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Interactions between climate and the marine nitrogen cycle on glacial-interglacial timescales Galbraith, Eric Douglas

Abstract

Nitrogen is a principal component of living organisms and comprises the majority of the atmosphere, yet the scarcity of biologically reactive nitrogen in the ocean limits growth and appears to have varied with past changes in physical climate. This thesis takes a multi-faceted approach, including fine-scale sediment analysis, modern field observations and global numerical modeling, to contribute an integrated view of the marine nitrogen cycle and its climatic sensitivity. Nitrogen bound in diatom frustules, extracted from laminated sediments of the Guaymas Basin, has greater seasonal δ¹⁵N variability than corresponding bulk sediment. Downslope transport of frustules from the shelf contributes δ¹⁵N -depleted nitrogen, while pelagic diatom frustules display great sensitivity to seasonal growing conditions. Bulk sedimentary δ¹⁵N represents a reliable integrated monitor of the local nitrogen substrate. Records of bulk sedimentary δ¹⁵N from the subarctic Pacific reflect the tripartite imprints of diagenesis, variable nitrate utilization and changes in δ¹⁵N -nitrate. Modern subsurface δ¹⁵N -nitrate is homogenous across the open subarctic Pacific. Diagenesis introduces δ¹⁵N -enrichments at core tops and there is a gradual decrease of δ¹⁵N with burial. The Gulf of Alaska record does not sample HNLC waters, but records changes in δ¹⁵N -nitrate and diagenesis. This is used to correct for regional 8I 5Nnitrate variability, revealing rapid increases in nitrate utilization, likely due to Fe fertilization, in the western subarctic Pacific during glacial periods. The δ¹⁵N -nitrate record suggests denitrification may have occurred in the deep ocean during the last glacial maximum, and almost certainly in the upper water column of the deglacial subarctic Pacific. A global compilation of δ¹⁵N records evokes co-ordinated changes in denitrification throughout the global thermocline, implying large increases in aggregate denitrification rate, matched by changes in N₂ fixation, under warming conditions. A global, physically-driven modulation of subsurface oxygen concentrations is proposed as the primary relevant forcing on glacial-interglacial timescales. Simple schemes for denitrification and N₂ fixation, based on widely accepted ecological principles, are quantified and embedded in a General Circulation Model of intermediate complexity. The model highlights the competition between diazotrophs and other phytoplankton for phosphorus as a key element of the marine biosphere. The model confirms a pronounced sensitivity of denitrification rates to the physical climate state, with more rapid rates of nitrogen cycling and expanded nitrogen limitation under warmer climates.

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