Low late Pliocene sedimentary \delta15N from a 10 site survey

Comparison of sediment samples from 10 ODP sites in the Pacific, Atlantic and Indian Oceans reveals a widespread change in the \delta¹⁵N of sedimentary organic matter between late Pliocene and Holocene times. Late Pliocene \delta¹⁵N values from the Pacific, Indian and southwestern Atlantic are 2.3 \permil lower than their modern counterparts while sites in the north and tropical Atlantic show no difference. These results pose interesting questions for the paleoceanographic community and for extending the use of \delta¹⁵N as a paleoceanographic proxy back in time. The lower late Pliocene values may reflect reduced rates of denitrification and a lower \delta¹⁵N of nitrate in the ocean. Alternatively, the modern balance between nitrogen fixation and denitrification, inputs and outputs of nitrate, may not have existed during this period of warmer-than-modern sea-surface temperatures. Lesser rates of denitrification and/or enhanced rates of fixation, the main sink and source, respectively, would generate higher nitrate concentrations, increasing export production and the draw down of atmospheric CO₂. The lower \delta¹⁵N values may therefore indicate a fundamental change in the marine N cycle that is of interest to paleoclimate studies It is also possible that these low \delta¹⁵N values may reflect a post-burial diagenetic change. High resolution records from the late Pliocene show systematic relations between \delta¹⁵N and other paleoceanographic parameters, suggesting that the short term variability is real. If there is a slow, continuous decrease in \delta¹⁵N values due to diagenesis then absolute values of \delta¹⁵N from bulk sedimentary organic matter can not be evaluated with respect to modern baseline values.