The Effect of Seawater Carbonate Chemistry on the Stable Isotope Composition of Cibicidoides wuellerstorfi and Other Cibicidoides Species

The δ13C composition of Cibicidoides wuellerstorfi and other Cibicidoides spp is an important tool to reconstruct past changes in the deep ocean carbon cycle. The species are expected to match the δ13C of ambient dissolved inorganic carbon (DIC), although it has been recognized that substantial offsets can occur. Here, I present a compilation of modern δ13C and δ18O data for named Cibicidoides species in combination with fully resolved carbonate chemistry at each core location. The data show for C. wuellerstorfi that the offset from the expected value in both carbon (∆13C) and oxygen (∆18O) is correlated with seawater carbonate chemistry. The result is comparable to, but not identical with, published culture experiments in which marine organisms were grown under variable pH‐conditions. Overall, ∆13C in C. wuellerstorfi correlates positively with carbonate saturation, [DIC], and temperature. The three variables together explain 47.1% of the variation in ∆13C. The trend for ∆18O is similar, except that the effect of temperature has been removed through correction with a published δ18O‐temperature equation. Up to 35% of the remaining variation in ∆18O can be explained by ambient carbonate chemistry. Data for other named Cibicidoides species are broadly similar, but are too sparse for a detailed analysis. The results indicate that strongly negative ∆13C occurs predominantly in the deep Atlantic in response to a combination of low [DIC], low temperature, and undersaturation within the lysocline. Implications for paleoceanographic reconstructions are discussed.