Evolution of Antarctic Intermediate Water during the Plio-Pleistocene and implications for global climate: Evidence from the South Atlantic
Introduction
Antarctic Intermediate Water (AAIW) is a cool (∼2–5 °C) and low-salinity (∼34.4 psμ) water mass that originates from an area between 45 and 55°S, extending from the Antarctic Polar Front towards the north of the Subantarctic Front (McCartney, 1977; Talley, 1996). There it is formed by vertical mixing with upwelled Circumpolar Deep Water (CDW) during the Austral winter, and after subduction its core flows northward at intermediate (700–1200 m) water depths in all three ocean basins (McCartney, 1977; Piola and Georgi, 1982; Tsuchiya, 1989; Talley, 1996; Sarmiento et al., 2004; Pellichero et al., 2018 and refs. therein). Hence, AAIW is an important part of the thermohaline ocean circulation and represents the shallow arm of the cold water route from the Southern to the Northern Hemisphere for balancing the southward flow of North Atlantic Deep Water (NADW) from the North Atlantic Ocean (e.g., Talley, 2013).
Two interlinked main processes have been proposed to drive the formation, and northward drift of AAIW. One is the well-known wind-driven upwelling and Ekman transport occurring during the present-day, and the last glacial and interglacial (Wainer et al., 2012 and refs therein). The other is seasonal sea ice formation and melting close to Antarctica that drive buoyancy changes in the Southern Ocean (Abernathey et al., 2016; Evans et al., 2018; Pellichero et al., 2018). In the Southwest Atlantic Ocean, AAIW is mainly derived from the Southeast Pacific Ocean, flowing through the Drake Passage and then northward towards the area of DSDP Site 516 (McCartney, 1977; Piola and Matano, 2001, Fig. 1a; location: 30°17′S; 35°17′W; 1313 m water depth).
Large amounts of anthropogenic CO2 are stored in Southern Ocean surface and subsurface waters through AAIW formation as well as its subsurface flow domains in the Atlantic, Pacific, and Indian Ocean sectors (Sabine et al., 2004). Even though there is upwelling of CDW close to Antarctica that releases CO2 into the atmosphere (not fully sequestered by bio-productivity) there is still a net transport of atmospheric CO2 into the Southern Ocean (Gruber et al., 2009). Hence, during past times of positive ocean-atmosphere CO2 disequilibria, AAIW formation can be expected to have been an important water mass for facilitating carbon uptake and sequestration on short time scales.
AAIW is also a nutrient rich water mass that ventilates the thermocline of upwelling regions, together with shallower subantarctic mode waters, in the tropics and subtropics (Toggweiler et al., 1991; Gordon et al., 1992). The high nutrient content of these waters fuel biological productivity in these regions where the biologic pump can sequester carbon through sedimentary carbon burial, a process active both today and in the geologic past (Sarmiento et al., 2004; Cortese et al., 2004; Etourneau et al., 2012; Winckler et al., 2016).
During the ‘Plio-Pleistocene climate transition’, which is the gradual climatic change from Pliocene greenhouse conditions towards the onset of Northern Hemisphere Glaciation, resulting in larger and bipolar glacial cycles since around 3 Ma, several studies have proposed that there is a linkage to increased subsurface delivery of cooler, nutrient-rich waters from the Southern Ocean via AAIW to lower-latitude upwelling sites (Marlow et al., 2000; Lawrence et al., 2006; Etourneau et al., 2009, 2012). These nutrients would have fuelled primary productivity of diatoms and resulting in enhanced CO2 uptake in these regions during this Plio-Pleistocene cooling period between 3 and 2 Ma (Etourneau et al., 2012). Moreover, by ∼1.8 Ma, distinct cooling of AAIW source regions has been linked to a coeval cooling of the equatorial East Pacific Ocean, indicating that AAIW may have directly cooled this region by upwelling as a result of increased wind speeds (Martinez-Garcia et al., 2010).
