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In this study we set out to measure the δ¹⁵N values bound to diatom organic matter and contained in bulk sediments in two cores from the Southern Ocean spanning the late Miocene to Pliocene climate transition (~6-2 Ma). Down-core diatom-bound δ¹⁵N was to provide early Pliocene surface water nitrate δ¹⁵N values and concentrations as a proxy for past nutrient utilization and paleoproductivity. Coupled with bulk sediment δ¹⁵N values we also aimed to investigate the degree of sediment diagenesis. The early Pliocene is one of the most recent intervals of time characterized by prolonged relative global warmth and sea level high stand, and we test the hypothesis that this particular interval of warmth was associated with enhanced atmospheric CO₂ levels due to weaker polar ocean stratification and decreased nitrate utilization.

Thus far we have conducted a pilot study focusing on Site 745 using a subset (n=15) of the down-core samples. In addition to measuring bulk sediment δ¹⁵N values, in this pilot study we have developed a new labor-saving method for more efficiently isolating diatom frustules from bulk sediments.  The δ¹⁵N of diatom bound organic matter (DBOM) has been suggested as a more direct proxy for nutrient utilization than bulk sediment δ¹⁵N, as the organic matter inside of the diatom’s frustule has a better-constrained source and is well-protected against diagenesis.   However, DBOM is difficult to analyze because it occurs in low concentrations and requires labor-intensive isolation.  We modified existing methods of DBOM isolation to chemically disaggregate the sediments prior to physical separation (hydrochloric acid/hydrogen peroxide decalcification, sodium dithionite treatment to remove Fe/Al oxides, hydrogen peroxide oxidation, wet sieving, hydrogen peroxide oxidation).  Microscope pictures were taken of samples at each processing step and they demonstrated that our method was isolating the diatoms from the remaining sediment and that the end product was a very clean diatom sample.  Our modified method increases sample throughput while maintaining sample quality. 

For the measurement of δ¹⁵N of DBOM, we started with ~4 g of sediment to ensure sufficient opal for analysis.  Sample analysis was completed on a Thermo-Finnigan DeltaPlus XP Isotope Ratio Mass Spectrometer (IRMS) with Conflo III interfaced in continuous flow mode to Costech 4010 CHNS-O Elemental Analyzer (EA).  Previous studies had indicated that the use of EA-IRMS combustion approaches have revealed high N₂ blanks attributed to air trapped in frustules.  We have made modifications to our EA-IRMS system to reduce the N₂ blanks.  Results from a test with acid-washed diatom frustule pieces did not demonstrate the N₂ blank that other labs have found.  Utilizing an EA-IRMS system instead of wet chemical oxidation and GC-IRMS analysis also reduces labor, sampling time, and cost.  Overall, our method is more effective at separating clays and organic matter from diatom frustules and is cost, labor, and time saving when compared to similar methods.

For the measurement of bulk sediments ~1 gram of bulk sediments was oven dried for 48 hours, ground with a mortar and pestle into a fine powder, and homogenized. About 90-100 mg of the thus prepared bulk sediment was weighed out into tin cups. Samples were analyzed on a Thermo-Finnigan DeltaPlus XP Isotope Ratio Mass Spectrometer (IRMS) with Conflo III interfaced in continuous flow mode to Costech 4010 CHNS-O Elemental Analyzer (EA). 

Results from the pilot study show that there are marked differences between the δ¹⁵N of the bulk sediments and the δ¹⁵N of isolated diatoms. The bulk sediment values are higher on average by ~4 per mil and display lower overall variations. The distinct difference between the bulk and diatom values is consistent with results from other studies, however, a wide range of differences are reported in the literature. The C/N ratios vary around the expected ratio of ~7, with higher variations in the bulk consistent with the fact that the bulk consists of a mixture of sources. The %N is, as expected, low in the bulk fraction and very low in the diatom fraction, necessitating the large sample volume.  

Work in progress: We have generated a complete down-core bulk sediment δ¹⁵N record at one of the proposed sites (Site 745 in the Indian Ocean Sector of the Southern Ocean). The corresponding DBOM δ¹⁵N record is almost complete. We will present the down-core results from Site 745 at the upcoming Fall Meeting of the AGU (December 2011).

This research project was conducted by post doctoral researcher Dr. Rebecca Hays. Dr. Hays presented first results from the methods test at the Geological Society of America Northeastern and North-Central Section Meeting in Pittsburg, PA, in March 2011 and has submitted an abstract to the annual Fall Meeting of the AGU in San Francisco in December, 2011. Manuscripts summarizing the methods as well as the results from the down-core bulk sediment δ¹⁵N record are currently in preparation. During her co-tenure as a post doc at the University of Delaware and the Stroud Water Research Center, Dr. Hays was appointed to a faculty position at Eastern University. We believe that the PRF funded post doctoral research opportunity greatly enhanced her research experience contributing to her success in securing a much sought-after academic position.