[1] We compare a compilation of 220 sediment core d 13 C data from the glacial Atlantic Ocean with three-dimensional ocean circulation simulations including a marine carbon cycle model. The carbon cycle model employs circulation fields which were derived from previous climate simulations. All sediment data have been thoroughly quality controlled, focusing on epibenthic foraminiferal species (such as Cibicidoides wuellerstorfi or Planulina ariminensis) to improve the comparability of model and sediment core carbon isotopes. The model captures the general d 13 C 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 that mimics enhanced northward sea ice export and melting away from the zone of sea ice production. This results in a shoaled and weakened North Atlantic Deep Water flow and intensified Antarctic Bottom Water export, hence confirming previous reconstructions from paleoproxy records. Moreover, the modeled abyssal ocean is very cold and very saline, which is in line with other proxy data evidence.

The δ 13 C value measured on benthic foraminiferal tests is widely used by palaeoceanographers to reconstruct the distribution of past water masses. The biogeochemical processes involved in forming the benthic foraminiferal δ 13 C signal (δ 13 C foram ), however, are not fully understood and a sound mechanistic description is still lacking. We use a reaction-diffusion model for calcification developed by Wolf-Gladrow et al. (1999) and Zeebe et al. (1999) in order to quantify the effects of different physical, chemical, and biological processes on δ 13 C foram of an idealised benthic foraminiferal shell. Changes in the δ 13 C value of dissolved inorganic carbon (δ 13 C DIC ) cause equal changes in δ 13 C foram in the model. The results further indicate that temperature, respiration rate, and pH have a significant impact on δ 13 C foram . In contrast, salinity, pressure, the δ 13 C value of particulate organic carbon (δ 13 C POC ), total alkalinity, and calcification rate show only a limited influence. In sensitivity experiments we assess how combining these effects can influence δ 13 C foram . We can potentially explain 33 to 47% of the interglacial-to-glacial decrease in δ 13 C foram by changes in temperature and pH, without invoking changes in δ 13 C DIC . Furthermore, about a quarter of the −0.4‰ change in δ 13 C foram observed in phytodetritus layers can be accounted for by an increase in respiration rate and a reduction in pH.

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