Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific

Abstract

Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the δ15N and δ18O of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time-series Station ALOHA shows less of an increase in δ18O than in δ15N as one ascends from abyssal waters into the denitrification-impacted mid-depth waters. A box model of the ocean nitrate N and O isotopes indicates that this is the effect of the low latitude nitrate assimilation/regeneration cycle: organic N sinking out of the surface spreads the high-δ15 N signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate δ18O signal of denitrification can only be transmitted by circulation in the interior. Copyright 2009 by the American Geophysical Union.