The Argentine Continental Margin (ACM), one of the largest margins worldwide, is a complex geological feature where geotectonic evolution, as well as the post ocean-opening history, configured three types of margins (Fig. 1): passive volcanic rifted (red line), transcurrent (orange line) and mixed convergent, and sheared (yellow line). Apart from its implications for the evolution of the Southern Ocean, the ACM constitutes a key region in the global oceanographic–climatic system as it is the only place in the Southern Ocean with a net water-mass exchange between the equatorial and southern polar regions (Fig. 2). Strong Antarctic-sourced currents run along the Argentine margin, driven by the Coriolis force, from 56∘ S and reaching to at least 34∘ S, or even farther north. Also, waters of Northern Hemisphere origin flow south along the Argentine margin north of about 38∘ S. The ACM evolution has been affected by climate, current, sea level, and tectonic processes, as well as by sediment input patterns and history along the roughly 5000 km long coast line of Argentina. All of these characteristics, coupled with the fact that this is a critically important, yet under-sampled portion of the World Ocean, makes the ACM an important region for using IODP (International Ocean Discovery Program) scientific drilling to explore and discover the potential benefits of such operations within the “passive” sector of the margin (here called the Argentine Passive Volcanic Continental Margin – APVCM).

The IODP workshop event, Developing Scientific Drilling Proposals for the Argentina Passive Volcanic Continental Margin (APVCM) – Basin Evolution, Deep Biosphere, Hydrates, Sediment Dynamics and Ocean Evolution, was held in Buenos Aires on 8–11 September 2015. The 3.5-day event was conducted in the Ministerio de Relaciones Exteriores Comercio Internacional y Culto (Ministry of Foreign Affairs, International Trade and Worship) in the city of Buenos Aires, comprising 45 scientists from 8 countries and 34 organizations or institutions, who discussed scientific drilling on the APVCM (Fig. 1) to determine the composition of and reconstruct the history of the sedimentary deposits under the impact of climatic and tectonic events. Breakout discussion groups were dedicated to tectonics, paleoceanography, sedimentology and seismic stratigraphy, and deep-Earth life forms, biosphere and geochemistry.

The workshop aimed to bring together a diverse group of scientists to explore and discover the merits of and thereby develop a strategy for scientific drilling operations on the APVCM. The goal of a scientific drilling campaign along and across the APVCM is to significantly contribute to our understanding of the evolution of the South Atlantic and its role and influence on global ocean circulation and the climate history of our planet. Sediments on the APVCM margin range from Late Jurassic–Cretaceous to Holocene in age, and depositional units from approximately the Eocene to Pliocene are particularly well developed. Records from this margin obtained through scientific drilling will be important to resolve key questions of the evolution of Earth's oceans and climate through this period.

IODP workshop topics were introduced to the audience through key note presentations on

the evolution of the southwestern Atlantic Ocean;

Workshop sponsorship was provided by the National Science Foundation US Science Support Program (USSSP), Argentina's Ministry of Science, Technology and Productive Innovation (MINCYT-CONICET), the Argentine Ministry of Foreign Affairs, COPLA (National Commission of the Outer Limit of the Shelf – CONVEMAR), the Pampa Azul Initiative, YPF S.A. (Argentina's National Petroleum Company), and CIG (Geological Research Center, University of La Plata – CONICET), Argentina. This was also a European Consortium for Ocean Research Drilling (ECORD) MagellanPlus workshop.

Seismic data are particularly important for planning and executing scientific drilling programs, and for the Argentine margin it is appropriate to mention early on in this report that three significant seismic data sets were shown and discussed at the workshop. The primary data set for the Argentine margin consists of mostly dip lines collected by BGR (Bundesanstalt für Geowissenschaften und Rohstoffe) in Hanover, Germany. The second primary data set consists of the primarily dip seismic lines collected by COPLA, which build on the BGR lines by extending the BGR lines offshore and by filling in between the BGR lines where they are widely spaced. The COPLA lines were collected in support of Argentina's application to set the outer limit of the Argentine continental shelf and there will be limited access to these lines, at least until that process has been concluded. Workshop organizers met with members of COPLA several times before the workshop to discuss the goals of the IODP workshop and the kind of data needed to support IODP scientific drilling. We were told that portions of the lines were expected to be available to support scientific drilling on a case-by-case basis. Indeed, four potential drill sites were proposed during the meeting based on the COPLA lines, and a pre-proposal currently active in the IODP system uses COPLA and BGR lines to define two potential sites (903-Pre, Fig. 3). The ArgentineSPAN-(TM) lines collected on the Argentine margin by ION Geophysical, Inc., were the third set of lines presented and discussed. These deep-penetration, proprietary lines are both strike lines and dip lines, and may also be available to support scientific drilling. Indeed, 911-Pre (Fig. 3) uses ArgentineSPAN-(TM) and BGR lines to define several sites. Other important data sets may exist on the margin, but they were not discussed at this meeting.

