Integrated investigation of rare earth elements, molecular fossils, and stable isotopes in drill cores from deep-buried strata in the Yangtze Block: Implications for the Early Cambrian sedimentary environments

Abstract

Geological records of the Lower Paleozoic indicate the co-occurrence of large-scale geo- and bio-events, including the diversification of marine complex life and widespread phosphogenesis, making the Early Cambrian aquatic environments a long-standing subject of global interest. Despite extensive research, the interplay between paleoclimate, continental chemical weathering, seawater redox conditions, and salinity during the Cambrian Explosion remains debated. To address this, we conducted geochemical analyses (e.g., XRF, ICP-MS, MRM GC–MS, and IRMS) and optical microscopy on drill cores from the Early Cambrian deep-buried strata (>4 km depth) in an intrashelf basin within the Yangtze Block. Geochemical proxies, including pronounced negative cerium anomalies (as low as 0.20) and a positive pristane-to-phytane ratio excursion, indicate an expansion of oxic waters between ∼531 and 526 Ma, while widespread anoxic conditions prevailed around 531 and 526 Ma. Climate fluctuations between arid and humid periods are inferred from major elements (e.g., Al, Fe, Mn), trace element ratios (e.g., Rb/Sr, Sr/Cu), and rare earth element parameters (e.g., ΣLREE/ΣHREE, Y/Ho). High gammacerane index values (up to 0.41), the presence of β-carotene, elevated Sr/Ba ratio, and δ¹⁸O values (up to −4.87 ‰) suggest periods of high salinity and evaporative conditions during arid phases. Organic geochemical signatures, including a high carbon preference index (CPI > 1) and highly negative δ¹³C values (<−24 ‰), indicate a biological origin of organic matter, with measurable 2-methylhopane (up to 2.48 %) and 4-methylsterane indices (up to 20 %) suggesting contributions from primary producers such as cyanobacteria, alphaproteobacteria, and dinoflagellates. The co-occurrence of oxygenated, highly saline, and evaporative Ca-P-rich waters with the diversification of small shelly fossil assemblages (SSFs 2 and 3) suggests that the studied intrashelf basin provided a favorable setting for the emergence and diversification of complex life. These findings offer new insights into the paleoenvironmental conditions that shaped marine ecosystems during the Cambrian radiation.