Award: OCE-1416700

The processes that lead to export of organic matter from the surface to the deep ocean remain poorly understood. This project has investigated the role of different particles in sinking of organic matter in a model. The global model of ocean biogeochemical cycles and isotopes (MOBI) was improved by adding two interacting size classes of dead organic matter (detritus). Small detritus aggregates to larger particles, which sink faster in the water column. A first working model based on the previous work by Burd (2013; Journal of Geophysical Research: Oceans, doi:10.1002/jgrc.20255, their Model C) was constructed that fits the global distributions of different ocean biogeochemical tracers such as phytoplankton nutrients (nitrate, phosphate and dissolved iron) and dissolved oxygen, dissolved inorganic carbon and alkalinity. The resulting model displays small concentrations of large detritus and large concentrations of small detritus at the surface (Fig. 1). However, at depths below about 2,200 m concentrations of large detritus are larger than concentrations of small detritus. This is because small particles continue to aggregate into larger particles as they sink. This model will be used in future numerical experiments to study the effects of ocean acidification, which we hypothesize may influence particle density and sinking by changing the production and dissolution of calcium carbonate, on organic matter fluxes from the surface to the deep ocean. Numerical experiments performed with a model without the above-mentioned particle dynamics have shown nonlinear behavior of certain metrics related to ocean acidification. Eyre et al. (2018, Science, doi:10.1126/science.aao1118) present evidence for a threshold in aragonite saturation state of 2.9 below which coral reefs show net dissolution of their calcium carbonate. Our model simulations show that the global surface ocean area above that threshold depends strongly and non-linearly on the carbon emission scenario (Fig. 2). Essentially, all emission scenarios above the lowest (RCP2.6) result in large decreases in the area suitable for coral reef growth. The two high emission scenarios RCP6.0 and RCP8.5 show essentially zero area remaining suitable for coral growth after 2090 and 2070, respectively. This suggests large benefits for coral reefs of carbon emission reductions. The project has supported the training of an early career scientist and it has contributed to enhancing the software infrastructure for ocean carbon cycle research by improving a widely-used model. Last Modified: 07/08/2019 Submitted by: Andreas Schmittner