Providing a Palaeoecological and Geochemical Context for Cloudina in Western North America
Kirk L. Domke1†, David J. Bottjer1, Sean J. Loyd1, Frank A. Corsetti1 and Timothy W. Lyons2
1 University of Southern California, Los Angeles, California, USA
2 University of California, Riverside, California, USA
Biomineralized skeletons of benthic organisms are first recorded in the latest Neoproterozoic with the appearance of Cloudina, a small (length up to 1 cm; 2–4 mm diameter) tube-dwelling organism. Cloudina were examined from the Precambrian Reed Formation, and the overlying lower member of the Deep Spring Formation, at several localities in eastern California and western Nevada. These western North America Cloudina localities contrast with other Cloudina localities in that the assemblages occur in level-bottom sedimentary settings, unlike the biohermal settings found elsewhere. Cloudina preservation varies significantly at the study sites, from relatively complete tests to fractured debris, occurring primarily in shell beds. Microstratigraphic and diagenetic analyses in thin section give further evidence for transportation before final deposition.
In addition to palaeoecological analyses, this study also provides concentration and isotopic analyses of carbonate-associated sulfate, which gives insight to the redox conditions of the oceans during this critical interval in Earth history. Samples collected at high stratigraphic resolution from the Cloudina localities in the Reed and Deep Spring Formations provide a record of latest Neoproterozic δ 34S. Using sulfate as a proxy for changing oceanic conditions, we gain insight into how oxygenation of the oceans may have played a role in the evolution of Cloudina and the subsequent Cambrian radiation.
A broader context of these cloudinids is necessary for establishing a baseline towards understanding how early biomineralizers evolved in response to the ongoing “agronomic revolution” of increasing bioturbation that was occurring during the Neoproterozoic-Cambrian transition. Our ability to provide this broad context is enhanced by the physical and microbial structures within the interbedded siliciclastic strata in this region. The Cloudina in western North America occur before the Precambrian-Cambrian boundary, and hence before the appearance of significant vertical bioturbation. The thick Lower Cambrian sections in the study area provide numerous carbonate intervals from which to assess the context of succeeding early biomineralizers. This stratigraphic interval in eastern California and western Nevada thus provides a basis on which to determine how the first biomineralizing organisms evolved as part of the “Cambrian substrate revolution”.