Early Pleistocene Sediment Record from the Antarctic Zone of the Southern Ocean Dominated by Obliquity

We present an orbital-scale record of percent biogenic silica (opal) at Ocean Drilling Program Site 745B situated in the Antarctic Zone of the Indian Ocean sector of the Southern Ocean spanning a majority of the early Pleistocene (1.1-2.6 Ma). By investigating the relative importance of obliquity versus precession-paced variability in our record, we seek to contextualize the apparent dominance of obliquity pacing in early Pleistocene d¹⁸O records. Notably, between 1.1 and 1.8 Ma, both the site’s shipboard magnetic susceptibility record and our biogenic silica record principally exhibit obliquity-related spectral peaks at a periodicity of 41 kyr, with relatively minor spectral power at precessional periodicities (23-19 kyr). During the older part of the record (1.8-2.6 Ma), only d¹⁸O and magnetic susceptibility vary at the 41 kyr obliquity periodicity, while the biogenic silica record does not show prominent orbital pacing at any of the major periodicities. We suggest that the surprising dominance of obliquity-paced variability in all records between 1.1 and 1.8 Ma indicate a lack of response of the proxies to precessional forcing during this period. The notable lack of orbital forcing in the opal record before 1.8 Ma may reflect both a more southerly location of the polar frontal zone with respect to the site and thus outside the region of wind-driven upwelling and waters undersaturated with respect to silica prior to the establishment of the opal belt at about 2 Ma.