Late Oligocene-Miocene proto-Antarctic Circumpolar Current dynamics off the Wilkes Land margin, East Antarctica

Abstract

At present, the Southern Ocean plays an important role in the global climate system and in modern Antarctic ice sheet dynamics. Past Southern Ocean configurations are however poorly understood. This information is yet important as it may provide important insights into the climate system and past ice-sheet behavior under warmer than present day climates. Here we study Southern Ocean dynamics during the Oligocene and Miocene when reconstructed atmospheric CO2 concentrations were similar to those expected during this century. We reconstruct snapshots of late Oligocene to earliest Miocene (24.2−23 Ma) paleoceanographic conditions in the East Antarctic Wilkes Land abyssal plain. For this, we combine marine sedimentological, geochemical (X-ray fluorescence, TEX86,), palynological and isotopic (εNd) records from ocean sediments recovered at Deep Sea Drilling Project (DSDP) Site 269. Overall, we find that sediments, delivered to the site by gravity flows and hemipelagic settling during glacial−interglacial cycles, were persistently reworked by a proto-Circumpolar Deep Water (CDW) with varying strengths that result from climatically controlled frontal system migrations. Just prior to 24 Ma, terrigenous input of predominantly fine-grained sediments deposited under weak proto-CDW intensities and poorly ventilated bottom conditions dominates. In comparison, 24 Ma marks the start of episodic events of enhanced proto-CDW current velocities, associated with coarse-grained deposits and better-ventilated bottom conditions. In particular, the dominance of P-cyst and low Calcium (Ca) in the sediments between 24.2 Ma and 23.6 Ma indicate the presence of an active open ocean upwelling associated with high nutrient conditions. This is supported by TEX86-derived sea surface temperature (SST) data pointing to cool ocean conditions. From 23.6 to 23.2 Ma, our records reveal an enrichment of Ca in the sediments related to increased calcareous microfossil preservation, high amounts of G-cysts and increasing TEX86-SSTs. This implies warmer water masses reaching the Antarctic margin as the polar front migrated southward. Together with the radiogenic Nd isotope data indicating modern-like CDW values, our records suggest a prominent poleward expansion of proto-CDW over our study site and reduced AABW formation during the latest Oligocene (i.e. 23.2 Ma ago). Our findings support the notion of a fundamentally different Southern Ocean, with a weaker proto-ACC than present during the late Oligocene and the earliest Miocene.

Subduction initiation in the Scotia Sea region and opening of the Drake Passage: when and why?

“…Ocean connections remained strongly constricted until <26 Ma, by obstruction through continental blocks and narrow deep-ocean passageways. This agrees with interpretations of a weak ACC and relatively flat latitudinal sea surface temperature gradients across the Southern Ocean during the Oligocene (Bijl et al, 2018;Evangelinos et al, 2020;Hartman et al, 2018;Salabarnada et al, 2018), which continued into the Miocene (~10 Ma; Sangiorgi et al, 2018). Although this may not be all due to the development of the Scotia Sea and other factors may have contributed (climate deterioration and ice sheet expansion), the Drake Passage was strongly constricted for most of the late Paleogene-early Neogene, and strengthening of the ACC coincides with major tectonic widening of the Scotia Sea ( Fig.…”

Magnetostratigraphic Chronology of a Cenozoic Sequence From DSDP Site 274, Ross Sea, Antarctica

“…Regardless of the exact explanation of the unconformity, these insights are invaluable because they demonstrate that the Early Miocene is characterized by long-term fluctuations of the Antarctic Ice Sheets (Levy et al, 2016;Levy et al, 2019;Jovane et al, 2019;Evangelinos et al, 2020) and that, prior to the Middle Miocene Climatic Optimum (∼15 Ma), there was a period of climatic instability modulated by long-term variations with phases of bottom current intensification which either eroded or did not allow the deposition of sediments in the deep parts of the basin.…”

Temperate Oligocene surface ocean conditions offshore of Cape Adare, Ross Sea, Antarctica

“…Deppeler and Davidson, 2017). Although our data suggest that ocean conditions were colder inshore than further offshore, they remain warm considering their proximity to marine-terminating outlet glaciers and ice caps in the Ross Sea area (De Santis et al, 1999;Galeotti et al, 2016;Levy et al, 2019;Kulhanek et al, 2019;Evangelinos et al, 2021). Levy et al (2019) provided a model for ice-proximal to ice-distal oceanographic conditions in the Ross Sea during the Oligocene.…”

“…Levy et al (2019) interpreted a prominent increase in the sensitivity of benthic oxygen isotope variations to obliquity forcing (termed "obliquity sensitivity") between 24.5 and 24 Ma, synchronous with the first occurrence of ice-proximal glaciomarine sediments at DSDP Site 270, disconformities in CRP-2/2A and a large turnover in Southern Ocean phytoplankton. The major expansion of the ice sheet close to the Oligocene-Miocene boundary in the Ross Sea (Levy et al, 2019;Kulhanek et al, 2019;Evangelinos et al, 2021) argues in favour of Ross Sea bottom water strengthening, leading to the slowdown of the sedimentation rates above 192.7 m b.s.f. and the formation of the >7 Myr duration hiatus at ∼ 181 m b.s.f.…”