Late Pliocene cooling, sea ice and the establishment of a Ross Sea polynya: Geochemical and diatom assemblage constraints from McMurdo Sound, Antarctica
Abstract
The marine sediment cores collected by the Antarctic Geological Drilling (ANDRILL) Program from sites beneath the McMurdo Ice Shelf (MIS; Core AND-1B) and in Southern McMurdo Sound (SMS; Core AND-2A) represent the most complete record to date of Neogene climate evolution proximal to the Antarctic continent. Diatom-rich lithologic units alternate with glacial sediments throughout the Pliocene and early Pleistocene of AND-1B; each diatom-rich unit within this oscillating record has distinctive geochemical and diatom assemblage characteristics and most are interpreted to preserve single interglacial intervals of 40-thousand-year glacial/interglacial cycles. Though the dramatic Pliocene glacial/interglacial oscillations recorded at the MIS site are absent in the shallower SMS record, AND-2A preserves a single diverse late Pliocene diatom assemblage, providing an additional constraint on Ross Sea Pliocene climate. Here, we focus on the reconstruction of sea surface conditions from four discrete AND-1B interglacial units deposited ~3.2, 3.0, 2.9, and 2.6 Ma. Diatom assemblages record the onset of Plio-Pleistocene cooling in the Ross Sea at 3.2 Ma, intensifying at 3.0 Ma, and suggest spring blooms in a surface ocean seasonally stratified by sea ice melt. Following the initial cooling, an increase in warm-water species at 2.9 and 2.6 Ma records a temporary late Pliocene reversal in the cooling trend. The Pliocene diatom-bearing interval in AND-2A is equivalent to the 2.6 Ma diatomite, providing further evidence for late Pliocene reversion to warmer open ocean conditions. Cooling resumes in the early Pleistocene, but sea-ice related diatoms, which dominate late Pleistocene and recent Antarctic sediments, are present only as minor components throughout the ANDRILL records. Sedimentary δ13C and δ15N in the AND-1B diatomite units provide additional insights into Pliocene evolution of sea ice, stratification, and primary productivity. For AND-1B diatomite units younger than 3.2 Ma, δ13C and δ15N vary in phase, the amplitude of δ13C fluctuations increases progressively up section, and peak interglacial δ15N trends toward more positive values, reflecting increasing variability in surface-water stratification within individual glacial minima. During the peaks of Late Pliocene glacial minima, negative δ13C values likely indicate enhanced wind mixing and summer polynya formation over AND-1B, while more positive values during the start and end of glacial minima indicate increased stratification due to sea ice melt or reduced wind stress. In sequence, these units offer a unique perspective on the changing character of the interglacial environment in the Ross Embayment spanning the transition from the mid-Pliocene climatic optimum into modern cold-polar conditions.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMPP12B..04R
- Keywords:
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- 1041 GEOCHEMISTRY / Stable isotope geochemistry;
- 4944 PALEOCEANOGRAPHY / Micropaleontology;
- 9310 GEOGRAPHIC LOCATION / Antarctica;
- 9605 INFORMATION RELATED TO GEOLOGIC TIME / Neogene