Carbonate-δ18O paleothermometry is used in many diagenetic studies to unravel the thermal history of basins. However, this approach generally requires an assumed pore-water δ18O (δ18Opw) 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 δ18Opw and can lead to erroneous temperature estimates. Here, we use clumped-isotope thermometry, a proxy based on the 13C–18O 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 δ18Opw.

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 δ18Opw values are assumed to equal Miocene seawater values. Calculated δ18Opw values range from +0.3 to +2.5‰ (VSMOW)—higher than coeval Miocene seawater, likely due to δ18Opw 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 δ18Opw 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 δ18Opw during diagenesis, addressing a longstanding conundrum in basin-evolution research.

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