The Xingu River is a large clearwater river in eastern Amazonia and its downstream sector, known as the Volta Grande do Xingu (“Xingu Great Bend”), is a unique fluvial landscape that plays an important role in the biodiversity, biogeochemistry and prehistoric and historic peopling of Amazonia. The sedimentary dynamics of the Xingu River in the Volta Grande and its downstream sector will be shifted in the next few years due to the construction of dams associated with the Belo Monte hydropower project. Impacts on river biodiversity and carbon cycling are anticipated, especially due to likely changes in sedimentation and riverbed characteristics. This research project aims to define the geological and climate factors responsible for the development of the Volta Grande landscape and to track its environmental changes during the Holocene, using the modern system as a reference. In this context, sediment cores, riverbed rock and sediment samples and greenhouse gas (GHG) samples were collected in the Volta Grande do Xingu and adjacent upstream and downstream sectors. The reconstruction of past conditions in the Volta Grande is necessary for forecasting future scenarios and defining biodiversity conservation strategies under the operation of Belo Monte dams. This paper describes the scientific questions of the project and the sampling surveys performed by an international team of Earth scientists and biologists during the dry seasons of 2013 and 2014. Preliminary results are presented and a future workshop is planned to integrate results, present data to the scientific community and discuss possibilities for deeper drilling in the Xingu ria to extend the sedimentary record of the Volta Grande do Xingu.
The Xingu River is the third largest tributary of the Amazon River and the second largest clearwater river system in South America. Its downstream reach comprises an anomalous bedrock anastomozing system (Wohl and Merritt, 2001) known as the Volta Grande do Xingu (“Xingu Great Bend”). The channel morphology of the Xingu River in the Volta Grande is characterized by multiple flow-path channels with rapids flowing over fractured basement rocks. The tremendous size and morphological complexity of the rapids, combined with a high variation in water level between the dry and wet seasons, make the Volta Grande do Xingu a unique environment for Amazonian biodiversity (Zuanon, 1999; Camargo et al., 2004; Acselrad et al., 2009; Camargo and Ghilardi, 2009; Nogueira et al., 2010). Compared to other Amazonian rivers, the Xingu River also stands out due to its relatively high spatial and temporal variability in methane emissions (Sawakuchi et al., 2014) and changes in land use as a result of historic and prehistoric settlements (Heckenberger et al., 2003).
The main channel of the Volta Grande has been impounded and will be partially
diverted for operation of the Belo Monte hydropower plant (Fearnside, 2006;
Sousa and Reid, 2010; Sabaj Pérez, 2015), its construction expected to be
complete during the second half of 2016. That project has prompted great
debate surrounding the tradeoffs between energy generation and
socioenvironmental impacts. The seasonal water level variation and flood
pulse in the Volta Grande do Xingu will be determined by the dam's operation
and energy production. Changes in river substrate and loss of environmental
diversity are expected under the Belo Monte scenario. The trapping of
sediments upstream and decrease in the water flow downstream of the
impoundment dam will negatively affect biodiversity through the loss of
various river substrates and benthic habitats. The trapping of fine-grained
sediment in the in-stream and off-stream reservoirs can stimulate the
production of greenhouse gases (GHG). The shift from bedrock/sand to mud
substrates is expected to increase GHG emissions, based on comparisons of
CH
Simplified geological map of the Volta Grande do Xingu and Xingu ria, eastern Amazonia (Bahia et al., 2004). Lithologies: (A) Archean gneisses, granodiorites and granitoids (Xingu complex) and metavolcanic and metasedimentary rocks; (PP) intrusive suites: Paleoproterozoic granites, granodiorites and charnockites; (Ou) Ordovician–Devonian organic-rich shales and sandstones (Trombetas group); (Dm1–Dm2–Du) Middle–Upper Devonian shales, siltstones and sandstones (Urupadi and Curuá groups); (J) Triassic–Jurassic diabase (Penatecaua formation); (K) Alter do Chão formation: sandstones and conglomerates; (EN) Eocene–Neogene undifferentiated sediments and laterite crusts; (Q) undifferentiated Quaternary sediments. The red bar indicates the position of the main Belo Monte dam (Pimental site).
