Calcium isotopes in deep time: Potential and limitations
Calcium is an essential element in the biogeochemical cycles that regulate the long-term climate state of Earth. The removal of CO2 from the ocean-atmosphere system is controlled by the burial of carbonate sediments (CaCO3), ultimately linking the global calcium and carbon cycles. This fundamental link has driven the development of the stable calcium isotope proxy with applica-tion to both ancient skeletal and non-skeletal bulk carbonate sediments. Calcium isotope ratios (44/40Ca) have been used to track long-term changes in seawater chemistry (e.g., aragonite vs. calcite seas) and to elucidate short-term climatic perturbations associated with mass extinction events. However, developments in the calcium isotope proxy have shown that 44/40Ca values in carbonate minerals also are sensitive to changes in precipitation rates, mineralogy and diagene-sis, thereby complicating the application of the proxy to the reconstruction of global cycles. First, inorganic carbonate precipitation experiments have demonstrated that carbonate 44/40Ca values are sensitive to precipitation rates with higher rates generally leading to larger fractiona-tion. Second, 44/40Ca values are sensitive to carbonate mineralogy with inorganic aragonite and calcite being on average ~1.5‰ and ~0.9‰ depleted relative to contemporaneous seawater, re-spectively. The effects of both changes in carbonate mineralogy and precipitation rates affect primary and secondary minerals, but are particularly pronounced during carbonate diagenesis where relatively slow rates of recrystallization and neomorphism can lead to significant changes in bulk sediment 44/40Ca values. Third, changes in faunal composition expressed in skeletal fossil archives can lead to large changes in carbonate 44/40Ca values that are decoupled from changes in global cycles. Nevertheless, when these factors are appropriately considered the ap-plication of calcium isotopes in ancient carbonate sediments becomes a powerful tool for under-standing biogeochemical processes that operate over many scales; from diagenetic changes with-in the sediment pore-space, to regional changes across ancient carbonate platforms, and to global changes in seawater chemistry through time. Importantly, the processes that contribute to varia-bility in carbonate 44/40Ca values are likely to impact other carbonate-bound proxies, highlight-ing the potential for calcium isotopes as a hint to better understand the variability of other iso-tope systems.