Obliquity Dominance in Early Pleistocene Sediments From the Antarctic Zone of the Southern Ocean (Indian Ocean Sector)

We constructed a record of percent biogenic silica (opal) accumulation at Ocean Drilling Program Site 745B located in the Indian Ocean sector of the Antarctic Zone of the Southern Ocean. The record spans the majority of the early Pleistocene (1.1-2.6 Ma). Orbital-scale sampling affords a look at the relative importance of obliquity versus precession variability through a time interval that is characterized by obliquity pacing in early Pleistocene delta 18O records. Variations in the site's magnetic susceptibility record closely resemble those in the benthic foraminiferal delta 18O stack (Lisiecki & Raymo, 2005, ) and provide orbital-scale age control. Between 1.1 and 1.8 Ma, obliquity-related 41 kyr spectral peaks dominate with relatively little power at precession periods (23-19 kyr) in all records. Between 1.8 and 2.6 Ma, only the delta 18O and magnetic susceptibility data display a distinct 41 kyr peak, while the opal lacks spectral power at any of the orbital periodicities. The lack of more pronounced precession-scale variations in the two proxy records is consistent with observations in foraminiferal delta 18O records. A low or absent response to precession in these records appears to be due to environmental control. 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 the site's position outside the region of wind-driven upwelling, and/or upwelling waters undersaturated with respect to silica prior to the establishment of the opal belt at about 2 Ma. Orbital-scale variations in magnetic susceptibility resemble those in benthic foraminiferal delta 18O and provide orbital-scale age control Biogenic silica lacks a consistent response to precession between 1.1 and 2.6 Ma; obliquity-scale variability begins at 1.8 Ma Results support that lack of more pronounced precession in early Pleistocene delta 18O records may not be solely due to a muting of the signal