| The stable oxygen isotope composition (delta18O) of freshwater turtle bone biophosphate is a delta18Ow proxy useful for modern- and paleoenvironmental reconstruction. However, it remains unclear whether marine turtles can be used as an oceanic delta 18Ow proxy. Analysis of the delta18O PO4 systematics of three modern species of marine turtles ( Caretta caretta, Dermochelys coriacea, and Chelonia mydas ) demonstrates that these taxa possess the same correlation between delta 18O values of their bone phosphate and delta18O of ambient waters as that documented in certain freshwater taxa. The correlation does not vary among taxa or with body size in two growth series of Caretta caretta. The low delta18OPO4 variations within and among skeletal elements from any given individual suggest that bioapatite is formed in these taxa, on average, over a narrow (32 +/- 2°C) body temperature range. Thus, variations in delta18OPO4 are primarily due to variations in ambient water isotopic composition. Comparison of bone-derived delta18Ow values with maps of oceanic delta18Ow help identify region(s) the turtle frequented during its life (note that no individuals were sacrificed for this study).;Because the delta18OPO4 value of skeletal elements depends on both ambient temperature and delta18O w value, independent delta18Ow proxies are needed to ensure accurate paleotemperature calculation from fossils. Combining turtle bone analyses with sampling of coeval fish and shark specimens allows both temperature and delta18Ow to be calculated, rather than assuming one in order to calculate the other. This technique was applied to a series of marine turtle, fish, and shark fossils from the Western Interior Seaway of North America (WIS) in order to determine paleoceanic conditions during Late Coniacian to Early Campanian time (ca. 87-82 Ma). Previous studies based on delta18OCO3 analyses of invertebrates assumed a mean global delta18Ow value of --1‰ V-SMOW (the value calculated for an ice-free earth) yielded anomalous results, with bottom water temperatures of ca. 30°C and inverse thermal stratification of the water column.;Analyses of WIS marine turtles indicate delta18O w values were significantly lower than --1‰ (ca. --3.45‰), likely due to inputs of low delta18Ow waters from runoff and the boreal ocean. Applying the turtle-derived delta18 Ow value to fish and shark analyses yield more realistic water temperatures (14.25 +/-2.5°C and 14.51 +/-1.9°C, respectively). Similar analyses of fish and shark specimens from coeval deposits in the Mississippi Embayment (located near the boundary between the WIS and the Tethyan Ocean) indicate warmer temperatures (18.01 +/-1.0°C and 18.80 +/-1.5°C, respectively) and delta18Ow values (ca. --1.37‰) closer to the calculated global average. The significant difference between these regions implies a relatively steep latitudinal thermal gradient existed locally (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. The study provides new, useful information regarding paleoenvironmental reconstruction of the WIS and demonstrates the need for reliable delta18O w proxies to ensure accurate paleotemperature calculations. |
