Southeastern Atlantic deep-water evolution during the late-middle Eocene to earliest Oligocene (Ocean Drilling Program Site 1263 and Deep Sea Drilling Project Site 366)

Abstract

Comparison of new benthic foraminiferal δ 18 O and δ 13 C records from Ocean Drilling Program (ODP) Site 1263 (Walvis Ridge, southeast Atlantic, 2100 m paleodepth) and Deep Sea Drilling Project (DSDP) Site 366 (Sierra Leone Rise, eastern equatorial Atlantic, 2200-2800 m paleodepth) with published data from Atlantic and Southern Ocean sites provides the means to reconstruct the development of deep-water circulation in the southeastern Atlantic from the late-middle Eocene to the earliest Oligocene. Our comparison shows that in the late-middle Eocene (ca. 40 Ma), the South Atlantic was characterized by a homogeneous thermal structure. Thermal differentiation began ca. 38 Ma. By 37.6 Ma, Site 1263 was dominated by Southern Component Water; at the same time, warm saline deep water filled the deeper South Atlantic (recorded at southwest Atlantic ODP Site 699, paleodepth 3400 m, and southeast Atlantic ODP Site 1090, paleodepth 3200 m). The deep-water source to eastern equatorial Site 366 transitioned to Northern Component Water ca. 35.6-35 Ma. Progressive cooling at Site 1263 during the middle to late Eocene and deep-water thermal stratification in the South Atlantic may be attributed at least in part to the gradual deepening and strengthening of the proto-Antarctic Circumpolar Current from the late-middle Eocene to the earliest Oligocene, as the Drake and Tasman gateways opened. Our isotopic comparisons across depth and latitude provide evidence of the development of deep-water circulation similar to modern-day Atlantic Meridional Overturning Circulation.

No Change in Southern Ocean Circulation in the Indian Ocean From the Eocene Through Late Oligocene

“…An increase in the flux of NCW into the Southern Ocean would cause a decrease in ε Nd ( t ) values into the Oligocene, due to the mixing of unradiogenic NCW ( ɛ Nd = −10.07 ± 0.10; ferromanganese crust sample ALV‐539; O'nions et al, ) with proto‐CDW (i.e., ε Nd ( t ) = −7.5 to −8 in the late Eocene; Scher et al, ). Comparison of benthic foraminiferal δ 18 O records suggests the inception of NCW formation around the middle to late Eocene (~38.5 Ma) in the Labrador Sea (Borrelli et al, ), strengthening around 35 Ma (Langton et al, ). The presence of NCW in the deep Atlantic by the earliest Oligocene is also supported by the onset of drift deposition in the northeast Atlantic (Davies et al, ) and δ 13 C deep‐water aging gradients (Elsworth et al, ), though Abelson and Erez () suggest NCW and the onset of modern‐like Atlantic meridional overturning circulation (AMOC) occurred during the late Eocene, immediately before the EOT.…”

Evidence for a heavily glaciated Antarctica during the late Oligocene “warming” (27.8–24.5 Ma): Stable isotope records from ODP Site 690

“…This was occurring at a time when equatorial seaways (i.e., Panama and Tethys) were still open, allowing water mass flow from the Atlantic to Pacific Oceans. Foraminiferal studies have suggested a warm, saline deepwater mass existed during the Eocene and Oligocene in the Atlantic and Pacific Oceans and had a low‐latitude source [ Borrelli and Katz , ; Kennett and Stott , ; Langton et al , ; Mead et al , ; Pak and Miller , ; Pekar et al , ; Via and Thomas , ].…”

“…This gradient began developing in the middle Eocene [ Cramer et al , ], reaching full extent in the early Oligocene. The development of this gradient has been linked to the strengthening and deepening of the Antarctic Circumpolar Current (ACC) during the late Eocene to early Oligocene [ Borrelli et al , ; Borrelli and Katz , ; Cramer et al , ; Katz et al , ; Langton et al , ; Scher et al , ; Scher and Martin , ], and development of a multilayered ocean structure by ~30 Ma as the high‐latitude Southern Ocean became thermally isolated [ Cramer et al , ; Katz et al , ; Scher et al , ]. This led to a differentiation between southern and northern deepwater sources [ Via and Thomas , ] and allowed the Atlantic and Pacific Ocean basins to be bathed by a new bottom water mass while the Southern Ocean became isolated and homogenized [ Scher et al , ].…”

Quantifying the Effect of the Drake Passage Opening on the Eocene Ocean

“…In the Atlantic Ocean, several studies have documented thermal differentiation between the North and South Atlantic with an~2°C cooling of southern high latitudes, represented by añ 0.5‰ δ 18 O bf difference, starting between 38.5 and 35 Ma (Borrelli et al, 2014;Coxall et al, 2018;Cramer et al, 2009;Katz et al, 2011;Langton et al, 2016;Liu et al, 2018). Southern Hemisphere cooling in the Atlantic basin has often been interpreted as an indicator of the onset of a proto-ACC and of a decreased southward OHT (Borrelli et al, 2014;Katz et al, 2011;Langton et al, 2016). Borrelli et al (2014) further describe brief warming of North Atlantic deep waters occurring contemporaneously to the thermal differentiation (~38.5 Ma) and interpret this warming as the signature of deepwater formation in the Northern Hemisphere.…”

“…Further, DP opening is often correlated with the contemporaneous onset of a marked difference between northern and southern latitude temperatures in the Atlantic Ocean, with cooler temperatures in the high southern latitudes suggested by δ 18 O data (Borrelli et al, 2014;Coxall et al, 2018;Cramer et al, 2009;Katz et al, 2011;Langton et al, 2016). On the other hand, different proxies (δ 13 C, εNd, δ 18 O, and contourites) suggest the onset of North Atlantic Deep Water formation (NADW) during the late Eocene, which may also have contributed to the thermal differentiation mentioned above (Borrelli et al, 2014;Coxall et al, 2018;Hohbein et al, 2012;Katz et al, 2011;Langton et al, 2016;Scher & Martin, 2008).…”