This project comprises a multidisciplinary research team working to improve
knowledge on the origin of the Volta Grande do Xingu, the reconstruction of
its Late Quaternary changes in vegetation, hydrology and biogeochemistry, and
related effects on biodiversity. Additionally, we expect that a better
understanding of the fluvial dynamics of the Volta Grande do Xingu on scales
from hundreds to thousands of years before present will yield valuable
insights to forecast future environmental scenarios. The Belo Monte Dam
complex will directly affect the Volta Grande by flooding approximately
382 km
The Xingu is an Amazonian clearwater river (Sioli, 1985), with a bedload
dominated by fine to coarse sand, low suspended load and neutral to slightly
alkaline waters (pH approximately 7.3). After the confluence with the Iriri
River, the Xingu flows NE and bends 90
This project deals with the characterization of the present sedimentary
dynamics and reconstruction of past environmental conditions of the Volta
Grande during the Quaternary. The description of the present state of the
Volta Grande will provide a reference scenario to track past environmental
changes. It is also a fundamental exercise to evaluate future changes due to
the Belo Monte Dam complex. The reconstruction of paleogeography,
paleovegetation and paleohydrology will shed light on how the Volta Grande do
Xingu achieved its present geomorphological and ecological complexity,
allowing us to evaluate the long-term variability of river dynamics. The main
scientific questions motivating this project are the following.
What is the age of the rapids of the Volta Grande? The rapids of the Volta Grande
do Xingu play a major role in aquatic diversity and endemism, particularly
for fishes. The uplift and exhumation histories of the bedrock channels will
be studied at multiple timescales through cosmogenic nuclides ( What effect did Quaternary precipitation changes have on river sediment
supply? Changes in sediment composition and sedimentation rate through time
will be compared with the precipitation changes in the South American Monsoon
System (SAMS). This will allow us to evaluate the response of
fluvio-hydrological variables such as water turbidity and river level
seasonality to Quaternary climate changes. Age models to constrain
environmental indicators (e.g., geochemistry, environmental magnetism) will
be supported by How did riparian vegetation respond to Holocene climatic and
anthropogenic changes? Various palynological studies carried out in Amazonia
suggest that during the Late Holocene, drought episodes and human
interference in the landscape were non-uniform in time and space, suggesting
that the period may have been unusually dynamic with respect to climate (Bush
et al., 2014). Other studies also point to evidence of increase in frequency
of anthropogenic and climate-related fires, with significant changes in
vegetation (McMichael et al., 2012a, b). Archaeological evidence suggests
that human settlement of the Xingu River catchment dates to the Middle
Holocene (e.g., Silva and Rebellato, 2003). How do the carbon budget and GHG emissions respond to changes in sediment
supply and river level? Riverbed sediment type, suspended sediment
concentration and changes in water depth throughout the year can drastically
influence GHG emissions from Amazonian rivers. The lower Xingu River (ria
sector) has the greatest CH
Field surveys were performed during the dry seasons of 2013 and 2014. Sites
for coring in the Xingu ria were selected based on water depth profiles
coupled with riverbed sediment sampling. Cores were collected in deeper
portions of the channel covered by muddy sediments. Deeper zones of perennial
floodplain lakes and the Xingu ria were the targets for coring, since they
represent accumulation sites characterized by relatively continuous
deposition of fine-grained sediments throughout the year. Eight sediment
cores were retrieved from the Xingu ria and from nearby floodplain lakes.
Sediment cores were collected in water depths from 1 m (floodplain lakes) to
18 m (ria), the latter depths (ria) by divers. The cores were collected
using PVC tubes of up to 6 m in length and percussion into the substrate.
Samples of riverbed bedrock surfaces and sands were collected in rapids and
waterfalls for determination of surface exposure ages, erosion rates and
basement cooling and exhumation history. Parallel surveys by a team of
ichthyologists will allow us to evaluate ecological relationships between
fishes and riverbed characteristics. Floating chambers were used to measure
CO
Sediment cores retrieved in the Xingu ria and floodplain lakes of the Xingu and Iriri rivers.
