaciation. Stronger monsoons are associated with faster continental
erosion but not weathering intensity, which either shows no trend or
a decreasing strength since the middle Miocene in Asia. Marine productivity
proxies and terrestrial chemical weathering, erosion, and vegetation proxies
are often seen to diverge. Future work on the almost unknown Paleogene is
needed, as well as the potential of carbonate platforms as archives of
paleoceanographic conditions.
Introduction
Monsoon climatic systems exist in most continents where large seasonal
temperature differences develop between the continental interior and the
surrounding oceans. The Asian monsoon is the strongest such system because
of the great size of the Asian continent and the height of the topography
associated with the Himalayan mountains and Tibetan Plateau, whose
development is tightly linked with the climate (Molnar et al., 1993;
Prell and Kutzbach, 1992). The monsoon is split into two distinct seasons,
one wet and one dry. In the summer, onshore winds bring moisture from the
Bay of Bengal into South Asia and from the South China Sea and western
Pacific into southern China and former Indochina (Wang, 2006;
Webster et al., 1998). During northern hemispheric winter the winds reverse,
with cold dry air blowing from the atmospheric high-pressure area in Siberia
towards the ocean. The seasonal advance of the monsoon rain front into the
continent represents a migration in the intertropical convergence zone
(ITCZ) and is mirrored by a similar system that brings heavy rain to
northern Australia and parts of Indonesia during the southern hemispheric
summer (Suppiah, 1992).
There has been significant scientific interest in monsoon climates because
the Asian monsoon has come to symbolize the archetypal example of how the
solid Earth and atmosphere coevolve, with feedbacks between the two over
various timescales (Clift et al., 2008; Whipple, 2009).
The societal significance of the Asian monsoon has also made it the target
of research given the high population density that is sustained by the
agriculture permitted by summer rainfall today, as well as its role in
controlling the rise and subsequent decline of early urban civilizations
(Madella and Fuller, 2006; Clift and d'Alpoim Guedes,
2021). Although much is known about the atmospheric physics that controls
the intensity of the monsoon in the present day, the long-term development
of this climatic system is less well characterized, especially prior to the
Quaternary.
Scientific drilling in the 1980s played a crucial role in the first attempt
to constrain the timing of Asian monsoon intensification, particularly
through records of oceanic upwelling and productivity along the Arabian
margin (Kroon et al., 1991; Prell et al., 1992), where strong summer
winds are linked to the monsoon in the present day. The winds blow to the
northeast offshore Arabia, bringing nutrient-rich water to the surface that
causes a seasonal bloom in planktic foraminifers in the modern Arabian Sea
during the summer (Curry et al., 1992). In turn
an oxygen minimum zone (OMZ) forms in response to the high marine
productivity driven by monsoon-induced upwelling (Altabet et al.,
1995) and because of limited vertical mixing. This phenomenon is
particularly well developed along the NE margin offshore India and Pakistan.
The OMZ is greater when the productivity is high and when winter mixing of
the water column is shallow (Reichart et al., 1998).
Correlation of these records with terrestrial vegetation proxies in the
Himalayan foreland basin played a key role in leading to an estimate for
initial intensification at ∼ 8 Ma (Quade et al., 1989),
which at that time was believed to correlate with a phase of rapid Tibetan
uplift (Harrison et al., 1992). Subsequently, re-examination of
the Oman margin cores has resulted in recognition of an initial monsoon wind
system starting at 12.9 Ma and intensifying ∼ 7 Ma
(Gupta et al., 2015). Drilling in the South China Sea by
the Ocean Drilling Program (ODP) Leg 184 in 1999 established a contrasting
chemical weathering and salinity record that implied a much earlier
intensification of heavy rains in southern China, starting around 24 Ma and
again at 15 Ma (Clift et al., 2002; Wan et al., 2007) and indicated a
drying of the climate in the late Miocene (Steinke et al., 2010).
It is apparent that a number of different processes control the strength of
the monsoon systems by influencing the temperature of the continent as well
as the oceans. In particular, the development of high topography in central
Asia has been invoked to cause long-term strengthening of the monsoon system,
and while early efforts focused mostly on the Tibetan Plateau
(Manabe and Terpstra, 1974), greater emphasis has recently
been placed on the height of the Himalayas and their role in controlling the
South if not East Asian monsoon (EAM) (Boos and Kuang, 2010). Climate
models have, however, also invoked the importance of topography in the
Iranian Plateau (Acosta and Huber, 2020), as well as the
opening and closure of marine gateways, most notably in the western Tethys
Ocean between Arabia and Eurasia (Gülyüz et al., 2020; Torfstein
and Steinberg, 2020). In addition, constriction of the Indonesian
Throughflow (ITF), the oceanic gateway between Indian and Pacific oceans,
has increased as Australia collided with Indonesia and New Guinea starting
in the Miocene (Van Ufford and Cloos, 2005). Added to this, it has
become apparent that the monsoon rainfall is sensitive to global climate in
being generally heavier when Earth is hotter. On shorter
glacial–interglacial timescales, it has been recognized that monsoon rains
tend to be stronger during interglacial times in Asia and become drier
during periods of extensive northern hemispheric glaciation. Multi-proxy
reconstructions from recent drilling indicate that coupled ice volume and
greenhouse gas forcing is a critical factor driving changes in monsoonal
rainfall at orbital timescales as well (Clemens et al., 2021; Gebregiorgis
et al., 2018; McGrath et al., 2021). On shorter timescales oceanic phenomena
such as the El Niño–Southern Oscillation (ENSO) are often linked to the
strength of monsoon rains in Asia (Wang et al., 2013; Lau and Wang, 2006).
The Australian monsoon is linked to the Asian system but has different
sensitivities. A positive Indian Ocean Dipole (IOD), when the western Indian
Ocean warms but the sea offshore western Australia is colder than normal, is
associated with droughts in SE Asia and Australia, especially in southeast
Australia (Cai et al., 2009). Likewise, the El Niño state in
the Pacific, when colder waters dominate in the western Pacific and the
eastern Pacific is warmer than normal, weakens spring rains in Australia
(Ashcroft et al., 2016). The IOD in particular has been affected by
the closure of the ITF (Kajtar et al., 2015), although ENSO
appears to be more dependent on the atmospheric Walker circulation
(Sprintall et al., 2014). Nonetheless, restriction of the gateway
is one of the factors driving long-term aridification of Australia
(Krebs et al., 2011). Both ENSO and IOD are also affected by
shorter-term orbital-related processes that affect the strength and
seasonality of the rains and control the environment. El Niño conditions
and thus weak Australian rains are more common when the Earth is generally
warmer. Likewise, the IOD has become more positive, and thus Australia has
been drier since the Last Glacial Maximum (LGM; 20 ka) (Abram et al., 2020).
Model-based testing of what controls monsoon intensity has been hampered by the
lack of long-duration marine records. Although shallow piston coring has
been effective in collecting sequences to examine how the monsoon has
changed over millennial timescales, scientific ocean drilling is required to
look at how this system has evolved on orbital (104) to tectonic
(> 106 years) timescales. The work necessarily involves
correlation of marine records with terrestrial sediment archives, as well as
tectonic models derived from studying the mountains, whose uplift has
influenced the atmospheric dynamics across the continents. Although
continental records are important in providing some local control over
environmental conditions and erosion of mountain belts, it is the marine
depocenters that comprise the more continuous, better dated records that
are required to develop sophisticated models of monsoon evolution and its
impacts. The marine record is also critical if we are to relate the
continental environmental conditions with the oceanography of the
surrounding seas because understanding of the modern monsoon would indicate
that they should be tightly coupled (Fasullo, 2012; Tada and Murray,
2016). Before the start of this most recent campaign of scientific drilling,
there were major gaps in our data coverage, hampering better understanding.
Although there were Miocene to Recent records from the Arabian and South
China seas, no high-resolution records were available over many other parts
of the region, at least spanning the Neogene, let alone the Paleogene. Since
the onset of the monsoon has been estimated to be as old as the late Eocene
(Licht et al., 2014; Sorrel et al., 2017), this made the marine record of
rather limited use to study long-term evolution. Although it has long
been argued that erosion and weathering of the mountains might be a primary
control over global climate during the Cenozoic (Raymo and Ruddiman,
1992), the lack of a long-term erosional record from either of the large
submarine fans in the Indian Ocean, let alone the major rivers of former Indochina
prevented these models being tested.
Regional drilling campaigns
From 2013 to 2016, the International Ocean Discovery Program (IODP) conducted
drilling around Asia and Australia that was specifically designed to
document variations in monsoon intensity on millennial and longer tectonic
timescales. This has resulted in significant revision of our reconstructions
of when the monsoon intensified and weakened across a range of timescales,
and in turn this contributes to our understanding of what the primary
driving factors controlling wind and rainfall intensity in South and East
Asia have been.
Shaded bathymetric map of the regions affected by the Asian and
Australian monsoon systems showing the area of operations of the expeditions
completed by IODP in the most recent campaign. Areas of older ODP and DSDP
drilling are shown as blue dots. IBR is the Indo-Burman Range. Route of IFT
after Gordon (2005). Dashed pink lines show outline of Indus and
Bengal fans. ITCZ locations are from Yan (2005). Base map is
from GeoMapApp.
