Shallow Calcium Carbonate Cycling in the North Pacific Ocean

Key Points: - High resolution carbonate chemistry, δ13C-DIC, and particle flux measurements in the NE Pacific sheds light on the upper oceancalcium carbonate and alkalinity cycles. - Based on this sampling campaign, there isevidence for substantial CaCO3 dissolution in the mesopelagic zone above the saturation horizon. - Dissolution experiments, observations, and modeling suggest that shallow CaCO3 dissolutionis coupled to the consumption of organic carbon, through a combination of zooplankton grazing and oxic respiration within particle microenvironments. \ud The cycling of biologically produced calcium carbonate (CaCO3) in the ocean is a fundamental component of the global carbon cycle. Here, we present experimental determinations of in situcoccolith and foraminiferal calcite dissolution rates.We combine these rates with solid phase fluxes,dissolved tracers, and historical data to constrain the alkalinity cycle in the shallow North Pacific Ocean.The in situ dissolution rates of coccolithophores demonstrate a nonlinear dependence on saturation state. Dissolution ratesof all three major calcifying groups (coccoliths, foraminifera, and aragonitic pteropods)aretoo slow to explainthe patternsofboth CaCO3sinking fluxand alkalinity regenerationin the NorthPacific.Usinga combination of dissolved and solid-phase tracers, we document a significant dissolution signal in seawater supersaturated for calcite. Driving CaCO3dissolutionwith acombination of ambient saturation state and oxygen consumption simultaneously explainssolid-phase CaCO3flux profiles and patterns of alkalinity regeneration across the entire N. Pacific basin. Wedo not need to invokethe presence ofcarbonate phases with higher solubilities.Instead, biomineralization and metabolic processesintimately associatethe acid (CO2) and the base (CaCO3) in the same particles,driving the coupled shallow remineralization of organic carbonand CaCO3.The linkage of these processes likely occurs through a combination of dissolution due to zooplankton grazing and microbial aerobic respiration withindegrading particle aggregates.The coupling of these cyclesacts as a major filter on the export of both organic and inorganic carbon to the deep ocean