Reports: ND249945-ND2: Diatom-Bound Nitrogen Isotopes as a Diagenetically Stable Proxy for Southern Ocean Nutrient Utilization and Early Pliocene Climatic Warmth

Katharina Billups, PhD, University of Delaware

Anthony Aufdenkampe, PhD, Stroud Water Research Center

In this study we set out to measure the δ15N values of bulk sediments and bound to diatom organic matter in cores from the Southern Ocean spanning the late Miocene to Pliocene climate transition (~6-2 Ma). 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. Measuring the δ15N in the two phases should allow us to investigate potential effects of sediment diagenesis.

Our results from the bulk sediment analyses indicate that δ15N values increase to above late Holocene levels from the late Miocene through the late Pliocene (6.5 to 2 Ma). Opal and carbon mass accumulation rates (MAR) (taken from Ehrmann et al., 1991) are low during the early Pliocene. Relatively high δ15N together with low export production is consistent with a more southerly position of the Polar Frontal Zone (PFZ) allowing the expansion of nitrate depleted, low nutrient upper waters south toward Site 745. The interpretation is supported by a relatively small δ15N gradient between Site 745 and a site in the Subantarctic Zone of the Southern Ocean (Site 1090, Etourneau et al., 2009). During the late Pliocene to early Pleistocene climate transition (between ~2.1 and at 1.7 Ma), δ15N values display large variations approaching those observed between the last glacial and interglacial transition in this latitude band. Opal and carbon MARs also show large fluctuations, but in the opposite sense. The pattern of high δ15N values associated with low export production may reflect changes in nutrient utilization in response to changes in water column stratification once the PFZ has moved north of the location of Site 745. Our results provide a mechanism for enhancing early Pliocene CO2 level via reduced uptake of CO2 due to low productivity in the Southern Ocean. Once the PFZ has moved north, the region may have become sensitive to changes in water column stratification potentially contributing to fluctuations in CO2.

Results from the diatom bound organic matter δ15N measurements are inconclusive. Although the long-term trend parallels the trend in the bulk sediment record, the absolute δ15N values are lower than expected based on published records spanning this interval of time. Therefore we believe that it is the diatom bound δ15N values that have been compromised either by diagenesis or other artifacts. Work is currently underway to verify these measurements.

All δ15N data were generated by post-doctoral researcher Dr. Hays at the Stroud Water Research Center under the supervision of A. Aufdenkampe. A first manuscript focusing on the results from the bulk sediments is currently in review in the journal Global and Planetary change. PI K Billups is taking the lead on this article as it focuses on the Paleoceanographic interpretations of the data. Dr. Hays developed the method for the diatom cleaning, and she is currently writing a first draft of this manuscript. 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. We also presented a poster at the annual Fall Meeting of the AGU in San Francisco in December, 2011. During her co-tenure as a post-doc at the University of Delaware and the Stroud Water Research Center, Dr. Hays received 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 an academic position. The grant also contributed to the achievements of PI K. Billups as an Associate Professor, who asked to be considered for promotion to Professor during the grant period.