Sampled sediments from lateral bars in the Volta Grande (Fig. 3a) that were
targeted for cosmogenic
Fluvial sand
The morphology and composition of Xingu River substrates are extremely important for aquatic ecology and biodiversity. The Volta Grande is dominated by substrates consisting of fractured bedrocks, iron oxide crusts, gravels and sands. Iron oxide crusts allow for the formation of complex morphologies in the riverbed. The Xingu ria has a more homogeneous riverbed mainly covered by organic-rich mud, with sand deposition in the ria head and on the shallow marginal portions of the channel. The substrate complexity in the Volta Grande offers more niche space, and is hypothesized to be a driver of diversity among rheophilic fishes in the Volta Grande compared to the Xingu ria. An old and stable system of clearwater and complex braids with rocky rapids might account for the exceptionally diverse fish fauna, especially with respect to lithophilic and rheophilic species like loricarid catfishes (Fig. 4). Furthermore, the clarity of the water may enhance the effects of substrate composition and coloration on the color patterns of fishes, especially lithophilic species. Thus, the concentration of suspended sediments and its variation due to changes in hydrology and vegetation cover may play an important role in fish evolution and ecology. Changes in riverbed complexity and fish diversity will be monitored during operation of Belo Monte dams. Digital elevation models (DEMs) such as SRTM (Farr et al., 2007) and ASTER GDEM (Tachikawa et al., 2011) and bathymetric surveys will provide information for morphometric analysis (Grohmann, 2004; Grohmann et al., 2007) of the Volta Grande region and correlation with niche segregation among fishes.
Diversity of sucker mouth armored catfishes (Loricariidae) in the
Xingu River, with emphasis on lithophilous species. Clockwise from upper
left:
To date, two sediment cores (XC01-2 and XC05) have been opened for
description and sub-sampling. Sediment core XC01-2 was collected in a
floodplain lake under 1 m water depth, and it consists of
The samples at 120 and 84 cm depth from core XC01-2 were prepared for quartz
OSL dating in the Luminescence and Gamma Spectrometry Laboratory of the
Institute of Geosciences of the University of São Paulo. These samples
represent the sand bed (bar top) underlying muddy sediments of the floodplain
lake. Equivalent doses (De) were estimated using the single-aliquot
regenerative (SAR) dose protocol (Murray and Wintle, 2003) in multigrain
aliquots (180–250
Environmental magnetism techniques are used to investigate the formation, transportation, and depositional and post-depositional alterations of magnetic minerals in sediments (Thompson and Oldfield, 1986; Evans and Heller, 2003). Changes in the concentration, grain size and shape of magnetic minerals are related to climate through processes affecting sediment composition and texture, such as weathering conditions, vegetation type and erosion rates in the sediment source areas.
Magnetic susceptibility measurements of core XC05 were performed at the
Paleomagnetic Laboratory of the Institute of Astronomy, Geophysics and
Atmospheric Sciences, University of São Paulo (IAG/USP). Paleomagnetic
specimens from the XC05 sediment core were collected using cubic plastic
boxes (8 cm
Low-field magnetic susceptibility by mass normalized of sediments
and concentration CH
The Xingu ria has lake-like sedimentary dynamics and thereby serves as an
analogous setting for the future of Belo Monte's reservoirs. Many insights
about future changes in GHG emissions can be obtained through the study of
CH
Analyses of pollen, spores (plants and algae), diatoms, sedimentary pigment
degradation units (SPDU) and charred microparticles will be employed from the
Xingu sediments as proxies for the identification of climatic forcing on
forest dynamics versus human manipulation of the local vegetation in close
interval sampling in the sediments. Diatom analysis in conjunction with SPDU
will provide an indirect measurement of former lake levels, as well as
determine possible episodes of marine water incursions into the ria system.
