Collaborative Research: Isotopic Constraints on the Nitrogen Dynamics of the Bering Sea

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ABSTRACTOCE-0136449OCE-0136584This project will consider the mechanisms of denitrification - the chemical reduction of dissolved nutrient NO3- largely to inert N2 gas- in the polar oceans and specifically the Bering Sea. Denitrification in the ocean, the loss of nutrient N essential for biological processes, is conventionally thought to take place only in conditions of relative dissolved oxygen deficiency ( [O2] 5-10mM). The northern Polar ocean shows significant nitrate depletion relative to phosphate despite the concentration of oxygen rarely falling below 15uM throughout much of the region. It has been known for many years that the Bering Sea in particular exhibits a sizeable nitrate deficit, but whether this arises in the water column or in the sediments, or perhaps is even advected in from low-latitude oxygen minimum zones in the Pacific remains unclear. Isotopic measurements of nitrate will be used to assess the relative role of sedimentary vs. water column denitrification based on observed distinct isotopic signatures accompanying these two processes. Using a recently developed isotopic analytical method, the d 18O of oceanic nitrate will also be developed as a tracer to complement the interpretation of d 15N differences to study these aspects of the oceanic nitrogen cycle. A further line of enquiry of the study is to consider the proposition that the modern polar Pacific ocean operates as an analogue of the glacial Southern Ocean, a key unknown in interpreting Holocene variations in atmospheric CO2 and thus broadening our understanding of geologically recent climate change.