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1. Dynamics of the Pacific Antarctic Circumpolar Current

   https://doi.org/10.14379/iodp.proc.383.2021  

   Lamy, F.; Winckler, G.; Alvarez Zarikian, C.A. (July 2021, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   The Antarctic Circumpolar Current (ACC), the world’s strongest zonal current system, connects all three major ocean basins of the global ocean and therefore integrates and responds to global climate variability. Its flow is largely driven by strong westerly winds and is constricted to its narrowest extent in the Drake Passage. Fresh and cold Pacific surface and intermediate water flowing through the Drake Passage (cold-water route) and warm Indian Ocean water masses flowing through the Agulhas region (warm-water route) are critical for the South Atlantic contribution to Meridional Overturning Circulation changes. Furthermore, physical and biological processes associated with the ACC affect the strength of the ocean carbon pump and therefore are critical to feedbacks linking atmospheric CO2 concentrations, ocean circulation, and climate/cryosphere on a global scale. In contrast to the Atlantic and Indian sectors of the ACC, and with the exception of drill cores from the Antarctic continental margin and off New Zealand, there are no deep-sea drilling paleoceanographic records from the Pacific sector of the ACC. To advance our understanding of Miocene to Holocene atmosphere-ocean-cryosphere dynamics in the Pacific and their implications for regional and global climate and atmospheric CO2, International Ocean Discovery Program Expedition 383 recovered sedimentary sequences at (1) three sites in the central South Pacific (CSP) (U1539, U1540, and U1541), (2) two sites at the Chilean margin (U1542 and U1544), and (3) one site from the pelagic eastern South Pacific (U1543) close to the entrance to the Drake Passage. Because of persistently stormy conditions and the resulting bad weather avoidance, we were not successful in recovering the originally planned Proposed Site CSP-3A in the Polar Frontal Zone of the CSP. The drilled sediments at Sites U1541 and U1543 reach back to the late Miocene, and those at Site U1540 reach back to the early Pliocene. High sedimentation rate sequences reaching back to the early Pleistocene (Site U1539) and the late Pleistocene (Sites U1542 and U1544) were recovered in both the CSP and at the Chilean margin. Taken together, the sites represent a depth transect from ~1100 m at Chilean margin Site U1542 to ~4070 m at CSP Site U1539 and allow investigation of changes in the vertical structure of the ACC, a key issue for understanding the role of the Southern Ocean in the global carbon cycle. The sites are located at latitudes and water depths where sediments will allow the application of a wide range of siliciclastic-, carbonate-, and opal-based proxies to address our objectives of reconstructing, with unprecedented stratigraphic detail, surface to deep-ocean variations and their relation to atmosphere and cryosphere changes. 

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2. Expedition 383 Scientific Prospectus: Dynamics of the Pacific Antarctic Circumpolar Current (DYNAPACC)

   https://doi.org/10.14379/iodp.sp.383add.2018  

   Lamy, F.; Winckler, G.; Alvarez Zarikian, C.A. (August 2018, Scientific prospectus)

   null (Ed.)

   The Antarctic Circumpolar Current (ACC) is the world’s strongest zonal current system that connects all three major ocean basins of the global ocean and therefore integrates and responds to global climate variability. Its flow is largely driven by strong westerly winds and constricted to its narrowest extent in the Drake Passage. Transport of fresh and cold surface and intermediate water masses through the Drake Passage (cold-water route) strongly affect the Atlantic Meridional Overturning Circulation (AMOC) together with the inflow of Indian Ocean water masses (warm-water route). Both oceanographic corridors are critical for the South Atlantic contribution to AMOC changes. In contrast to the Atlantic and Indian sectors of the ACC, and with the exception of drill cores from the Antarctic continental margin and off New Zealand, deep-sea drilling records of the Pacific sector of the ACC lack information on its Cenozoic paleoceanography. To advance our knowledge and understanding of Plio-Pleistocene atmosphere-ocean-cryosphere dynamics in the Pacific and their implications for regional and global climate and atmospheric CO2, International Ocean Discovery Program Expedition 383 proposes the recovery of 180 to 500 m long high-resolution Plio-Pleistocene sediment sequences at (1) three primary sites located on a cross-frontal transect in the central South Pacific (CSP) between the modern Polar Front (Site CSP-3A) and the Subantarctic Zone (Sites CSP-1A and CSP-2B), (2) two sites (CHI-1C and CHI-4B) at the Chilean margin, and (3) one site from the pelagic eastern South Pacific (ESP; Site ESP-1A) close to the entrance to the Drake Passage. The planned sites represent a depth transect from ~1100 m at the Chilean margin (Site CHI-4B) to >5000 m in the Bellingshausen Sea (Site CSP-3A) that will allow investigation of Plio-Pleistocene changes in the vertical structure of the ACC—a key issue for understanding the role of the Southern Ocean in the global carbon cycle. All of the six primary and eight alternate sites were surveyed with seismic lines in 2009–2010 and most recently in 2016. The sites are located at latitudes and water depths where sediments will allow the application of a wide range of siliciclastic, carbonate, and opal-based proxies to address our objectives of reconstructing, with unprecedented stratigraphic detail, surface to deep ocean variations and their relation to atmosphere and cryosphere changes through stadial-to-interstadial, glacial-to-interglacial, and warmer-than-present time intervals. 

