PALEO-FE INPUTS TO THE SOUTHERN OCEAN ON GLACIAL/INTERGLACIAL TIME SCALES

Comparisons of terrigenous accumulation in sediments and dust flux estimates from Antarctica differ by more than an order of magnitude, supporting hemipelagic inputs as the main source of terrigenous material to the Southern Ocean. Furthermore, a similar mismatch in Fe accumulation points to a hemipelagic source rather than an eolian source for Fe. However, the sources of the terrigenous material and related Fe and the cause of the glacial/interglacial variability are not adequately quantified.

The high resolution and long records (~1Ma) recovered during ODP Leg 177 in the southeastern Atlantic Ocean provide an excellent opportunity to study terrigenous loading, both temporally and with latitude across the polar front zone (PFZ). This enables a more comprehensive view of Fe accumulation and cycling in the Southern Ocean. These records support the previously observed large changes in terrigenous loads during glacial intervals (5-100 times greater) compared to interglacial intervals. The high resolution of these records allows us to evaluate the timing of peak terrigenous loading compared to glacial state. For example, maxima in Fe accumulation rates at Site 1094 (53S), which was likely ice covered during glacial intervals, are focused at terminations and probably include an ice-rafted component. Maxima at Site 1089 (40S) tend to occur during full glacial conditions.

To examine provenance, we used Al/Ti and Fe/Ti ratios as proxies for changes in sources of detrital material. We found significant differences across the PFZ, averages ranging between basalt-like ratios (Al/Ti<10) and ratios indicative of continental crust (Al/Ti=15-27, Fe/Ti=11-14). Sites with basalt-like ratios reflect proximity to either a volcanic island or seamount. Downcore records further suggest glacial/interglacial changes in provenance. For example, glacial intervals at Site 1089 tend to be characterized by crustal ratios and interglacials by more basaltic ratios, possibly reflecting changes in the intensity of the Agulhus Current.

The records of Fe accumulation provide further support for a hemipelagic source of Fe to the Southern Ocean. Furthermore, glacial/interglacial variability in delivery and provenance is likely related to changes in ocean circulation on the same time scales.