Expedition 383 summary

Abstract

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 (warmwater 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 opalbased proxies to address our objectives of reconstructing, with unprecedented stratigraphic detail, surface to deep-ocean variations and their relation to atmosphere and cryosphere changes. Introduction Our prior knowledge of southern high latitude paleoceanography comes from conventional sediment coring and Deep Sea Drilling Project (DSDP)/Ocean Drilling Program (ODP) drilling in the Atlantic and Indian sectors of the Antarctic Circumpolar Current G. Winckler et al. Expedition 383 summary (ACC). A prominent example is ODP Leg 177, which drilled sites along a north–south transect across the major ACC fronts and documented the Cenozoic history of the Atlantic sector of the Southern Ocean. These sites revealed, for example, linked changes in dust supply, marine productivity, and biological nutrient consumption (Martínez-Garcia et al., 2009, 2011, 2014), opposing changes in productivity in the Antarctic compared to the Subantarctic Zone on glacial–interglacial cycles (Jaccard et al., 2013) and in the ACC compared to the Antarctic margin on longer timescales (Cortese et al., 2004), marked steps in sea-surface temperature (SST) evolution since the Pliocene (Martínez-García et al., 2010), and Antarctic Ice Sheet (AIS) dynamics during the late Quaternary (Kanfoush et al., 2002; Teitler et al., 2010, 2015; Venz and Hodell, 2002; Hayes et al., 2014). In contrast, studies of the Pacific sector of the ACC have been limited mainly to regions close to the Antarctic continental margin south of the ACC (e.g., Ross Sea during DSDP Leg 28 and Antarctic Peninsula during ODP Leg 178). Integrated Ocean Discovery Program Leg 318 drilled Cenozoic sediments off Wilkes Land (eastern Indian Southern Ocean) and revealed significant vulnerability of the East Antarctic Ice Sheet (EAIS) to Pliocene warming (Cook et al., 2013; Expedition 318 Scientists, 2010). A substantial advance in constraining the history and stability of the West Antarctic Ice Sheet (WAIS) and Ross Ice Shelf came from the Antarctic Geological Drilling (ANDRILL) project (e.g., Naish et al., 2009; Florindo et al. 2008), which documented ice sheets that were expanded and stable between 13 and 11 Ma and more dynamic in the late Miocene– late Pliocene (10–2.5 Ma) with cyclic expansion of grounded ice sheets over the coring site after 2.5 Ma. These onand near-shore drilling programs have significantly advanced our understanding of AIS dynamics during the Cenozoic. However, to understand how these ice sheet changes are ultimately linked to climate, atmospheric CO2 levels, and ocean circulation requires constraints on the strength, latitudinal extent, and overall nature of the ACC (i.e., the current system that connects Antarctica to the rest of the globe). In this context, the results of Expedition 383 in the Pacific ACC will be closely linked to the recently completed International Ocean Discovery Program (IODP) expeditions to the Ross Sea (Expedition 374), Amundsen Sea (Expedition 379), and Scotia Sea (Expedition 382). These expeditions targeted Antarctic near-shore records, which will likely be incomplete due to glacial erosion, and the potential for analyses of paleoceanographic proxies may be limited. Therefore, Expedition 383, Dynamics of the Pacific Antarctic Circumpolar Current (DYNAPACC), pelagic sites were selected to provide critical paleoceanographic baseline information, including rates of change, for improving the understanding of reconstructed AIS changes and testing ice sheet models. Background The ACC, the world’s largest current system, connects all three major basins of the global ocean (i.e., the Pacific, Atlantic, and Indian Oceans; Figure F1) and therefore integrates and responds to climate signals throughout the globe (e.g., Talley, 2013). Through deep mixing, upwelling, and water mass formation, the ACC is fundamentally tied to the global Meridional Overturning Circulation (MOC; Marshall and Speer, 2012) and the stability of Antarctica’s ice sheets. The ACC has long been recognized as a key player in regulating atmospheric CO2 variations and therefore global climate based on the tight coupling between Southern Hemisphere temperatures and atmospheric CO2 concentrations observed in Antarctic ice cores (e.g., Parrenin et al., 2013). Strong, zonally symmetric westerly winds drive a northward Ekman component of flow that promotes ventilation of deep water, which provides a direct hydrographic link between the large deep-ocean reservoir of dissolved inorganic carbon and the surface ocean (e.g., Kuhlbrodt et al., 2007; Marshall and Speer, 2012). The ACC therefore acts as a window through which the interior ocean and atmosphere interact and as a key player in regulating atmospheric CO2 variations and therefore global climate. Biological utilization of nutrients in the Southern Ocean is particularly important in relation to changes in atmospheric CO2 concentration because it regulates the preformed nutrient inventory for most of the deep ocean and therefore the global average efficiency of the biological pump (e.g., Sigman and Boyle, 2000; Sigman et al., 2010). Nutrient utilization is inefficient in the Southern Ocean today in part because phytoplankton growth is limited by the scarcity of bioavailable iron (e.g., de Baar et al., 1995). Martin (1990) proposed that dust-borne iron fertilization of Southern Ocean phytoplankton caused the ice age reduction in atmospheric CO2. However, the role of iron in explaining variations in opal export to the Southern Ocean sediments is complex. Chase et al. (2015) found a significant correlation between annual average net primary production and modeled dust deposition but not between dust deposition and opal burial. On glacial–interglacial timescales, proxy records from the Subantarctic region support a positive relationship between dust flux and opal production (e.g., Chase et al., 2003; Bradtmiller et al., 2009; Lamy et al., 2014; Kumar et al., 1995; Anderson et al., 2014). The opposite is true south of the Antarctic Polar Front (APF) (e.g., Chase et al., 2003; Jaccard et al., 2013), indicating that factors other than dust regulate the production and export of opal in the Southern Ocean and, by association, the strength and efficiency of the biological carbon pump. Leading candidates for these other factors include changes related to shifts in the hydrographic fronts, the flow of the ACC, the spatial and temporal variability of wind-driven upwelling that supplies nutrients and CO2 to surface waters, and sea ice extent. Oceanographic setting Expedition 383 drilled sites located in the ACC system in the central South Pacific (CSP), the eastern South Pacific (ESP) and at the southern Chilean margin close to the Drake Passage. The flow of the ACC is largely driven by Southern Westerly Winds (SWW) and is constricted to its narrowest extent in the Drake Passage. This socalled “cold-water route” through the Drake Passage is one important pathway for the return of fresh and cold waters from the Pacific to the Atlantic, which strongly affects the strength of the Atlantic MOC (AMOC), in concert with the “warm-water route” inflow of warm and salty Indian Ocean water masses through the Agulhas Current system (Beal et al., 2011; Gordon, 1986). The Drake Passage is ~800 km wide, and it is located between Cape Horn and the western Antarctic Peninsula (Figure F1). Numerous hydrographi