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The Atlantic Meridional Overturning Circulation (AMOC) impacts temperatures, ecosystems, and the carbon cycle. However, AMOC effects on Earth's carbon cycle remains poorly understood, in part because contributions of different physical and biological mechanisms that impact carbon storage in the ocean are not typically diagnosed in climate models. Here, we explore modeled effects of AMOC shutdowns on ocean Dissolved Inorganic Carbon (DIC) by applying a new decomposition that explicitly calculates preformed and regenerated DIC components and separates physical and biological contributions. An extensive evaluation in transient simulations finds that the method is accurate, especially for basin‐wide changes, whereas errors can be significant at global and local scales. In contrast, estimates of respired carbon based on Apparent Oxygen Utilization lead to large errors and are generally not reliable. In response to a shutdown of the AMOC under Last Glacial Maximum (LGM) background climate, ocean carbon increases and then decreases, leading to opposite changes in atmospheric carbon dioxide (CO2). DIC changes are dominated by opposing changes in biological carbon storage. Whereas regenerated components increase in the Atlantic and dominate the initial increase in global ocean DIC until model year 1000, preformed components decrease in the other ocean basins and dominate the long‐term DIC decrease until year 4000. Biological disequilibrium is an important contribution to preformed carbon changes. Biological saturation carbon decreases in the Pacific, Indian, and Southern Oceans due to a decrease in surface alkalinity. The spatial patterns of the DIC components and their changes in response to an AMOC collapse are presented.
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Carbon and carbon-13 in the preindustrial and glacial ocean
Despite their importance for Earth’s climate and paleoceanography, the cycles of carbon (C) and its isotope13C in the ocean are not well understood. Models typically do not decompose C and13C storage caused by different physical, biological, and chemical processes, which makes interpreting results difficult. Consequently, basic observed features, such as the decreased carbon isotopic signature (δ13CDIC) of the glacial ocean remain unexplained. Here, we review recent progress in decomposing Dissolved Inorganic Carbon (DIC) into preformed and regenerated components, extend a precise and complete decomposition to δ13CDIC, and apply it to data-constrained model simulations of the Preindustrial (PI) and Last Glacial Maximum (LGM) oceans. Regenerated components, from respired soft-tissue organic matter and dissolved biogenic calcium carbonate, are reduced in the LGM, indicating a decrease in the active part of the biological pump. Preformed components increase carbon storage and decrease δ13CDICby 0.55 ‰ in the LGM. We separate preformed into saturation and disequilibrium components, each of which have biological and physical contributions. Whereas the physical disequilibrium in the PI is negative for both DIC and δ13CDIC, and changes little between climate states, the biological disequilibrium is positive for DIC but negative for δ13CDIC, a pattern that is magnified in the LGM. The biological disequilibrium is the dominant driver of the increase in glacial ocean C and the decrease in δ13CDIC, indicating a reduced sink of biological carbon. Overall, in the LGM, biological processes increase the ocean’s DIC inventory by 355 Pg more than in the PI, reduce its mean δ13CDICby an additional 0.52 ‰, and contribute 60 ppm to the lowering of atmospheric CO2. Spatial distributions of the δ13CDICcomponents are presented. Commonly used approximations based on apparent oxygen utilization and phosphate are evaluated and shown to have large errors.
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- Award ID(s):
- 1924215
- PAR ID:
- 10521991
- Editor(s):
- Carré, Matthieu
- Publisher / Repository:
- PLOS
- Date Published:
- Journal Name:
- PLOS Climate
- Volume:
- 3
- Issue:
- 7
- ISSN:
- 2767-3200
- Page Range / eLocation ID:
- e0000434
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1371/journal.pclm.0000434