The Argentine margin contains important sedimentological, paleontological and chemical records of regional and local tectonic evolution, sea level, climate evolution and ocean circulation that date from the opening of the South Atlantic in the Late Jurassic–Early Cretaceous as well as of the present-day results of post-depositional chemical and biological alteration. Despite its important location, which underlies the exchange of southern- and northern-sourced water masses, the Argentine margin has not been investigated in detail using scientific drilling techniques (Fig. 1). This low level of scientific drilling activity in the region may in part be due to the reports of Maurice Ewing and co-workers (e.g., Lonardi and Ewing, 1971), which, based on widely spaced and low-resolution seismic profiles, noted that the margin had an erosional character as it was crossed by numerous large canyon systems, some of which were likely altered by strong currents. However, a number of papers published since 2009 reported new high-resolution and/or multichannel seismic surveys (Fig. 4), often combined with multi-beam bathymetric data, which show the common occurrence of layered sediments and prominent sediment drifts on the Argentine and adjacent Uruguayan margins (e.g., Hernández-Molina et al., 2009, 2010, 2015; Violante et al., 2010, Krastel et al., 2011; Lastras et al., 2011; Muñoz et al., 2012; Grützner et al., 2011, 2012, 2016; Preu et al., 2012, 2013; Voigt et al., 2013; Uenzelmann-Neben et al., 2016; see also Hinz et al., 1999). There has also been significant progress studying the climatic records in surficial and near-surface sediments recovered in sediment cores from the Argentine margin (e.g., Chiessi et al., 2007; Bozzano et al., 2011; Govin et al., 2012; Bender et al., 2013; Razik et al., 2013; Razik, 2014; García Chapori et al., 2014, 2015), demonstrating that this margin also contains important modern sedimentary deposits.

Encouraged by these recent results, our 3.5-day IODP workshop in Buenos Aires (8–11 September 2015) focused on opportunities for scientific drilling on the APVCM as a significant contribution to several of IODP's research themes described in the program's Science Plan (IODP-SP) for 2013–2023 Illuminating Earth's Past, Present and Future (http://www.iodp.org/program-documents). Future drilling in this region is likely to be of high priority because this margin lies beneath a key portion of the global ocean conveyor belt of thermohaline circulation (Broecker, 1991).

Interest is high in the southern South Atlantic Ocean, and there is currently much being learned about the details of the evolution of this key ocean basin (Torsvik et al., 2009; Moulin et al., 2010; Heine et al., 2013; Granot and Dyment, 2015). The rift phase of the Gondwana breakup extended from the Triassic/Jurassic(?) to the Early Cretaceous. Seaward dipping reflectors (SDRs) are observed on seismic profiles near the continental–ocean boundary (COB), indicating the presence of massive volcanism (a large igneous province, or LIP) at the transition from rifting to drifting (Gladczenko et al., 1997; Hinz et al., 1999). The volcanic layers that make up the SDRs may be subaerial and interbedded with terrestrial sediments. Slumps are observed in the prograding Cretaceous sediment off the Colorado Basin that may thin the sediments over SDRs in some areas. Cretaceous shelf sediments here are expected to be black shales, sandstones and coarser-grained deposits (Loegering et al., 2013) while Cretaceous sediments in deeper water are likely to be pelagic shales, marls and fine-grained sands (R. Gerster, personal communication, 2015). At about 95 Ma the Equatorial Atlantic Gateway opened, and the gateway continued to enlarge allowing for enhanced exchange of southern waters with the North Atlantic Basin, perhaps leading to a global cooling of bottom water and the end of the Cretaceous greenhouse period (Friedrich et al., 2012; Granot and Dyment, 2015). However, basin-to-basin differences in water properties are not well resolved, including the character of the South Atlantic waters that flooded the North Atlantic Basin (Friedrich et al., 2012).

