U-PB AND B ISOTOPE MEASUREMENTS USING IN SITU LA-ICP-MS IN CARBONATE SEDIMENTS ACROSS THE PROTEROZOIC-PHANEROZOIC TRANSITION

Carbonate sediments precipitated from seawater can be valuable records of ancient seawater chemistry and environmental conditions. Such records are especially valuable at the Proterozoic-Phanerozoic transition, an interval in which observed changes in animal life, including animal diversification, the rise of biomineralization, and habitat expansion, might plausibly be linked with environmental and geochemical changes. Carbonate archives may record those changes. However, the fact that carbonates are also susceptible to post-depositional processes like diagenesis, burial, lithification, deformation, dissolution and reprecipitation, and/or oxidative weathering is a major complication. These processes may set and re-set the primary geochemical signals recorded in carbonate sediments. For this reason, identifying which carbonate phases are likely to preserve a primary geochemical signal is an important step in developing robust geochemical records of Earth’s ancient environments. One geochemical archive of particular interest, boron (B) isotopes measured in marine carbonates, may allow us to understand changes in ocean pH associated with the rise of carbonate biomineralization.

Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) offers an important tool by combining in situ geochemical characterization with petrographic and sedimentological context. Uranium-lead (U-Pb) dating of carbonate phases in petrographic context also allows us to robustly date the phases other geochemical records are being sourced from, increasing our confidence about the primary or secondary nature of the signals being recovered. This contribution examines B isotopes and U-Pb dating in tandem in carbonate sediments that span the Proterozoic-Phanerozoic transition from multiple locations globally. We demonstrate how integrating sedimentological and petrographic context with these geochemical measurements increases the confidence we place in these records.