Juxtaposed Permian and Pleistocene isotopic archives: Surfi cial environments recorded in calcite and goethite from the Wichita Mountains, Oklahoma

A paleokarst fi ll deposit from the Wichita Mountains, south-central Oklahoma, United States, consists primarily of sparry calcite, Fe-sulfi des, and goethite. Previous cement-stratigraphic studies and paleontological fi nds suggest that calcite mineralization was initiated during Permian time, whereas goethite and other oxides apparently formed from oxidation of preexisting Fe-sulfi des during Pleistocene time. Therefore, these deposits have the potential to offer insight into surfi cial hydrology and paleoenvironment in an upland setting from two time periods at a single site. δC PDB and δO SMOW measurements of 17 samples from growth bands in a single karst-fi ll calcite crystal range from –10.7‰ to –6.6‰ (mean = –8.6‰) and 27.1‰ to 28.3‰ (mean = 28‰), respectively. Large oscillations in the δC values through the growth series may originate from seasonal changes in the magnitude of biological productivity during Permian time. These δC oscillations contrast with the relative stability of the δO values, which are more positive than would be expected for isotopic equilibrium with local modern waters. The δO values of the calcite may refl ect the δO values of ambient meteoric groundwaters in the Permian that were isotopically similar to waters in modern, seasonally dry, low-latitude coastal regions. Goethites are not in equilibrium with modern waters or coexisting calcites in the fi ssure-fi ll deposit as determined from δO and δD values of the goethites. Furthermore, the combined δO and δD values of the goethites are indicative of formation from meteoric waters at a temperature of ~9 °C ± 3 °C. This inferred temperature is 7 °C ± 3 °C cooler than local modern mean annual temperature and corresponds well with independent studies that propose temperatures ~6 °C cooler in this region during Pleistocene time. The mole fraction and δC values of the Fe(CO 3 )OH component in solid solution in the goethite sample are 0.0103 and –10.1‰, respectively. In combination, these values suggest that goethite formed in an environment characterized by mixing of three isotopically distinct CO 2 components: (1) oxidized biological carbon, (2) atmospheric CO 2 , and (3) CO 2 from dissolution of carbonate in the karst system. Oxidized biological carbon may have originated either from fl ora characterized by C3 or mixed C3:C4 photosynthesizers. Mass balance calculations between these three CO 2 end members correspond to an inferred soil CO 2 concentration [CO 2 contributed from (1) and (2) above] ranging from ~8,000 ppmV to ~16,000 ppmV for a local ecosystem dominated by C 3 fl ora. This inferred range of soil CO 2 concentrations is typical of grasslands characterized by relatively high biological productivity. If C4 fl ora were a signifi cant 56 N.J. Tabor and C.J. Yapp source of oxidizing carbon, the higher calculated ambient CO 2 concentration at the time of goethite crystallization in the cave (~20,000 ppmV) might be interpreted to correspond to an unusually productive C4 soil present at a time of generally cooler and drier conditions across the southern Great Plains of North America.