Oxygen isotopic data from carbonate cements in concretions have been used to infer the isotopic composition of meteoric fluids present at the time of concretion growth in terrestrial sediments that were deposited within the early Cretaceous South Polar Circle at 75-80°S. Carbon and oxygen isotope compositions have been determined on over 135 samples of carbonate from 45 concretions taken from 24 localities (Aptian-Albian in age) in the terrestrial sedimentary basins associated with the Otway and Strzelecki groups, southeastern Australia. The carbonate cements include calcite having -26.4 ≤ δ13C ≤ 19.6 and 3.6 ≤ δ18O ≤ 29.6 or siderite having 17.6 ≤ δ18O ≤ 30.8. Calcite-cemented concretions are more abundant and are interpreted to represent early near-surface cementation events on the basis of textural evidence such as high ( > 30%) porosities at the time of cementation and mineralogical evidence such as the preferential preservation within concretions of labile detrital grains including plagioclase, pyroxene, and amphibole. The oxygen isotopic data indicate that meteoric fluids with very low δ18O, certainly less than -15‰ and probably on the order of -20‰, were involved in the precipitation of the early calcites. The extremely low δ18O values of the fluids involved in the early diagenesis of both the Otway and Strzelecki groups suggest that the catchment area of the river system that carried sediments to these basins had a cold high-latitude climate (with mean annual temperatures less than 5°C and quite possibly below freezing). By analogy with the relationship between modern 18O distribution of meteoric fluids and climate, these new data suggest that the early Cretaceous polar regions may not have been ice-free.