Changes in river dynamics and vegetation will be studied through a proxy
approach applied to the organic particles (charcoal, pollens, spores and
diatoms) in sediment cores. Samples for palynological and diatom analysis
were collected in intervals of 2 cm. Establishment of vegetation dynamics as
influenced by global patterns of climatic change and local human manipulation
of forest elements will also help to trace local surface processes, as they
share a common temporal sequence. Lake levels, inferred from diatom and SPDU
profiles, will allow for a better understanding of water depth variation in
the lake system, and will provide means for a better interpretation of pollen
and spore signals in lake sediments. In general, the combination of different
biological proxies will permit the evaluation of human impact on the local
Late Holocene landscape and the possible occurrence of cultural forests,
which in turn may help explain the distribution pattern of
A comprehensive suite of geochemical analyses will be applied to sediment cores XC01-2 (floodplain lake site) and XC05 (southern Xingu ria site) in order to infer patterns of environmental change for the Xingu region. Downcore variability in organic and inorganic sedimentary components are commonly paired with biological proxy information (e.g., from pollen, diatoms, benthic invertebrates, and fish fossils) in the study of evolving floodplain lake systems (McGlue et al., 2012; Moreira et al., 2013). Together, these data demonstrate how climate or human activities may influence water levels, hydrochemistry, marine connectivity, and ecological relationships. For cores XC01-2 and XC05, inorganic geochemistry will utilize carbonate coulometry and energy dispersive X-ray fluorescence (XRF) measurements collected at a 2 cm interval. Considering sedimentation rates expected for the coring sites, the 2 cm sampling interval is sufficient to capture transitions in depositional processes that may be responding to natural or human modification of the surrounding land surface in the decadal to millennial timescales. XRF provides both major and trace element sediment chemistry, which allows a robust mineralogical model to be constructed and potentially affords new insights into dynamic Holocene limnological processes. Organic geochemistry will focus on elemental analysis and stable isotopes of carbon and nitrogen, which will be used to deduce trends in primary productivity, organic matter preservation, and provenance. The synthetic multi-proxy approach we have adopted will greatly expand paleo-record development for the region, which may hold promise for predicting the response of this unique aquatic ecosystem to future disturbances (Gell and Reid, 2014).
A workshop is planned for 2016 to integrate and discuss results of environmental proxies in core sediments, cosmogenic nuclides, thermochronology data, greenhouse gases, and geological correlations with fish diversity. In addition to its unique ecological-landscape character, the Volta Grande do Xingu is also the first clearwater river threatened by the new round of hydropower expansion in the Brazilian Amazon. Thus, lessons from the Volta Grande act as a reference for evaluation of hydropower projects planned for other analogous rivers like the Tapajós River. Environmental impacts of the Belo Monte dams on the Volta Grande were evaluated based on sedimentation data for short time intervals (a few years), considering the size and complexity of the ecological system. Understanding past changes in hydrology, sedimentation and vegetation in decadal to millennial timescales will support more reliable predictions of future ecological scenarios. The major challenge of the project that will be addressed in the workshop is the integration among studies dealing with different timescales, from the geological evolution of the Volta Grande and its role in fish diversity to modern and future influence of anthropogenic activities on river substrates and GHG emissions. To extend the age of environmental reconstructions, future drilling to obtain deep sediment cores in the Xingu ria system will be discussed. Researchers and students interested in the Xingu project are welcome to join the workshop.
H. O. Sawakuchi and D. J. Bertassoli
performed greenhouse gas sampling and analyses. J. L. Antinao contributed to
rock and sediment sampling for cosmogenic nuclides analyses and is performing
analysis of
We are very grateful for the outstanding support of divers Dani and Ronca, boat pilots Tonho and Nelson and cook Ilma during the field surveys. We thank the Editor Thomas Wiersberg and the reviewers, Antje Schwalb and Hella Wittmann-Oelze, for comments that substantially improved this paper. A. O. Sawakuchi thanks the FAPESP (grant no. 2011/06609-1). G. A. Hartmann thanks CAPES (grant AUXPE 2043/2014) and CNPq (grant 454609/2014-0). F. N. Pupim thanks FAPESP (grant no. 2014/23334-4). C. H. Grohmann is a research fellow of CNPq (306294/2012-5) and is co-funded by a collaborative Dimensions of Biodiversity BIOTA grant supported by grant no. 2012/50260-6, São Paulo Research Foundation (FAPESP), the National Science Foundation (NSF DEB-1241066), and the National Aeronautics and Space Administration (NASA). M. Sabaj Pérez and fieldwork were supported in part by the iXingu project, NSF DEB-1257813. J. F. Savian thanks CNPq (grant 457802/2014-6) and FAPERGS (grant 2329-2551/14-1). Edited by: T. Wiersberg Reviewed by: A. Schwalb and H. Wittmann-Oelze