Hernández-Molina et al. (2010) and Grützner et al. (2012) suggested that sediments from about the Cretaceous–Tertiary boundary to the Eocene–Oligocene boundary are thick along the margin and are characterized by being parallel to sub-parallel reflections of low to moderate amplitude. This is generally a time of low to moderate bottom current activity and a warm climate.

Lastras et al. (2011) and Muñoz et al. (2012) sampled relatively thick sections of fine-grained Eocene sediments at about 45 to 47∘ S outcropping at water depths of from 900 to 2500 m in the walls of large canyons. Eocene sediments found in cores from this section of the slope consist of benthic diatoms to the south and a carbonate facies to the north. Ewing and Lonardi (1971) also noted the presence of Eocene sediments in this region of large canyons, as well as canyons on the southern Argentine margin, which may be particularly well developed because they are cut into thick, fine-grained Eocene sediments by persistent, strong currents. Cursory analysis of the ArgentineSPAN-(TM) seismic lines in the area suggest that these layered sediments also exist in somewhat shallower water south of the zone of canyons.

Hanna et al. (1976) and Ross (1976) described several new, non-reworked Eocene diatom species from Vema cores collected in this area. Core VM12-46 (47.483∘ S, 59.35∘ W; water depth 1167 m), described by D. Ericson and available on GeoMapApp (http://www.geomapapp.org), contains a fine-grained Eocene sediment with abundant coccoliths, diatoms, silico-flagellates, radiolarian and sponge spicules as well as common benthic foraminifera.

Eocene sediments on the upper slope are overlain by a prominent but now buried sediment drift in deep water, which is likely of Oligocene to early late Miocene age (Fig. 4; the “giant drift” of Hernández-Molina et al., 2010 and Grützner et al., 2012). This drift is buried to the west by the flanks of younger and shallower deposits (termed “mounded drifts”) of mid- to late Miocene age, which developed sequentially within the south-central portion of the Argentine margin. Sediments likely of Pliocene to the Holocene age overlay the mounded drifts, although they are often more localized. These younger sediments are generally interpreted as drifts, perhaps associated with the levees of channels, or deposits within channels, where the channels intersect flow along the margin. Drifts are also present farther north along the margin (e.g., Hernández-Molina et al., 2015) with drifts apparently associated with flows of Antarctic Intermediate Water (AAIW), Upper Circumpolar Deep Water (UCDW), North Atlantic Deep Water (NADW), Lower Circumpolar Deep Water (LCDW) and Antarctic Bottom Water (AABW).

Violante et al. (2010) and Grützner et al. (2011, 2012, 2016) suggested that increased sediment flux to the margin during the Miocene may in part be related to an uplift in the Andes that in turn is due to increased Miocene Pacific Ocean crustal spreading and subduction rates, which peaked at about 10 to 20 Ma (Pardo-Casas and Molnar, 1987; Martinod et al., 2010). However, the routes or processes by which Andean sediments reach the margin and are redistributed within the margin are not well understood.

The shift in deposition from primarily shallow-water Eocene sediments to deep-water, drifted Oligocene sediments appears to mark the deepening of the Antarctic Circumpolar Current (ACC) in the Oligocene (Katz et al., 2011) and the first entry of northward-flowing deep waters into the Argentine Basin (Uenzelmann-Neben et al., 2016). The establishment of a shallow ACC in the Eocene played an important role in isolating Antarctica and allowing for the growth of continental ice sheets (Katz et al., 2011), and the thick, shallow-water (now the upper slope) Eocene sediments described along the southern Argentine margin may have accumulated in response to the formation of a shallow ACC and associated shallow northward flow along the Argentine margin. The ACC apparently strengthened and deepened into the Oligocene as the Tasman Gateway and then the Drake Passage deepened, leading to the development of the modern four-layer structure as well as deep northward flow in the Argentine Basin (Katz et al., 2011; Figs. 2 and 4).

