Abstract 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 δ 18 O 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 δ 18 O w at high latitudes due to this phenomenon. This study shows the utility of another palaeo- δ 18 O w proxy, turtle phosphate oxygen from turtle carapace and plastron. Turtle δ 18 O p was sampled from several Early Cretaceous sites. We find that turtle-derived δ 18 O w values range from −3.1 ± 0.9‰ at a palaeolatitude of 29.5° N to −7.2‰ at 40° N. Turtles at the lowest latitude (Arkansas) plot off the carbonate-derived δ 18 O w gradient; however, turtles at 34° N and 40° N plot within the range of carbonate-derived δ 18 O w . Turtles from Arkansas (29.5° 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 δ 18 O w gradient. These data are encouraging steps towards using turtles to estimate palaeohydrological dynamics; however, additional analysis of lower- and higher-latitude specimens is necessary to confirm this trend.