Radiogenic Sr and Stable C and O Isotopes Across Precambrian‐Cambrian Transition in Marine Carbonatic Phosphorites of Malyi Karatau (Kazakhstan)—Implications for Paleo‐environmental Change

Abstract

AbstractThe Precambrian‐Cambrian (PЄ‐Є) transition is a unique period in Earth's history known for the “Cambrian Explosion.” Here we present a continuous radiogenic and stable isotope (87Sr/86Sr, δ13CVPDB, carb, and δ18OVPDB, carb) and elemental (including rare earth elements) record across the PЄ‐Є transition, preserved in phosphatic and carbonate shallow‐water deposits from two sections (Koksu and Kyrshabakty) in Kazakhstan. Microstructure, trace, and rare earth elements concentrations reflect near‐primary preservation. δ13CVPDB, carb data (−12.1‰ to 3.3‰) record formation conditions at a time of severe geodynamic changes. Negative δ13CVPDB, carb at the PЄ‐Є boundary resemble the Basal Cambrian Carbon Anomaly. Average δ18OVPDB, carb values of −11.7 ± 3.8‰ are suggested to represent near‐primary signals and can be explained by isotopically lighter past sea water and elevated temperature during precipitation compared to present‐day conditions. Initial 87Sr/86Sr of the carbonate and phosphate fraction are in the range of 0.7086–0.7112 at the Koksu section and 0.7082–0.7136 at the Kyrshabakty section. This is in general agreement with the seawater 87Sr/86Sr evolution during the Early Cambrian. In this study, we present a refined 87Sr/86Sr seawater evolution, highlighting an extreme positive Sr isotope excursion at the PЄ‐Є transition, which is suggested to be indicative for enhanced continental weathering and an increase in nutrient runoff into shallow ocean waters. This extreme geodynamic event may have served as a driving force for enhanced biologic activity in the lead up to the Cambrian Explosion, where phosphatic shallow‐water deposits from Kazakhstan constitute a valuable paleo‐environmental archive.