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

Abstract

The difference between nitrate δ15N and δ18O, or Δ(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 Δ(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 latitudes—nitrate δ15N and δ18O are both higher than CDW, while nitrate Δ(15–18) is as low as +1.5‰. The lower SAMW nitrate Δ(15–18) results from the production of low δ15N organic matter during partial assimilation of the surface nitrate pool followed by its sinking and remineralization back to nitrate, lowering the δ15N of thermocline nitrate more than its δ18O. As SAMW flows toward the lower latitudes, nitrate Δ(15–18) is expected and observed to increase because complete surface ocean nitrate consumption produces sinking nitrogen with a δ15N similar to that of the nitrate supplied from below such that remineralization lowers the δ18O of nitrate, but not its δ15N. Nitrate Δ(15–18) is also used to estimate a surprisingly high low latitude sinking nitrogen δ15N of ≈9.0‰, suggesting a major effect of 14NO3− loss by tropical denitrification on middepth nitrate basin‐wide. The remineralization of this high sinking nitrogen δ15N increases nitrate Δ(15–18) as deep as the southward‐moving Pacific Deep Water, which supplies CDW. This relatively high Δ(15–18) is then lowered to the observed CDW value by the remineralization of Southern Ocean sinking nitrogen with a low δ15N.