Quantification of a North American greenhouse hydrological cycle: using oxygen isotopic composition of phosphate from Early Cretaceous (Aptian–Albian) turtles

The oxygen isotopic composition of turtle phosphate from Early (Aptian–Albian) Cretaceous turtles from several North American palaeolatitudes was measured to estimate the isotopic compositions of groundwater δ18O_w. In Earth’s history, under greenhouse conditions, researchers have recorded an intensification of the hydrological cycle as global heat was distributed from the equator to the poles via latent heat transport. Previous studies, using pedogenic carbonate nodules, show an increased rainout effect, causing intense depletion in δ18O_w at high latitudes due to this phenomenon. This study shows the utility of another palaeo-δ18O_w proxy, turtle phosphate oxygen from turtle carapace and plastron. Turtle δ18O_p was sampled from several Early Cretaceous sites. We find that turtle-derived δ18O_w values range from ––3.1 ± 0.9 ‰ at a palaeolatitude of 29.5^o N to –7.2‰ at 40^oN. Turtles at the lowest latitude (Arkansas) plot off the carbonate-derived δ18O_w gradient; however, turtles at 34^o N and 40^o N plot within the range of carbonate-derived δ18O_w. Turtles from Arkansas (29.5^o N) were likely to have been influenced by marine water and/or enriched Gulf of Mexico water vapour resulting in a deviation from the expected δ18O_w gradient. These data are encouraging steps towards using turtles to estimate palaeo-hydrological dynamics; however, additional analysis of lower- and higher-latitude specimens is necessary to confirm this trend.