SEDIMENTARY NITROGEN ISOTOPIC RATIO AS A RECORDER FOR SURFACE OCEAN NITRATE UTILIZATION

In two contrasting regions of the ocean, the equatorial Pacific and the southern ocean, the deltaN-15 of core top sediments were strongly related to [NO3-] in surface waters. With distance from the equator in the equatorial Pacific, deltaN-15 increased from 7 parts per thousand to 16 parts per thousand as [NO3-] decreased from 8 muM to < 0.1 muM. Going from 60-degrees to 30-degrees S in the SE Indian Ocean, core top deltaN-15 increased from 5 parts per thousand to 11 parts per thousand as surface [NO3-] decreased from 25 muM to < 0.1 muM. These results are strong evidence that sedimentary deltaN-15 in these regions is recording the increasing isotopic enrichment of near-surface NO3- with its depletion by phytoplankton. In the case of the equatorial Pacific, deltaN-15 values for sinking particles collected at 150 m matched well the core top sediment values, demonstrating little diagenetic alteration of the near-surface generated isotopic signal. These equatorial Pacific data sets have variations with near-surface [NO3-] consistent with Rayleigh fractionation kinetics for a fractionation factor (epsilon(u) of 2.5 parts per thousand This value is substantially lower than previously found for temperate or polar regions, perhaps as a result of differences in phytoplankton species assemblage or growth condition. In the southern ocean south of the polar front, comparison of deltaN-15 values for opal-rich sediments south and sinking particles indicates an apparent +5 parts per thousand diagenetic enrichment relative to the surface-generated signal that requires further investigation. This exception aside, our observations show that the surface-water relationship of increasing deltaN-15 with increasing NO3-depletion is generally transmitted to and preserved in the sediments, an important requirement for further development and application of this important paleoceanographic tool.