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Variable C∕P composition of organic production and its effect on ocean carbon storage in glacial-like model simulations

Variable C∕P composition of organic production and its effect on ocean carbon storage in glacial-like model simulations
Variable C∕P composition of organic production and its effect on ocean carbon storage in glacial-like model simulations
During the four most recent glacial maxima, atmospheric CO2 has been lowered by about 90–100 ppm with respect to interglacial concentrations. It is likely that most of the atmospheric CO2 deficit was stored in the ocean. Changes in the biological pump, which are related to the efficiency of the biological carbon uptake in the surface ocean and/or of the export of organic carbon to the deep ocean, have been proposed as a key mechanism for the increased glacial oceanic CO2 storage. The biological pump is strongly constrained by the amount of available surface nutrients. In models, it is generally assumed that the ratio between elemental nutrients, such as phosphorus, and carbon (C∕P ratio) in organic material is fixed according to the classical Redfield ratio. The constant Redfield ratio appears to approximately hold when averaged over basin scales, but observations document highly variable C∕P ratios on regional scales and between species. If the C∕P ratio increases when phosphate availability is scarce, as observations suggest, this has the potential to further increase glacial oceanic CO2 storage in response to changes in surface nutrient distributions. In the present study, we perform a sensitivity study to test how a phosphate-concentration-dependent C∕P ratio influences the oceanic CO2 storage in an Earth system model of intermediate complexity (cGENIE). We carry out simulations of glacial-like changes in albedo, radiative forcing, wind-forced circulation, remineralization depth of organic matter, and mineral dust deposition. Specifically, we compare model versions with the classical constant Redfield ratio and an observationally motivated variable C∕P ratio, in which the carbon uptake increases with decreasing phosphate concentration. While a flexible C∕P ratio does not impact the model's ability to simulate benthic δ13C patterns seen in observational data, our results indicate that, in production of organic matter, flexible C∕P can further increase the oceanic storage of CO2 in glacial model simulations. Past and future changes in the C∕P ratio thus have implications for correctly projecting changes in oceanic carbon storage in glacial-to-interglacial transitions as well as in the present context of increasing atmospheric CO2 concentrations.
1726-4170
2219-2244
Ödalen, Malin
34d587e6-279c-4112-bbff-a8fdcf14bc21
Nycander, Jonas
b9cd5227-3f9e-4957-a430-9d2f5e695d3f
Ridgwell, Andy
769cea5c-e033-456a-8b53-51dfa307dc35
Oliver, Kevin I. C.
588b11c6-4d0c-4c59-94e2-255688474987
Peterson, Carlye D.
4b60547f-1758-4902-92a5-ecc9e118e221
Nilsson, Johan
983c8a87-dd62-4d68-a60b-fe0e9e5e00e3
Ödalen, Malin
34d587e6-279c-4112-bbff-a8fdcf14bc21
Nycander, Jonas
b9cd5227-3f9e-4957-a430-9d2f5e695d3f
Ridgwell, Andy
769cea5c-e033-456a-8b53-51dfa307dc35
Oliver, Kevin I. C.
588b11c6-4d0c-4c59-94e2-255688474987
Peterson, Carlye D.
4b60547f-1758-4902-92a5-ecc9e118e221
Nilsson, Johan
983c8a87-dd62-4d68-a60b-fe0e9e5e00e3

Ödalen, Malin, Nycander, Jonas, Ridgwell, Andy, Oliver, Kevin I. C., Peterson, Carlye D. and Nilsson, Johan (2020) Variable C∕P composition of organic production and its effect on ocean carbon storage in glacial-like model simulations. Biogeosciences, 17 (8), 2219-2244, [121]. (doi:10.5194/bg-17-2219-2020).

Record type: Article

Abstract

During the four most recent glacial maxima, atmospheric CO2 has been lowered by about 90–100 ppm with respect to interglacial concentrations. It is likely that most of the atmospheric CO2 deficit was stored in the ocean. Changes in the biological pump, which are related to the efficiency of the biological carbon uptake in the surface ocean and/or of the export of organic carbon to the deep ocean, have been proposed as a key mechanism for the increased glacial oceanic CO2 storage. The biological pump is strongly constrained by the amount of available surface nutrients. In models, it is generally assumed that the ratio between elemental nutrients, such as phosphorus, and carbon (C∕P ratio) in organic material is fixed according to the classical Redfield ratio. The constant Redfield ratio appears to approximately hold when averaged over basin scales, but observations document highly variable C∕P ratios on regional scales and between species. If the C∕P ratio increases when phosphate availability is scarce, as observations suggest, this has the potential to further increase glacial oceanic CO2 storage in response to changes in surface nutrient distributions. In the present study, we perform a sensitivity study to test how a phosphate-concentration-dependent C∕P ratio influences the oceanic CO2 storage in an Earth system model of intermediate complexity (cGENIE). We carry out simulations of glacial-like changes in albedo, radiative forcing, wind-forced circulation, remineralization depth of organic matter, and mineral dust deposition. Specifically, we compare model versions with the classical constant Redfield ratio and an observationally motivated variable C∕P ratio, in which the carbon uptake increases with decreasing phosphate concentration. While a flexible C∕P ratio does not impact the model's ability to simulate benthic δ13C patterns seen in observational data, our results indicate that, in production of organic matter, flexible C∕P can further increase the oceanic storage of CO2 in glacial model simulations. Past and future changes in the C∕P ratio thus have implications for correctly projecting changes in oceanic carbon storage in glacial-to-interglacial transitions as well as in the present context of increasing atmospheric CO2 concentrations.

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Accepted/In Press date: 27 February 2020
Published date: 22 April 2020

Identifiers

Local EPrints ID: 444432
URI: http://eprints.soton.ac.uk/id/eprint/444432
ISSN: 1726-4170
PURE UUID: 6fb12cbc-7463-48ac-ac5c-a769431c82fb

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Date deposited: 19 Oct 2020 16:31
Last modified: 25 Nov 2021 17:39

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Contributors

Author: Malin Ödalen
Author: Jonas Nycander
Author: Andy Ridgwell
Author: Carlye D. Peterson
Author: Johan Nilsson

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