Evaluating Glacial CO2 System Reconstructions From Benthic Proxies in Earth System Simulations

Reconstructing marine dissolved inorganic carbon (DIC) across glacial cycles is critical for understanding the sensitivity of the marine carbon sink to natural climatic change. Published estimates of DIC invoke linear relationships between DIC and , apparent oxygen utilization (AOU), or C. These relationships are based on conceptual models and correlations from modern spatial tracer gradients. However, it remains unclear whether the spatial correlations also hold for temporal change. Here, we apply these empirical methods to Earth system model results to test their applicability to transient glacial‐interglacial changes. The model uses established, experimentally‐constrained carbonate system solvers and explicitly tracks the various components of the carbon cycle (e.g., DIC, alkalinity, temperature, ). Predicting simulated DIC from simulated , AOU, or C often results in large prediction errors. The interplay of the carbonate system, ocean circulation, and biologically‐mediated DIC and alkalinity re‐distributions creates a system too complex to be captured by existing empirical methods. Specifically, large local alkalinity changes can arise due to circulation and export production changes even without substantial changes in global mean alkalinity. Consequently, reconstructed constrains DIC changes insufficiently. Similarly, marine AOU or C are not reliable proxies of remineralized DIC. Furthermore, DIC changes are not a direct metric for atmospheric drawdown even without considering changes in global mean alkalinity because of net carbon exchange with sediments and the land biosphere. We suggest that spatially‐resolved, transient Earth system simulations may provide a more reliable means of estimating carbon cycle shifts observed in proxy data than current empirical methods.