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

Abstract 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 17Oexcess) has only recently been analyzed in CaCO3. Carbonate 17Oexcess 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 17Oexcess 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 17Oexcess in carbonates is only weakly temperature dependent. On the other hand, carbonate 17Oexcess values directly record variability in 17Oexcess of the parent water, making it a useful paleo-hydrological proxy.