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

The Cedars is an area in Northern California with a chain of highly alkaline springs resulting from CO2-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 Ca2+ and OH(-)enriched water to the surface, creating an environment which promotes rapid precipitation of CaCO3 at surface springs. This environment enables us to apply the recently developed Delta(47)-Delta(48) dual clumped isotope analysis to probe kinetic isotope effects (KIEs) and timescales of CO2 transformation in a region with the potential for geological CO2 sequestration. We analyzed CaCO3 recovered from various localities and identified significant kinetic fractionations associated with CO2 absorption in a majority of samples, characterized by enrichment in Delta(47) values and depletion in Delta(48) values relative to equilibrium. Surface floes exhibited the largest KIEs (Delta(47): 0.163%,Delta Delta(48): -0.761%). Surface floe samples begin to precipitate out of solution within the first hour of CO2 absorption, and the dissolved inorganic carbon (DIC) pool requires a residence time of >100 h to achieve isotopic equilibria. The Delta(48)/Delta(47) slope of samples from the Cedars ( -3.223 +/- 0.519) is within the range of published theoretical values designed to constrain CO2 hydrolysis-related kinetic fractionation ( -1.724 to -8.330). The Delta(47)/delta O-18 slope ( -0.009 +/- 0.001) and Delta(47)/delta C-13 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 Delta(47)-Delta(48) dual carbonate clumped isotope framework can be applied to study CO2-absorption processes in applied systems, including sites of interest for geological sequestration.

Automated summary

The Cedars in Northern California is an area with highly alkaline springs formed by the interaction of CO2-charged meteorological water with a peridotite body. This study applied the Delta(47)-Delta(48) dual clumped isotope analysis to investigate the kinetic isotope effects and timescales of CO2 transformation in this region, which has the potential for geological CO2 sequestration. Significant kinetic fractionations associated with CO2 absorption were identified, and the study provides important insights into the CO2 sequestration processes.