Geochemical evidence for oxygenated bottom waters during deposition of fossiliferous strata of the Burgess Shale Formation

摘要

Deciphering the oxygenation potential of bottom waters during deposition of the fossiliferous strata of the Burgess Shale is key to understanding the paleoecology of the organisms preserved as fossils and the processes involved in their preservation. Methods of paleo-redox determination that are based upon trace fossils, organic carbon content, or size analysis of pyrite framboids are problematic when applied to the Burgess Shale. Fortunately, patterns in redox-sensitive trace elements hold great potential for determining paleo-redox conditions for these greenschist-facies rocks because they are independent of both age and metamorphic grade. Four Burgess Shale-Type (BST) fossil-bearing sections were studied; three lie within the Burgess Shale Formation, whereas one lies within the slightly younger Duchesnay unit of the Middle Chancellor Formation. The geochemical proxies of seawater redox conditions that were used are Mo, U/Th, V/Cr, V/(V+Ni), Ni/Co and V/Sc. Each of these redox-sensitive indices suggests that oxygenated conditions prevailed in the overlying seawater during deposition of beds within which BST preservation developed. Non-fossiliferous beds at the base of the Mt. Stephen Trilobite Beds yield three geochemical indices (V/[V+Ni], V/Sc, Ni/Co) that indicate deposition under dysoxic to anoxic conditions. Thus the only beds to have been deposited under anoxic conditions are barren of fossils of soft-bodied taxa, further contradicting the notion that bottom-water anoxia is a factor in BST preservation. The laminated nature of the Burgess Shale and evidence for subsurface anoxia, coupled with evidence for oxygenated bottom waters, presence of bacterial mats, and surficial trace fossils suggest that the Burgess Shale was deposited in the exaerobic zone (oxic–anoxic boundary at the water–sediment interface). Implications of this reinterpretation of the depositional environment of the Burgess Shale include: (1) taphonomic models for BST preservation must account for the presence of oxygen in bottom waters; (2) paleoecological interpretations of the Burgess Shale need to account for a probable autochthonous benthic component in what is a complex assemblage of fossils that includes elements derived from the platform; and (3) the possibility of Burgess Shale communities based upon chemosynthesis and/or bacteria grazing must be considered.