Abstract
<p>Marine dissolved organic matter (DOM) is quantitatively important for the biogeochemical cycling of carbon and nitrogen. It exerts its influence through the ocean’s biological pump, with the fate of dissolved organic nitrogen (DON) and carbon (DOC) impacting the ocean’s fertility and its capacity for storing carbon on climate-relevant timescales, respectively. This dissertation identifies and quantifies important mechanisms and timescales for DOM degradation in the marine environment by combining observations from ship-based studies, assessments of ocean circulation, and incubation experiments to interrogate the relevant processes. The fate of terrigenous DOC (tDOC), delivered to the ocean by rivers, was investigated during its transit across the broad Siberian continental shelves. Observations of DOC coupled with tracers of freshwater (δ18O) and shelf water (228Ra/226Ra) identified an aged riverine component present in the Transpolar Drift over the central Arctic basins. Residency on the shelf reduced the DOC content relative to conservative mixing with marine water, indicating significant removal of tDOC during river-to-ocean transport. The 228Ra/226Ra age tracer was used to constrain the timescale of tDOC removal, finding a removal rate several times higher than previously reported in the western Arctic Beaufort gyre (holding more highly aged shelf water). These findings highlight the enhanced lability of fresh tDOC upon delivery to the Arctic Ocean. tDOC mineralization is important in that it mitigates the strength of the surface Arctic Ocean atmospheric CO2 sink; a sink that will be further reduced with an increase in labile tDOC flux and mineralization due to Arctic warming and permafrost thaw. Extending the analysis to include terrigenous DON (tDON), evidence for significant tDON mineralization was found as well, however riverine delivery of both inorganic and organic nitrogen had only a minor (<15%) impact on Arctic shelf export production. The global distribution of DON in the surface ocean in conjunction with DON mineralization incubation experiments were employed to understand the fate of DON and its role as a source of nitrogen supporting export production in oligotrophic systems. Inputs of nitrate to the euphotic zone at equatorial and eastern boundary upwelling centers fuel net production (accumulation) of DON that resists rapid remineralization. This material was found to be recalcitrant to degradation by surface microbial communities; instead microbial DON mineralization is a slow process (months) that occurs once surface DON is exposed to microbial communities found in the upper mesopelagic zone. DON transported towards the oligotrophic ocean by surface currents is vertically mixed to depths within the deep euphotic zone (~50 to 100 m) at the eastern edges of the subtropical gyres. These results suggest the primary fate of surface DON to be removal via vertical mixing and subsequent mineralization below the mixed layer, implying a limited role for direct DON support of gyre export production from the surface layer. DON may contribute to export production at the eastern edges of the subtropical gyres, but after its mineralization within the deep euphotic zone.</p>