Controls on the termination of the massive chert system in the Ediacaran-Cambrian Liuchapo Formation, South China

Abstract

The widespread deposition of marine siliceous rocks marks a fundamental shift in the ancient ocean’s biogeochemical dynamics, transitioning from Precambrian systems dominated by direct inorganic silica precipitation as the primary silicon sink to modern environments where microbial processes strongly govern silica dissolution. The massive Liuchapo chert in the Nanhua Basin, South China, formed during the Ediacaran–Cambrian transition (ECT, ∼550–521 Ma), a critical interval of silicon cycle perturbations, rapid multicellular diversification, and significant environmental change. Its abrupt termination, followed by the widespread deposition of Niutitang black shales without a depositional hiatus, reflects a fundamental shift in the oceanic silicon cycle, linked to profound marine environmental evolution and dynamic biotic responses during the key period of the ECT. However, the biogeochemical processes driving the cessation of chert deposition remain poorly understood. Here, we present high-resolution geochemical data from the XAD1 borehole, situated in an an outer slope setting of the Nanhua Basin to assess controls on the termination of this widespread marine chert system. Our multi-proxy approach, including iron speciation, redox-sensitive trace metal analyses, and rare earth elements plus yttrium (REE + Y) systematics documents a highly fluctuated redox condition between oxic to ferruginous throughout the Liuchapo Formation, before giving way to persistent euxinic conditions in the lower Niutitang Formation. Hydrothermal activity likely played a key role in the Liuchapo chert formation, while its subsequent decline, coupled with the onset of euxinic conditions, weakened the Fe-Si loop essential for chert deposition. Furthermore, the emergence of silica-secreting organisms may have reduced dissolved silicon levels, further driving the cessation of chert deposition. These findings provide new insights into the environmental and biological factors controlling the termination of the Liuchapo massive chert system, contributing to a deeper understanding of the complex geochemical and ecological dynamics during this pivotal period in Earth’s history.