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Biogeosciences An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Jun 2019

Research article | 03 Jun 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).

Variable C/P composition of organic production and its effect onocean carbon storage in glacial model simulations

Malin Ödalen1, Jonas Nycander1, Andy Ridgwell2,3, Kevin I. C. Oliver4, Carlye D. Peterson2, and Johan Nilsson1 Malin Ödalen et al.
  • 1Department of Meteorology, Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden
  • 2Department of Earth Sciences, University of California–Riverside, Riverside, CA 92521, USA
  • 3School of Geographical Sciences, Bristol University, Bristol BS8 1SS, UK
  • 4National Oceanography Centre, Southampton, University of Southampton, Southampton SO14 3ZH, United Kingdom

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 of 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, e.g. phosphorus, and carbon (C/P ratio) in organic material is fixed according to the classical Redfield ratio. The constant Redfield ratio appears to hold approximately when averaged over basin scales, but observations document highly variable C/P ratios on regional scales and between species. If the C/P ratio decreases when nutrient 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, remineralisation depth of organic matter, and mineral dust deposition. Specifically, we compare model versions with 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 d13C 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.

Malin Ödalen et al.
Interactive discussion
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Status: open (until 15 Jul 2019)
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Malin Ödalen et al.
Model code and software

cGENIE release v0.9.5 A. Ridgwell, M. Ödalen, and K. I. C. Oliver

Malin Ödalen et al.
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Short summary
In glacial periods, ocean uptake of carbon is likely a key player for achieving low atmospheric CO2. In climate models, ocean biological uptake of carbon (C) and phosphorus (P) are often assumed to occur in fixed proportions. In this study, we allow the proportion C:P to vary, and simulate, to first approximation, the complex biological changes that occur in the ocean over long time scales. We show here that, for glacial--interglacial cycles, this complexity contributes to low atmospheric CO2.
In glacial periods, ocean uptake of carbon is likely a key player for achieving low atmospheric...