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3. Expedition 383 Scientific Prospectus: Dynamics of the Pacific Antarctic Circumpolar Current (DYNAPACC)

   https://doi.org/10.14379/iodp.sp.383.2018  

   Lamy, F; Winckler, G; Alvarez_Zarikian, CA (August 2018, International Ocean Discovery Program)

   The Antarctic Circumpolar Current (ACC) is the world’s strongest zonal current system that connects all three major ocean basins of the global ocean and therefore integrates and responds to global climate variability. Its flow is largely driven by strong westerly winds and constricted to its narrowest extent in the Drake Passage. Transport of fresh and cold surface and intermediate water masses through the Drake Passage (cold-water route) strongly affect the Atlantic Meridional Overturning Circulation (AMOC) together with the inflow of Indian Ocean water masses (warm-water route). Both oceanographic corridors are critical for the South Atlantic contribution to AMOC changes. In contrast to the Atlantic and Indian sectors of the ACC, and with the exception of drill cores from the Antarctic continental margin and off New Zealand, deep-sea drilling records of the Pacific sector of the ACC lack information on its Cenozoic paleoceanography. To advance our knowledge and understanding of Plio-Pleistocene atmosphere-ocean-cryosphere dynamics in the Pacific and their implications for regional and global climate and atmospheric CO2, International Ocean Discovery Program Expedition 383 proposes the recovery of 180 to 500 m long high-resolution Plio-Pleistocene sediment sequences at (1) three primary sites located on a cross-frontal transect in the central South Pacific (CSP) between the modern Polar Front (Site CSP-3A) and the Subantarctic Zone (Sites CSP-1A and CSP-2B), (2) two sites (CHI-1C and CHI-4B) at the Chilean margin, and (3) one site from the pelagic eastern South Pacific (ESP; Site ESP-1A) close to the entrance to the Drake Passage. The planned sites represent a depth transect from ~1100 m at the Chilean margin (Site CHI-4B) to >5000 m in the Bellingshausen Sea (Site CSP-3A) that will allow investigation of Plio-Pleistocene changes in the vertical structure of the ACC—a key issue for understanding the role of the Southern Ocean in the global carbon cycle. All of the six primary and eight alternate sites were surveyed with seismic lines in 2009–2010 and most recently in 2016. The sites are located at latitudes and water depths where sediments will allow the application of a wide range of siliciclastic, carbonate, and opal-based proxies to address our objectives of reconstructing, with unprecedented stratigraphic detail, surface to deep ocean variations and their relation to atmosphere and cryosphere changes through stadial-to-interstadial, glacial-to-interglacial, and warmer-than-present time intervals. 

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4. The Effect of Southern Ocean Topography on the Global MOC and Abyssal Water Mass Distribution

   https://doi.org/10.1175/JPO-D-23-0253.1  

   Monkman, Tatsu; Jansen, Malte F (December 2024, Journal of Physical Oceanography)

   We investigate the role of Southern Ocean topography and wind stress in the deep and abyssal ocean overturning and water mass composition using a suite of idealized global ocean circulation models. Specifically, we address how the presence of a meridional ridge in the vicinity of Drake Passage and the formation of an associated Southern Ocean gyre influence the water mass composition of the abyssal cell. Our experiments are carried out using a numerical representation of the global ocean circulation in an idealized two-basin geometry under varying wind stress and Drake Passage ridge height. In the presence of a low Drake Passage ridge, the overall strength of the meridional overturning circulation is primarily influenced by wind stress, with a topographically induced weakening of the middepth cell and concurrent strengthening of the abyssal cell occurring only after ridge height passes 2500 m. Passive tracer experiments show that a strengthening middepth cell leads to increased abyssal ventilation by North Atlantic water masses, as more North Atlantic Deep Water (NADW) enters the Southern Ocean and then spreads into the Indo-Pacific. We repeat our tracer experiments without restoring in the high-latitude Southern Ocean in order to identify the origin of water masses that circulate through the Southern Ocean before sinking into the abyss as Antarctic Bottom Water. Our results from these “exchange” tracer experiments show that an increasing ridge height in Drake Passage and the concurrent gyre spinup lead to substantially decreased NADW-origin waters in the abyssal ocean, as more surface waters from north of the Antarctic Circumpolar Current (ACC) are transferred into the Antarctic Bottom Water formation region. Significance StatementThe objective of this study is to investigate how topographic features in the Southern Ocean can affect the overall structure of Earth’s large-scale ocean circulation and the distribution of water masses in the abyssal ocean. We focus on the Southern Ocean because the region is of central importance for exchange between the Atlantic and Indo-Pacific Ocean basins and for CO₂and heat uptake into the abyssal ocean. Our results indicate that Southern Ocean topography plays a major role in the overall circulation by 1) controlling the direct transfer of abyssal waters from the Atlantic to the Indo-Pacific via its influence on the Atlantic meridional overturning circulation and 2) controlling the coupling between the abyssal ocean and surface waters north of the Antarctic Circumpolar Current via the Southern Ocean gyre. 

