Data_ Constraints on Recent Variations of Sedimentary Nitrogen Isotopic Composition on the East China Sea Inner Shelf

Nitrogen isotopic composition of sediments is a key tracer for nitrogen processes in the ocean and has been widely used to reconstruct biogeochemical processes. However, the mechanism responsible for δ¹⁵N variation in recently accumulated sediments on the marginal sea has received little attention. In this study, we measured total nitrogen and organic carbon contents (TN and TOC), their stable isotopes (δ¹⁵N and δ¹³C), and mean grain size in five box-cores retrieved from the inner shelf of the East China Sea (ECS) to explore constraints on sedimentary δ¹⁵N variation under intensified human activities. Heterogeneities in δ¹⁵N downcore profiles were found in different sites. Sedimentary δ¹⁵N in the northern inner shelf has decreased since the 1970s due to increasing anthropogenic nitrogen input from the Yangtze River, as supported by the results of a steady state model. Additionally, a downcore decline of δ¹⁵N in the mud-sand transitional zone on the ECS inner shelf was caused by increasing coarser Yangtze River Estuary-derived sediments with low δ¹⁵N, mediated by either the strengthening East Asian winter monsoon (EAWM) or delta erosion due to the reduction of the Yangtze River sediment discharge. In contrast, less affected by human activities, sedimentary δ¹⁵N record in the northwest of Taiwan Island showed a positive link with the Pacific Decadal Oscillation (PDO), as increased supply of phosphate-abundant Kuroshio water promoted nitrate assimilation and weakened isotopic fractionation during PDO positive periods. These findings may provide useful insights for reconstructing the centennial-millennial scale nitrogen cycle on the river-dominated marginal sea in future studies.