Ancient carbonates as archives for global environmental changes during Ediacaran-Cambrian time: a geochemical perspective

Aim: Using geochemical proxies to record changes in environmental conditions during the Ediacaran-Cambrian time

Methods: Thin section petrography combined with high-resolution analyses of trace elements and U-Pb, Sr, B and Li isotopes by LA-ICPMS (single and multi-collector)

Problem: Identification of pristine early diagenetic carbonate phases and their distinction from late diagenetic and altered carbonates. 

In this study, we use high spatial resolution multi-element and isotope analyses to identify pristine carbonate phases in samples from recent drill cores from Namibia, drilled by International Continental Scientific Drilling Program project GRIND-ECT. GRIND-ECT aims to capture, in drill core, the Ediacaran-Cambrian transition in key successions worldwide.

The method applied allows for relatively high sample throughput (>200) and precise detection of heterogeneous altered domains. Exclusion criteria are the presence or increased concentration of various trace elements (Al, Rb, LREE ...), heterogeneity of the ⁸⁷Sr/⁸⁶Sr isotope ratio, a lack of discernible populations in U/Pb isotope space, resulting in non-correlated scattering, and too young U-Pb dates.

To date, about 10-15% of the samples analyzed yielded a U-Pb date consistent with depositional age or an early post-depositional age. Geochronologic control of ancient sedimentary sequences is critical for evaluating cause and effect and rates of change in ancient environments. However, precision of LA-ICPMS ages (ca. 1-2%, e.g., 6-11 Ma) is insufficient to resolve changes in the Ediacaran-Cambrian oceans on timescales relevant for biological and environmental change. Therefore, the ages of these pre-characterized samples need to be refined using high-precision methods (Cantine et al., EGU 2023).

Analyses of Li and B isotopes are in progress on the pre-characterized, potentially pristine carbonate areas. We expect to see a change in the Li, B and Sr isotope records through Ediacaran-Cambrian time hopefully with significant anomalies. The correlation of the three isotope systems would be a confirmation for us that we are targeting pristine, early diagenetic carbonates.