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

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 δ ¹⁸O. 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. Key Points Nitrate N and O isotopes are sensitive to organic matter remineralization Coupled isotopes track nitrate modification from Southern to North Pacific Nitrate N and O isotopes allow us to quantify regenerated N isotopes