DETERMINING THE DIAGENETIC CONDITIONS OF CONCRETION FORMATION: ASSESSING TEMPERATURES AND PORE WATERS USING CLUMPED ISOTOPES

Carbonate-delta O-18 paleothermometry is used in many diagenetic studies to unravel the thermal history of basins. However, this approach generally requires an assumed pore-water delta O-18 (delta O-18(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 delta O-18(pw) and can lead to erroneous temperature estimates. Here, we use clumped-isotope thermometry, a proxy based on the C-13-O-18 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 delta O-18(pw). Results demonstrate that diagenetic processes can modify the delta O-18 of ancient pore water, confounding attempts to estimate diagenetic temperatures using standard approaches. Clumped-isotope-based temperature estimates for Monterey Formation concretions range from similar to 17 to 35 degrees C, up to similar to 12 degrees C higher than traditional delta O-18 carbonate-water paleothermometry when delta O-18(pw) values are assumed to equal Miocene seawater values. Calculated delta O-18(pw) values range from +0.3 to +2.5 parts per thousand (VSMOW)-higher than coeval Miocene seawater, likely due to delta O-18(pw) modification accompanying diagenesis of sedimentary siliceous phases. Clumped-isotope temperatures for the Holz Shale concretions range from similar to 33 to 44 degrees C, about 15 to 30 degrees C lower than temperatures derived using the traditional method. Calculated delta O-18(pw) values range from -5.0 to -2.9 parts per thousand 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 delta O-18(pw) during diagenesis, addressing a longstanding conundrum in basin-evolution research.