Pelagic calcium carbonate production and shallow dissolution in the North Pacific Ocean

Planktonic calcifying organisms play a key role in regulating ocean carbonate chemistry and atmospheric CO₂. Surprisingly, references to the absolute and relative contribution of these organisms to calcium carbonate production are lacking. Here we report quantification of pelagic calcium carbonate production in the North Pacific, providing new insights on the contribution of the three main planktonic calcifying groups. Our results show that coccolithophores dominate the living calcium carbonate (CaCO₃) standing stock, with coccolithophore calcite comprising ~90% of total CaCO₃ production, and pteropods and foraminifera playing a secondary role. We show that pelagic CaCO₃ production is higher than the sinking flux of CaCO₃ at 150 and 200 m at ocean stations ALOHA and PAPA, implying that a large portion of pelagic calcium carbonate is remineralised within the photic zone; this extensive shallow dissolution explains the apparent discrepancy between previous estimates of CaCO₃ production derived from satellite observations/biogeochemical modeling versus estimates from shallow sediment traps. We suggest future changes in the CaCO₃ cycle and its impact on atmospheric CO₂ will largely depend on how the poorly-understood processes that determine whether CaCO₃ is remineralised in the photic zone or exported to depth respond to anthropogenic warming and acidification.