Reduction in Marine Primary Productivity in the Early Cambrian Nanhua Basin, South China.

The Cambrian explosion is represented by a rapid diversification of early animals in which the role of marine primary productivity remains obscure. In this study, we analyzed multiple geochemical data, including TOC, major, and trace elements, in the basinal Yuanjia section, South China. Covariations among TOC, P/Al, CuEF, and NiEF suggest that they could be taken as effective marine productivity proxies in the early Cambrian Nanhua Basin. The similarities of CdEF and Cd/Mo in the Nanhua Basin and modern upwelling settings suggest that they might be effective to track upwelling, where Cd and Mo were mainly controlled by plankton biomass and redox conditions, respectively. Our results indicate that CoEF and Co × Mn were invalid in evaluating upwelling because of the significant effects of water-column redox conditions on Co enrichments in the Nanhua Basin. The decreased TOC, P/Al, CuEF, and NiEF reflect a long-term decline in marine productivity from late age 2 to age 3. In comparison with the published results in the outer shelf (Jinsha, TZS drill core, YJK drill core, and GDM-1 well) and slope areas (TX-1 well), the fall in marine productivity might be common in the early Cambrian Nanhua Basin. Our results exhibit that the reduced marine productivity was accompanied by weakened upwelling, quiet hydrothermal activities, and enhanced local terrestrial fluxes, indicating that variations in marine productivity might be mainly driven by the development of upwelling in the early Cambrian Nanhua Basin. Comparison of marine productivity with fossil records suggests that food availability was sufficient to sustain the Cambrian explosion in the Nanhua Basin. We infer that marine productivity might indirectly stimulate early animal evolution through its significant impact on water-column oxygen levels in the early Cambrian Nanhua Basin.

Uncovering the Ediacaran phosphorus cycle.

Phosphorus is a limiting nutrient that is thought to control oceanic oxygen levels to a large extent1-3. A possible increase in marine phosphorus concentrations during the Ediacaran Period (about 635-539 million years ago) has been proposed as a driver for increasing oxygen levels4-6. However, little is known about the nature and evolution of phosphorus cycling during this time4. Here we use carbonate-associated phosphate (CAP) from six globally distributed sections to reconstruct oceanic phosphorus concentrations during a large negative carbon-isotope excursion-the Shuram excursion (SE)-which co-occurred with global oceanic oxygenation7-9. Our data suggest pulsed increases in oceanic phosphorus concentrations during the falling and rising limbs of the SE. Using a quantitative biogeochemical model, we propose that this observation could be explained by carbon dioxide and phosphorus release from marine organic-matter oxidation primarily by sulfate, with further phosphorus release from carbon-dioxide-driven weathering on land. Collectively, this may have resulted in elevated organic-pyrite burial and ocean oxygenation. Our CAP data also seem to suggest equivalent oceanic phosphorus concentrations under maximum and minimum extents of ocean anoxia across the SE. This observation may reflect decoupled phosphorus and ocean anoxia cycles, as opposed to their coupled nature in the modern ocean. Our findings point to external stimuli such as sulfate weathering rather than internal oceanic phosphorus-oxygen cycling alone as a possible control on oceanic oxygenation in the Ediacaran. In turn, this may help explain the prolonged rise of atmospheric oxygen levels.

Ediacaran Marine Redox Heterogeneity and Early Animal Ecosystems.

Oxygenation has widely been viewed as a major factor driving the emergence and diversification of animals. However, links between early animal evolution and shifts in surface oxygen levels have largely been limited to extrapolation of paleoredox conditions reconstructed from unfossiliferous strata to settings in which contemporaneous fossils were preserved. Herein, we present a multi-proxy paleoredox study of late Ediacaran (ca. 560-551 Ma) shales hosting the Miaohe Konservat-Lagerstätte of South China and, for comparison, equivalent non-fossil-bearing shales at adjacent sections. For the fossiliferous strata at Miaohe there is geochemical evidence for anoxic conditions, but paleontological evidence for at least episodically oxic conditions. An oxygen-stressed environment is consistent with the low diversity and simple morphology of Miaohe Biota macrofossils. However, there is no evidence for euxinic (anoxic and sulphidic) conditions for the fossiliferous strata at Miaohe, in contrast to adjacent unfossiliferous sections. Our results indicate that Ediacaran marine redox chemistry was highly heterogeneous, even at the kilometre-scale. Therefore, our study provides direct-rather than inferred-evidence that anoxia played a role in shaping a landmark Ediacaran ecosystem. If the anoxic conditions characteristic of the studied sections were widespread in the late Neoproterozoic, environmental stress would have hindered the development of complex ecosystems.