Marine particles and their remineralization buffer future ocean biogeochemistry response to climate warming

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Abstract. Transport and fate of particulate organic carbon (POC) and nutrients through marine particles co-determine the future response of ocean biogeochemistry and oceanic carbon uptake under climate warming. This makes the parametrization of the biological carbon pump in Earth system models (ESMs) an important model component and motivates us to compare a recently developed new sinking scheme (M4AGO; Maerz et al. 2020) to the current CMIP6 default Martin curve-like sinking scheme in MPI-ESM1.2-LR (see Mauritsen et al. 2019) under the future shared socio-economic pathway high-emission scenario SSP5-8.5. In their global response, the two model versions are similar, showing a decrease of integrated net primary production between the historical (1985–2014) and future (2070–2099) period of about 8.1 % and 9.7 % for the CMIP6 and M4AGO version, respectively. However, the models response differs latitudinally. In M4AGO, the temperature-dependent remineralization offsets the future increase in sinking velocity caused by a higher CaCO3 to POC ratio in the low latitudes. There, M4AGO thus buffers the export loss of nutrients to the mesopelagic, visible in little future changes of the export to net primary production ratio (the p ratio), while the CMIP6 version shows more pronounced changes with regionally declining or increasing p ratio. In the Arctic Ocean, the projected future increase of net primary production in the CMIP6 version is diminished with M4AGO through its higher POC transfer efficiency in high latitude regions. Hence, the more mechanistic and to environmental changes-responding M4AGO scheme shows a stronger buffering regional response to climate warming than the CMIP6 model version. The higher transfer efficiency also impinges on higher CO2 uptake in high latitude regions while the tropical regions turn later into a net sink with M4AGO compared to the standard CMIP6 version. Next to ballasting, we identified the particle microstructure as vigorous determinant for future changes of POC sinking velocity. Microstructure co-determines particle porosity and particle density. Processes governing the microstructure thus can be regarded as decisive to understand for reducing uncertainty of future POC fluxes.

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