No detectable influence of the carbonate ion effect on changes in stable carbon isotope ratios (<i>δ</i>¹³C) of shallow dwelling planktic foraminifera over the past 160&thinsp;kyr

<p>Laboratory experiments showed that the isotopic fractionation of <span class="inline-formula"><i>δ</i>¹³</span>C and of <span class="inline-formula"><i>δ</i>¹⁸</span>O during calcite formation of planktic foraminifera are species-specific functions of ambient CO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="490d7ecf09fa5682b13318d48526c23f"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-20-991-2024-ie00001.svg" width="13pt" height="17pt" src="cp-20-991-2024-ie00001.png"/></svg:svg></span></span> concentration. This effect became known as the carbonate ion effect (CIE), whose role for the interpretation of marine sediment data will be investigated here in an in-depth analysis of the <span class="inline-formula">¹³</span>C cycle. For this investigation, we constructed new 160 kyr long mono-specific stacks of changes in both <span class="inline-formula"><i>δ</i>¹³</span>C and <span class="inline-formula"><i>δ</i>¹⁸</span>O from either the planktic foraminifera <i>Globigerinoides ruber</i> (rub) or <i>Trilobatus sacculifer</i> (sac) from 112 and 40 marine records, respectively, from the wider tropics (latitudes below 38°). Both mono-specific time series <span class="inline-formula">Δ(<i>δ</i>¹³C_rub)</span> and <span class="inline-formula">Δ(<i>δ</i>¹³C_sac)</span> are very similar to each other, and a linear regression through a scatter plot of both data sets has a slope of <span class="inline-formula">∼</span> 0.99 – although the laboratory-based CIE for both species differs by a factor of nearly 2, implying that they should record distinctly different changes in <span class="inline-formula"><i>δ</i>¹³C</span>, if we accept that the carbonate ion concentration changes on glacial–interglacial timescales. For a deeper understanding of the <span class="inline-formula">¹³</span>C cycle, we use the Solid Earth version of the Box model of the Isotopic Carbon cYCLE (BICYLE-SE) to calculate how surface-ocean CO<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M13" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi/><mn mathvariant="normal">3</mn><mrow><mn mathvariant="normal">2</mn><mo>-</mo></mrow></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="13pt" height="17pt" class="svg-formula" dspmath="mathimg" md5hash="96f0b496066d50de98e398addfd99620"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="cp-20-991-2024-ie00002.svg" width="13pt" height="17pt" src="cp-20-991-2024-ie00002.png"/></svg:svg></span></span> should have varied over time in order to be able to calculate the potential offsets which would by caused by the CIE quantified in culture experiments. Our simulations are forced with atmospheric reconstructions of CO<span class="inline-formula">₂</span> and <span class="inline-formula"><i>δ</i>¹³</span>CO<span class="inline-formula">₂</span> 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 in the deep ocean largely agrees with the carbon isotope ratio of dissolved inorganic carbon (DIC), <span class="inline-formula"><i>δ</i>¹³C_DIC</span>, as reconstructed from benthic foraminifera. We find that both <span class="inline-formula">Δ(<i>δ</i>¹³C_rub)</span> and <span class="inline-formula">Δ(<i>δ</i>¹³C_sac)</span> agree better with changes in simulated <span class="inline-formula"><i>δ</i>¹³C_DIC</span> when ignoring the CIE than those time series which were corrected for the CIE. The combination of data- and model-based evidence for the lack of a role for the CIE in <span class="inline-formula">Δ(<i>δ</i>¹³C_rub)</span> and <span class="inline-formula">Δ(<i>δ</i>¹³C_sac)</span> suggests that the CIE as measured in laboratory experiments is not directly transferable to the interpretation of marine sediment records. The much smaller CIE-to-glacial–interglacial-signal ratio in foraminifera <span class="inline-formula"><i>δ</i>¹⁸</span>O, when compared to <span class="inline-formula"><i>δ</i>¹³</span>C, prevents us from drawing robust conclusions on the role of the CIE in <span class="inline-formula"><i>δ</i>¹⁸</span>O as recorded in the hard shells of both species. However, theories propose that the CIE in both <span class="inline-formula"><i>δ</i>¹³</span>C and <span class="inline-formula"><i>δ</i>¹⁸</span>O depends 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 is restricted to the two species investigated here needs to be checked with further data from other planktic foraminiferal species.</p>