Southern Ocean warming and Wilkes Land ice sheet retreat during the mid-Miocene

Abstract

Observations and model experiments highlight the importance of ocean heat in forcing ice sheet retreat during the present and geological past, but past ocean temperature data are virtually missing in ice sheet proximal locations. Here we document paleoceanographic conditions and the (in)stability of the Wilkes Land subglacial basin (East Antarctica) during the mid-Miocene (~17–13.4 million years ago) by studying sediment cores from offshore Adélie Coast. Inland retreat of the ice sheet, temperate vegetation, and warm oligotrophic waters characterise the mid-Miocene Climatic Optimum (MCO; 17–14.8 Ma). After the MCO, expansion of a marine-based ice sheet occurs, but remains sensitive to melting upon episodic warm water incursions. Our results suggest that the mid-Miocene latitudinal temperature gradient across the Southern Ocean never resembled that of the present day. We demonstrate that a strong coupling of oceanic climate and Antarctic continental conditions existed and that the East Antarctic subglacial basins were highly sensitive to ocean warming.

Paleoceanography and ice sheet variability offshore Wilkes Land, Antarctica – Part 2: Insights from Oligocene–Miocene dinoflagellate cyst assemblages

“…The latter is strongly coupled to polar surface water temperature, as deep-water formation was predominantly at high latitudes at that time (Herold et al, 2011). High-amplitude variations in benthic δ 18 O thus suggest either (i) strong climate dynamics in the high latitudes with relatively minor ice volume change (which would be in accordance with numerical modelling experiments (Barker et al, 1999) and the interpretation of Foster and Rohling, 2013), or (ii) strong fluctuations in Antarctic ice volume, with relatively subdued temperature variability (which would be in accordance with indications for unstable Antarctic ice sheets under warmer-than-present climates Greenop et al, 2014;Rovere et al, 2014;Sangiorgi et al, 2018). If one assumes a presentday δ 18 O composition (−42 ‰ versus standard mean ocean water) for Oligocene-Miocene Antarctic ice sheets and modern deep-water temperature (2.5 • C), the benthic δ 18 O fluctuations during the Oligocene-Miocene suggest long-term icesheet variability to have fluctuated considerably (Liebrand et al, 2017).…”

“…These assemblages prevail in surface waters with mean annual temperatures of 8-17 • C (Prebble et al, 2013) and therefore suggest relatively warm surface water temperatures close to the Wilkes Land margin. In support of this, a Bayesian approach on the TEX 86 index values at Site U1356 (presented in Sangiorgi et al, 2018;Hartman et al, 2017) also suggests the Southern Ocean mid-latitudes as a modern-analogue region and reconstructs a paleotemperature range of 8-20 • C for the Oligocene-Miocene at Site U1356, with values in excess of 24 • C for the late Oligocene (Hartman et al, 2017). Further, supporting evidence for temperate Oligocene-Miocene surface waters comes from the abundance of nannofossils encountered in the sediments (Escutia et al, 2011;Salabarnada et al, 2018).…”

“…This approach takes advantage of studies on the present-day relationship between Southern Ocean microplankton in general and dinoflagellate cysts in particular and their surface water characteristics (e.g. Eynaud et al, 1999;Zonneveld, 2002, 2007;Prebble et Sangiorgi et al, 2018) and lithostratigraphic units (Escutia et al, 2011). Samples taken for palynology and age-depth plot (tie points were derived from Tauxe et al, 2012, which has been recalibrated to the GTS2012 timescale of Gradstein et al, 2012, and modified based on Crampton et al, 2016).…”

“…Figure 4. Core recovery, lithostratigraphic facies (after Salabarnada et al, 2018, andSangiorgi et al, 2018), lithologic units (Escutia et al, 2011), chronostratigraphic epochs (E: Eocene) and stages (L: Lutetian; Burd. : Burdigalian; Ser.…”

“…A marked increase in abundance of gonyaulacoid cysts is associated with the Mid-Miocene Climatic Optimum (MMCO, between ∼ 17 and 15 Ma; Figs. 4, 5; Sangiorgi et al, 2018). Of the gonyaulacoid taxa, Nematosphaeropsis labyrinthus is associated with frontal systems of the present-day Southern Ocean (Prebble et al, 2013) and of the North Atlantic Ocean (Boessenkool et al, 2001;Zonneveld et al, 2013).…”