Paleoceanography and ice sheet variability offshore Wilkes Land, Antarctica – Part 1: Insights from late Oligocene astronomically paced contourite sedimentation

Abstract

Abstract. Antarctic ice sheet and Southern Ocean paleoceanographic configurations during the late Oligocene are not well resolved. They are however important to understand the influence of high-latitude Southern Hemisphere feedbacks on global climate under CO2 scenarios (between 400 and 750 ppm) projected by the IPCC for this century, assuming unabated CO2 emissions. Sediments recovered by the Integrated Ocean Drilling Program (IODP) at Site U1356, offshore of the Wilkes Land margin in East Antarctica, provide an opportunity to study ice sheet and paleoceanographic configurations during the late Oligocene (26–25 Ma). Our study, based on a combination of sediment facies analysis, magnetic susceptibility, density, and X-ray fluorescence geochemical data, shows that glacial and interglacial sediments are continuously reworked by bottom currents, with maximum velocities occurring during the interglacial periods. Glacial sediments record poorly ventilated, low-oxygenation bottom water conditions, interpreted as resulting from a northward shift of westerly winds and surface oceanic fronts. Interglacial sediments record more oxygenated and ventilated bottom water conditions and strong current velocities, which suggests enhanced mixing of the water masses as a result of a southward shift of the polar front. Intervals with preserved carbonated nannofossils within some of the interglacial facies are interpreted as forming under warmer paleoclimatic conditions when less corrosive warmer northern component water (e.g., North Atlantic sourced deep water) had a greater influence on the site. Spectral analysis on the late Oligocene sediment interval shows that the glacial–interglacial cyclicity and related displacements of the Southern Ocean frontal systems between 26 and 25 Ma were forced mainly by obliquity. The paucity of iceberg-rafted debris (IRD) throughout the studied interval contrasts with earlier Oligocene and post-Miocene Climate Optimum sections from Site U1356 and with late Oligocene strata from the Ross Sea, which contain IRD and evidence for coastal glaciers and sea ice. These observations, supported by elevated sea surface paleotemperatures, the absence of sea ice, and reconstructions of fossil pollen between 26 and 25 Ma at Site U1356, suggest that open-ocean water conditions prevailed. Combined, this evidence suggests that glaciers or ice caps likely occupied the topographic highs and lowlands of the now marine Wilkes Subglacial Basin (WSB). Unlike today, the continental shelf was not overdeepened and thus ice sheets in the WSB were likely land-based, and marine-based ice sheet expansion was likely limited to coastal regions.

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

“…2; Escutia et al, 2011). Salabarnada et al (2018) present a detailed lithologic column of the Oligocene and Miocene sediments. The lithologic facies described in Salabarnada et al (2018) will help us compare paleoceanographic differences among climatic extremes.…”

“…The lithologic facies described in Salabarnada et al (2018) will help us compare paleoceanographic differences among climatic extremes. Salabarnada et al (2018) distinguished various lithologies along with interpretations of their depositional settings which can be summarised as (1) laminated silty clay sediments (interpreted as glacial deposits; hereafter Fg), (2) bioturbated siltstones and claystones that in some intervals are carbonate cemented (interpreted as interglacial deposits, hereafter Fi), and (3) perturbed mass transport deposits (MTDs): slumps and debris flows. We refer to Salabarnada et al (2018;Fig.…”

“…Salabarnada et al (2018) distinguished various lithologies along with interpretations of their depositional settings which can be summarised as (1) laminated silty clay sediments (interpreted as glacial deposits; hereafter Fg), (2) bioturbated siltstones and claystones that in some intervals are carbonate cemented (interpreted as interglacial deposits, hereafter Fi), and (3) perturbed mass transport deposits (MTDs): slumps and debris flows. We refer to Salabarnada et al (2018;Fig. S2 in their Supplement) for a detailed description of these facies, and to the supplementary datasets on PANGAEA (Bijl et al, 2018b) for more detailed separation of our palynological results per facies type.…”

“…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.…”

“…We reconstruct the oceanographic regimes during the Oligocene and mid-Miocene and evaluate their implications. We further compare the palynological data with lithological observations and their interpretations from Salabarnada et al (2018). Pairing the sedimentological interpretations and biomarker-derived absolute sea-surface tem- perature (SST) reconstructions from Site U1356 (Hartman et al, 2017) with our dinocyst assemblage data, we reconstruct the paleoceanographic conditions off Wilkes Land and assess their variability on both glacial-interglacial and longer-term timescales.…”