PRESERVATION OF THE BURGESS SHALE BIOTA FROM MARBLE CANYON

The Marble Canyon biota of the Burgess Shale is one of the richest new Cambrian fossil deposits to be discovered in decades. Here, we review new geochemical, sedimentologic and paleontologic datasets to explore the circumstances surrounding the preservation of the Marble Canyon biota. Comprehensively, the data support the unique biostratinomic setting at the front of the Cathedral Escarpment as the most important control on the remarkable density and fidelity of preservation of soft-bodied fossils relative to other deposits. Taphonomic analyses confirm that gross morphology is captured by thin carbonaceous remains and mineral films, most prominently represented by Fe-Mg oxide, a phase that is absent from Burgess Shale sites on and near Fossil Ridge. New cross-section analyses from thin sections reveal carbonaceous films of soft-bodied fossils envelop the “aluminosilicate-like” mineral films, revealing that these phases did not grow around the outsides of organisms during early burial, but instead filled voids created by high temperature volatilization of carbonaceous remains. These patterns confirm a late-stage, metamorphic origin of fossil-associated mineral films. Mechanistic models for carbonaceous preservation invoking direct physical protection of organic carcasses from microbial decay are not supported. Geochemical and sedimentologic data further exclude the possibility that soft-bodied fossils were buried below the zone of sulfate reduction either during initial burial or subsequently during early fossilization. Sulfur isotope data illustrate that intensity of sulfate reduction in the early burial environment was more strongly influenced by intermittent weak bioturbation -which corresponds to a sharp reduction in the abundance and fidelity of soft-tissue preservation- than to bed thickness. Preservation of the most labile tissues occurs in beds as thin as 0.7 mm, characterized by pauses in sediment accumulation and emplacement of carbonate cements at the seafloor. New data from Marble Canyon clearly reveal the role of early cements in impeding oxidant flux into sediments, thereby imposing kinetic restrictions on rates of sulfate reduction and leading to carbonaceous preservation of Burgess Shale-type fossils.