Testing selenite gypsum crystals from caves for reconstructing the stable isotope composition of paleo-aquifers

Abstract

Selenite is a variety of gypsum (CaSO₄·2H₂O) that typically forms large, transparent prismatic crystals. In recent decades, meter-sized selenite crystals have been found in several caves worldwide. These gypsum speleothems formed underwater, in highly stable, epithermal or low-temperature aquifers. Here we investigate the U-Th ages of decimeter-sized selenite crystals from Cueva de los Cristales (Naica mine, northern Mexico) and the Giant Geode of Pulpí (Almería, southeastern Spain). Furthermore, the hydrogen and oxygen isotope compositions of gypsum hydration water measured along the crystal growth axes were used to reconstruct the isotopic evolution of the Naica and Pulpí paleo-aquifers. Extremely low uranium concentrations (<2 ppb in Pulpí and < 0.1 ppb in Naica) make obtaining reliable age models challenging. However, repeated independent measurements of each sample and using large sample sizes can yield reliable and accurate ages. A Naica crystal that grew over the last 31 ± 6 kyr recorded a stepwise change in the isotope composition of paleo-aquifer water, likely reflecting a shift in recharge isotopic composition (rainfall) due to Glacial–Holocene climate change in northern Mexico. A crystal from the Giant Geode of Pulpí that began growing at 191 ± 26 kyr recorded relatively constant δ²H groundwater values, likely reflecting stable climatic conditions during its formation. We critically discuss the caveats of using selenite gypsum crystals from caves as paleohydrological archives and conclude that, despite the challenges, they have strong potential for reconstructing the isotopic composition of paleo-aquifers and past rainfall.