Reduced calcification in modern Southern Ocean planktonic foraminifera

Abstract

Anthropogenic carbon dioxide has been accumulating in the oceans, lowering both the concentration of carbonate ions and the pH (ref. 1), resulting in the acidification of sea water. Previous laboratory experiments have shown that decreased carbonate ion concentrations cause many marine calcareous organisms to show reduced calcification rates2,3,4,5. If these results are widely applicable to ocean settings, ocean acidification could lead to ecosystem shifts. Planktonic foraminifera are single-celled calcite-secreting organisms that represent between 25 and 50% of the total open-ocean marine carbonate flux6 and influence the transport of organic carbon to the ocean interior7. Here we compare the shell weights of the modern foraminifer Globigerina bulloides collected from sediment traps in the Southern Ocean with the weights of shells preserved in the underlying Holocene-aged sediments. We find that modern shell weights are 30–35% lower than those from the sediments, consistent with reduced calcification today induced by ocean acidification. We also find a link between higher atmospheric carbon dioxide and low shell weights in a 50,000-year-long record obtained from a Southern Ocean marine sediment core. It is unclear whether reduced calcification will affect the survival of this and other species, but a decline in the abundance of foraminifera caused by acidification could affect both marine ecosystems and the oceanic uptake of atmospheric carbon dioxide.

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Figure 1: Location of Holocene core-top samples, sediment-trap mooring and sediment core GC17 used in this study.
Figure 2: SAZ Holocene core-top (filled circles) and average flux-weighted sediment-trap shell weights (open squares) for G. bulloides.
Figure 3: G. bulloides shell weight and shell flux for 47 S sediment trap at 2,000 m below sea surface and the distribution of individual G. bulloides shell weights from core SO136-140BX.
Figure 4: Measured shell weights for G. bulloides (300–355 μm) in GC17 (filled diamonds) and Vostok pCO2 record (grey solid line; ref. 20).

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Acknowledgements

This work was supported by the Australian Government’s Cooperative Research Centres Programme through the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC) and the Australian Government Department of Climate Change. The Australian Antarctic Division (ASAC grants to T.W.T.) supports the ongoing sediment trap program. The Australian Institute of Nuclear Science and Engineering provided radiocarbon dates (AINSE Grant 07/072) and Geoscience Australia provided sediment samples.

Author information

A.D.M. and W.R.H. conceived the foraminiferal shell-weight experiment; A.D.M. carried out laboratory processing; T.W.T. and S.G.B. run the Australian Antarctic Sciences Subantarctic Zone sediment trap program, including handling of moorings and laboratory processing of bulk sediment-trap samples; A.D.M. and W.R.H. wrote the manuscript, with all authors commenting.

Correspondence to William R. Howard.

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Moy, A., Howard, W., Bray, S. et al. Reduced calcification in modern Southern Ocean planktonic foraminifera. Nature Geosci 2, 276–280 (2009). https://doi.org/10.1038/ngeo460

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