Deciphering environmental processes in sedimentary carbonates, a dynamic isotope box model approach

Carbon and oxygen isotope records of sedimentary carbonates (δ 13 C carb and δ 18 O carb ) are among the most widely used proxies for paleoenvironmental reconstructions, although their interpretation can be complicated by complex and overprinting processes. Here we present an innovative dynamic box model to unravel the environmental implications of δ 13 C carb and δ 18 O carb , that we apply and constrain in a modern lacustrine setting. We compare the simulated isotopic trends with those measured in the lake sedimentary carbonates over the last 60 years and confront our modeled dissolved inorganic carbon concentrations in the lake with reported measurements for the same period. While most of the relevant environmental parameters of the studied volcanic Lake Alchichica (Mexico) are well constrained, tuning the remaining inputs of the model allows a quantitative assessment of the environmental processes governing the lake's hydrological, carbon and oxygen cycles. Most notably, our results reveal that a clear imbalance in the lake's C budget and δ 13 C data can only be resolved if the model includes a substantial – but previously unrecognized – volcanic CO 2 degassing flux to the lake. Our results further support that groundwater extraction and agriculture in the area have increasingly affected the lake's hydrological and C balance since the 1980s. Overall, we show that this alkaline lake, despite high biological productivity and massive carbonate deposition, is a net CO 2 source to the atmosphere. Through a thorough parametrization and characterization of the C and O fluxes in a modern setting, this new modeling approach hold promises for reconstructing ancient environments from carbonate isotope archives.