Global carbonate production and accumulation – Updated present-day estimates and future projections

There are two primary mechanisms for carbon transfer into the geological record within the ocean: the organic sink and the inorganic carbon sink. The inorganic carbon sink (i.e., the long-term preservation of carbonate minerals in the geologic record), in particular, is not well constrained. Over the past several years, theoretical and experimental work has significantly improved our understanding of the inorganic carbon cycle, particularly in the context of carbonate minerals. This work reviews the state-of-the-art of marine calcium carbonate production, dissolution, and accumulation, representing a comprehensive update on the works of Milliman (1993) and Smith and Mackenzie (2016). We combine updated estimates with recent advances in understanding the carbonic acid system and the global ocean carbon cycle. These processes are supplemented by organismal to ecosystem-level responses of calcifying organisms to future climate change, enabling estimates of future global marine carbonate budget development. Our synthesis offers a comprehensive perspective on the long-term balance of carbonate preservation under current boundary conditions and projected future climate scenarios.

Present-day net global annual gross production of CaCO₃ throughout all carbonate-producing ecosystems is estimated between 176.1*10¹² mol yr^-1 (= 17.61 Gt yr^-1) and 437.1*10¹² mol yr^-1 (= 437.1 Gt yr^-1). However, only 8 to 20% of CaCO₃ sediment is preserved each year, according to literature data. This leads to ~35*10¹² mol yr^-1 (= 3.5 Gt yr^-1) of CaCO₃ accumulation per year. Today, nearly all CaCO₃ production is estimated to be biologically mediated or biologically controlled, while contributions from abiogenic carbonates are assumed to be minor. Dissolution of CaCO₃ in the marine realm buffers excess CO₂ from organic matter oxidation in the water column and sediment. The carbonate buffer effect within the carbonic acid system is expected to decline in future climate change scenarios, likely leading to more intense carbonate dissolution.

Anthropogenic climate change, including excess CO₂ in surface oceans, is further proposed to impact the CaCO₃ production capabilities of all CaCO₃-producing ecosystems in the ocean by affecting the diversity, abundance, and calcification potential of organisms. Predicting the future development of CaCO₃ accumulation in the oceans is an evolving process. With the currently available data, estimating future CaCO₃ accumulation is challenging due to multiple unknowns regarding disruptions and adaptations of the producers. The herein-reviewed data suggest that the global net CaCO₃ accumulation will decrease by over 50 % to 19*10¹² mol yr^-1 (= 1.90 Gt yr^-1) by 2050 AD, and further to 10.3*10¹² mol yr^-1 (= 1.03 Gt yr^-1) by 2100 AD based on a “business as usual” CO₂ emission model.

 

References

Milliman, J. D.: Production and accumulation of calcium carbonate in the ocean: Budget of a nonsteady state, Global Biogeochem Cy, 7, 927–957, https://doi.org/10.1029/93gb02524, 1993.

Smith, S. V. and Mackenzie, F. T.: The Role of CaCO3 Reactions in the Contemporary Oceanic CO2 Cycle, Aquat Geochem, 22, 153–175, https://doi.org/10.1007/s10498-015-9282-y, 2016.