The onset of modern‐like Atlantic meridional overturning circulation at the Eocene‐Oligocene transition: Evidence, causes, and possible implications for global cooling

Abstract

A compilation of benthic δ18O from the whole Atlantic and the Southern Ocean (Atlantic sector) shows two major jumps in the interbasinal gradient of δ18O (Δδ18O) during the Eocene and the Oligocene: one at ∼40 Ma and the second concomitant with the isotopic event of the Eocene‐Oligocene transition (EOT), ∼33.7 Ma ago. From previously published circulation models and proxies, we show that the first Δδ18O jump reflects the thermal isolation of Antarctica associated with the proto‐Antarctic circumpolar current (ACC). The second marks the onset of interhemispheric northern‐sourced circulation cell, similar to the modern Atlantic meridional overturning circulation (AMOC). The onset of AMOC‐like circulation slightly preceded (100–300 kyr) the EOT, as we show by the high‐resolution profiles of δ18O and δ13C previously published from DSDP/ODP sites in the Southern Ocean and South Atlantic. These events coincide with the onset of antiestuarine circulation between the Nordic seas and the North Atlantic which started around the EOT and may be connected to the deepening of the Greenland‐Scotland Ridge. We suggest that while the shallow proto‐ACC supplied the energy for deep ocean convection in the Southern Hemisphere, the onset of the interhemispheric northern circulation cell was due to the significant EOT intensification of deepwater formation in the North Atlantic driven by the Nordic antiestuarine circulation. This onset of the interhemispheric northern‐sourced circulation cell could have prompted the EOT global cooling.

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

“…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. We interpret the long‐term decreasing trend at our study sites to be related to the increase in NCW contribution to proto‐CDW.…”

Transient temperature asymmetry between hemispheres in the Palaeogene Atlantic Ocean

“…The marked long-term increase in alkenone abundance across the EOT at Site U1404 (Fig. 2, S3) does not appear to be directly associated with surface temperature changes and may reflect surface ocean productivity in response to circulation changes [11][12][13][14][15]21 . For most of the study period, short-term warming events are associated with modest transient increases in alkenone concentration (Fig.…”

“…The EOT is marked by substantial climatic and oceanic reorganization, including atmospheric CO 2 drawdown 7,8 , high-latitude cooling 9 , widespread Antarctic glaciation, and deepening of the carbonate compensation depth 6 and northward migration of the Intertropical Convergence Zone 10 in the Pacific. It has also been suggested that the EOT marked the initiation of Atlantic Meridional Overturning Circulation (AMOC) [11][12][13] , whereas the Antarctic Circumpolar Current 14 and modern ocean structure 15 are suggested to have developed later around 30 Ma. Once established, the early Antarctic ice sheets appear to have been extremely dynamic in response to orbitally paced changes in high latitude climate forcing 1,4,6 .…”

“…3b). Increasing evidence [11][12][13] suggests that the 37-34 Ma interval is critical to understanding the early development of AMOC. Our incomplete record during this interval prevents us from confidently linking surface temperature changes to the detailed early history of AMOC.…”

Phosphogenesis during the Cenozoic transition from greenhouse to icehouse conditions: Upper Oligocene to lower Miocene siliceous, phosphate, and organic‐rich sediments near La Purísima, Baja California Sur, Mexico

“…A further circumstance that may have catalysed phosphorite formation during these time periods is the focused return of phosphorus-enriched deep waters to the continental margin by an increase in the efficiency of coastal upwelling (Berger, 2007). This is often related to the isolation of the Antarctic continent and the onset of the Antarctic Circumpolar Current (ACC; Barker, Filippelli, Florindo, Martin, & Scher, 2007;Lyle, Gibbs, Moore, & Rea, 2007), in tandem with the progressive shift to North Atlantic Deep Water formation (NADW; Scher & Martin, 2008;Abelson & Erez, 2017;Coxall et al, 2018). These oceanographic changes may have led to an overall increase in bottomwater circulation, which improved the exchange between high-latitude and low-latitude deep waters.…”