Towards a mechanistic understanding of the palaeoclimatological proxy δ13C in benthic foraminifera

Tilman.Hesse [ at ]


The proxy δ13C as derived from benthic foraminiferal shells is widely used by palaeoceanographers to reconstruct the distribution of past water masses. The biogeochemical processes involved in forming the benthic foraminiferal δ13C signal (δ13C_foram), however, have not been fully understood yet, and a sound mechanistic description is still lacking. This thesis attempts to make progress towards the long-standing goal of a mechanistic understanding and description of δ13C in benthic foraminifera. Furthermore, the still debated state of the glacial ocean circulation and water mass distribution is assessed using δ13C. First, a compilation of 220 sediment core δ13C reconstructions from the glacial Atlantic Ocean is compared with three-dimensional ocean circulation simulations including a marine carbon cycle model. Second, a reaction-diffusion model for calcification in foraminifera is adapted for the use in benthic foraminifera. This model is able to quantify the effects of different physical, chemical and biological processes on the δ13C signal of an idealised benthic foraminiferal shell (δ13C_foram). Sensitivity experiments with the stand-alone calcification model are performed. Third, the three-dimensional ocean circulation simulations are used to drive the foraminifera calcification model in order to have a spatial representation of δ13C_foram in the glacial ocean. The results are employed in another model-data comparison in the glacial Atlantic Ocean. The ocean model captures the general δ13C pattern indicated by present-day water column data and Late Holocene sediment cores but underestimates intermediate and deep water values in the South Atlantic. The best agreement with glacial reconstructions is obtained for a model scenario with an altered freshwater balance in the Southern Ocean, which has a shoaled and weakened North Atlantic Deep Water flow and intensified Antarctic Bottom Water export. Results from the foraminifera calcification model indicate that temperature, respiration rate, and pH have a significant impact on δ13C_foram. The results from the coupled ocean circulation/carbon cycle model and the foraminifera calcification model improve the correlation with glacial reconstructions for all simulations considered. Knowledge of vital parameters such as the respiration rate are important for constraining uncertainties in the formation of the δ13C_foram signal. The results show that an interdisciplinary approach to assessing palaeoclimate is both valu- able and useful for advancing our understanding of the climate system.

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Hesse, T. (2013): Towards a mechanistic understanding of the palaeoclimatological proxy δ13C in benthic foraminifera , PhD thesis, Universität Bremen.

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