The drilling campaigns addressed all the marginal seas around Asia,
stretching from the Arabian Sea in the southwest (Betzler et al., 2017; Pandey et al., 2016) to the Sea of Japan in the northeast (Tada et al., 2015), continuing southward through Indonesia, to the west coast of Australia
(Gallagher et al., 2017) (Fig. 1). As well, both the major
Indian Ocean submarine fans, the Indus and the Bengal
(Clemens et al., 2016; France-Lanord et al., 2016),
which together represent the bulk of the sediment derived from the Himalayan
orogen, were drilled together with the Nicobar fan located on the east side
of the Ninety-East Ridge and thus provide a relatively complete history of
Himalayan erosion, at least covering much of the Neogene (McNeill et al.,
2017). Furthermore, the eastern Indian Margin and Andaman Sea were drilled
to reconstruct the Miocene to present monsoonal paleoclimate and
paleoceanography (Clemens et al., 2016). In the South China Sea,
three expeditions recovered sections of sediment mostly from the northern
margin, related to the Pearl River catchment of southern China
(Li et al., 2015; Sun et al., 2018), enhancing
records obtained during the earlier ODP Leg 184 (Wang et al., 2000).
Although drilling was largely focused on Neogene targets, for the first time the regional
character of the campaign provided a wide sampling of
many of the large continental drainage systems of Asia from which
environmental, weathering, and erosion records can be derived, thus
constraining both South and East Asian monsoons and providing the high-resolution records
necessary for comparison between systems. Moreover, these are now
accompanied by similarly high-resolution records from Western Australia
looking at the evolution in climate of this continent and constraining the
development of the monsoon system in that region, providing a means of
comparison with the Asian monsoons (Gallagher et al., 2017). A
related expedition offshore NW Australia and north of New Guinea provides an
important record of the Western Pacific Warm Pool
(WPWP) (Rosenthal et al., 2018), which is closely correlated
with the intensity of the EAM system and additional Neogene records of the
NW Australian system. In this study we summarize the primary findings in
each of these critical areas and then assess how they may be related to one
another in order to understand how different parts of the Asian–Australian
monsoon system may be linked.
Arabian Sea
Some of the earliest constraints on the geological evolution of the monsoon
were derived in the Arabian Sea through scientific drilling (Fig. 1). This
is a particularly good area to look at the monsoon because of the strong
oceanographic effects that the wind system has on upwelling and marine
production in the area, as well as on the delivery of sediments from the
Indian peninsula and particularly the Indus River, which represents the
largest drainage system in the NW Himalaya. Prior to IODP drilling, the
southwestern Indian margin was sampled by Indian National Gas Hydrate
Expedition 01 Site NGHP-01-01A, located offshore Goa. Sedimentary and
geochemical data indicate stronger water mass mixing associated with a
winter monsoon circulation after 23.7 Ma (Beasley et
al., 2021). The same study also argued for the first summer monsoon after 23 Ma based on increases in Ti / Ca and dissolution of the biogenic carbonate fraction, as well as formation of an OMZ in the eastern Arabian Sea.
Expedition 355: environmental records in the Laxmi Basin
Expedition 355 recovered two long sections (1109 m penetration at Site U1456 and 1008 m at Site U1457) from the central Laxmi Basin, offshore western India, forming the eastern side of the Indus submarine fan. Recovery was 92 % with the advanced piston corer at Site U1456, 93 % at Site U1457 and 57 %, and 48 % with the Rotary Core Barrel (RCB) at each site respectively. The occurrence of a large mass transport complex (MTC) disrupted plans to recover sediment older than around 11 Ma (Dailey et
al., 2019), but operations did retrieve a relatively continuous record of
continental erosion spanning the last 11 Myr and covering the critical climatic transition around 8 Ma, albeit disrupted by a number of hiatuses (Routledge et al., 2020). Work within the NW Himalayan foreland had used carbon isotopes to identify changes in vegetation
(Quade et al., 1989), and these same methods were applied to detrital
organic carbon in the cores to assess how vegetation had evolved in the
Indian peninsula and Himalayan foreland since 11 Ma. Studies of sedimentary
organic carbon implied a change in the vegetation in the source regions
starting at 7 Ma (Khim et al., 2020). The δ13C
of long-chain n-C32 fatty acids shifted from -34 ‰ to
-22 ‰ between 10 and 6.3 Ma, and this was interpreted to
indicate the progressive increase in C4 grasses at the expense of C3 plants (e.g., trees), especially between around 8.2 and 6.3 Ma (Suzuki et al., 2020) (Fig. 2d). This climatic transition was furthermore supported by a high-resolution study that focused on the period of climatic transition (Feakins et al., 2020). That study employed a multi-proxy
approach involving bulk organic carbon and plant wax alkanes and acids, as well
as a variety of pollen, charcoal, and lignin proxies, to assess how the
vegetation changed under the influence of the evolving monsoon.
Temporal evolution in weathering proxies from the Laxmi Basin
together with possible forcing factors. (a) 565 spectra intensity (hematite) and (b) CIA*, from Zhou et al. (2021), (c)δ13C of n-C32 fatty acids (Suzuki et al., 2020) and from n-C31n-alkanes (Feakins et al., 2020), and (d)δ15N and total organic carbon (TOC) from Tripathi et al. (2017).
The Feakins et al. (2020) study was able to separate the influence of
the Indus River compared to regional rivers draining the Indian peninsula.
δ13C values from n-C31n-alkanes together with the other
proxies supported the idea of expansion of grasslands through the late
Miocene in northwest India, with change especially noted between 7.2 and 7.4 Ma. Interestingly, there was no clear change in δD values, which
have been used as proxies for rainfall intensity in the Arabian Sea
(Huang et al., 2007). This implied that there has been a relatively
constant monsoon system across the area that was not disrupted during the
time of transition in the vegetation and in turn raises the possibility
that it was cooling, reconstructed from TEX86 data, and not drying that
was responsible for the shift in the environment at that time.
Chemical weathering data has also been employed to constrain the evolving
environment in the Indus catchment. Hematite / goethite values measured by
color spectroscopy were used to constrain relative humidity, with hematite
favored during times of drier climate. These records indicated a drying, or
at least an increase in the duration of the dry season, after
∼ 7.7 Ma, and while there was a phase of increased humidity
between 6.3 and 5.9 Ma, a long-term trend was towards less chemical
weathering and slower and drier conditions as the Miocene progressed into the
Pliocene (Clift et al., 2020).
Bulk sediment major element geochemical data, color spectral data, and clay
mineralogy confirmed a long-term decrease in weathering intensity that is
often associated with drier, colder conditions (Zhou et al., 2021).
The role of hematite in highlighting dry conditions is particularly
noteworthy in implying a long-term decrease in humidity and thus summer
monsoon rains (Fig. 2a). Clay minerals are sensitive to environmental
conditions (Thiry, 2000) but can also be used as provenance
proxies under certain conditions. Clays have been used to argue that since
3.7 Ma, changes in monsoon strength have caused the sediment supplied to the
Indian Ocean to alternate with more supply from the peninsula and Deccan
Traps during times of heavier summer rains (Cai et al., 2020).
It is noteworthy that there is a disconnect between monsoon intensity
inferred from oceanic productivity records and those related to the climate.
In the modern day, strong summer winds are associated both with heavy rain
and with upwelling (Curry et al., 1992). This
linkage extends to orbital timescales (Clemens et al., 2021) but does not
appear to have been the case at longer timescales.
Nitrogen isotope compositions of sediment track marine nitrogen cycling,
which can be related to biological productivity. N isotopes reflect the
source of dissolved N to phytoplankton, and in nutrient-replete
environments, the relative utilization. In the Arabian Sea, N isotopes
reflect the upwelling of partially denitrified nitrate from the OMZ.
Increases above the oceanic mean are taken as an increase in the intensity
of the OMZ, often related to oxygen demand due to monsoon-driven upwelling
(Altabet et al., 1995). N isotope and organic carbon contents of
Laxmi Basin sediments indicate that the first sign of denitrification
occurred at 3.2–2.8 Ma and that the modern OMZ was not established until
∼ 1 Ma (Tripathi et al., 2017). Subsequent higher-resolution studies spanning 800 ka indicated a persistent OMZ in the eastern
Arabian Sea but with a breakdown of the OMZ in the western Arabian Sea
during glacial times when the summer monsoon weakened (Kim et al.,
2018). Geological evidence for marine production and continental erosion
implies that the summer monsoon strengthened at 2.95 Ma, possibly linked to
closure of the ITF but then progressively weakened independently of the
glacial climatic cycles (Sarathchandraprasad et al., 2021).
The lack of correlation between the OMZ history and the terrestrial
weathering and vegetation proxies is a clue that the paleoceanography and
terrestrial rainfall are not tightly coupled over the longer-term geological
past. This conclusion is consistent with a number of recent modeling
studies (Nilsson-Kerr et al., 2021; Acosta and Huber, 2020).
Expedition 359: Maldives monsoon winds
The Maldives archipelago in the northern central Indian Ocean has acted for
over 25 Myr as a giant natural sediment trap and contains a record of
monsoon-related environmental changes in that region. The carbonate
platforms record sea-level fluctuations, while the drift and periplatform
deposits carry the record of monsoon-driven changes of the surface and
intermediate water mass current regime and of wind-driven dust influx.
IODP Expedition 359 cored sediments from eight locations in the Inner Sea of
the Maldives (Betzler et al., 2017). Penetration was up to 1097 m below the seafloor at Site U1467, located in the center of the Inner Sea, with
high degrees of recovery (> 90 %) over many of the intervals
drilled by the APC. The expedition was designed to reconstruct the evolution of
the South Asian monsoon (SAM) and related fluctuations of sea level. The
timing of these changes is assessed by dating sedimentary alterations that
mark stratigraphic turning points in the Neogene Maldives platform–basin
system. The first turning points, dated as early and middle Miocene, are
related mostly to sea-level changes. These are reliably recorded in the
stratigraphy of the carbonate sequences in which sequence boundaries provide
the ages of the sea-level lowstands (Vail et al., 1977).
