No detectable influence of the carbonate ion effect on changes in stable carbon isotope ratios (δ¹³C) of shallow dwelling planktic foraminifera over the past 160 kyr

Abstract.

Laboratory experiments showed that the isotopic fractionation of δ¹³C and of δ¹⁸O during calcite formation of planktic foraminifera are species-specific functions of oceanic CO₃^2--concentration. This effect became known as the carbonate ion effect (CIE), whose role during the interpretation of marine sediment data will be investigated here in an in-depth analysis of the ¹³C cycle. For that effort we compiled new 160 kyr-long mono-specific stacks of changes in both δ¹³C and of δ¹⁸O from either the planktic foraminifera G. ruber (rub) or T. sacculifer (sac) from 112 and 40 non-polar marine records, respectively. Both mono-specific time series Δ(δ¹³C_rub) and Δ(δ¹³C_sac) are very similar to each other and a linear regression through a scatter plot of both data sets has a slope of ∼0.99 — although the laboratory-based CIE for both species differ by nearly a factor of two, implying that they should record distinctly different changes in δ¹³C, if we accept that the carbonate ion concentration changes on glacial/interglacial timescales. For a deeper understanding we use the global carbon cycle model BICYCLE-SE to calculate how surface ocean CO₃^2- should have varied over time in order to be able to calculate the potential corrections which would follow the laboratory-based CIE. Our simulations are forced with atmospheric reconstructions of CO₂ and δ¹³CO₂ derived from ice cores to obtain a carbon cycle which should at least at the surface ocean be as close as possible to expected conditions and which agrees in the deep ocean in the carbon isotope of dissolved inorganic carbon (DIC), δ¹³C_DIC, with reconstruction from benthic foraminifera. We find that both Δ(δ¹³C_rub) and Δ(δ¹³C_sac) agree better with changes in  simulated δ¹³C_DIC when ignoring the CIE than those time series which where corrected for the CIE. The combination of data- and model-based evidence for the lack of a role for the CIE in Δ(δ¹³C_rub) and Δ(δ¹³C_sac) suggests to us that the CIE as measured in laboratory experiments is not directly transferable to the interpretation of marine sediments records. We hypothesise that both foraminifera species can optimise their light environments via vertical motion and therefore calcify under nearly stable CO₃^2--concentration. The much smaller CIE-to-glacial/interglacial-signal-ratio in δ¹⁸O, when compared to δ¹³C prevents us to draw robust conclusions on the role of the CIE on δ¹⁸O as recorded in the hard shells of both species. However, theory proposes that the CIE in δ¹³C and δ¹⁸O depends both on the pH in the surrounding water, suggesting that the CIE should be detectable in neither or both of the isotopes. Whether this lack of role of the CIE in the interpretation of planktic paleo data is a general feature, or restricted to the two species investigated here, needs to be checked with further data from other planktic foraminiferal species.