Carbonate ¹⁷O_excess as a paleo-hydrology proxy: Triple oxygen isotope fractionation between H₂O and biogenic aragonite, derived from freshwater mollusks

Carbonates are common paleoclimate archives in a variety of environments with land carbonates recording both temperatures and hydrological conditions. Whereas δ¹⁸O has long been used for that purpose, triple oxygen isotopes (given as ¹⁷O_excess) has only recently been analyzed in CaCO₃. Carbonate ¹⁷O_excess is expected to reflect the temperature, in which the carbonate formed, and the triple oxygen isotopic composition of its parent water. The reconstruction of past water ¹⁷O_excess values from carbonate records requires characterization of the ¹⁷O fractionation between water and CaCO₃. Here, we use freshwater mollusk shells grown in well constrained spring environments, at a temperature range of 15–28 °C, to derive the fractionation factor ¹⁷α between CO₂ extracted from CaCO₃ and the carbonate habitat water. The spring water temperature and isotopic composition are constant year-round, avoiding uncertainty associated with seasonality. The derived ¹⁸α and ¹⁷α between CO₂ extracted from mollusk aragonite and spring water are used to calculate the fractionation slope θ (=ln¹⁷α/ln¹⁸α), resulting in 0.5231 ± 0.0003, with no discernible temperature dependence. Using this value, we evaluated the fractionation slope of the acid digestion of CaCO₃ as 0.5170 ± 0.0004. The minor difference between the fractionation slope between aragonite and parent water and the reference slope of 0.528, implies that ¹⁷O_excess in carbonates is only weakly temperature dependent. On the other hand, carbonate ¹⁷O_excess values directly record variability in ¹⁷O_excess of the parent water, making it a useful paleo-hydrological proxy.