The Maldives archipelago, located in the Indian Ocean, is affected
by the seasonally reversing monsoonal winds and currents. The archipelago
has two rows of drowned and active atolls that line the Inner Sea. Seismic
line across the Maldives Inner Sea with line drawing showing the platform
sequence (PS) boundaries and the drift sequence (DS) boundaries. Carbonate
platform sedimentation is controlled by sea level, while the drift sequences
are controlled by strong currents which start at 13 Ma. Major turning points
in sea level (blue circles) and current control (reddish circles) are
indicated. Position of seismic line corresponds to white line in the map.
Maps were produced using the program Esri ArcMap 10.1 (http://www.esri.com, last access: 29 May 2022). Bathymetric data were exported as Geotiffs from GeoMapApp 3.6.0 (http://www.geomapapp.org, last access: 29 May 2022). Worldwind satellite images (http://worldwind.arc.nasa.gov/java, last access: 29 May 2022) were merged with multibeam data acquired during cruises M74/4 and SO236.
An abrupt change in sedimentation patterns is recognized across the entire
archipelago at a sequence boundary dated as 12.9–13.0 Ma
(Betzler et al., 2016). At this turning point, the
platform sedimentation switched to a current-controlled mode when the
monsoon-wind driven circulation started in the Indian Ocean (Fig. 3).
Several areas of the platform drowned in response to physical current
effects, i.e., erosion and sediment re-deposition (Ling et al., 2021;
Lüdmann et al., 2018; Reolid et al., 2020, 2019). The
similar age of the onset of drift deposition from monsoon-wind driven
circulation across the entire archipelago indicates an abrupt onset of
strong monsoon winds in the Indian Ocean (Betzler et al., 2016, 2018). Ten unconformities dissect the drift sequences, attesting to
changes in current strength and/or direction that were likely caused by the
combined impact of changes in monsoon wind intensity and sea-level
fluctuations over the last 13 Myr. One major shift in the drift packages is
dated with 5.8 Ma and coincided with a long-term sea-level rise that
transformed the focused delta drift deposition to more widespread sheeted
drifts (Fig. 3). A second major shift at 3.8 Ma coincided with the end of
stepwise platform drowning and a reduction of the OMZ in the Inner Sea.
The strata of the Maldives platform provide a detailed record of the
extrinsic controlling factors on carbonate platform growth through time.
This potential of carbonate platforms for dating Neogene climate and current
changes has been exploited in other platforms drilled by earlier scientific
drilling. For example, Great Bahama Bank, the Queensland Plateau, and the
platforms on the Marion Plateau show similar histories with sediment
architectures driven by sea level in their early history (early to middle
Miocene) replaced by current-driven drowning or partial drowning during
their later history (late Miocene) (Betzler and Eberli, 2019). In
all three platform systems, the influence of currents on sedimentation is
reported to be between 11 and 13 Ma.
The lithogenic fraction of the Maldives carbonate drifts provides a unique
record of atmospheric dust transport during the past 4 Myr because grain
size can act as a proxy for dust flux, as well as wind transport capacity
(Lindhorst et al., 2019). Entrainment and long-range
transport of dust in the medium to coarse silt size range is linked to the
strength of the Arabian Shamal winds and the occurrence of convective storms
that prolong dust transport. Dust flux and the size of dust particles
increased between 4.0 and 3.3 Ma, corresponding to the closure of the ITF
seaway and the intensification of the SAM. Between 1.6 Ma and the Recent,
dust flux again increased but shows higher variability, especially during
the last 500 kyr. Eolian transport capacity based on grain size increased
between 1.2 and 0.5 Ma but has slightly decreased since that time. Dust
transport varied on orbital timescales, with eccentricity control being the
most prominent (400 kyr throughout the record and 100 kyr between 2.0 and 1.3 Ma and since 1.0 Ma). Higher-frequency cycles (obliquity and precession) are most pronounced in wind transport capacity.
Using XRF scans of the cores recovered at Site U1467, wavelet and spectral
analyses of the Fe / K record show increased dominance of 100 kyr cycles
after the Mid-Pleistocene Transition (MPT) at 1.25 Ma in tandem with the
global ice volume inferred from calculated seawater δ18O data
(LR04 record) (Kunkelova et al., 2018). In contrast to the LR04 record,
the Fe / K profile from Site U1467 resolves cycles that are similar to 100 kyr cycles around the 130 kyr eccentricity frequency band in the interval from 1.25 to 2.0 Ma. These cycles similar to 100 kyr cycles likely formed through the bundling of two or three
obliquity cycles, indicating that low-latitude Indian–Asian climate
variability reflected an increased tilt sensitivity to regional eccentricity
insolation changes (pacing tilt cycles) prior to the MPT. The implication of
appearance of the 100 kyr cycles in the LR04 and the Fe / K records since the MPT suggests strengthening of a climate link between the low and high latitudes during this period of climate transition.
Bay of BengalExpedition 353: South Asian monsoon
Expedition 353 (Clemens et al., 2016) drilled the Ninety-East
Ridge, Bengal fan, northeast Indian Margin, and Andaman Sea. The expedition
recovered 4.28 km of sediment from six sites with 97 % recovery on
average. Double and triple coring was conducted in order to produce complete
sections that would allow for reconstruction of continental erosion and
monsoonal hydroclimate at a range of timescales from sites with
sedimentation rates ranging 2–15 cm kyr-1 (Robinson et al., 2016), with recovery spanning Campanian to Recent, although much of the Eocene was not recovered.
Paleocene–Oligocene: start of Himalayan erosion
Barnet et al. (2020) examined the interval of Earth history
containing the well-known hyperthermal events of the Paleocene–Eocene
Thermal Maximum (PETM) and Eocene Thermal Maximum (ETM), often studied as
potential analogues for future anthropogenic climate change. Trace element
and isotopic records from Ninety-East Ridge IODP Site U1443 and ODP Site 758
spanning ∼ 58–53 Ma place these hyperthermal events in the
context of a long-term warming of the water column on the order of
4–5 ∘C. These results are comparable to those reconstructed from
the low-latitude Pacific, demonstrating global-scale synchronous warming of
the low-latitude and high-latitude regions of deep-water formation. These
new findings support the idea that atmospheric CO2 was the primary
driver of global climate during this time of climatic transition.
Ali et al. (2021) used a newly-developed isotope chronostratigraphy from
Site U1443 (Lübbers et al., 2019) coupled with samples from
ODP Site 758 (at the same location) to evaluate the radiogenic Sr, Nd, and
Pb isotopic composition of clay minerals produced from silicate weathering
and deposited in the Bay of Bengal since 27 Ma, the longest such marine
record in South Asia. They demonstrate remarkable source consistency,
indicating dominance of supply from Himalayan rocks and the Indo-Burman
Range, implying that the spatial pattern of weathering associated with
monsoon rainfall has varied little over the past 27 Ma.
(a, e) Global δ18O and δ13C from
bottom-living (benthic) foraminifera compiled from more than 40 DSDP and ODP
sites (Zachos et al., 2008) representing global ice volume and
deep-sea temperature, highlighting the global climate changes of the last 30 Myr. Periods of global cooling, after Zachos et al. (2008), are marked
with blue bars, and the MMCO is marked with a red bar (b, f). Clay
mineralogy of ODP Site 758, relative abundances (% of smectite and
kaolinite, illite, and chlorite), I CRAT record of ODP Site 1148
(Clift et al., 2008) from the South China Sea as a record of
East Asian monsoon development, (e) Nd and Pb isotope composition of ODP Site 758 clays, (g) mineral flux record (Hovan and Rea, 1992) of
ODP Site 758 recalculated using updated linear sedimentation rates, and
sedimentation rates from the Bengal fan DSDP Site 218 and IODP Site U1451
(France-Lanord et al., 2016; Galy et al., 2010), and at
the bottom a summary of the regional tectonic events (Allen and
Armstrong, 2012). 1 – high Himalayan uplift, Main Central Thrust (MCT); 2 –
onset of normal fault, southern Tibet; 3 – surface uplift, eastern Tibet; 4 – initial thrusting on main boundary Thrust; and 5 – fast exhumation at Himalayan syntaxes, outward growth of NE Tibet. This figure is from Ali et al. (2021).
A number of investigators have worked to differentiate provenance,
weathering, tectonics, and climate change signals at Expedition 353 sites,
located both in proximal and distal positions relative to the source
drainages and using both hemipelagic and turbiditic sequences.
Bretschneider et al. (2021) assessed high-resolution records of
radiogenic Sr, Nd, and Pb isotopic composition of clay minerals deposited on
the Ninety-East Ridge at Site U1443 across five time slices within the
middle to late Miocene (15.8–9.5 Ma). This is the same site where Ali et al. (2021) recorded an increase in clay mineral abundance at
∼ 13.9 Ma that would signify an increase in physical
weathering intensity in the sources, coincident with middle Miocene global
cooling and regional tectonic reorganization (Fig. 4). Despite Himalayan
tectonic reorganization, the erosional sources remained remarkably
consistent across the five time slices. However, shorter (orbital)
timescale variability shows significant fluctuations in all three isotope
systems, likely linked to monsoon intensity. Variability within the middle Miocene Climatic Optimum (MMCO; 16–15 Ma) and the interval of global
cooling at 13.9–13.8 Ma was larger than during younger intervals, a change
attributed to movement of the precipitation locus from the high Himalaya to
the frontal Himalayan ranges and to the Indo-Burman Range.
