The marine calcium carbonate (CaCO3) cycle is a crucial component of the global carbon cycle, intricately linked to atmospheric CO2. Marine organism calcification largely controls CaCO3 formation. Planktonic calcifiers, ranging from primary producers to zooplankton, have played a significant role since the Mesozoic, regulating surface water alkalinity, ballasted organic matter and alkalinity export, and establishing a pelagic carbonate buffer influencing major CO2 changes. While numerous studies have attempted to quantify total pelagic CaCO3 production and the contributions of specific plankton groups, the relative contributions of the main calcifying taxa remain unclear. Estimates of total pelagic CaCO3 production vary widely (0.7–4.7 Pg C yr−1), with satellite-based or modeled estimates generally higher than those based on export fluxes. Understanding the relative contributions of different planktonic calcifying groups (coccolithophores, foraminifera, pteropods, heteropods), the proportion of aragonite versus calcite, and the magnitude of CaCO3 production compared to export, is critical. The North Pacific is particularly important for this research due to its large volume, diverse biogeochemical conditions, and undersaturation with respect to calcite and aragonite, making its calcifying organisms vulnerable to ocean acidification.

Existing literature suggests that coccolithophores and planktonic foraminifera contribute approximately equally to global pelagic CaCO3 production and sedimentation, based on sediment trap export fluxes and sediment data. However, recent studies highlight the potential importance of shelled pteropods. These planktonic calcifiers have distinct biogenic calcification mechanisms and varying vulnerabilities to ocean acidification, with shell solubility depending on polymorph mineralogy and Mg content. Their particulate inorganic carbon to particulate organic carbon (PIC/POC) ratios also influence the integrated carbon export rain ratio, a key term in ocean carbon cycling and atmospheric CO2. The association of PIC and POC within calcifying organisms may play a critical role in CaCO3 dissolution above the saturation horizon. Despite the importance of pelagic calcification, significant uncertainties remain regarding production rates, standing stocks, export fluxes, and the contributions of different groups. While relative distributions of pelagic calcifiers in the North Pacific have been studied, estimates of their contributions to CaCO3 standing stock and production rates are lacking.

This study involved a research cruise across subtropical to subpolar North Pacific waters to assess pelagic living CaCO3 standing stock. Five survey stations were sampled, deploying plankton nets for zooplankton and Niskin bottles for phytoplankton. Coccolithophores, foraminifera, pteropods, and heteropods were quantified, estimating CaCO3 biomass. Annual production was estimated using turnover time for each group. This was compared to aragonite and calcite export fluxes estimated from floating sediment traps deployed during sampling and historical time series. A 0.5 m diameter net with 90 µm mesh size was used to collect integrated samples of calcifiers. Samples were preserved in formalin, split, and analyzed. Foraminifera were wet-picked, counted, and weighed. Pteropods and heteropods were quantified, and shell diameter measured. CaCO3 biomass was estimated using PIC/POC ratios. Coccolithophore analysis involved filtering seawater and analyzing filters under a microscope. The total living coccolithophore calcite standing stock was obtained from the surface to a depth of 1% of the fluorescence peak. Production rates were calculated by dividing CaCO3 standing stock by turnover time estimates for each group. Seasonal bias was corrected using satellite-derived PIC and chlorophyll data, and zooplankton time series data for pteropods/heteropods. Pteropod CaCO3 biomass and production were also estimated using the MAREDAT database.

Total CaCO3 standing stock was lower in the nutrient-poor subtropical gyre and higher in the nutrient-rich subpolar gyre, reflecting the ecological shift. Coccolithophores dominated the CaCO3-producing standing stock at all stations (~79% on average), followed by pteropods (~14%) and foraminifera (~6%). Calcite (from coccolithophores and foraminifera) made up ~86% of the standing stock. Coccolithophore CaCO3 standing stock profiles followed chlorophyll fluorescence. Loose coccoliths contributed significantly to the total CaCO3 standing stock. Pteropod standing stocks were broadly consistent with previous estimates in the northwestern Pacific and Gulf of Alaska, showing significant seasonal and interannual variability. Heteropod presence was limited to the subtropics and transition zone. Foraminiferal CaCO3 standing stocks were similar to or slightly higher than previous estimates in the North Pacific. CaCO3 production was estimated using turnover times and standing stocks. Coccolithophores accounted for ~86% of total CaCO3 annual production, followed by pteropods (~10%), foraminifera (~2%), and heteropods (~0.3%). About 89% of CaCO3 production was calcite. Seasonally corrected annual CaCO3 production ranged from 0.2–0.4 mol m−2 yr−1 in the subtropical gyre and 0.9–1.0 mol m−2 yr−1 in the subpolar gyre. Production estimates were higher than export fluxes at 100–200 m in floating sediment traps and long-term sediment traps at ALOHA and PAPA stations, suggesting significant in situ remineralization in the photic zone (~80%). Analysis using the MAREDAT database showed typical pteropod CaCO3 biomass in the upper 250 m of 0.5 mg m−3 and a daily production of 12 mg m−2 day−1 in the North Pacific. Globally, estimates were much lower than previously reported means due to skewness in the global data. A simple regression of production against satellite PIC data provided a first order approximation of global CaCO3 production (~3.1 × 1014 mol yr−1; 3.7 Pg C yr−1), considerably higher than export flux estimates.

The findings show that coccolithophores are the dominant contributors to CaCO3 production in the North Pacific, and that a significant proportion of the CaCO3 produced is remineralized within the photic zone. This challenges previous assumptions about the relative importance of different calcifying groups and highlights the need to consider shallow dissolution processes. The large discrepancy between production and export rates suggests that shallow dissolution plays a crucial role in regulating the CaCO3 cycle. This in situ remineralization explains the previously observed differences between estimates of CaCO3 production derived from different methods. The study's estimates of global CaCO3 production are consistent with some previous high-end estimates but significantly higher than those based on export fluxes. The discrepancy between the study’s results and those of Buitenhuis et al (2019) on CaCO3 polymorph production is attributed to differences in model parametrization, turnover time assumptions, and assumptions about CaCO3 dissolution. Future changes in the CaCO3 cycle and its effects on the atmospheric CO2 will likely depend on the poorly understood processes that determine whether CaCO3 is remineralized in the photic zone or exported to depth.

This study provides the first comprehensive quantification of pelagic CaCO3 production in the North Pacific, revealing the dominance of coccolithophores and the substantial role of shallow dissolution. The results highlight the importance of considering both production and remineralization processes when assessing the CaCO3 cycle's response to climate change. Future research should focus on improving our understanding of shallow dissolution processes and their response to environmental changes.

The study's snapshot in time (August 2017) may not fully capture the seasonal variability of pteropod abundances. The approach of using turnover times to estimate production assumes a steady state, which might not always hold true. The method used to correct for seasonal bias relies on correlations with satellite data and other indirect estimates, introducing uncertainties.