PALEOTEMPERATURE RECONSTRUCTION OF THE LATE CRETACEOUS MISSISSIPPI EMBAYMENT AND WESTERN INTERIOR SEAWAY USING OXYGEN ISOTOPES FROM MARINE VERTEBRATE FOSSILS

The Late Cretaceous Western Interior Seaway (WIS) affected North American climate and global oceanic circulation by serving as a conduit between the boreal and Tethyan oceans. However, quantitative reconstruction of WIS temperatures and thermal gradients has been problematic. Because skeletal δ¹⁸O values depend on both ambient temperature and δ¹⁸O_w value, independent δ¹⁸O_w proxies are needed to ensure accurate paleotemperature calculations from fossil δ¹⁸O data. However, independent δ¹⁸O_w proxies are often not available. Thus, previous paleotemperature reconstructions based on fossil shell δ¹⁸O_CO3 analyses often applied a mean global δ¹⁸O_w value of –1‰ V-SMOW (the value calculated for an ice-free earth) to the WIS. This approach has yielded anomalous results such as WIS bottom water temperatures of ca. 30 °C and inverse thermal stratification of the water column.

A new method employing analyses of both bone phosphate- and bone carbonate oxygen isotopes from marine vertebrate fossils independently resolves water temperature and isotopic composition and also provides checks for diagenetic alteration of the bone isotope signals. This approach was applied to late Coniacian-early Campanian (ca. 87-82 Ma) marine turtle, fish, and shark fossils from Gulf Coast & Western Interior strata. Analyses of Western Interior marine turtles indicate δ¹⁸O_w values were significantly lower than –1‰ (ca. –3.45‰), likely due to inputs of low δ¹⁸O_w waters from runoff and the boreal ocean. Applying the turtle-derived δ¹⁸O_w value to fish and shark analyses yield mean water temperatures of 14-15 °C. Analyses of specimens from coeval Mississippi Embayment deposits (located near the boundary between the WIS and the Tethyan Ocean) indicate warmer temperatures of 18-19 °C and δ¹⁸O_w values closer to the calculated global average (ca. –1.37‰). The significant difference between these regions implies a relatively steep latitudinal thermal gradient existed between the Mississippi Embayment and the central WIS (0.6-0.8 °C/° latitude), while the open-ocean gradient was likely much lower. There is little evidence of thermal or isotopic stratification of the water column in either region, although small sample sizes and uncertainties about the depth habitats of some taxa obfuscate the matter.