At the same time it has been suggested that the Southern Ocean upwelling of CDW and AAIW formation considerably strengthened (McKay et al., 2012; Hill et al., 2017). There is evidence for polar sea surface cooling close to Antarctica and sea ice extension in the Ross Sea around 3.3 Ma, as recorded by sediment core AND-1B in the Ross Sea and other opal records (a proxy for ocean stratification) from the Atlantic sector of the Southern Ocean such as ODP Site 1096 (McKay et al., 2012; Hillenbrand and Cortese, 2006; locations in Fig. 1). Cooling of mid-polar latitudes compared to the tropics caused an increase in the meridional temperature gradient, resulting in increased zonal westerly wind speeds (Brierley et al., 2009; Fedorov et al., 2013), which in turn was likely important for AAIW formation and subduction (Wainer et al., 2012 and refs therein). In fact, climate model simulations (Hill et al., 2017) show that expanded Ross Sea sea ice extent invigorated and broadened austral westerly wind circulation. The stronger westerlies promoted vertical mixing of water masses in the Southern Ocean and enhanced upwelling of CDW (Hill et al., 2017). The same relationship, that is, between increased westerly wind speeds and increased circulation in the Southern Ocean, has been suggested by an eddy-resolving simulation for the present-day (Bishop et al., 2016).
Despite these climate simulation studies and proposed changes in AAIW provenance and properties across the Plio-Pleistocene climate transition, there are until now no proxy records of late Neogene changes in AAIW in the Atlantic Ocean. To elucidate both the development of AAIW in the South Atlantic Ocean and the telecommunication of AAIW with subtropical upwelling regions, we here present the first proxy records of Atlantic AAIW over the last four million years. Using sediments from South Atlantic DSDP Site 516, we present a multiproxy approach that tracks water mass provenance using neodymium (Nd) isotope ratios in Fe-Mn encrusted foraminifera, bottom water temperatures (BWT) from benthic foraminiferal (Uvigernina peregrina and Cibicidoides wuellerstorfi) Mg/Ca, and salinity derived from benthic oxygen isotopes and the Mg/Ca-derived BWT.
Section snippets
Benthic stable isotope and Mg/Ca analysis of Site 516 and 516 (A)
For preparation of samples for δ18O and Mg/Ca analyses we followed previous studies (Karas et al., 2009, 2017). Our Site 516 samples span the time period over the last 4 Myr. We used the >250 μm size fraction. Each sample represents 4–34 specimens of Uvigerina peregrina. The samples were gently crushed and visually separated under a microscope. Approximately two-thirds of each initial sample was used for Mg/Ca analyses with one-third reserved for stable isotope analyses. One to three specimens
Interpretation of εNd signatures of Site 516
The main source of the Nd in the foraminifera is authigenic Fe-Mn-oxide on the foraminiferal tests. While diagenetic porewaters can influence the εNd signatures on foraminiferal tests, especially in regions with high input of volcanic ashes like in the North Pacific close to Alaska (e.g. Abbott et al., 2015; Du et al., 2016), extracted εNd signatures from foraminifera from many (South) Atlantic sites have been shown to faithfully record ancient bottom water masses in different ocean settings
Conclusions
We document the emergence of modern-property AAIW during the Plio-Pleistocene in the South Atlantic Ocean at DSDP Site 516. Using a multi proxy-approach, including εNd in Fe-Mn-oxide encrusted foraminifera, benthic foraminiferal BWTMg/Ca, and stable isotopes, we reconstruct AAIW source water provenance, temperature, and salinity changes.
Our results indicate that prior to Northern Hemisphere Glaciation, during the Pliocene warm interval between 4 and 3 Ma, modern-property AAIW (cold, fresh, and
Author contributions
C.K. performed the analyses and wrote the first draft. All authors were involved in interpreting the data and bringing the manuscript to completion.
Conflicts of interest
The authors declare no competing interests.
Data and materials availability
Data is available at the NOAA National Center for Environmental Information (https://www.ncdc.noaa.gov/paleo/study/27750).
Acknowledgments
We thank the IODP for providing sample material. For technical support in the lab we thank L. Bolge. For further help in the lab and discussions we thank Y. Wu, W. Huang, K. Esswein, A. Dial, G. B. Whitlock, Y. Kiro, M. Yehudai, A. Bahr and D. Nürnberg. We thank the Max Kade Foundation, the German Research Foundation (project: KA3461/1-2), Columbia's Center for Climate and Life, and the Storke Endowment of the Columbia Department of Earth and Environmental Sciences for financial support for
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