Dual carbonate clumped isotopes (Δ47-Δ48)  constrains kinetic effects and timescales in peridotite-associated springs at  The Cedars, Northern California

Abstract

The Cedars is an area in Northern California with a chain of highly alkaline springs resulting from CO-charged meteorological water interacting with a peridotite body. Serpentinization resulting from this interaction at depth leads to the sequestration of various carbonate minerals into veins accompanied by a release of Ca and OH enriched water to the surface, creating an environment which promotes rapid precipitation of CaCO at surface springs. This environment enables us to apply the recently developed Δ-Δ dual clumped isotope analysis to probe kinetic isotope effects (KIEs) and timescales of CO transformation in a region with the potential for geological CO sequestration. We analyzed CaCO recovered from various localities and identified significant kinetic fractionations associated with CO absorption in a majority of samples, characterized by enrichment in Δ values and depletion in Δ values relative to equilibrium. Surface floes exhibited the largest KIEs (ΔΔ­: 0.163‰, ΔΔ­: -0.761‰). Surface floe samples begin to precipitate out of solution within the first hour of CO absorption, and the dissolved inorganic carbon (DIC) pool requires a residence time of >100 hours to achieve isotopic equilibria. The Δ/Δ slope of samples from the Cedars (-3.223±0.519; 1 SE) is within the range of published theoretical values designed to constrain CO hydrolysis-related kinetic fractionation (-1.724 to -8.330). The Δ/δO slope (-0.009±0.001) and Δ/δC slope (0.009±0.001) are roughly consistent with literature values reported from a peridotite in Oman of -0.006±0.002 and -0.005±0.002, respectively. The consistency of slopes in the multi-isotope space suggests the Δ-Δ dual carbonate clumped isotope framework can be applied to study CO-absorption processes in applied systems, including sites of interest for geological sequestration.