Coupled nitrate nitrogen and oxygen isotopes and organic matter remineralization in the Southern and Pacific Oceans

The difference between nitrate N-15 and O-18, or (15-18), is sensitive to organic matter remineralization and tracks the modification of nitrate as it passes from the deep Pacific Ocean, through the Southern Ocean surface, and into the intermediate-depth Pacific. Circumpolar Deep Water (CDW) is upwelled with a nitrate (15-18) of +3.0 and appears unaltered by nitrate assimilation in the Antarctic surface. However, within Subantarctic Mode Water (SAMW)the primary conduit of deep-sea nutrients to the lower latitudesnitrate N-15 and O-18 are both higher than CDW, while nitrate (15-18) is as low as +1.5. The lower SAMW nitrate (15-18) results from the production of low N-15 organic matter during partial assimilation of the surface nitrate pool followed by its sinking and remineralization back to nitrate, lowering the N-15 of thermocline nitrate more than its O-18. As SAMW flows toward the lower latitudes, nitrate (15-18) is expected and observed to increase because complete surface ocean nitrate consumption produces sinking nitrogen with a N-15 similar to that of the nitrate supplied from below such that remineralization lowers the O-18 of nitrate, but not its N-15. Nitrate (15-18) is also used to estimate a surprisingly high low latitude sinking nitrogen N-15 of approximate to 9.0, suggesting a major effect of (NO3-)-N-14 loss by tropical denitrification on middepth nitrate basin-wide. The remineralization of this high sinking nitrogen N-15 increases nitrate (15-18) as deep as the southward-moving Pacific Deep Water, which supplies CDW. This relatively high (15-18) is then lowered to the observed CDW value by the remineralization of Southern Ocean sinking nitrogen with a low N-15.