The Role of Temperature and Oxygen for the Remineralization of Sinking Organic Matter

An accurate representation of the remineralization of sinking particles of organic matter is crucial for reliable projections of the ocean's biogeochemical cycles, particularly that of carbon. Both water temperature and oxygen concentration are thought to influence remineralization, but particularly temperature effects are difficult to constrain with field observations. We develop a new parameterization of POC flux to depth which includes a Michaelis-Menten-type oxygen dependence and an exponential temperature dependence. To test the parameterization, we use a compilation of POC flux measurements at different depths from 19 different sites, covering a wide range of temperature- and oxygen conditions. Best results are achieved with a temperature coefficient that results in a Q₁₀ of 1.8 and an oxygen half-saturation constant between 5-10 μmol O₂/L. Using those coefficients, the RMSE between the normalized POC measurements and the fit is 23% percent smaller than without a temperature and oxygen dependence. While the oxygen dependence is crucial to reproduce the flux in the oxygen minimum zones, the temperature dependence is of higher importance at the remaining sites. We test the new parameterization in the ecosystem model COBALT coupled to GFDL’s ESM2M Coupled Climate–Carbon Earth System Model. In a present-day simulation, the new version that includes a temperature-dependent remineralization (COBALT-T, both COBALT and COBALT-T feature an oxygen dependence) has a 0.1 GtC/yr lower global particle flux at 100m depth and a 0.02 GtC/yr higher particle flux at 2000m depth, caused by stronger remineralization in the warmer surface water of the low latitudes and weaker remineralization in the colder intermediate-depth water. Particularly in the low latitudes, the deep ocean oxygen concentration is higher and the oxygen minimum zone smaller. We discuss differences between COBALT and COBALT-T in a future run until 2100 under RCP8.5.