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5. Iceberg Alley and Subantarctic Ice and Ocean Dynamics

   https://doi.org/10.14379/iodp.proc.382.2021  

   Weber, M.E.; Raymo, M.E.; Peck, V.L.; Williams, T. (May 2021, Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   International Ocean Discovery Program Expedition 382, Iceberg Alley and Subantarctic Ice and Ocean Dynamics, investigated the long-term climate history of Antarctica, seeking to understand how polar ice sheets responded to changes in insolation and atmospheric CO2 in the past and how ice sheet evolution influenced global sea level and vice versa. Five sites (U1534–U1538) were drilled east of the Drake Passage: two sites at 53.2°S at the northern edge of the Scotia Sea and three sites at 57.4°–59.4°S in the southern Scotia Sea. We recovered continuously deposited late Neogene sediments to reconstruct the past history and variability in Antarctic Ice Sheet (AIS) mass loss and associated changes in oceanic and atmospheric circulation. The sites from the southern Scotia Sea (Sites U1536–U1538) will be used to study the Neogene flux of icebergs through “Iceberg Alley,” the main pathway along which icebergs calved from the margin of the AIS travel as they move equatorward into the warmer waters of the Antarctic Circumpolar Current (ACC). In particular, sediments from this area will allow us to assess the magnitude of iceberg flux during key times of AIS evolution, including the following: • The middle Miocene glacial intensification of the East Antarctic Ice Sheet, • The mid-Pliocene warm period, • The late Pliocene glacial expansion of the West Antarctic Ice Sheet, • The mid-Pleistocene transition (MPT), and • The “warm interglacials” and glacial terminations of the last 800 ky. We will use the geochemical provenance of iceberg-rafted detritus and other glacially eroded material to determine regional sources of AIS mass loss. We will also address interhemispheric phasing of ice sheet growth and decay, study the distribution and history of land-based versus marine-based ice sheets around the continent over time, and explore the links between AIS variability and global sea level. By comparing north–south variations across the Scotia Sea between the Pirie Basin (Site U1538) and the Dove Basin (Sites U1536 and U1537), Expedition 382 will also deliver critical information on how climate changes in the Southern Ocean affect ocean circulation through the Drake Passage, meridional overturning in the region, water mass production, ocean–atmosphere CO2 transfer by wind-induced upwelling, sea ice variability, bottom water outflow from the Weddell Sea, Antarctic weathering inputs, and changes in oceanic and atmospheric fronts in the vicinity of the ACC. Comparing changes in dust proxy records between the Scotia Sea and Antarctic ice cores will also provide a detailed reconstruction of changes in the Southern Hemisphere westerlies on millennial and orbital timescales for the last 800 ky. Extending the ocean dust record beyond the last 800 ky will help to evaluate dust-climate couplings since the Pliocene, the potential role of dust in iron fertilization and atmospheric CO2 drawdown during glacials, and whether dust input to Antarctica played a role in the MPT. The principal scientific objective of Subantarctic Front Sites U1534 and U1535 at the northern limit of the Scotia Sea is to reconstruct and understand how intermediate water formation in the southwest Atlantic responds to changes in connectivity between the Atlantic and Pacific basins, the “cold water route.” The Subantarctic Front contourite drift, deposited between 400 and 2000 m water depth on the northern flank of an east–west trending trough off the Chilean continental shelf, is ideally situated to monitor millennial- to orbital-scale variability in the export of Antarctic Intermediate Water beneath the Subantarctic Front. During Expedition 382, we recovered continuously deposited sediments from this drift spanning the late Pleistocene (from ~0.78 Ma to recent) and from the late Pliocene (~3.1–2.6 Ma). These sites are expected to yield a wide array of paleoceanographic records that can be used to interpret past changes in the density structure of the Atlantic sector of the Southern Ocean, track migrations of the Subantarctic Front, and give insights into the role and evolution of the cold water route over significant climate episodes, including the following: • The most recent warm interglacials of the late Pleistocene and • The intensification of Northern Hemisphere glaciation. 

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