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Variable C/P composition of organic production and its effect on ocean carbon storage in glacial model simulations
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0003-4855-7767
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0002-4414-6859
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2019 (English)In: Biogeosciences Discussions, ISSN 1810-6277, E-ISSN 1810-6285Article in journal (Refereed) Submitted
Abstract [en]

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.

Place, publisher, year, edition, pages
2019.
Keywords [en]
Ocean biogeochemistry, Glacial climate
National Category
Climate Research
Research subject
Atmospheric Sciences and Oceanography
Identifiers
URN: urn:nbn:se:su:diva-172892DOI: 10.5194/bg-2019-149OAI: oai:DiVA.org:su-172892DiVA, id: diva2:1350636
Conference
Joint IAPSO-IAMAS-IAGA Assembly 2017, Cape Town, South Africa, Au­gust 27 - Septem­ber 1, 2017; Ocean Sciences Meeting 2018, Portland, OR, USA, February 11 - 16, 2018
Note

Discussion paper under review for publication in Biogeosciences.

Available from: 2019-09-11 Created: 2019-09-11 Last updated: 2020-03-19
In thesis
1. Model analysis of ocean carbon storage and transport across climate states
Open this publication in new window or tab >>Model analysis of ocean carbon storage and transport across climate states
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The ocean carbon cycle plays a fundamental role in the Earth’s climate system, on decadal to multi-millennial timescales. Of the carbon held in the ocean, the atmosphere, and the terrestrial biosphere combined, more than 90% resides in the ocean. Carbon enters the surface ocean through air-sea gas exchange and from terrestrial sources. It is transported to the deep ocean with the ocean circulation and through the so-called biological pump, where carbon is taken up in the surface ocean by photosynthetic organisms that fall down and decompose at depth. This thesis contributes to the understanding of the processes involved in ocean carbon storage and transport. It examines how these processes respond to model perturbations, and how this response influences our attempts to simulate glacial-interglacial fluctuations in atmospheric carbon dioxide (CO2).

The thesis investigates the response of the simulated ocean carbon storage, and distribution of the isotopic tracer δ13C, to changes in physical and biological parameters. In the included studies, we use observational as well as proxy records of oceanic properties to evaluate our model simulations. In addition, we use a climate model to interpret proxy evidence of glacial-interglacial changes in ocean δ13C. By using a separation framework, we identify the origin of the carbon in the model ocean, and attribute observed changes to the processes involved.

The results indicate a strong link between ocean carbon storage and the strength of the global ocean overturning circulation. Stronger circulation leads to less carbon storage through a weakening of the biological pump, and through reduced solubility due to an increase in global ocean average temperature.

In simulations of glacial climate, we find that biological adaptability to the surrounding nutrient conditions, through a flexible carbon-to-phosphorus ratio (C/P) in ocean photosynthesis, increases the ocean carbon storage compared to simulations where fixed C/P is applied. The biological flexibility improves the model’s ability to reproduce glacial atmospheric CO2. In line with previous research, we find freshwater input to the North Atlantic to be an important factor for reproducing glacial proxy records. The ensemble of simulations that achieve a good representation of glacial-interglacial δ13C indicates a deglacial whole-ocean change in δ13C of 0.28 ± 0.06‰.

The thesis underlines the importance of the initial state, and the choice of model parameterisations, for the outcome of model ensemble, and intercomparison studies. Finally, it proposes a new method for estimation of ocean carbon transport, and attribution of this transport to different water masses and carbon system processes.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University, 2019. p. 42
Keywords
Oceanography, Climate, Climate model, Carbon cycle, Paleoclimate
National Category
Climate Research Oceanography, Hydrology and Water Resources Geosciences, Multidisciplinary
Research subject
Atmospheric Sciences and Oceanography
Identifiers
urn:nbn:se:su:diva-172894 (URN)978-91-7797-829-9 (ISBN)978-91-7797-830-5 (ISBN)
Public defence
2019-10-25, William-Olssonsalen, Geovetenskapens hus, Svante Arrhenius väg 14, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 3: Manuscript. Paper 4: Manuscript.

Available from: 2019-10-02 Created: 2019-09-11 Last updated: 2019-09-24Bibliographically approved

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