Diagenesis of phosphatic hardgrounds in the Monterey Formation: A perspective from bulk and clumped isotope geochemistry

Understanding the authigenesis of carbonate fluorapatite through isotopic geochemistry can yield important information on fundamental geologic processes occurring on continental margins around the world. This is particularly true for phosphatic hardgrounds, which are often found in regions of upwelling, but of which questions remain about the initial formation and subsequent diagenesis. Here, we apply standard isotopes (delta C-13, delta O-18) alongside the novel clumped isotope (. 47) used in this study for the first time to reconstruct the temperature of formation of carbonate ions within the lattice of sedimentary carbonate fluorapatite. We investigated phosphatic hardgrounds of Miocene age (12.7-10.8 Ma) sampled at El Capitan State Beach in the Monterey Formation. The range of isotopic signatures observed is between +1.5% and +8.0% for delta C-13 relative to the Vienna-Peedee belemnite (VPDB) standard and -9.5% and -6.0% VPDB for delta O-18, and values range between 0.599% and 0.615% for Delta(47). The enriched delta C-13 and depleted delta O-18 signatures are suggestive of recrystallization within the methanogenic zone. Clumped isotope geochemistry further constrains this transformation as having taken place at a temperature of 61-66 degrees C +/- 5 degrees C, in line with previous estimates for maximum burial of the Monterey Formation based on the silica phase transition. The calculated delta O-18 for the connate fluid shows an expected range for seawater composition for the Miocene, suggesting only minor contribution of silica-derived oxygen to the delta O-18 of carbonate fluorapatite. The combined conventional and clumped isotope data set also points out that methanogenesis took place deeper within the sediment in the middle-Miocene than at present day within the Santa Barbara Basin. This study furthers our understanding of phosphogenesis and potential links to burial processes in the Monterey Formation, and it shows for the first time that the clumped isotope paleother-mometer could be used to under-stand fundamental geochemical processes in authigenic sedimentary phosphates.