24 Oct 2023
 | 24 Oct 2023
Status: this preprint is currently under review for the journal CP.

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

Peter Köhler and Stefan Mulitza


Laboratory experiments showed that the isotopic fractionation of δ13C and of δ18O during calcite formation of planktic foraminifera are species-specific functions of oceanic CO32--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(δ13Crub) and Δ(δ13Csac) 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 CO32- 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), δ13CDIC, with reconstruction from benthic foraminifera. We find that both Δ(δ13Crub) and Δ(δ13Csac) agree better with changes in  simulated δ13CDIC 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(δ13Crub) and Δ(δ13Csac) 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 CO32--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.

Peter Köhler and Stefan Mulitza

Status: open (until 19 Dec 2023)

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  • RC1: 'Comment on cp-2023-84', Anonymous Referee #1, 13 Nov 2023 reply

Peter Köhler and Stefan Mulitza

Data sets

Mono-specific non-polar stacks of d13C and d18O from the planktic foraminifera G. ruber and T. sacculifer and simulation results of the 13C cycle across the last glacial cycle P. Köhler and S. Mulitza

Peter Köhler and Stefan Mulitza


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Short summary
We compile 160-kyr long non-polar mono-specific stacks of δ13C and of δ18O from the planktic foraminifera G. ruber and or T. sacculifer and compare them with carbon cycle simulations using the BICYCLE-SE model. Simulations are forced with  atmospheric CO2 and δ13CO2. In our stacks and our model-based interpretation we can not detect a carbonate ion effect, the species-specific isotopic fractionation during hard shell formation as function of carbonate chemistry in the surounding sea water.