UC San Diego

This thesis uses a variety of natural abundance isotope measurements to expand our understanding of nitrogen and organic carbon cycling within the ocean. Chapters 2 & 3 use stable isotopes of nitrate to investigate recent trends in nitrogen cycling in the southern California Current System (sCCS). In Chapter 2 we show that surface ocean warming during the 2015-2016 El Niño event led to reduced nitrate supply and increased reliance on recycled nitrate produced via euphotic zone nitrification. We also identified spatial and temporal variability in euphotic zone nitrate utilization associated with iron limitation of phytoplankton. Chapter 3 uses a time series of stable isotopes of nitrate along with other physical and chemical measurements to examine recent biogeochemical changes in a partially isolated marine basin off the coast of California, whose bottom waters become deoxygenated between annual flushing events. Our data confirm an unprecedented increase in water column denitrification in basin bottom waters in recent years that was likely caused by a long-term trend of decreasing oxygen concentrations outside the basin. Chapter 4 uses a variety of chemical and isotopic measurements to characterize organic matter across oxygen gradients in the eastern tropical North Pacific (ETNP). This work describes unique organic matter dynamics along the oxycline and shows that optical properties of dissolved organic matter effectively identify locations where intense oxic remineralization takes place, whereas most other bulk measurements of organic matter do not provide insight into unique processes within these environments. We also document the impact of a passing hurricane on water column properties. Chapter 5 uses thermal oxidation in conjunction with a chemical processing method to show that the current paradigm of marine DOC cycling needs revision. Our results show that even the most 14C-depleted DOC fraction exhibits a vertical radiocarbon gradient, suggesting the addition and removal of DOC from this reservoir on shorter timescales than current models predict. We propose a new hypothesis for marine DOC cycling that would revise the water column lifetime of marine DOC to only 2,300 years, approximately doubling the estimated flux of carbon through the deep ocean.