Orbital-scale variability in biogenic silica accumulation in the Indian Ocean sector of the Southern Ocean during the early Pleistocene
Date
2023
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Publisher
University of Delaware
Abstract
The early Pleistocene (0.7 – 2.5 million years ago, Ma) is characterized by δ18O and inferred global ice volume variability dominantly at the 41 kyr obliquity periodicity. Despite being hypothesized to substantially influence global ice sheet variability, precessional-scale orbital forcing is barely present in the global δ18O record. To further investigate proposed mechanisms responsible for this apparent mismatch and, in particular, to test whether the percent opal record indicates the presence of an interhemispheric δ18O cancellation mechanism as proposed by Raymo et al. (2006) during the early Pleistocene, I generated an orbital-scale record of biogenic opal deposition from Ocean Drilling Program Site 745B in the Indian Ocean sector of the Southern Ocean between 1.6 – 2.47 Ma. A robust age model was constructed for this and an unpublished portion of the record between 1.07 and 1.6 Ma by orbitally tuning variations in magnetic susceptibility to variations in the global δ18O stack LR04 (Lisiecki & Raymo, 2005). Throughout the study period (1.07 – 2.47 Ma), changes in percent opal trace glacial-interglacial changes in the LR04 δ18O stack, highlighting the general sensitivity of primary productivity at the site to orbital insolation forcing. The fit between percent opal and the LR04 is particualry good in the interval between 1.2 and 1.6 Ma, during which both the LR04 and percent opal are particularly dominated by obliquity-period variability. I interpret this surprising dominance of the obliquity-period over precession in the opal record to reflect obliquity-driven changes in latitudinal temperature gradients. Due to a relatively minor contribution from precession-paced variability and the misalignment of precessional peaks between LR04 and percent opal, this record does neither support nor question the presence of an interhemispheric cancellation mechanism. I submit that changes in the overall pacing of orbital-scale productivity changes at Site 745, from more precessional variability between 2.0 and 2.4 Ma to more obliquity-focused variability between 1.2 and 1.6 Ma, may have been driven by changes in the sensitivity of the Antarctic Ice Sheet to orbital insolation forcing at these periods, due to an extension in summer sea-ice melting season resulting from the particularly warm interglacials during this time period. Moreover, I identify a remarkable maximum in percent opal and opal mass accumulation rates at 2.1 Ma (MIS 78) as indicating the establishment of a modern-like Polar Front system in the Southern Ocean region.
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Keywords
Obliquity, Pleistocene, Precession, Productivity, Southern Ocean