Constraining Uncertainties in Marine Calcifier Oxygen Isotope Values (δ18O ${\boldsymbol{\delta }}^{\mathbf{18}}\mathbf{O}$) Across Latitudes and Kingdoms Using a Proxy System Modeling Framework

Paleoceanographic proxy archives encode information about the marine environment, which can yield key insights into past climate variability. In particular, marine calcifiers' stable oxygen isotopic composition (δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$) tells us about seawater temperature and oxygen isotope composition. Here, we use a proxy system model (PSM) framework to systematically evaluate the drivers of skeletal/shell δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$ in three taxa of fast‐growing marine calcifiers (crustose coralline algae, bivalves, and sclerosponges) from disparate locations, including high latitudes and deeper waters. We evaluate the impact of the quality of environmental data, the recording season in which the calcifier might document the environmental variability, and the importance of uncertainties on the PSM. Whereas the overall PSM‐modeled δ18Opseudocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{pseudocarb}}$ captured the measured δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$ well at some locations, local environmental variability derived from a reanalysis product and chronological uncertainties limit the ability to effectively model δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$ at other locations. Using the PSM approach we highlight the complexity of interpreting δ18Ocarb ${{\delta }^{18}\mathrm{O}}_{\mathrm{carb}}$ as seawater temperature and oxygen isotope composition in these remote locations. Plain Language Summary Marine stony algae, clams, and sponges, form hard skeletons or shells and can live for hundreds of years, making them important recorders of their environment. Chemical measurements of the hard parts of these marine organisms capture changes in seawater temperature and how water cycles between the ocean and atmosphere as rainfall, both of which are changing due to human activities. By measuring the chemistry throughout the lifespan of the organisms, we can understand environmental variability before and since these human pressures. We test a simple model that relates the environmental changes to the chemical composition recorded in the hard parts of these marine organisms across a geographical range of ocean environments. We evaluate the importance of the quality of the environmental data, biological information about the growth of the organism, and uncertainty in the measurement itself on the model's effectiveness. We find that the model performs well at some locations, supporting