Modeling oceanic nitrate and nitrite concentrations and isotopes using a 3-D inverse N cycle model

Nitrite (inline-formula $M1inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg2010dd2111c7c15e48605a77755bb515 bg-16-347-2019-ie00001.svg25pt16ptbg-16-347-2019-ie00001.png ) is a key intermediate in the marine nitrogen (N) cycle and a substrate in nitrification, which produces nitrate (inline-formula $M2inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg4c315b3ea451cf26923ad12993612b33 bg-16-347-2019-ie00002.svg25pt16ptbg-16-347-2019-ie00002.png ), as well as water column N loss processes denitrification and anammox. In models of the marine N cycle, inline-formula $M3inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg77e75b8e3421a260344892322cb22e26 bg-16-347-2019-ie00003.svg25pt16ptbg-16-347-2019-ie00003.png is often not considered as a separate state variable, since inline-formula $M4inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimge16cba38499a6a16cb1a10e488ec56da bg-16-347-2019-ie00004.svg25pt16ptbg-16-347-2019-ie00004.png occurs in much higher concentrations in the ocean. In oxygen deficient zones (ODZs), however, inline-formula $M5inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimgbcdbde0566f3e4be69cabaa2d184dd7a bg-16-347-2019-ie00005.svg25pt16ptbg-16-347-2019-ie00005.png represents a substantial fraction of the bioavailable N, and modeling its production and consumption is important to understand the N cycle processes occurring there, especially those where bioavailable N is lost from or retained within the water column. Improving N cycle models by including inline-formula $M6inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg512563fdf3290188d12e109ad19f7123 bg-16-347-2019-ie00006.svg25pt16ptbg-16-347-2019-ie00006.png is important in order to better quantify N cycling rates in ODZs, particularly N loss rates. Here we present the expansion of a global 3-D inverse N cycle model to include inline-formula $M7inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg5437eb9d658a57482749982e166d9cca bg-16-347-2019-ie00007.svg25pt16ptbg-16-347-2019-ie00007.png as a reactive intermediate as well as the processes that produce and consume inline-formula $M8inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimgacfef2545d5fc9db8c57cc631417bebf bg-16-347-2019-ie00008.svg25pt16ptbg-16-347-2019-ie00008.png in marine ODZs. inline-formula $M9inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg19cb9306f275cfd5c1fc7eac2dd7137a bg-16-347-2019-ie00009.svg25pt16ptbg-16-347-2019-ie00009.png accumulation in ODZs is accurately represented by the model involving inline-formula $M10inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimgdd23f13eb24280cbe650be4567ce8571 bg-16-347-2019-ie00010.svg25pt16ptbg-16-347-2019-ie00010.png reduction, inline-formula $M11inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg57110407e23879950960c6f561b96366 bg-16-347-2019-ie00011.svg25pt16ptbg-16-347-2019-ie00011.png reduction, inline-formula $M12inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimgc033a67d9119098cc5416343a6e1ef33 bg-16-347-2019-ie00012.svg25pt16ptbg-16-347-2019-ie00012.png oxidation, and anammox. We model both inline-formula¹⁴N and inline-formula¹⁵N and use a compilation of oceanographic measurements of inline-formula $M15inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimgecc3e6dd5af0ffb1da8bfbfcb16b8e8b bg-16-347-2019-ie00013.svg25pt16ptbg-16-347-2019-ie00013.png and inline-formula $M16inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg27e568876ed278ec5c17db65eed8f3ba bg-16-347-2019-ie00014.svg25pt16ptbg-16-347-2019-ie00014.png concentrations and isotopes to place a better constraint on the N cycle processes occurring. The model is optimized using a range of isotope effects for denitrification and inline-formula $M17inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg3e662158e7d6a0c69e79cdad3103a280 bg-16-347-2019-ie00015.svg25pt16ptbg-16-347-2019-ie00015.png oxidation, and we find that the larger (more negative) inverse isotope effects for inline-formula $M18inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimgaf0a8140822a727076a64a0cd661cca4 bg-16-347-2019-ie00016.svg25pt16ptbg-16-347-2019-ie00016.png oxidation, along with relatively high rates of inline-formula $M19inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimg5b351cc4bebabeb32f4334881ca7cf41 bg-16-347-2019-ie00017.svg25pt16ptbg-16-347-2019-ie00017.png , oxidation give a better simulation of inline-formula $M20inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg827b0fe0e97f70953101fc9e20cd0031 bg-16-347-2019-ie00018.svg25pt16ptbg-16-347-2019-ie00018.png and inline-formula $M21inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 2}}^{-}$ 25pt16ptsvg-formulamathimga1a44ad94a8c8dc93a0d1674031afc05 bg-16-347-2019-ie00019.svg25pt16ptbg-16-347-2019-ie00019.png concentrations and isotopes in marine ODZs.