Isotopic Signature of Organic Nitrogen in Authigenic Carbonates: Implications for Dynamic Nitrogen Cycling at Submarine Cold Seeps

Abstract Active nitrogen cycling, including microbially mediated nitrogen fixation and nitrogen loss, occurs at marine cold seeps and may influence nitrogen budgets in the deep ocean. However, the record of these processes in seep environments remains poorly constrained. Here, we investigate the nitrogen isotopic composition of organic nitrogen ( δ 15 N ON ) along with carbon isotopes and elemental composition of authigenic carbonates from four methane‐seep sites and three brine‐seep sites in the Gulf of Mexico and the South China Sea. Methane‐seep carbonates exhibit significantly lower δ 13 C values of total organic carbon ( δ 13 C TOC ; mean: −34.7 ± 8.0‰, n = 32) than authigenic carbonates from brine seeps (mean: −27.9 ± 8.4‰, n = 24). In contrast, δ 15 N ON values are generally low for both types of seeps, with mean values of 0.1 ± 2.2‰ ( n = 32) for methane seeps and −1.2 ± 0.8‰ ( n = 24) for brine seeps. δ 15 N ON values below zero are common and δ 15 N ON values are largely decoupled from δ 13 C TOC values at most sites, which is interpreted to reflect dynamic nitrogen cycling near the sediment‐water interface at seeps, involving processes such as dissimilatory nitrate reduction to ammonium, denitrification, and anaerobic ammonium oxidation. The nitrogen involved in these processes is subsequently taken up by methanotrophic consortia and associated microorganisms, preserving its isotopic signature in the organic matter incorporated into seep carbonates. Our results demonstrate that δ 15 N ON values of seep carbonates represent a novel archive of a hotspot of benthic nitrogen cycling, providing new insight into nitrogen cycling in deep‐sea seep environments.