Impact of long-term diagenesis on δ15N of organic matter in marine sediments: Sites 1227 and 1230

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This study addresses the problem of diagenetic fractionation of δ15N in sedimentary organic matter by constructing isotopic mass balances for the sedimentary nitrogen and pore water ammonium at two Ocean Drilling Program (ODP) sites, 1227 and 1230. At Site 1230, ammonium production flux integrated through the sedimentary column indicates that >60% of organic matter is lost to decomposition. The δ15N of pore water ammonium is <0.7‰ different from that of the sedimentary organic matter, which implies that very little isotopic fractionation is associated with degradation of organic matter at this site. The constant δ15N of the solid-phase sedimentary nitrogen through the whole profile supports this conclusion. Atomic C/N ratios (9-12) indicate that organic matter at this site is primarily of marine origin. At Site 1227, the sedimentary organic matter appears to be a mixture of terrestrial and marine components. Ammonium is ∼4‰ heavier than the organic matter. The observed isotopic enrichment of pore water ammonium relative to the sedimentary nitrogen might indicate either the preferential decomposition of isotopically heavier marine fraction of the organic matter, or possibly, a nonsteady-state condition of the ammonium concentration and δ15N profiles. Interpretation of the results at Site 1227 is further complicated by the contribution of ammonium with δ15N of ∼4‰ that is diffusing upward from Miocene brines.

Publication Title, e.g., Journal

Proceedings of the Ocean Drilling Program: Scientific Results