The older intervals from pelagic sections on the Ninety-East Ridge indicate
relatively stable sediment provenance over long intervals of time. However,
reconstructions from the middle–northern Bengal fan IODP Site U1444A, spanning
the late Miocene to present (∼ 7–0 Ma), do indicate
provenance changes. Chang and Zhou (2019) used optically and
thermally stimulated luminescence (OSL and TL) of quartz and K-feldspar
grains to characterize the sediment mineral composition through time. These
authors interpret a distinct increase in quartz luminescence sensitivity at
∼ 3.5–0.5 Ma as increased hemipelagic contribution from
Indian peninsular rivers relative to Himalayan-sourced pulses of turbidite
sediments which dominated at 7–6 and 3.8–3.5 and since 0.5 Ma. In this case,
changes between hemipelagic (Indian peninsula) and turbiditic (Himalayan)
deposition were attributed more to changes in tectonic activity in the
Himalayan region as opposed to the monsoonal climate.
Peketi et al. (2021) studied the time interval since
∼ 6 Ma at the eastern Indian continental margin Site U1445A
using Sr and Nd isotopic analysis of the lithogenic fraction, elemental
Fe / Al ratios, and clay mineralogy. They documented variable Ganga and
Brahmaputra provenance at Site U1445A, with little input from the proximal
Mahanadi drainage basin. The interval since 1.8 Ma indicates increased flux
from the Brahmaputra River (supplied by erosion from the Trans-Himalayan
batholiths) during periods of monsoon intensification and more from the
Ganga during times of weakened monsoons. Variability within the interval
6–1.8 Ma was found to be influenced by both climate and tectonic forcings,
the relative effects of which could not be differentiated.
Dunlea et al. (2020) also investigated the sequence recovered at Site U1445 to assess the expansion of C4 vegetation, linked to drying of the
environment. Their assessment of provenance was based on major, trace, and
rare earth elemental concentration data, which were interpreted to indicate
a Mahanadi drainage provenance, contrary to the conclusions of Peketi et al. (2021). The associated bulk organic and compound specific
biomarkers, whether reflective of the Mahanadi drainage within the core
monsoon zone of India or larger drainages to the north, document the
existence of C4 vegetation before the end of the Miocene but with an
expansion to higher abundances at ∼ 3.5–1.5 Ma, all
superimposed on an overall long-term decrease in monsoon precipitation since
the late Miocene. These findings build upon the existing evidence for
regionally heterogeneous responses in the timing of C4 expansions and
contractions, indicating sensitivity to regional climate changes in addition
to global pCO2 forcing (Feakins et al., 2020).
Hemipelagic sediment from western Andaman Sea Site U1447 spans the past 10 Myr and was analyzed by Lee et al. (2020b) for Sr and Nd isotopes,
clay mineralogy, and δ13C of sediment organic matter. The Nd
and Sr data indicate sources in the Myanmar region, including major river
drainages (e.g., Irrawaddy, Salween, Sittang) and smaller drainages from the
Indo-Burman Range. Like the Ninety-East Ridge Site U1443 studies, the
results indicate no significant changes in provenance since the late Miocene,
and hence clay mineralogical changes can be interpreted in the context of
monsoonal environmental change. A decreasing trend of smectite / (illite + chlorite) [S / (I + C)] implies stronger physical and weakened chemical weathering since the late Miocene, consistent with global cooling at that time. Climatologically, this is interpreted as a
strengthening of the winter monsoon or weakening of the summer monsoon over
this time period. Distinct events at 9.2–8.5, 3.6, 2.4, and 1.2 Ma were
interpreted to result from the combined effects of global cooling and
Tibetan Plateau uplift, the relative impacts of which cannot yet be
differentiated. Initial results from spectral natural gamma ray (NGR)
sediment core-logging and benthic foraminiferal stable isotope analyses of
the upper Miocene record at Site U1447 indicate that an important long-term
increase in physical weathering and erosion coincided with the globally
recognized late Miocene cooling trend between ∼ 7.0 and 5.5 Ma
(Kuhnt et al., 2020).
Lübbers et al. (2019) examined the critical interval of time from
13.5 to 8.2 Ma at Ninety-East Ridge Site U1443 using O and C stable isotopes
from benthic foraminifera, XRF elemental data, and carbonate accumulation
rates. At this equatorial site, a marked decrease in carbonate deposition
took place between ∼ 13.2 and 8.7 Ma, coinciding with the
middle to late Miocene “carbonate crash”. Synthesizing the timing of this
event at a global array of sites led the authors to hypothesize changes in
chemical weathering and riverine influx of calcium and carbonate ions as
fundamental mechanisms driving the carbonate crash and recovery. After 11.2 Ma, elemental ratio data (Ba / Ti) implied increased primary production and
organic carbon burial. This timing, somewhat earlier than the global onset
of the biogenic bloom, is attributed to intensification of upper-ocean
mixing associated with changes in the seasonality and intensity of SAM winds
and precipitation.
Jöhnck et al. (2020) produced a set of multi-proxy records from Site U1448 in the Andaman Sea spanning 6.24–4.91 Ma. Their benthic and planktic foraminiferal stable isotopes, combined with paired planktic carbonate Mg / Ca
elemental ratio data, yield the first high-resolution orbital-scale
reconstructions of monsoon variability across the Miocene–Pliocene
transition. They found a 4 ∘C increase in mixed-layer temperature
between 5.55 and 5.28 Ma, coincident with a change from precession-dominated
to obliquity-dominated variability in planktic δ18O and
seawater δ18O. This suggests that intensified cross-equatorial
heat and moisture transport paced by obliquity resulted in increased summer
monsoon precipitation during warm stages. In contrast, cold stages were
characterized by colder mixed-layer temperatures and reduced monsoon
rainfall, resembling Late Pleistocene stadials. The interval 5.55–4.91 Ma
was one showing strong coherence of seawater δ18O with orbital
precession, indicating that seawater δ18O minima lag precession
minima by 119∘ (7.6 kyr). This lag is consistent with that
measured in the Pleistocene from the same region (Gebregiorgis et al., 2018) and at Site U1446 on the northeast Indian margin (Clemens et al., 2021).
Most recently, a study of sediment from Ninety-East Ridge Site U1443 spanning
the period 9 to 5 Ma reconstructed changes in biogenic production at high
resolution and highlighted variance over cycles of 19–23 kyr, similar to
that seen in the Late Pleistocene (Bolton et al.,
2022). This work confirmed the important of insolation forcing of monsoon
wind strength in the Indian Ocean and demonstrated that the wind system did
not intensify significantly during the late Miocene.
Pleistocene: orbital forcing of monsoon productivity and rainfall
Monsoon variability has been assessed across the MPT, as well as orbital-scale variability over the past million years and high-resolution
variability across marine isotopic stage 5 (MIS5). Lee et al. (2020a) evaluated paleo-productivity over the past 2.3 Myr at
northeast Indian margin Site U1445 using the mass accumulation rate (MAR) of
biogenic opal, total organic carbon (TOC), and total nitrogen to assess links
between productivity and monsoon forcing across the MPT. These authors
identified a regime change from a dominance of biogenic opal prior to the
MPT to biogenic carbonate after this time. These changes were interpreted in
the context of riverine silicate supply, with a strengthened monsoon-induced
supply at 2.3–1.5 Ma, prior to the MPT, resulting in enhanced biogenic opal
productivity. Across the MPT and thereafter, weakened monsoon runoff reduced
stratification and enhanced nitrate supply from upwelling, leading to a
carbonate-dominated productivity regime. The inferred reduction in monsoonal
runoff is supported by the 0.19 ‰ shift in seawater
δ18O across the MPT observed at Site U1446 (Clemens et al., 2021).
Orbital-scale investigations of the monsoon at Andaman Sea Site NGHP-17/U1448 and northeast Indian margin Site U1446 have used water-related
isotopes (speleothem δ18O, leaf wax δD, and seawater
δ18O), leaf wax δ13C, and elemental XRF ratios to
differentiate changes in the isotopic composition of rainfall from rainfall
amount. McGrath et al. (2021) showed that variability in leaf wax
δD is strongly coherent with that of speleothem δ18O,
with variability in both proxies being coherent and in phase with ice-volume
minima and pCO2 maxima. In contrast, seawater δ18O from
the Andaman Sea (Gebregiorgis et al., 2018) and Indian margin
(Clemens et al., 2021) indicates that maximum rainfall/runoff
occurred significantly later and, in the case of precession, in phase with
maximum summer-monsoon wind strength proxies. These relationships indicate
that speleothem δ18O and leaf wax δD predominantly
reflect the isotopic composition of rainfall, varying as a function of
changing moisture source areas and transport path dynamics, whereas seawater
δ18O predominantly reflects monsoonal rainfall and runoff amount.
Nilsson-Kerr et al. (2019) focused on millennial-scale seawater
δ18O and elemental ratio reconstructions of ice volumes during
Termination II (TII; 139–127 ka) in the northeast Indian margin at Site U1446.
They found that the TII is characterized by a transient monsoon
intensification associated with the polar seesaw. The deglacial progression
is characterized first by southern hemispheric warming, then by warming in
the tropics, coincident with monsoon intensification, followed by northern
hemispheric warming. These temporal relationships imply that the monsoon
served as a conduit for the transport of heat across the Equator into the
Northern Hemisphere, promoting deglaciation. This work was followed by a
low-latitude synthesis of MIS 5 (130–70 ka) reconstructions combined with
modeling (Nilsson-Kerr et al., 2021). Results document strong
regional variability beyond that which can be ascribed to simple meridional
migration of the ITCZ. Dipole-like patterns are pervasive across monsoon
regions, highlighting the importance of mechanisms internal to the climate
system, as opposed to monsoon systems responding simply external radiation
forcing.
