Chronometry and formation pathways of gypsum using Electron Spin Resonance and Fourier Transform Infrared Spectroscopy

Abstract

Gypsum is an authigenic precipitate that forms under periods of accentuated aridity and occurs widely in arid zones. However its use in quantitative paleoclimatology has been limited due to the absence of a method to determine the timing of its formation. We present here the results of a feasibility study that demonstrates that the timing of the formation event of gypsum can be estimated using Electron Spin Resonance (ESR) analysis. We used well documented samples from White Sands in New Mexico, USA, the Thar Desert, India and lakes in the Simpson Desert and Mallee Region, Australia and found that ESR ages could be obtained using radiation sensitive SO 4 −, SO 3 − radicals and a photobleachable signal O 3 −. ESR signals were consistent with control ages based on contextual information. These suggest that the dating signals (SO 4 −, SO 3 −) are stable over time scales >100 ka. We propose that this stability of the SO 4 − signals over geological time scales arises due to hydrogen bonding between the water proton and the SO 4 − radical and that the suitability of these radiation-induced radicals comes from their being a part of the host matrix. Further, ESR along with Fourier Transform Infrared (FT-IR) Spectroscopy methods additionally inform on the geochemical pathways for gypsum formation and help elucidate complex formation processes even in samples that appeared unambiguous gypsum precipitates. Thus, the presence of Hannebachite (CaSO 3.½H 2O) and Mn 2+ in Thar and Australian samples suggested a reducing environment such that low valence sulfur reacted with CaCO 3 to form hannebachite and eventually gypsum. The presence of sulfur, partially as sulfite in Thar gypsum samples suggested that redox cycles were mediated by microbial activity. Absence of these features in White Sands samples suggested oxic conditions during gypsum precipitation.