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Abstract
Recent studies have enhanced our understanding of the linkage of oxygenation and metazoan evolution in Early Cambrian time. However, little of this work has addressed the apparent lag of animal diversification and atmospheric oxygenation during this critical period of Earth history. This study utilizes the geochemical proxy and N isotope record of the Ediacaran–Cambrian boundary preserved in intra-shelf basin, slope, and slope basin deposits of the Yangtze Sea to assess the ocean redox state during the Early Cambrian metazoan radiation. Though ferruginous conditions appear to have prevailed through the water column during this time, episodes of local bottom-water anoxia extending into the photic-zone impacted the slope belt of the basin. Heterogenous oceanic redox conditions are expressed by trace element concentrations and Fe speciation, and spatial variation of N isotopes. We propose that the coupling of ocean chemistry and Early Cambrian animal diversification was not a simple cause-and-effect relationship, but rather a complex interaction. Specifically, it is likely that animal diversification expanded not only temporally but also spatially from the shallow shelf to deep-water environments in tandem with progressive oxygenation of the extensive continental margin.
Highlights
The Cambrian explosion of diverse bilaterian clades is considered to have proceeded in three phases preserved in the fossil record: (1) the first phase is recognized by the appearance of basal metazoan phyla in Late Ediacaran time; (2) the second phase begins with the first occurrence of biomineralization of plates, spines and shells, and is recorded by the widespread occurrence of small shelly fossils (SSFs); (3) the final, perhaps the most profound, phase is defined by the emergence of the three supraphylogenetic clades during Cambrian stage 31
Sedimentary δ15N values of investigated samples range from −7.5‰ to +5.9‰, showing the greatest variation for all studied sections near the Ediacaran-Cambrian boundary (Fig. 2; refer to supplementary text for the stratigraphic and sedimentological context of the investigated intervals)
Results of the present study suggest that areas of the Yangtze Sea experienced rapid fluctuations of oceanic redox conditions following the Cambrian explosion that may reflect lower than expected oceanic and atmospheric oxygen levels at this time
Summary
The Cambrian explosion of diverse bilaterian clades is considered to have proceeded in three phases preserved in the fossil record: (1) the first phase is recognized by the appearance of basal metazoan phyla in Late Ediacaran time; (2) the second phase begins with the first occurrence of biomineralization of plates, spines and shells, and is recorded by the widespread occurrence of small shelly fossils (SSFs); (3) the final, perhaps the most profound, phase is defined by the emergence of the three supraphylogenetic clades during Cambrian stage 31 Though these biological events have been attributed to environmental modifications, notably oxygenation of the global ocean, specifics of the relationship between the emergence and diversification of metazoans and atmospheric-oceanic oxygenation remain unresolved. A principal goal of the present study, is an improved understanding of the interaction of ocean redox state and the Cambrian metazoan explosion
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