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

Culture studies of denitrifying bacteria predict that denitrification will generate equivalent gradients in the delta N-15 and delta O-18 of deep ocean nitrate. A depth profile of nitrate isotopes from the Hawaii Ocean Time-series Station ALOHA shows less of an increase in delta O-18 than in delta N-15 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-delta N-15 signal of pelagic denitrification into waters well below and beyond the suboxic zone, whereas the nitrate delta O-18 signal of denitrification can only be transmitted by circulation in the interior. Citation: Sigman, D. M., P. J. DiFiore, M. P. Hain, C. Deutsch, and D. M. Karl (2009), Sinking organic matter spreads the nitrogen isotope signal of pelagic denitrification in the North Pacific, Geophys. Res. Lett., 36, L08605, doi: 10.1029/2008GL035784.