Ocean Drilling Program, United States

This study addresses the problem of diagenetic fractionation of delta (super 15) N 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 delta (super 15) N 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 delta (super 15) N 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 approximately 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 delta (super 15) N profiles. Interpretation of the results at Site 1227 is further complicated by the contribution of ammonium with delta (super 15) N of approximately 4% that is diffusing upward from Miocene brines.