Seminar by Yeongjun Ryu from Postdoctoral Research Fellow, Department of Geosciences, Princeton University, New Jersey, US
13 November 2025
KST 14:00
The Seminar is being held in Room 1010 (Jasmin) – Integrated mechanical engineering building. Click here for the campus map.
Nitrogen is a crucial nutrient for biological productivity in the upper ocean, and its cycling is sensitive to global climate. The stable nitrogen isotopic ratio (i.e., 15N/14N ratio, or δ15N) is a powerful tool for tracing nitrogen cycling. However, the δ15N of organic nitrogen forms is relatively underexplored, despite their importance, especially in the upper ocean. This study investigates the marine nitrogen cycle through δ15N measurements of organic nitrogen, focusing on chlorophyll and its degradation products (collectively, “chlorin”) and on dissolved organic nitrogen (DON).
First, the δ15N measurements of chlorin (δ15NChl) are used to infer the phytoplanktonic origins of suspended particles. The data indicate a higher contribution from cyanobacteria-derived material than expected from contemporaneous phytoplankton communities, suggesting the persistence of prokaryotic biomass and preferential export of eukaryotic particles, which can induce a bias toward eukaryotes in sedimentary palaeoceanographic proxies.
Second, meridional distributions of surface DON concentration and δ15N are examined across the Pacific and Atlantic. A tight correlation between surface DON δ15N and subsurface nitrate δ15N implies rapid cycling of a labile fraction of DON, suggesting its role in supporting biological production in the oligotrophic gyres. Incorporation of DON into a nitrogen budget for the Atlantic Ocean yields revised spatial distributions of N2 fixation, with elevated rates inferred in the northern tropics. These findings suggest that DON degradation serves as an alternative nitrogen source where N2 fixation is suppressed by phosphorus-rich but iron-limited conditions.
In contrast to the surface DON, deep oceanic DON reveals an inter-basin gradient of decreasing concentration and increasing δ15N from the North Atlantic to the North Pacific, consistent with progressive microbial degradation. Regional variation in the δ15N of deep DON also suggests a contribution from solubilization of sinking particles, whose inclusion leads to acceleration of turnover of deep oceanic DON by up to sevenfold. These findings indicate two distinct sources of deep DON with different turnover timescales.