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 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.