Expedition 354: Himalayan erosion and tectonics
Expedition 354 recovered long-duration erosion records going back to the
Miocene in the central part of the submarine fan (Fig. 1). Coring followed a
program of short APC cores, followed by extended core barrel (XCB) and
finally RCB to the base of the section. The 17 holes drilled at Sites
U1449–U1455 penetrated a total of 5167.2 m sub-seafloor. Coring spanned
2889.7 m of this penetration and recovered 1727.12 m of sediment and rocks
(60 % average recovery). This expedition was designed to retrieve a
complete record of turbidite deposition spanning the Neogene, together with
more detail over the Pleistocene. The main objectives were to provide a
record of erosion, both in terms of distribution and rate across the
Himalayan arc, and a record of continental monsoon precipitation and vegetation
and to estimate the impact of Himalayan erosion on the global carbon cycle.
The E–W transect approach employed allowed the migration of the depocenter
through time to be reconstructed and is the basis for reconstructing the
paleo-erosion record (Fig. 5).
(a) Interpreted seismic section of the Bengal fan at 8∘ N (Schwenk and Spieß, 2009) with position of the
Expedition 354 sites. The upper fan comprises a stacking of channel levee
systems, with alternations of high accumulation of fan deposits and hemipelagic intervals that are 2 orders of magnitude lower. The easternmost Site U1451 drilled ∼ 1200 m on the west flank of the 90∘ E Ridge, where the onset of turbiditic deposition at this position has been
dated around 18 Ma. Two other 900 m penetration sites (U1550 and U1455)
complete the Neogene record. (b) Mass accumulation rates for the three deep records of Expedition 354. Age models are from France-Lanord et al. (2016), Lenard et al. (2020a), Cruz et al. (2021),
and Reilly et al. (2020).
Turbidites of the Bengal fan
Sediments retrieved at all sites of Expedition 354 are dominated by
turbidites composed of massive sand lobes and silty sand to clayey silt
turbidites deposited by channel levee systems (Adhikari et al., 2018).
Hemipelagic, calcareous clay layers mark intervals with no turbidite
deposition. Sand deposition is estimated to represent up to 60 % of the
sediments in the Pleistocene section (Bergmann et al., 2020) and
was already widespread during the Miocene, reflecting strong erosion under a
monsoon climate. In the absence of sand recovery in the deeper sections,
high penetration rates and hole instability implied the presence of
unconsolidated sand throughout the Miocene (France-Lanord et
al., 2016). Petrologic and geochemical characteristics of turbidites are
very similar to those of modern Ganga and Brahmaputra river sediments
(France-Lanord et al., 2016; Yoshida et al., 2021). Their Sr–Nd isotopic
compositions and heavy mineral geochronological characteristics further
demonstrate the Himalayan lineage of these detrital sediments (Blum et
al., 2018; Lenard et al., 2020b; Huyghe et al., 2020), making them suitable
archives of how Himalayan erosion has responded to the changing climate.
Erosion of the Himalaya
The geochemical characteristics of the Bengal fan turbidites demonstrate
their Himalayan origin. While this was known from earlier Deep Sea Drilling
Project (DSDP) and ODP records (Galy et al., 2010) or
Pleistocene cores (Hein et al., 2017; Joussain et al., 2017), for the first Expedition 354 allows for the time study of a complete Neogene to Holocene
record, with minimum gaps in deposition and sediment transport bias. In
addition, the abundance of sand layers and their efficient recovery by
the half-length APC allowed for the sampling of up to 1 kg of sand and so access
to dense mineral extractions for thermochronology or large samples of quartz
grains for cosmogenic isotopes. Such methods are critical to reconstructing
erosion and seeing how this related to monsoon intensity.
The distribution of U–Pb ages of detrital zircon reveals that in addition to
the Himalaya, supply by erosion of the Asian plate, north of the
Indus-Yarlung suture, was already as significant during the early Miocene, as
it is today via the southern Tibetan connection of the Yarlung Tsangpo to
the Brahmaputra (Blum et al., 2018). A multi-proxy reconstruction of
provenance and exhumation rates employed apatite and rutile U–Pb, mica
Ar–Ar, and zircon fission-track data. For sediments older than 10 Ma, the rutile
and zircon fission-track thermochronometry shows lag times between cooling
and sedimentation that imply derivation from the Greater Himalaya, which
were exhuming rapidly from 17 to 14 Ma, but then these slowed. Over the
interval 5.6–3.5 Ma, lag times shortened to < 1 Myr, and only these
are found since that time (Najman et al., 2019). This implies a
speeding up of erosion since 5.6 Ma, especially from the eastern syntaxis
centered on Namche Barwa. Najman et al. (2019) ascribe variations in
erosion to tectonic forces in the Himalaya and syntaxis rather than the
evolving monsoon climate, although we note that the SAM rainfall likely
peaked around 15 Ma and then weakened in the late Miocene (Clift et al.,
2008; Yang et al., 2020; Molnar and Rajagopalan, 2012).
Apatite fission-track lag times are more stable through time and translate
in erosion rate to 1 to 3 mm yr-1 since the Miocene (Huyghe et al., 2020). Finally, for the first time on IODP cores, quartz in situ cosmogenic 10Be concentrations were measured since 6 Ma (Lenard et al., 2020b). This study reported steady erosion rates of ∼ 1 mm yr-1 since the Miocene, implying that the onset of Pleistocene climate variability had little effect on the erosion regime.
Link to the carbon cycle
Himalayan erosion is potentially a globally significant actor in the carbon
cycle. While silicate weathering is moderate in the drainage basin, the most
important sink for carbon is thought to be via the burial of organic carbon
(Galy et al., 2007). Detailed study of the weathering history is
still in progress, but shipboard data on major element geochemistry and clay
mineralogy already show that Bengal turbidites recovered at 8∘ N
have relatively stable compositions and reflect essentially moderate
intensity of silicate weathering. Overall K / Al ratios (a proxy for
alteration intensity) of turbidites from Miocene to Holocene are similar to
or higher than those of the modern rivers (Lupker et al., 2013),
which indicates comparable to lower weathering conditions relative to today,
consistent with results from the Arabian and South China seas
(Clift and Jonell, 2021a). Similarly, clay mineral assemblages
are dominated by illite and chlorite, which derives from physical erosion,
not chemical weathering.
Shipboard organic carbon data confirm the general negative relationship
between grain size and TOC in turbidites. Previous studies on Bengal fan
turbidites demonstrated that they are dominated by organic matter exported
from land and also carry indications of the evolution of vegetation.
Shipboard data indicate that the organic carbon loading of the turbidites is
slightly lower than observed in recent sediment in the northern part of the
fan (Galy et al., 2008). However, a number of samples carry millimeter to
centimeter organic particles that locally lead to very high organic carbon
concentrations (Lee et al., 2019). These are derived from
wood debris, whose δ13C composition can be used to constrain
the dominant vegetation. Miocene δ13C values have a mean of
-26.6 ‰, indicating C3 dominance, but from about 4 Ma to
present, the δ13C of the wood is -20.5 ‰,
which is 3.3 ‰ more positive than the most 13C-enriched sample during the Miocene. This suggests a mixture of C3 and C4 fragments. More 13C-enriched values appear since 1 Ma, where
δ13C wood values are bimodal with a C4-like cluster (mean =-13.1 ‰) as well as the typical C3-like values (mean =-26.3 ‰). This suggests a change in the ecosystems
from which the wood is being exported, with one-third of the wood derived
from C4 plants in the last 1 Myr. In addition to the permanent transfer of organic carbon with fine grained particles, low-frequency wood export
contributed significantly to the carbon burial in Bengal turbidites.
On shorter timescales, Weber et al. (2018) investigated the role of
orbital forcing in monsoon rainfall since 200 ka at Site U1452. The
variability of TOC, total nitrogen, and the δ13C composition of
organic matter was used to indicate the marine origin of the organic matter,
and this showed that primary marine productivity likely increased during
times of enhanced NE monsoon during glacial periods. At the same time, there
was faster delivery of sediment to the Bay of Bengal caused by higher soil
erosion on land. Similarities between the sediment record and the Antarctic
climate record spanning multiple glacial cycles imply a close relationship
between high-southern-latitude and monsoonal Asian climate driven by shifts
in position of the ITCZ.
South China Sea: chemical weathering and fluvial runoff
Since the first scientific oceanic drilling in 1999 (ODP Leg 184) in the
South China Sea which focused on the theme of “Exploring the Asian
monsoon”, four more IODP expeditions (349, 367, 368, and 368X) have been
completed since 2014. This later campaign was designed to examine the
tectonic evolution of the South China Sea (Sun
et al., 2018; Li et al., 2015). Nonetheless, these new expeditions recovered
long sequences of sediment that can be used to study the Asian monsoon over
geologic timescales.
During Expedition 349, a total of 703 and 611 m of sediment/sedimentary
rock were recovered at the two deepest sites, Site U1431 in the central east
subbasin and U1433 in the southwest subbasin (Li et al.,
2015). Sedimentary magnetic parameters (magnetic susceptibility and ARM, anhysteretic remanent magnetization) and hematite / goethite values of
sediment from Hole U1431D were used to infer EAM variation since 6.5 Ma
(Gai et al., 2020). The magnetic results indicate that the EAM
was stable between 6.5 and 5.0 Ma and intensified between 5.0 and 3.8 Ma,
possibly due to closure of the Central American Seaway, and then weakened
gradually starting after 3.8 Ma in response to the onset of Northern
Hemisphere glaciation (NHG) and global cooling.
