Trace elements and stable isotopic geochemistry of two sedimentary sections in the lower Cambrian strata from the Tarim Basin, northwest China: Implications for silicification and biological evolution

Abstract

Early animal innovations were coupled with profound atmospheric oxidation and oceanic disturbances during the Cambrian explosion; however, the relationship between evolutionary breakthroughs and ocean oxygenation remains controversial. In addition to the sufficiently investigated South China Craton, detailed research on other regions is needed to expand our understanding of ocean oxygenation dynamics and their relationships with animal radiation. Paleoenvironment reconstruction in the Tarim Basin is expected to yield additional contributions to the relevant literature. In this study, rare earth elements, redox-sensitive proxies, organic and inorganic carbon isotopic compositions (δ13CCarb and δ13CKer) of samples from lower Cambrian in the Shiairike and Yaerdangshan Sections of the Tarim Basin were analyzed. Our results showed that the cherts of the studied strata were not directly deposited from hydrothermal fluids but formed in a rather anoxic diagenetic environment where pH drawdown by organic matter degradation facilitated the precipitation of dissolved silica. Yuertusi and Xishanbulake cherts may have derived predominantly from direct seawater precipitation, among which cherts with negative Eu anomalies may have formed near the sulfate–methane transition zone, whereas cherts without Eu anomalies may have formed near the sediment-water interface. The negative δ13CCarb excursion (from 2.47‰ to −11.11‰) detected in the Yuertusi chert unit showed covariations with the enrichment factor of trace elements and other geochemical redox proxies. Therefore, the isotope excursion might be constrained by the anoxic diagenetic pore water and cannot be appropriately utilized for stratigraphic correlation. Multiple geochemical analyses suggest that the Tarim Basin is characterized by a redox-stratified ocean comprising a dynamic oxygenated continental shelf and a persistent anoxic deep basin, which is consistent with the ocean redox structure in South China. Phosphogenesis, enhanced biological pump efficiency and the development of photosynthesis in benthic algae might have comprehensively contributed to the dynamic oxygenation of the marine continental shelf and accelerated the diversification of small shelly fauna in the early Cambrian.