Extensive Oceanic Anoxia during the early Cambrian: Evidence from Mn-Carbonate and Alabandite Deposition in the Northwestern Yangtze margin, South China

Abstract

The Cambrian Explosion, which is characterized by complex organisms radiation after ∼521 Ma, has led to speculation about low levels of ocean oxygenation during the Early Cambrian (∼541–521 Ma). Recently, the genesis of sedimentary Mn-deposits has provided a new perspective for understanding the redox status of ancient oceans. The present study reevaluates the redox conditions of the Early Cambrian Ocean in South China using sedimentary records of Mn-deposits. The Mn-deposits developed in the Lower Cambrian Qiujiahe Formation on the northwestern Yangtze margin, covering slope/basin-to-shelf environments. The combined sedimentological, mineralogical, and geochemical evidence indicates that Mn-deposits lack of Mn-oxide precursors and exhibited hydrothermal-type REE patterns and δ¹³C distributions. Mn‑carbonates and partially alabandite (MnS) were directly precipitated in Mn-rich anoxic waters. The Mn-bearing rocks exhibited high total organic matter content (TOC, average 3.96 %), and pyrite layers developed, indicating active anaerobic sulfate reduction with the removal of Fe before Mn-deposition. The increased of the pH and sulfurization degree, promoting the Mn-minerals precipitation in water columns. The widespread Mn deposition across slope/basin-to-shelf areas represents the sedimentary response to the extensive ocean anoxia during the Early Cambrian. Such a scenario aids in elucidating the evolution of early life, suggesting that extensive ocean anoxia likely contributed to delaying the radiation of complex animals during the Cambrian Explosion.