Thallium Isotopes Suggest the Global Deep Ocean Did Not Approach Modern Oxygenation During Cambrian Age 3 Metazoan Radiation.

The geologically rapid appearance of most extant animal groups in the Cambrian fossil record is often linked to enhanced ocean oxygenation. However, conflicting reconstructions of the Cambrian redox landscape make it difficult to determine the extent of ocean oxygenation during this significant biotic event, particularly regarding the redox state of the global deep ocean. In this study, we present authigenic thallium isotope compositions (εTl) for two shale sequences from South China (Qingjiang and Weng'an) that span the Cambrian Stage 2-3 boundary to the appearance of the Qingjiang biota, approximately 521-518 million years ago (Ma), a timeframe that chronicles a particularly rapid interval of metazoan diversification and radiation in the broader Cambrian explosion. If this event occurred amid modern-like extents of global ocean oxygenation, we would expect a significant increase in the global extent of seafloor Mn-oxide burial to drive lower εTl values near the modern open-ocean composition of -6‱. Instead, we observe broadly stable εTl values of around -3 to -4‱ in both studied sections. The lack of any significant Tl isotope shifts in our dataset argues against a short-term global ocean oxygenation event and suggests the global deep ocean was not characterized by modern extents of oxygenation 521-518 Ma. We reinterpret contemporaneous near-modern Mo and U isotope compositions to signal a relatively minor increase in marine oxygenation, likely limited to the continental shelves. However, εTl lower than the average isotopic composition of approximately -2‱ in Ediacaran shales suggests a shift to comparatively better-oxygenated conditions sometime between ~555 Ma and 521 Ma. If diversification at this time was linked to increased ocean oxygen levels, these changes were likely more dominant in the relatively shallow-water settings of continental shelves most densely populated by Cambrian animals and were incapable of dramatically altering seawater Tl isotope mass balance through seafloor Mn-oxide burial.