A STREAMLINED METHOD FOR DETERMINATION OF δ15N OF DIATOM-BOUND ORGANIC MATTER: APPLICATION TO THE PLIOCENE SOUTHERN OCEAN

We have developed an efficient labor-saving method for analyzing δ¹⁵N diatom-bound organic matter (DBOM) in order to analyze a large number of samples from cores in the Southern Ocean, which span the late Miocene to early Pliocene climate transition. We aim to test the hypothesis that early Pliocene relative global warmth was associated with enhanced atmospheric CO₂ levels due to weaker nutrient utilization. Down-core sediment δ¹⁵N has been used as a proxy for surface water nitrate δ¹⁵N values and past nutrient utilization. Recently, δ¹⁵N of DBOM has been suggested as a more direct proxy, as the organic matter inside of the diatom’s frustule has a better-constrained source and is well-protected against diagenesis. However, DBOM is more difficult to analyze because it occurs in low concentrations and requires labor-intensive isolation. Previous use of Elemental Analysis (EA) Isotope Ratio Mass Spectrometry (IRMS) combustion approaches have revealed high N₂ blanks attributed to air trapped in frustules. To avoid these blanks, these studies utilize a wet chemical oxidation method, converting N to NO₃^-, followed by denitrification of NO₃^- to N₂O for analysis by GC-IRMS.

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.