A STREAMLINED METHOD FOR DETERMINATION OF δ15N OF DIATOM-BOUND ORGANIC MATTER: APPLICATION TO THE PLIOCENE SOUTHERN OCEAN
We have modified existing methods of DBOM isolation and δ15N analysis to improve throughput while maintaining quality. We returned to EA analytical approaches, making several improvements to reduce N2 blanks. Results from a test with acid-washed frustule pieces indicated no detectable N2 blank. Additionally, we optimized current diatom-cleaning protocols to more efficiently separate clays and associated organic matter early in the process, thus increasing sample throughput.
Here we present results from a preliminary down-core bulk sediment δ15N record from Southern Ocean DSDP Site 745 (60S, 86E, 4082m water depth). Over the long term, bulk sediment δ15N values increase from late Miocene through early Pleistocene (~6.5-1.5 Ma) with an interval of relatively low values during early Pliocene (~5-3 Ma). To a first order, lower δ15N values during early Pliocene are consistent with reduced nitrate utilization due to weaker upper ocean stratification during the climatic warm interval. Results need to be confirmed with δ15N of DBOM and a higher resolution sampling strategy in order to support the hypothesis of a Southern Ocean role in Pliocene climate.