Proxies from land and sea showing the evolution of global climate
and East Asian summer monsoon since 25 Ma. (a) Global deep-sea δ18O (Westerhold et al., 2020); (b) chemical index of alteration at ODP Site 1146 (Wan et al., 2010) and 1148
(Wei et al., 2006) in the northern South China Sea; (c) seawater
δ18O at ODP Site 1146 (Steinke et al., 2010)
and IODP Site U1501 (Yang et al., 2021), as well as NGR
intensity at Site U1501 (Jian et al., 2018); (d) hematite / goethite ratio at ODP Site 1148 (Clift et al., 2008); (e) magnetic susceptibility of loess–paleosol sequences (Ding et al.,
1999; Qiang et al., 2011); (f) pollen-based humidity index from Liupan Mountain (Jiang and Ding, 2008); (g) pollen assemblage evolution at Xining Basin (Sun and Wang, 2005). All drilling sites are
located in the northern South China Sea on the continental margin.
Pollen from IODP Site U1433, mainly derived from the Mekong River, shows a long-term increase in herbaceous plants since 8 Ma and indicates a persistent
drying and weakening of precipitation in former Indochina (Miao et
al., 2017). Such an observation parallels trends in chemical weathering and
hematite proxies at the same site (Liu et al., 2019).
Interestingly, it remains debated as to whether the East Asian summer monsoon
(EASM) intensified or weakened since the late Miocene (see synthesized
proxies in Fig. 6). For example, magnetic susceptibility of loess–paleosol
sequences suggests a weaker EASM during the Miocene–early Pliocene relative
to late Pliocene–Pleistocene (An et al., 2001; Qiang et al., 2011; Zhao
et al., 2020). In contrast, pollen assemblages in North China (Jiang and
Ding, 2008; Sun and Wang, 2005) and weathering (Clift et al., 2014; Wan
et al., 2010; Wei et al., 2006) and paleoceanographic proxies in the South
China Sea (Holbourn et al., 2018; Steinke et al., 2010; Holbourn et al.,
2021) show a general weakening EASM since the middle Miocene, possibly
linked to global cooling.
Although IODP Expedition 368 recovered long sediment cores at Sites U1501
and U1505 on the continental margin in the NE South China Sea, most related
studies are still on-going. However, at Site U1501 a study of seawater
δ18O and Mg / Ca ratios in planktic foraminifera has been
completed. Differences between surface and thermocline records can be used
to track the thermal gradient between the surface and subsurface waters, and
the results imply that upper-water mixing was weaker at 9.4–7.3 Ma, which
may have related to increased fluvial runoff linked to higher rainfall.
These data also suggest a decrease in the intensity of EASM between 13.6 and
10.2 Ma and an increase during 10.2–7.3 Ma (Yang et al., 2021).
The trend in seawater δ18O at Site U1501 is a little different
from previous study at nearby ODP Site 1146 (Fig. 6), which shows a rapidly
decreasing sea surface salinity (SSS) and weakening of EASM since 7.5 Ma (Steinke et al., 2010;
Holbourn et al., 2018). In any case, a similar long-term decreasing trend in
the intensity of NGR (indicative of terrigenous clay input) is observed at
both sites (Jian et al., 2018), consistent with a coupled
evolution of continental erosion and monsoonal rainfall. This record implies
that late Miocene rainfall of South China might have become wetter, while that in
South Asia was drying after 7 Ma, a discrepancy that Yang et al. (2021) linked to formation of the WPWP at this time influencing
East Asia, while Indian Ocean surface water cooling and Tibetan uplift were
more influential in South Asia.
Sea of Japan: paleoceanography and Asian dust records
Expedition 346 targeted the upper Miocene to Holocene hemipelagic sediments
of the Sea of Japan (East Sea of Korea) and the northern East China Sea (ECS) (Fig. 7). Seven sites were drilled in the Sea of Japan, with two closely spaced
sites in the ECS. In total, the expedition recovered 6135.3 m of core by
the APC, with an average recovery of 101 %.
Bathymetric map of the area sampled by Expedition 346 with the
major bathymetric features labeled as well as the locations of the drilling
site. The red arrows show the major surface currents that affect the region.
Modified after Tada et al. (2015).
The primary objective was to explore the timing of onset and evolution of
millennial- and orbital-scale variabilities of the EAM based on the
hypothesis of Tada et al. (1999, 2015) that millennial- and
orbital-scale variabilities of the EAM were recorded in the hemipelagic
sediments of the Sea of Japan. These deposits are characterized by centimeter-
to decimeter-scale alternations of dark and light layers modulated by the
fresh-water discharge of the Yangtze River during summer that diluted the
surface water of the northern ECS and modulated the SSS and nutrient
concentration of the ocean water flowing into the Sea of Japan through the
Tsushima Strait. Changes in SSS and nutrient content of the ocean water
flowing into the Sea of Japan caused changes in ventilation and surface
productivity of the sea.
Clemens et al. (2018) analyzed δ18O and Mg / Ca ratios
of G. ruber at Site U1429 in the northern ECS and reconstructed δ18Osw, which reflects runoff-induced changes in SSS, during the last 400 kyr. They
demonstrated that local SSS changed in association with eccentricity and
obliquity cycles but not with the precession cycle, although precession is
clearly evident in the planktonic δ18O. This contrasts the work
of Cheng et al. (2016), who demonstrated that δ18O of Chinese
stalagmites shows a strong precession signal, with almost no evidence of
eccentricity and obliquity. Hence, the extent to which local precipitation
is reflected in stalagmite δ18O remains in question. Clemens et al. (2018) also demonstrated the presence of millennial-scale
variability of δ18Osw, which Kubota et al. (2019)
interpreted to reflect changes in precipitation of EASM in association with
Dansgaard–Oeschger cycles during the last glacial period. Kubota et al. (2019) also demonstrated that δ18Osw changes in
the northern ECS are closely associated with the changes in the gray scale
of the sediments in the deeper part of the Sea of Japan, consistent with the
hypothesis of Tada et al. (1999, 2015).
Irino et al. (2018) revised shipboard splices and constructed a
complete, continuous dark–light sedimentary sequence at the seven sites
drilled in the Sea of Japan, covering the last 3 Myr. Tada et al. (2018) used
this sequence to examine centimeter- to decimeter-scale dark layers for the
six sites deeper than ∼ 900 m water depth. They confirmed that
it is possible to correlate almost all of the dark layers between the six
sites in the deeper part of the basin, which could be traced back to 1.45 Ma
when the first distinct dark layer was deposited. It was concluded that the
Sea of Japan has responded to the orbital- and millennial-scale climatic
changes as a single system since 1.45 Ma and that intermittent occurrences
of millennial-scale variability of EAM can be traced back to at least 1.45 Ma. Based on XRF core scanning, Seki et al. (2019) demonstrated that gray-scale variation of Sea of Japan sediments basically reflects marine organic
carbon content and so in turn reflects millennial-scale variability of the
surface productivity.
Tada et al. (2018) also demonstrated that gamma ray attenuation density (GRA)
is controlled by diatom content and that this changes in association with
glacio-eustatic sea-level changes. Using this relationship, they constructed
an orbitally tuned age model covering the last 3 Myr. Kurokawa et al. (2019) extended this age model back to 11.7 Ma, allowing for precise
dating of paleoceanographic events across the basin. For example, they
identified the occurrence of a hiatus from 7.3 to 5.3 Ma at Site U1330 on
the South Korean Plateau, at water depths of 1072 m. It is possible that
this hiatus was caused by intensification of the Sea of Japan Intermediate
Water during the late Miocene global cooling (LMGC) interval from 7.8 to
5.8 Ma (Herbert et al., 2016).
Matsuzaki et al. (2020) examined radiolarian assemblages at Site U1425
in the middle of the Sea of Japan spanning the time interval 9.1–5.3 Ma and
used these to reconstruct annual mean sea surface temperature (SST). They
found a drastic decrease in annual SST from 24 to 16 ∘C from 7.9 to 6.6 Ma, which they attributed to
intensification of East Asian winter monsoon (EAWM) during the early half of
the LMGC. Based on comparison with NW Pacific high-latitude and midlatitude
alkenone-based SST (Herbert et al., 2016; LaRiviere et al., 2012), they
speculated that the later half of LMGC is characterized by summer cooling.
Shen et al. (2018) analyzed δ13C of black carbon of
probable eolian origin in sediments from Site U1430 and found a drastic
increase in δ13C that started at ∼ 5.3 Ma. They
argued that this increase most likely reflects expansion of C4 vegetation in Central Asia. However, they used a preliminary age model which did not take into account the hiatus from 7.3 to 5.3 Ma. According to a new age model of Kurokawa et al. (2019), the drastic increase in δ13C
occurred during the hiatus between 7.3 and 5.3 Ma. It is likely that
expansion of C4 vegetation in Central to East Asia occurred in association with LMGC.
Anderson et al. (2020) reconstructed the provenance of
aluminosilicate sediment at Site U1430 since 12 Ma using multivariate
partitioning of the major, trace, and rare earth element composition of bulk
samples. They identified four aluminosilicate components (Taklimakan, Gobi,
Chinese Loess, and Korean Peninsula) and demonstrated that the Taklimakan
Desert component was the most abundant component before 7.5 Ma, whereas Gobi
+ Chinese Loess components became dominant by 4 Ma (and maybe as early as
7.5 Ma when taking into account the hiatus between 7.3 and 5.3 Ma) (Fig. 8). Accumulation rates of these dust components were relatively high before 7.5 Ma and very low between 4 and 2 Ma and increased again after ∼ 2 Ma. It is possible that expansion of the Gobi Desert and Chinese Loess Plateau occurred in association with LMGC. Because sedimentation at Site U1430 was influenced by bottom current winnowing from 7.5 Ma to as young as 3.5 Ma, similar provenance studies should be conducted at other deeper-water sites to evaluate dust flux during the time interval between 7.5 and 1.5 Ma.
