| 1. |
- Schmittner, Andreas, et al.
(author)
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Calibration of the carbon isotope composition (δ13C) of benthic foraminifera
- 2017
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In: Paleoceanography. - 0883-8305. ; 32:6, s. 512-530
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Journal article (peer-reviewed)abstract
- The carbon isotope composition (δ13C) of seawater provides valuable insight on ocean circulation, air-sea exchange, the biological pump, and the global carbon cycle and is reflected by the δ13C of foraminifera tests. Here more than 1700 δ13C observations of the benthic foraminifera genus Cibicides from late Holocene sediments (δ13CCibnat) are compiled and compared with newly updated estimates of the natural (preindustrial) water column δ13C of dissolved inorganic carbon (δ13CDICnat) as part of the international Ocean Circulation and Carbon Cycling (OC3) project. Using selection criteria based on the spatial distance between samples, we find high correlation between δ13CCibnat and δ13CDICnat, confirming earlier work. Regression analyses indicate significant carbonate ion (-2.6 ± 0.4) × 10-3‰/(μmol kg-1) [CO3 2-] and pressure (-4.9 ± 1.7) × 10-3‰ m-1 (depth) effects, which we use to propose a new global calibration for predicting δ13CDICnat from δ13CCibnat. This calibration is shown to remove some systematic regional biases and decrease errors compared with the one-to-one relationship (δ13CDICnat = δ13CCibnat). However, these effects and the error reductions are relatively small, which suggests that most conclusions from previous studies using a one-to-one relationship remain robust. The remaining standard error of the regression is generally σ ≅ 0.25‰, with larger values found in the southeast Atlantic and Antarctic (σ ≅ 0.4‰) and for species other than Cibicides wuellerstorfi. Discussion of species effects and possible sources of the remaining errors may aid future attempts to improve the use of the benthic δ13C record.
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| 2. |
- Ödalen, Malin, et al.
(author)
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A revised Earth system model--based analysis of glacial--interglacial changes in ocean δ13C
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Other publication (other academic/artistic)abstract
- 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 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.
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| 3. |
- Ödalen, Malin, et al.
(author)
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Variable C∕P composition of organic production and its effect on ocean carbon storage in glacial-like model simulations
- 2020
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In: Biogeosciences. - : Copernicus GmbH. - 1726-4170 .- 1726-4189. ; 17:8, s. 2219-2244
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Journal article (peer-reviewed)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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