Paleoredox geochemistry and bioturbation levels of the exceptionally preserved early Cambrian Indian Springs biota, Poleta Formation, Nevada, USA.

摘要

The early Cambrian Indian Springs biota, western Nevada, USA exhibits Burgess Shale-type (BST) preservation of a diverse array of animal phyla, including the earliest definitive echinoderms. It therefore provides an important window on animal life during the Cambrian radiation. The objective of this study is to analyze the trace metal paleoredox geochemistry and bioturbation levels of this BST deposit in order to characterize the paleoenvironmental conditions in which these animals lived and their fossils were preserved. A total of 28 rock samples were collected from outcrops at three previously reported intervals of exceptional preservation at the Indian Springs locality, as well as at one interval not known to exhibit such preservation. An additional 20 random samples were collected from the talus for comparison. In the laboratory, the samples were geochemically analyzed for trace metal paleoredox indices (V/Cr and V/(V+Ni) ratios) and bioturbation levels were assessed through X-radiography and petrographic thin sections using the ichnofabric index (ii) method. Additional samples from coeval strata of the Poleta Formation in the White-Inyo Mountains, CA that lack BST preservation were also geochemically analyzed with the same methodology. Preliminary results indicate that oxic bottom water conditions dominated during deposition of these strata, despite consistently low bioturbation levels. This pattern holds for intervals with BST preservation and those without. Although ephemeral incursions of low-oxygen waters may have taken place, there is no evidence for persistent oxygen restriction in these paleoenvironments. The low levels of bioturbation indicate limited mixed layer development and a redox boundary near the sediment-water interface, likely allowing post-burial BST preservation to occur even in this setting dominated by oxic bottom waters. Paleoecological reconstructions and taphonomic hypotheses relating to the Indian Springs Lagerstatte should account for the paleoredox conditions revealed in this study. Several models propose the roles of clay minerals, the presence of hypersaline brines, and the actions of Fe-reducing bacteria as mechanisms for exceptional preservation.