The mid–late Miocene is a particularly important time in terms of climate history and global ocean circulation. During the early Miocene, the Antarctic Ice Sheet (AIS) appears to have fluctuated in size, with concomitant changes in sea level of about ±20 m (e.g., Foster et al., 2012). The mid-Miocene was characterized by the mid-Miocene Climatic Optimum (MCO), which extends from about 17 to 15 Ma. This was a time of reduced AIS volume (∼ 10–25 % of the modern AIS), high pCO2 (up to ∼ 450 ppm) and global temperatures warmer than today. Following the MCO, more complex oceanic circulation patterns developed during the mid-Miocene Climatic Transition (MMCT) from about 14.2 to 13.8 Ma. Possible important events that occurred at that time include the emplacement of large volcanic complexes (such as the Columbia River Basalts, CRB), which would have affected pCO2 (Armstrong McKay et al., 2014), the Andean uplift with an impact on atmospheric circulation and weathering patterns (Violante et al., 2010; Grützner et al., 2011 and 2012), and the closing of the Tethys seaway, which changed global ocean circulation patterns (Hamon et al., 2013). Climate deterioration continued into the late Miocene, leading to the initiation and growth of the West Antarctic Ice Sheet (Shevenell et al., 2004). Climatic deterioration has continued into the Pleistocene although the major current systems that were established between the end of the Miocene and the late Pliocene appear to have continued to the present day (Hernández-Molina et al., 2009; Preu et al., 2012, 2013). However, factors other than ocean currents have also been important to the development of the margin, including sea level change, climate variability and glaciation and uplift in the Andes (Violante et al., 2010; Grützner et al., 2011 and 2012). These kinds of factors can also control the timing, locations and rates of sediment input, which can also affect margin evolution.

Workshop participants agreed that scientific ocean drilling off Argentina (both in deep and shallow water) will contribute to the understanding of the role that the southern region has played in climate evolution and associated processes and will provide opportunities for focused studies.

The APVCM provides outstanding targets for investigating sedimentation and paleoceanographic conditions from the Cretaceous to the Holocene. The unique setting in the target region is linked to the tectonic evolution of Antarctica, the Southern and South Atlantic oceans, and the Andes. Specific questions and hypotheses were discussed in groups regarding several main sub-topics.

Development of APVCM-specific drilling proposals to address the objectives cited above will require the acquisition of additional data sets. Specifically, the following was suggested during the workshop:

Priority no. 1: obtain additional seismic data sets, including cross-lines to complement existing seismic data, for possible target sites. Such new seismic lines should be acquired using methods that resolve deeper sedimentary structures in addition to the upper sedimentary sequences. These data can be used to develop several drilling transects at different latitudes to target the collection of important Paleogene and Neogene sequences as well as to support drilling to deeper targets such as SDRs and Mesozoic sediments.

Action no. 1: augment existing high-quality seismic data (e.g., Grützner et al., 2012), for example through GEOMAR's three-dimensional (3-D) P-Cable (Planke et al., 2009; Planke and Berndt, 2002) and/or using other seismic systems, such as provided through the U.S. National Science Foundation (NSF) or other research grants.

Action no. 2: evaluate existing sediment samples and collect new gravity and piston cores near potential drill sites to document the age, nature and character of the near-surface material to support the potential transect-oriented IODP proposals.

Priority no. 2: coordinate with Pampa Azul, a strategic project of scientific research along the Argentine margin supported by the Argentine state (http://en.mincyt.gob.ar/accion/pampa-azul-9465). As stated on the Pampa Azul web site, the main objective of Pampa Azul is to help deepen scientific knowledge as the basis of conservation policies and management of natural resources, promote technological innovations applicable to the sustainable exploitation of natural resources and the development of industries related to the sea, strengthen the maritime awareness of Argentine society and support with scientific information and presence the country's sovereignty in the South Atlantic area. The Pampa Azul initiative was a one of the workshop sponsors and several of the attendees were associated with Pampa Azul. We encourage linkages between any scientific drilling campaign on the APVCM and Pampa Azul investigators. Argentina provides an excellent example for developing a vibrant, highly ambitious ocean research program (Pampa Azul). Coordinating with Pampa Azul through this IODP-sponsored workshop provided an important mechanism to begin important collaborations between the Argentine ocean research community and IODP investigators.

