Paleoceanography and ice sheet variability offshore Wilkes Land, Antarctica – Part 2: Insights from Oligocene–Miocene dinoflagellate cyst assemblages
2018
Abstract. Next to atmospheric CO 2 concentrations, ice-proximal oceanographic
conditions are a critical factor for the stability of Antarctic
marine-terminating ice sheets. The Oligocene and Miocene epochs
( ∼ 34–5 Myr ago) were time intervals with atmospheric CO 2
concentrations between those of present-day and those expected for the near
future. As such, these past analogues may provide insights into ice-sheet
volume stability under warmer-than-present-day climates. We present
organic-walled dinoflagellate cyst (dinocyst) assemblages from
chronostratigraphically well-constrained Oligocene to mid-Miocene sediments
from Integrated Ocean Drilling Program (IODP) Site U1356. Situated
offshore the Wilkes Land continental margin, East Antarctica, the sediments
from Site U1356 have archived the dynamics of an ice sheet that is today
mostly grounded below sea level. We interpret dinocyst assemblages in terms
of paleoceanographic change on different timescales, i.e. with regard to
both glacial–interglacial and long-term variability. Our record shows that a
sea-ice-related dinocyst species, Selenopemphix antarctica , occurs
only for the first 1.5 Myr of the early Oligocene, following the onset of full
continental glaciation on Antarctica, and after the Mid-Miocene Climatic
Optimum. Dinocysts suggest a weaker-than-modern sea-ice season for the
remainder of the Oligocene and Miocene. The assemblages generally bear strong
similarity to present-day open-ocean, high-nutrient settings north of the
sea-ice edge, with episodic dominance of temperate species similar to those
found in the present-day subtropical front. Oligotrophic and temperate
surface waters prevailed over the site notably during interglacial times,
suggesting that the positions of the (subpolar) oceanic frontal systems have
varied in concordance with Oligocene–Miocene glacial–interglacial climate
variability.
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