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Abstract:
The relationship between the evolution of early animal communities and oceanic
oxygen levels remains unclear. In particular, uncertainty persists in reconstructions
of redox conditions during the pivotal early Cambrian (541-510 million years
ago, Ma), where conflicting datasets from deeper marine settings suggest either
ocean anoxia or fully oxygenated conditions. By coupling geochemical palaeoredox
proxies with a record of organic-walled fossils from exceptionally well-defined
successions of the early Cambrian Baltic Basin, we provide evidence for the
early establishment of modern-type oxygen minimum zones (OMZs). Both innerand
outer-shelf environments were pervasively oxygenated, whereas mid-depth
settings were characterised by spatially oscillating anoxia. As such, conflicting
redox signatures recovered from individual sites most likely derive from sampling bias, whereby anoxic conditions represent
mid-shelf environments with higher productivity. This picture of a spatially restricted anoxic wedge contrasts with prevailing
models of globally stratified oceans, offering a more nuanced and realistic account of the Proterozoic-Phanerozoic ocean
transition.
