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A revised Earth system model--based analysis of glacial--interglacial changes in ocean δ13C
Stockholm University, Faculty of Science, Department of Meteorology .ORCID iD: 0000-0003-4855-7767
##### Abstract [en]

Across the latest deglaciation (from 21 to 0 ka), rearrangements in ocean circulation and carbon reservoirs occurred as climate changed. In the glacial state, atmospheric and terrestrial reservoirs of carbon were smaller, while carbon was stored in the deep ocean. The glacial-interglacial changes in atmospheric CO2 and deep ocean carbon storage are reflected by changes in the intermediate-to-deep ocean vertical δ13C gradient recorded in benthic foraminifera. However, sparse data coverage makes it difficult to infer ocean changes directly from the proxy records. In model studies, such records are often used to assess the validity of model simulations. In this study, we instead use a numerical model to interpolate and extrapolate the available benthic δ13C proxy records of Peterson et al. (2014) for the Holocene (HOL, 0-6 ka) and the Last Glacial Maximum (LGM, 19-23 ka). We apply appropriate boundary conditions for each time slice, and search for the best-possible fit to the proxy records by running ensembles, where we vary the wind stress scaling parameter, the amount of Atlantic-to-Pacific freshwater redistribution, and the fraction of brine relocated from the surface to the deep ocean. For both HOL and LGM, we find that the best fits are acheived when we apply a wind stress scaling of 0.8 and we apply a brine rejection relocation of 20%. However, the best fit for the LGM is found for weak freshwater redistribution, while a stronger redistribution is optimal for HOL. The best-fit simulations reproduce well the shift from a stronger to a weaker surface-to-deep ocean δ13C gradient across the deglaciation, as indicated by the proxy records. The differences in boundary conditions combined with the difference in freshwater redistribution result in a $\sim$50% weaker Atlantic Meridional Overturning Circulation (AMOC) at the LGM compared to HOL, while the Pacific oxygen minimum zone is found in the deep (LGM), rather than the intermediate (HOL), ocean. After using model-data data misfit to remove bias from the best-fit simulations, we find that the LGM is more depleted in δ13C compared to HOL, with a deglacial change in whole-ocean δ13C of 0.30‰. For both time slices, good fits to the proxy data are also achieved for other combinations of brine rejection relocation and freshwater redistribution. We therefore compute a bias-corrected ensemble average for the deglacial whole-ocean change in δ13C, to account for uncertainty in the analysis. After weighting the ensemble members by their skill in reproducing the proxy records, we estimate the deglacial whole-ocean change in δ13C (HOL-LGM) to 0.28 ± 0.06. This corresponds to 430 ± 90 Pg C transferred between the terrestrial carbon reservoir and the ocean. This should not be interpreted as an estimate of the overall change in terrestrial carbon storage during the glacial cycle, but as an estimate of the change in carbon with a terrestrial δ13C signature that could be accomodated in the LGM ocean.

##### Keywords [en]
Model study, Glacial ocean, Glacial climate
Climate Research
##### Research subject
Atmospheric Sciences and Oceanography
##### Identifiers
OAI: oai:DiVA.org:su-172893DiVA, id: diva2:1350637
##### Note

Manuscript in preparation for Climate of the Past.

Available from: 2019-09-11 Created: 2019-09-11 Last updated: 2019-09-16Bibliographically approved
##### 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)
##### 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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##### By author/editor
Ödalen, MalinPeterson, Carlye D.Ridgwell, AndyValdes, Paul J.
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