Resetting of shallow-water carbonate boron isotope values during marine burial diagenesis
The boron isotopic composition (11B) of bulk carbonates may provide an archive to reconstruct changes in ocean pH. Reconstructions from ancient carbonates typically assume that no significant resetting of 11B occurred during marine burial diagenesis. However, our understanding of B isotopic behavior associated with this process remains limited. Here we provide measurements of B/Ca and B isotopic composition (11B) from a modern peri-platform carbonate sequence near the Great Bahama Bank that has undergone only marine burial diagenesis. Our results reveal significant decreases in both 11B (~13‰) and B/Ca (~80 %) of bulk carbonates with depth. We attribute this pattern to the release of isotopically light B (11B ~20‰) to porewater during aragonite dissolution, with uptake of substantially isotopically lighter borate ions (11B ~-1‰) from porewater by newly forming low-Mg calcite. A quantitative model adds further support for this interpretation and provides a new estimate of average neomorphism rate (k_0) in the range of 110-6 to 510-6 yr-1, which is comparable to previous rate estimates for neomorphism and/or recrystallization during meteoric diagenesis. Our results demonstrate the strong potential for resetting 11B signatures in bulk carbonates during recrystallization, which must be considered in future attempts to reconstruct pH and pCO2 from these records and may require reinterpretation of existing records. Our results also suggest the potential of B isotopes as a proxy for carbonate recrystallization/neomorphism and original carbonate mineralogy. The attached file is the Supplementary Information of the manuscript.