New insights into the Sr isotope budget from stable-Sr isotope fractionation in gypsum

The geochemical cycle of strontium (Sr) is closely tied to the long-term inorganic cycle of carbon since the budgets of both elements are controlled by similar natural processes such as volcanism, continental weathering, and carbonates precipitation. As carbon dioxide is an important greenhouse gas, new insights into its cycle are fundamental for understanding climate variations in the past. Sr isotopes are a promising tool to advance our knowledge of the oceanic budget of Sr and, ultimately, the processes controlling long-term changes in atmospheric carbon dioxide and climate. While the widely used radiogenic strontium isotopes (⁸⁷Sr/⁸⁶Sr) can trace the oceanic Sr input fluxes, the stable-Sr isotope system (δ^88/86Sr) is responsive to both oceanic input and output fluxes of Sr. Gypsum (CaSO₄∙2H₂O) is an evaporitic mineral containing significant concentrations of Sr reaching up to thousands ppm. The geological record contains information about numerous events of the formation of marine evaporitic giants, in which gypsum is one of the most volumetrically important rocks. Given the remarkable sizes of evaporite reservoirs and the considerable content of Sr in gypsum, this rock can play an important role in the cycling of Sr and its oceanic isotope budget. This study focuses on quantifying the fractionation of stable Sr isotopes during gypsum precipitation to address open questions regarding 1) the suitability of gypsum as an archive for seawater δ^88/86Sr, 2) the potential impact of the evaporite sink on the global seawater δ^88/86Sr, and 3) the role of marine evaporite rocks in the continental cycle of Sr.

Gypsum samples were produced experimentally in the laboratory and outdoors by evaporating natural seawater, and Sr isotope fractionation was found empirically by analyzing δ^88/86Sr values of the precipitated solids and their respective solutions. Additionally, the experimental results were confirmed by studying natural samples, including modern gypsum and associated pore water from Dohat Faishakh Sabkha in Qatar and Messinian gypsum from Sicily.

The estimated typical Sr isotope fractionation in gypsum is 0.22±0.02‰. This positive value has the opposite direction compared to the negative Sr isotope fractionation in Ca-carbonate precipitation. However, the conducted experiments revealed a high variability of isotope fractionation values, ranging between 0.04‰ and 0.23‰ depending on the stirring environment. Therefore, the use of gypsum as an archive for δ^88/86Sr in past seawater must be approached with caution since robust reconstruction would require careful investigation of the studied natural samples and their precipitation environment. Furthermore, given the found isotope fractionation, during periods of intense evaporite formation, the removal of Sr to gypsum can serve as leverage for a detectable global seawater δ^88/86Sr change. Finally, weathering of gypsum is significantly affecting riverine δ^88/86Sr and can explain at least 25% of the gap between the δ^88/86Sr values of rivers (0.32‰ ^[3]) and source lithologies: carbonate (0.16-0.22‰ ^[2,4]) and silicate (~0.30‰ ^[1]) rocks.

[1] Charlier et al. (2012). EPSL. 329–330, 31–40.

[2] Krabbenhöft et al. (2010). GCA. 74, 4097–4109.

[3] Pearce et al. (2015). GCA. 157, 125–146.

[4] Vollstaedt et al. (2014). GCA. 128, 249–265.