Abstract

Carbonate-δ¹⁸ O paleothermometry is used in many diagenetic studies to unravel the thermal history of basins. However, this approach generally requires an assumed pore-water δ¹⁸O (δ¹⁸O_pw) value, a parameter that is difficult to quantify in past regimes. In addition, many processes can change the original isotopic composition of pore water, which further complicates the assignment of an initial δ¹⁸O_pw and can lead to erroneous temperature estimates. Here, we use clumped-isotope thermometry, a proxy based on the ¹³C–¹⁸ O bond abundance in carbonate minerals, to evaluate the temperatures of concretion formation in the Miocene Monterey Formation and the Cretaceous Holz Shale, California. These temperatures are combined with established carbonate–water fractionation factors to calculate the associated δ¹⁸O_pw.

Results demonstrate that diagenetic processes can modify the δ¹⁸O of ancient pore water, confounding attempts to estimate diagenetic temperatures using standard approaches. Clumped-isotope-based temperature estimates for Monterey Formation concretions range from ~ 17 to 35°C, up to ~ 12°C higher than traditional δ¹⁸O carbonate–water paleothermometry when δ¹⁸O_pw values are assumed to equal Miocene seawater values. Calculated δ¹⁸O_pw values range from +0.3 to +2.5‰ (VSMOW)—higher than coeval Miocene seawater, likely due to δ¹⁸O_pw modification accompanying diagenesis of sedimentary siliceous phases. Clumped-isotope temperatures for the Holz Shale concretions range from ~ 33 to 44°C, about 15 to 30°C lower than temperatures derived using the traditional method. Calculated δ¹⁸O_pw values range from −5.0 to −2.9‰ and likely reflect the influx of meteoric fluids. We conclude that the use of clumped isotopes both improves the accuracy of temperature reconstructions and provides insight into the evolution of δ¹⁸O_pw during diagenesis, addressing a longstanding conundrum in basin-evolution research.