Aluminosilicate contribution (wt % of aluminosilicate inventory) at Site U1430 through time. The end-member contributions are plotted as the total sum, with each color representing the sum of the specific end-member plus the end-members to the left. The shaded region denotes a hiatus, and the following box notes a period in which we did not overinterpret our data to fill the temporal gap between the end of the hiatus and the start of our Miocene-aged samples. From Anderson et al. (2020).
Results of Expedition 346 proved that dark and light alternations of the
Quaternary sediments from the Sea of Japan faithfully recorded millennial-scale
variability of EASM. Changes in salinity and nutrient concentration of the
influx through the Tsushima Strait into the Sea of Japan since 1.45 Ma
controlled surface productivity and ventilation in the sea so that
millennial-scale variability of EAM can be traced back to that time. The
work showed that distinct precession signals recorded in δ18O
of Chinese stalagmites were not caused by local precipitation changes of
EASM but more likely reflecting changes in the δ18O of
precipitation, whereas millennial changes in δ18O of Chinese
stalagmites probably reflect changes in EASM precipitation. It is highly
likely that EAWM intensified during the early half of the LMGC. On the other
hand, EASM seems to have been weakened later during LMGC. It is possible
that Gobi Desert expanded and C3 to C4 transition of vegetation occurred in Central to East Asia in association with the LMGC. However, the presence of a hiatus from 7.3 to 5.3 Ma in the sedimentary record of Site U1430 precludes precise dating of their timings. Similar examinations at Site 1425 are desirable.
Western Australian monsoon
North Australia is influenced by strong summer westerly and southwesterly
winds that source warm, moist equatorial air, leading to monsoonal rains and
cyclonic activity north of the monsoon shear line (Fig. 9). Seasonal
monsoonal runoff delivers substantial amounts of fluvial sediment to the
shelf via the Fitzroy, De Grey, Ashburton and Fortescue rivers. In contrast,
continental wind-blown dust is transported by the trade winds offshore
northwest Australia when the trade winds dominate during the winter dry
season (Fig. 9).
(a) Atmospheric circulation for January and July (Gentilli, 1972) with the mean monsoon shear line (McBride, 1986) and intertropical convergence zone (ITCZ). Base map adapted from the General Bathymetric
Chart of the Oceans (GEBCO) http://www.gebco.net (last access: 29 May 2022). IODP Expedition 356 sites are shown as stars. (b) Plate tectonic motion of Australia since 50 Ma at 10 Myr intervals, with the path line for Site U1459 (Expedition 356). Figure adapted from Gallagher and deMenocal (2019).
IODP Expedition 356 cored seven sites to determine the latitudinal variation
in climate and ocean conditions from ∼ 30 to 18∘ S over the last 5 myr (Gallagher et
al., 2017). The expedition recovered 5185.15 m of core, with 62 %
recovery. Sites were cored in shelf regions near to the shore to determine
the long-term history of the Australian monsoon and southwestern Australian
climate. Many sites yielded older sections, revealing a climate archive
extending as far back as ∼ 50 Ma. For example, Sites U1461 and
U1462 on the NW continental shelf at ∼ 22∘ S yielded
thick (up to 1 km) sections of upper Miocene to Recent strata that record
the southerly extent of the Australian monsoon and its intensity. Further
north, Sites U1463 and U1464 (∼ 18∘ S) cored middle
Miocene to Recent sections with contrasting facies ranging from Miocene
subaerial, arid, sabkha evaporitic facies (Groeneveld et al., 2017;
Tagliaro et al., 2018), and Pliocene deep-water carbonates (De
Vleeschouwer et al., 2018) to Quaternary aridity-related oolite facies
(Gallagher et al., 2018). These cores provide an unprecedented
opportunity to investigate the long-term history of the Australian monsoon
and aridity.
Previous studies suggested that at 23–14 Ma, Australia experienced
seasonally wetter, monsoonal rainfall compared to today when the monsoonal
front was in a similar position (Herold et al., 2011).
However, analyses of sediments from Site U1459 show that arid conditions
persisted from 16 to 6 Ma (Groeneveld et al., 2017), transitioning to a
wetter period with all-year-round rainfall at ∼ 5.5 Ma (Site U1463) (Christensen et al., 2017; De Vleeschouwer et al., 2018; Auer et
al., 2019) that became seasonal (monsoonal) starting at ∼ 3.3 Ma. The drying trend exhibited in Miocene NW Australia is broadly similar to
that seen in the Mekong basin, and there may be a relationship between the
northward migration of the westerlies from ∼ 12 Ma associated
with expanded Antarctic sea ice and its abrupt shift back associated with
LMGC (Groeneveld et al., 2017). The onset of humidity in NW Australia
occurred seemingly differentially, first at the more northerly Site U1464
∼ 6 Ma (Groeneveld et al., 2017) and later at Site U1463 at
∼ 5.5 Ma (Christensen et al., 2017). However,
Karatsolis et al. (2020) suggested the region was humid, probably since
∼ 7 Ma when the ITCZ moved southward. NW Australia remained in
the Humid Interval until ∼ 3.3 Ma (Christensen et al., 2017) when conditions became drier, although a major SST drop from ∼ 3.15 to 3 Ma based on TEX86 indicates temperature-driven drying may have begun slightly later (Smith et al., 2020). NW Australia achieved arid conditions
with a strong winter monsoon similar to today by ∼ 2.4 Ma at
the onset of the Arid Interval (Christensen et al., 2017) when
higher-amplitude interglacial–glacial fluctuations in SST
(Smith et al., 2020) led to a seasonal (monsoonal) regime.
Over the last 2 Myr, interglacial wetter (strong monsoon with clay influx)
and arid glacial (weak monsoon with limestone facies and dust input)
conditions persisted in Australia's northwest (Gallagher and
deMenocal, 2019). Evidence of the Holocene Australian summer monsoon (ASM)
activity has been interpreted at Site U1461 from K / Ca ratios, with the
percentage of potassium constrained from shipboard NGR (Ishiwa
et al., 2019). These data show increased fluvial terrigenous input after
11.5 ka, followed by a maximum at ∼ 8.5 ka due to enhanced
ASM-derived precipitation as a response to the southern migration of the
ITCZ. Subsequently, weakening of rainfall after 8.5 ka was caused by the
northern migration of the ITCZ.
Western Pacific Warm Pool
As a major source of heat and moisture to the atmosphere, the WPWP, often
defined by the 28 ∘C isotherm and located in the heart of the
Indo-Pacific Warm Pool, exerts a major role in influencing climate both in
the tropics and globally. Changes in the SST of the WPWP influence the
location and strength of convection in the rising limb of the Hadley and
Walker cells, affecting planetary-scale atmospheric circulation, atmospheric
heating, and tropical hydrology, including the Asian and Australian monsoons
(Neale and Slingo, 2003; Wang and Mehta, 2008). Likewise, an
important control on the WPWP and east Asian hydroclimate is the change in
the equatorial Pacific zonal and Equator to pole temperature gradients. A
primary goal of Expedition 363 was to assess changes in regional climate
variability, expressed in temperature, precipitation, and biological
productivity in the context of the global background state from the middle
Miocene through the Holocene.
Currently, there is an ongoing debate about the evolution of SST in the WPWP
since the late Miocene, due to substantial disagreement between proxy
records, whereas foraminiferal Mg / Ca suggests stable mixed-layer
temperatures throughout this period; records of the organic proxy
TEX86, interpreted to reflect SST, argue for a major cooling throughout
with a similar magnitude to the change observed in the eastern equatorial
Pacific (Zhang et al., 2014b; Ravelo et al., 2014; Zhang et al., 2014a).
These two contrasting scenarios of the evolution of the equatorial Pacific
zonal temperature gradient have very different implications on the
hydroclimate of the Indo-Pacific Warm Pool and likely the Asian monsoon
system. However, using samples from IODP Site U1488, Meinicke et al. (2021) show very good agreement between measurements of mixed-layer and thermocline planktic foraminiferal Mg / Ca and clumped isotopes (Δ47), two independent foraminiferal proxies of temperature, thereby supporting the view that the WPWP mixed-layer temperatures did not cool substantially since the early Pliocene, while subsurface temperatures cooled more strongly, a change analogous to a shift from El Niño-like to more La Niña-like conditions, which could have intensified regional precipitation in the WPWP and east Asia.
Seasonal to interannual climate variations in the WPWP are dominated by
fluctuations in precipitation associated with the seasonal march of the
monsoons, migration of the intertropical convergence zone (ITCZ), and
interannual changes associated with variability of the El Niño–Southern
Oscillation (ENSO). It has been argued that on orbital timescales, the ITCZ
and associated tropical precipitation belt migrate from a northern- to
southern-centered position, relative to the Equator. These hemispherically
asymmetric shifts are in pace with orbital variability. In contrast, new XRF
geochemical records from Site U1483 and nearby sites on the northwestern
Australia margin, when compared with published precipitation records from
the WPWP, suggest that precipitation changed nearly in phase between the two
hemispheres on the precession band, arguing for expansion and contraction in
the latitudinal extent rather than migration of the tropical precipitation
belt (Zhang et al., 2020). Furthermore, XRF records from other
sites further to the south on the Australian, including Site U1482, reveal
shorter periods of maximum Australian monsoon in the early Holocene
(∼ 10 ka), MIS 5e (∼ 130 ka), MIS 7
(∼ 200, ∼ 220, and ∼ 240 ka), and
MIS 9 (∼ 280, ∼ 305, and ∼ 330 ka),
when maxima in atmospheric greenhouse gases coincided with maxima in
Southern Hemisphere insolation (September). The intensification of the
regional monsoon is attributed to intensely heated low-pressure cells over the Pilbara region that trigger the southward shift of the ITCZ
(Pei et al., 2021). When compared with sites recovered in the
other expedition, it is clear that the ongoing research on sites recovered
during Expedition 363 will be important in testing hypotheses related to
regionality and globality of the monsoon system, both on orbital and shorter
timescales.