Priority no. 3: encourage the development of continent–ocean transect drilling proposals for joint evaluation by IODP and ICDP (International Continental Scientific Drilling Program). Important and complementary records come from both continental and marine settings, and a full understanding of margin evolution and requires working in both settings.

The main outcome of this workshop is the formation of several working groups addressing the various scientific topics briefly outlined in this report, and committed to preparing several pre-/full proposals to IODP that are appropriate for the R/V JOIDES resolution. As of January, 2017 two preliminary proposals are active in the IODP system and full proposals for these proposals have been invited; see https://www.iodp.org/proposals/active-proposals for full details. The 903-Pre “Argentine margin seaward dipping reflectors” considers the early opening of the South Atlantic and especially the sampling of the SDRs that formed in a non-hotspot setting and the overlying Cretaceous and Cenozoic sediments. Secondary objectives include the evolution of atmospheric circulation and the diversity and activity of microbial life under non-steady-state conditions. 911-Pre “Argentine margin paleoceanographic transects” develops a “transect” approach to better understand the evolution of Southern Ocean water structure over a broad vertical and temporal range. Secondary objectives include how the passive margin architecture responds to Cenozoic climate and tectonics and the microbial variability through time under different climate, deep-water circulation and sediment-flux regimes. Developing these and other pre-/full proposals for the time window when the R/V JOIDES resolution is likely to operate in the South Atlantic will probably require the collection of additional data and coordination amongst the international partners.

The scientific objectives for potential IODP drilling proposals are central aspects of the International Ocean Discovery Program as formulated in the Science Plan for 2013–2023 Illuminating Earth's Past, Present, and Future. Knowledge acquired as a result of this drilling project will significantly advance the IODP theme “climate and ocean change: reading the past, informing the future”, specifically challenges 1 “how does Earth's climate system respond to elevated levels of atmospheric CO2?”, 3 “what controls regional patterns of precipitation, such as those associated with monsoons or El Niño?” and 4 “how resilient is the ocean to chemical perturbations?”, as well as the “Earth connections: deep processes and their impact on Earth's surface environment” theme, specifically challenges 8 “what are the composition, structure, and dynamics of Earth's upper mantle?”, 9 “how are seafloor spreading and mantle melting linked to ocean crustal architecture?”, and 10 “what are the mechanisms, magnitude, and history of chemical exchanges between the oceanic crust and seawater?” Throughout the workshop we have been learning how to integrate objectives related to biosphere frontiers with those of other themes. This has been very successful and drilling resulting from this workshop will significantly advance the IODP theme “biosphere frontiers: deep life, biodiversity, and environmental forcing of ecosystems”, specifically challenges 5 “what are the origin, composition and global significance of subseafloor communities” and 7 “how sensitive are ecosystems and biodiversity to environmental change?”

The interconnection of this IODP project with Pampa Azul and corresponding initiatives (for example with the German DFG-BMBF) provides opportunities for early-career scientists among the various countries. ICDP at the German Research Centre for Geosciences (GFZ) in Potsdam actively fosters and sponsors such efforts and incentives via financial and logistical support. We also encourage syndicated, perhaps educational television, documentaries about scientific drilling on the Argentine margin as projects develop. This type of activity can help to inform the general public, whose tax dollars finance these studies, and raise favorable awareness for these types of international collaborations at a time of significant geo-political and economic challenges. This is just one aspect of interacting with the public at all levels in order to generate a deeper understanding and appreciation for the importance of scientific drilling both on land and in the ocean.

The evolution of the Argentine Continental Margin has been affected by climate, current, sea level and tectonic processes, as well as by sediment input patterns and depositional history along this roughly 5000 km long coast line. All of these characteristics, coupled with the fact that this is a critically important, yet under-sampled, portion of the World Ocean, makes the Argentine Continental Margin an important region for IODP scientific drilling. Workshop participants agreed that scientific ocean drilling off Argentina will contribute to the understanding of the role that the southern region has played in climate evolution and associated processes and will provide opportunities for focused studies. The APVCM provides outstanding targets for investigating sedimentation patterns, climatic, paleo-circulation and paleoceanographic conditions, biosphere and geochemistry from the Cretaceous to the Holocene. The evolution of this important setting is also linked to the evolution of the World Ocean as well as to the tectonic evolution of Antarctica, the Southern and South Atlantic oceans, and the Andes.