role of nutricline depth in regulating the ocean carbon cycle

Carbon uptake by marine phytoplankton, and its export as organic matter to the ocean interior (i.e., the "biological pump"), lowers the partial pressure of carbon dioxide (pCO₂) in the upper ocean and facilitates the diffusive drawdown of atmospheric CO₂. Conversely, precipitation of calcium carbonate by marine planktonic calcifiers such as coccolithophorids increases pCO₂ and promotes its outgassing (i.e., the "alkalinity pump"). Over the past [almost equal to]100 million years, these two carbon fluxes have been modulated by the relative abundance of diatoms and coccolithophores, resulting in biological feedback on atmospheric CO₂ and Earth's climate; yet, the processes determining the relative distribution of these two phytoplankton taxa remain poorly understood. We analyzed phytoplankton community composition in the Atlantic Ocean and show that the distribution of diatoms and coccolithophorids is correlated with the nutricline depth, a proxy of nutrient supply to the upper mixed layer of the ocean. Using this analysis in conjunction with a coupled atmosphere-ocean intermediate complexity model, we predict a dramatic reduction in the nutrient supply to the euphotic layer in the coming century as a result of increased thermal stratification. Our findings indicate that, by altering phytoplankton community composition, this causal relationship may lead to a decreased efficiency of the biological pump in sequestering atmospheric CO₂, implying a positive feedback in the climate system. These results provide a mechanistic basis for understanding the connection between upper ocean dynamics, the calcium carbonate-to-organic C production ratio and atmospheric pCO₂ variations on time scales ranging from seasonal cycles to geological transitions.