Nitrogen content and isotope ratios of Arabian Sea sediments

Water column and core-top d15N data show that Arabian Sea denitrification produces large nitrogen isotopic enrichments that are regionally recorded with fidelity in the sediments. These results facilitate interpretation of a 1 m.y. d15N record for Ocean Drilling Program site 722B on the Owen Ridge in terms of climatically linked oscillations in denitrification at the major orbital periods. As at present, denitrification was greatest during interglacial periods and, apparently, was not active during most glacial intervals. Cross-spectral analysis of d15N with foraminiferal d18O (global climate/sea level index) and lithogenic grain size (monsoon strength index) suggests forcings by changes in hydrography and productivity acting through the extent and intensity of the oxygen minimum zone. The data suggest that denitrification may be an internal forcing mechanism for climate change during major glacial/interglacial transitions through influence on marine N inventory and atmospheric CO2. However, compensation or amplification may occur from other sinks (sediment denitrification) or sources (nitrogen fixation).