Simulation of oxygen isotopes and circulation in a late Carboniferous epicontinental sea with implications for proxy records

Abstract Reconstructions of ancient ocean chemistry are largely based on geochemical proxies obtained from epicontinental seas. Mounting evidence suggests that these shallow inland seas were chemically distinct from the nearby open ocean, decoupling epicontinental records from broader ocean conditions. Here we use the isotope-enabled Community Earth System Model to evaluate the extent to which the oxygen isotopic composition of the late Carboniferous epicontinental sea, the North American Midcontinent Sea (NAMS), reflects the chemistry of its open-ocean sources and connect epicontinental isotope variability in the sea to large-scale ocean-atmosphere processes. Model results support estuarine-like circulation patterns demonstrated by past empirical studies and suggest that orographic runoff produced decreases in surface seawater δ 18 O ( δ 18 O w ) of up to ∼3‰ between the NAMS and the bordering ocean. Simulated sea surface temperatures are relatively constant across the sea and broadly reproduced from proxy-based δ 18 O paleotemperatures for which model-based values of epicontinental δ 18 O w are used, indicating that offshore-onshore variability in surface proxy δ 18 O is primarily influenced by seawater freshening. Simulated bottom water temperatures in the NAMS are also reproduced from biogenic calcite δ 18 O using model-based values of epicontinental δ 18 O w , suggesting that benthic marine fossil δ 18 O is also influenced by seawater freshening and coastal upwelling. In addition, glacial-interglacial variations in nearshore seawater freshening counteract the effects of temperature on marine biogenic δ 18 O values, suggesting that salinity effects should be considered in δ 18 O-based estimates of glacioeustatic sea level change from nearshore regions of the NAMS. Our results emphasize the importance of constraining epicontinental dynamics for interpretations of marine biogenic δ 18 O as proxies of paleotemperature, salinity, and glacioeustasy.