Global patterns and drivers of nitrogen isotope signal in modern marine sediments

Marine nitrogen cycle, shaped by nitrogen input and loss processes, regulates ocean productivity and leaves a distinct imprint on the ratio of stable nitrogen isotopes, δ¹⁵N. However, quantifying spatial patterns and centennial-scale variations in marine nitrogen cycling through δ¹⁵N fingerprints remains poorly constrained. Here, we present a global database of δ¹⁵N in marine sediments, comprising over 8400 seafloor samples, and create 148 time series from sediment cores spanning the past century. Our findings reveal that bulk δ¹⁵N in global marine sediments predominantly records organic matter, despite the non-negligible role of inorganic nitrogen. A widespread centennial δ¹⁵N increase, averaging 1.3‰ (interquartile range: 0.6 $-$ 3.3‰) per 100 years, coinciding with total nitrogen enrichment, indicates external nitrogen inputs, productivity, and accelerated denitrification across global coastal oceans. However, in river-influenced continental margins, terrestrial sedimentary processes can obscure the ¹⁵N-enriched signal, as riverine export of ¹⁵N-depleted refractory organic matter likely lowers sedimentary δ¹⁵N.