DID DYNAMIC REDOX CONDITIONS CONTROL LATE EDIACARAN METAZOAN ECOSYSTEMS?

The first appearance of skeletal metazoans in the late Ediacaran (~550 million years ago; Ma) has been linked to the widespread development of oxygenated oceanic conditions, but neither a precise spatial and temporal reconstruction of their evolution nor this relationship to ecology has been resolved. We consider the evolution of ocean chemistry from ~550 to ~541 Ma in the Nama Group, Namibia, the Dengying Formation on the Yangtze Platform, China, and the Manykaj and Emyaksin Formations, Siberia. Carbon isotope data are complemented with a reconstruction of water column redox dynamics utilizing Fe-S-C systematics and the distribution of skeletal and soft-bodied metazoans. Combined, these datasets provide insight into the potential role of ocean redox chemistry during this pivotal interval of major biological innovation

Anoxic and ferruginous water column conditions are noted in all three settings during and after the transition to positive d¹³C that marks the end of the Shuram/Wonoka excursion. In the Nama Group, both sub-basins reveal highly dynamic redox structures, where shallow, inner ramp settings experienced transient oxygenation. Anoxic conditions were caused either by episodic upwelling of deeper anoxic waters or higher rates of productivity. These settings supported short-lived and monospecific skeletal metazoan communities. By contrast, microbial (thrombolite) reefs, found in deeper inner- and mid-ramp settings, supported more biodiverse communities with complex ecologies and large skeletal metazoans. These long-lived reef communities, as well as Ediacaran soft-bodied biotas, are found particularly within transgressive systems, where oxygenation was persistent. We suggest that a mid-ramp position enabled physical ventilation mechanisms for shallow water column oxygenation to operate during flooding and transgressive sea-level rise. Our data support a prominent role for oxygen, and for stable oxygenated conditions in particular, in controlling both the distribution and ecology of Ediacaran skeletal metazoan communities.