Compilation and comparison of monsoon related proxies from across the Asia–Australia region. (a) Hematite content from Laxmi Basin measured by 565 nm spectral analysis (Zhou et al., 2021), (b)δ13C of wood from Bengal fan sediments (data from Lee et al., 2019), (c) source of wind-blown sediment in the Sea of Japan (Anderson et al., 2020), (d) hematite / goethite relative abundance tracked by 565/435 nm ratio from spectral analysis from northern South China Sea (Clift et al., 2008), (e) hematite / goethite relative abundance tracked by 565/435 nm ratio from SW South China Sea (data from Liu et al., 2019) and, (f) K (%) contents from western Australia acting a proxy for fluvial run-off at Site U1463 in NW Australia (data from Christensen et al., 2017) and Site U1459 in SW Australia (data from Groeneveld et al., 2017).
Synthesis
Comparison of the monsoon records in the different drilling areas targeted
in this campaign indicates drying trends in most parts of Asia since
∼ 10 Ma, which contrasts with the NW Australian wet phase at
5.0–2.5 Ma. However, this area too shows a trend to drier environments
after 2.5 Ma (Fig. 10). Decoupling of the Australian and Asian monsoons
reflects the greater influence of the ITF over regional climate in the
Southern Hemisphere. Even within Asia, drying of the continent has not
occurred in a uniform fashion. While most areas have seen increasing aridity,
southern China appears to have become wetter, possibly due to migration of
the ITCZ northwards since the late Miocene (Liu et al.,
2019). Drying trends elsewhere are not synchronous. In the Indus Basin of SW
Asia, drying started after 10 Ma, although the major change in vegetation to
being C4-dominated after around 7.2–7.4 Ma was not linked to changing
rainfall but rather cooling of the Arabian Sea (Feakins et al.,
2020). Drying started later, after 5 Ma in former Indochina and especially after 4 Ma in the Ganges Basin, although carbon isotope evidence from the Himalayan
foreland basin indicates that eastern parts of the basin have essentially
never made the C3 to C4 transition (Vögeli et al., 2017).
Summary of the temporal coverage now available across the
marginal seas of monsoonal Asia, distinguishing between carbonate, biogenic, and oceanic sites and those comprising clastic sediments with records of
continental erosion and weathering. Earliest monsoon age is from Licht et
al. (2014). India–Eurasia collision age is from Najman et al. (2010). Greater Himalayan exhumation is from Godin et al. (2006).
In central Asia, the Taklimakan Desert formed in the early Miocene
(Zheng et al., 2015), but further east desiccation of the Chinese
Loess Plateau appears to have occurred most strongly in the Pliocene based
on records from the Ulleung Basin (Anderson et al.,
2020). This migration in aridity may reflect the progressive northeastward
growth of the Tibetan Plateau, starting in the Eocene (Ji et al.,
2017) but with further uplift at 25–16 Ma and after 10 Ma
(Wang et al., 2022). Lack of correlation between SAM and EAM
is consistent with climate models that tie the SAM more closely with the
height of the Himalayan topographic barrier (Boos and Kuang, 2010) when
considering tectonic (> 106 years) timescales, while the EAM is
influenced more by the height and extent of the Tibetan Plateau and the
WPWP, at least by some studies (Tada et al., 2016). Recent climate
models emphasize how the northern expansion of the plateau has increased
rainfall in East Asia, especially during the drier winter season (Li et
al., 2021), at the same time that northeast Tibet and northern China dried
(Jiang et al., 2008; Zhang et al., 2021). In general the topography of
Asia, including the Iranian Plateau, and even East Africa, acts to steer
moisture inland and to focus precipitation, while the monsoon circulation
itself reflects seawater temperature gradients (Acosta and
Huber, 2020; Bordoni and Schneider, 2008). Thus, on shorter timescales when
orbital processes dominate there is a tendency for the EAM and SAM to wax
and wane together, as the surrounding oceans warm and cool.
The weakening monsoon is more generally caused by progressive global cooling
since the early Miocene, although the results from the new drilling now
indicate that this is not driven by more chemical weathering flux caused by
faster erosion in either the western (Clift and Jonell, 2021b)
or eastern Himalaya (France-Lanord et al., 2016). Instead, the
erosion of areas such as New Guinea under the influence of the Australian
monsoon may be critical (Macdonald et al., 2019), adding to the
effect of enhanced organic carbon burial (Galy et al., 2007).
However, earlier cooling may yet be related to Himalayan uplift in the
Eocene–early Miocene. The improved monsoon records clearly show that a
simple link between summer rains and topography in Asia is not viable, even
assuming the latter can be reconstructed, but must instead involve
additional tectonic and climatic processes.
Furthermore, combined records from across the region provide powerful tools
for understanding monsoon in a holistic way when combined with onshore
records. It is clear that proxies related to oceanographic productivity do
not simply track those of continental rainfall. Monsoon rainfall in South
Asia strengthened in the early Miocene (Clift and Webb, 2019),
while the upwelling records show monsoon winds starting after 13 Ma
(Betzler et al., 2016), linked to the stronger winds of
the Somali Jet. This is related to the Iranian and African topographic rise,
while Arabian Sea upwelling increased as the Arabian Peninsula became more
continental (Sarr et al., 2022). Wind strength and/or
stratification of ocean waters are key to controlling biological production
but are less important to supplying moisture to inland Asia. Warmer oceans
result in greater moisture supply, but the evolving topography diverts this
in different directions and focuses precipitation into different areas.
While some areas dry, others receive more rainfall, independently of how the
oceans evolve. Measuring continental rainfall is not easy and is probably
best tracked within limited drainage basins via the evolving vegetation
(i.e., C3 to C4 balance) coupled with δD measurements. Chemical
weathering proxies can play a part but are impacted by both temperature and
seasonality, as well as precipitation, so that they must be applied with
caution and over limited areas (cf., the contrasting evolution in
environments in former Indochina, southern China, and the Chinese Loess Plateau).
Future targets
While the monsoon drilling campaign has significantly advanced our
understanding of how this climate system has evolved, there remain
significant gaps in our comprehension. The almost total lack of Paleogene
sedimentary archives is especially noteworthy (Fig. 11). This is a serious
shortcoming because few of the marine sections even reach the time of
Greater Himalayan exhumation, when monsoon intensification likely happened,
let alone the older sections spanning the time of earliest monsoon activity.
Climate modeling (Farnsworth et al., 2019) supports sparse
terrestrial data (Licht et al., 2014; Sorrel et al., 2017) that indicate
the monsoon having initiated in the late Eocene or even earlier, but to date
few matching marine records have been cored, although suitable sequences
would be accessible in the distal Bengal fan, along the Owen Ridge in the
western Arabian Sea, and potentially in the Southwest Indian Margin.
Understanding the evolving continental topography is important when trying
to quantify the links between monsoon histories and mountain building in
Asia. As part of this effort there has been considerable research done on
the evolving river systems of SE Asia which are sensitive to the growth of
topography in the eastern Tibetan Plateau. Large-scale drainage capture has
been associated with plateau uplift and the re-tilting of east Asia towards
the east during the Cenozoic (Wang, 2004). Such studies are
hamstrung onshore because of difficulties in constraining sedimentation
ages and the lack of long-duration, semi-continuous records. The river
systems of SE Asia, especially the Irrawaddy and Mekong, are noteworthy in
being central to this debate but with few known sections that record their
development studied to date. Drilling in the delta or fan of these systems
would help resolve debates about drainage capture while also providing
environmental records for SE Asia.
The published and ongoing studies of IODP Expedition 359 Maldive cores show
that deposits surrounding carbonate platforms, in particular carbonate
drifts, bear a previously underestimated potential for the understanding of
the monsoon evolution on million-year timescales but also over shorter
intervals and would add substantial knowledge to monsoon fluctuations at all
timescales. Potential targets for further research on this topic exist in
the Laccadives, the Mascarene Plateau, or the South China Sea platforms.
Data availability
Data presented in this paper are available in the
articles cited throughout.
Author contributions
PDC led the overall writing, wrote the section about
the Indus fan, and edited the final paper. CB and GPE wrote the section about the Maldives. SCC, CFL, AH, and WK wrote the sections on the Bay of Bengal and Bengal fan. SW and AH wrote the section on the South China Sea. BC, YR, AH, and SG wrote the sections about the Australian monsoon and western Pacific Warm Pool. RWM and RT wrote the section about the Sea of
Japan. All authors contributed to the synthesis and future plans.
Competing interests
The contact author has declared that neither they nor their co-authors have any competing interests.
Disclaimer
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Acknowledgements
We thank Carl Brenner and Angela Slagle at USSSP for encouraging the writing of this synthesis. The manuscript benefited from reviews by Rebecca Robinson and Clara Bolton.
Review statement
This paper was edited by Will Sager and reviewed by Clara Bolton and Rebecca Robinson.
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