Abstract
The late Oligocene experienced atmospheric concentrations of CO2 between 400 and 750 ppm, which are within the IPCC projections for this century, assuming unabated CO2 emissions. However, Antarctic
ice sheet and Southern Ocean paleoceanographic configurations during the late Oligocene are not well
resolved, but are important to understand the influence of high-latitude Southern Hemisphere feedbacks
on global climate under such CO2 scenarios. Here, we present late Oligocene (26-25 Ma) ice sheet and
paleoceanographic reconstructions recorded in sediments recovered by IODP Site U1356, offshore of
the Wilkes Land margin in East Antarctica. Our study, based on a combination of sediment facies
analysis, physical properties, and geochemical parameters, 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 to represent a northward shift of westerly winds and surface oceanic fronts.
During interglacial times, more oxygenated and ventilated conditions prevailed, which suggests
enhanced mixing of the water masses with enhanced current velocities. Micritic limestone intervals
within some of the interglacial facies represent warmer paleoclimatic conditions when less corrosive
warmer northern component water (e.g. North Atlantic sourced deep water) had a greater influence on
the site. The lack of iceberg rafted debris (IRD) throughout the studied interval contrasts with early
Oligocene and post-Oligocene sections from Site U1356 and with late Oligocene strata from the Ross
Sea (CRP and DSDP 270), which contain IRD and evidence for coastal sea ice and glaciers. These
observations, supported by elevated paleotemperatures and the absence of sea-ice, suggest that between
26 and 25 Ma reduced glaciers or ice caps occupied the terrestrial lowlands of the Wilkes Land margin.
Unlike today, the continental shelf was not over-deepened, and thus marine-based ice sheet expansion
was likely limited to coastal regions. Combined, these data suggest that ice sheets in the Wilkes
Subglacial Basin were largely land-based, and therefore retreated as a consequence of surface melt
during late Oligocene, rather than direct ocean forcing and marine ice sheet instability processes as it
did in younger past warm intervals. Spectral analysis on late Oligocene sediments from the eastern
Wilkes Land margin show that the glacial-interglacial cyclicity and resulting displacements of the
Southern Ocean frontal systems between 26-25 Ma were forced by obliquity.
ice sheet and Southern Ocean paleoceanographic configurations during the late Oligocene are not well
resolved, but are important to understand the influence of high-latitude Southern Hemisphere feedbacks
on global climate under such CO2 scenarios. Here, we present late Oligocene (26-25 Ma) ice sheet and
paleoceanographic reconstructions recorded in sediments recovered by IODP Site U1356, offshore of
the Wilkes Land margin in East Antarctica. Our study, based on a combination of sediment facies
analysis, physical properties, and geochemical parameters, 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 to represent a northward shift of westerly winds and surface oceanic fronts.
During interglacial times, more oxygenated and ventilated conditions prevailed, which suggests
enhanced mixing of the water masses with enhanced current velocities. Micritic limestone intervals
within some of the interglacial facies represent warmer paleoclimatic conditions when less corrosive
warmer northern component water (e.g. North Atlantic sourced deep water) had a greater influence on
the site. The lack of iceberg rafted debris (IRD) throughout the studied interval contrasts with early
Oligocene and post-Oligocene sections from Site U1356 and with late Oligocene strata from the Ross
Sea (CRP and DSDP 270), which contain IRD and evidence for coastal sea ice and glaciers. These
observations, supported by elevated paleotemperatures and the absence of sea-ice, suggest that between
26 and 25 Ma reduced glaciers or ice caps occupied the terrestrial lowlands of the Wilkes Land margin.
Unlike today, the continental shelf was not over-deepened, and thus marine-based ice sheet expansion
was likely limited to coastal regions. Combined, these data suggest that ice sheets in the Wilkes
Subglacial Basin were largely land-based, and therefore retreated as a consequence of surface melt
during late Oligocene, rather than direct ocean forcing and marine ice sheet instability processes as it
did in younger past warm intervals. Spectral analysis on late Oligocene sediments from the eastern
Wilkes Land margin show that the glacial-interglacial cyclicity and resulting displacements of the
Southern Ocean frontal systems between 26-25 Ma were forced by obliquity.
| Original language | English |
|---|---|
| Journal | Climate of the Past |
| DOIs | |
| Publication status | Published - 5 Dec 2017 |