Antarctic climate, Southern Ocean circulation patterns, and deep water formation during the Eocene

Abstract

We assess early‐to‐middle Eocene seawater neodymium (Nd) isotope records from seven Southern Ocean deep‐sea drill sites to evaluate the role of Southern Ocean circulation in long‐term Cenozoic climate change. Our study sites are strategically located on either side of the Tasman Gateway and are positioned at a range of shallow (<500 m) to intermediate/deep (~1000–2500 m) paleowater depths. Unradiogenic seawater Nd isotopic compositions, reconstructed from fish teeth at intermediate/deep Indian Ocean pelagic sites (Ocean Drilling Program (ODP) Sites 738 and 757 and Deep Sea Drilling Project (DSDP) Site 264), indicate a dominant Southern Ocean‐sourced contribution to regional deep waters (εNd(t) = −9.3 ± 1.5). IODP Site U1356 off the coast of Adélie Land, a locus of modern‐day Antarctic Bottom Water production, is identified as a site of persistent deep water formation from the early Eocene to the Oligocene. East of the Tasman Gateway an additional local source of intermediate/deep water formation is inferred at ODP Site 277 in the SW Pacific Ocean (εNd(t) = −8.7 ± 1.5). Antarctic‐proximal shelf sites (ODP Site 1171 and Site U1356) reveal a pronounced erosional event between 49 and 48 Ma, manifested by ~2 εNd unit negative excursions in seawater chemistry toward the composition of bulk sediments at these sites. This erosional event coincides with the termination of peak global warmth following the Early Eocene Climatic Optimum and is associated with documented cooling across the study region and increased export of Antarctic deep waters, highlighting the complexity and importance of Southern Ocean circulation in the greenhouse climate of the Eocene.

Late Oligocene obliquity-paced contourite sedimentation in the Wilkes Land margin of East Antarctica: implications for paleoceanographic and ice sheet configurations

“…Under these conditions, bottom water formation and downwelling can still occur (with or without presence of sea ice) as a result of density contrasts related to seasonal changes in surface water temperature and salinity (Huber and 620 Sloan, 2001;Otto-Bliesner et al, 2002). Moreover, stable Nd isotopic composition in EoceneOligocene sediments from Site U1356 is consistent with modern day formation of bottom water from Adélie Land, as reported by Huck et al (2017). Our evidence above points to deposition of F1 during glacial cycles under poorly-ventilated, low-625 oxygenation conditions at the water-sediment interface (Fig.…”

Global Extent of Early Eocene Hyperthermal Events: A New Pacific Benthic Foraminiferal Isotope Record From Shatsky Rise (ODP Site 1209)

“…The gradients between basins diminish during hyperthermals due to reductions or reversals in overturning circulation (e.g., Zeebe & Zachos, ) triggered by warming (Kirtland Turner et al, ; Lunt et al, ). Convergence of δ 13 C from 52 to 47.25 Ma could point to increased deep‐water exchange (bidirectional mixing) between Atlantic and Pacific well before the widening of the circum‐polar gateways ~40 Ma ago (Scher & Martin, ; Thomas et al, ) or an increase in bottom water formation around Antarctica as reported from South Ocean Nd records (Huck et al, ). A reduction in the style of communication (bidirectional to uni‐directional) after 47.25 Ma could be the result of changes in deep‐water formation regions as climate cools after the end of the EECO.…”

Late Eocene Southern Ocean Cooling and Invigoration of Circulation Preconditioned Antarctica for Full‐Scale Glaciation

“…In the southwest Atlantic, at Sites 696 and 511 and the onshore Austral Basin, the effects of tectonic reorganization near the Drake Passage are expected to be potentially important. Although tectonic reconstructions for Drake Passage deepening remain ambivalent, there are no clear indications for a major reorganization in the late Eocene (e.g., Hill et al, ; Huck et al, ; Lagabrielle et al, ; Wright et al, ). We therefore ascribe enhanced bottom water flow in the southwest Atlantic to enhanced circulation of its western boundary as a likely westward propagation of the Antarctic Countercurrent.…”