COMPARISON OF TRACE-ELEMENT-BASED PALEO-REDOX AND PALEO-PRODUCTIVITY INDICATORS IN BURGESS SHALE-TYPE FOSSIL SITES

The geochemical character of five Burgess Shale-type (BST) fossil localities have been documented (Burgess Shale, Wheeler, Kinzers, Emu Bay, Lakhandra). Concentrations of redox-sensitive elements (V, U), and productivity-related elements (Ni, Ba, Cu) were Al-normalized to differentiate lithogenous contributions from precipitates and biogenic components. All BST-bearing units yield low values of U and Mo, whereas patterns of Al-normalized V concentration vary: 1) the Burgess Shale, Kinzers (Emigsville Mbr), and Emu Bay yield linear V/Al trends comparable to average upper crustal values, indicative of a lithogenous source; 2) Wheeler and Lakhandra display a more dispersed pattern; 3) the trilobite-rich, non-BST, pyritic Longs Park Mbr of the Kinzers displays elevated V/Al values. Ba/Al patterns also vary between BST localities: 1) the Burgess Shale, Kinzers, and Lakhandra sites yield non-linear patterns with values at or below average crustal values; 2) Ba at Emu Bay correlates with Al and average crustal concentration; and 3) the Wheeler suite display a linear base trend comparable to upper crustal averages, as well as a subset of samples with elevated Ba values. Ba-enrichment is an indication of increased productivity. However, sustained subsurface anoxia (e.g., matground environments) can liberate Ba, and result in below-average Ba values.

The geochemical patterns indicate varying chemical conditions between BST localities, consistent with interpretations based on field and paleontological data. Weakly elevated V and highly elevated Ba values in the Wheeler are consistent with deposition under a fluctuating oxycline associated with variable productivity, and lacking stable matgrounds, whereas the low-oxygen Proterozoic Lakhandra likely maintained a stable substrate. The Burgess Shale, Kinzers, and Emu Bay signatures are consistent with deposition under stable oxygenated conditions. The below-average Ba in sample suites from the Burgess Shale and Kinzers (Emigsville) indicate a stable substrate, and are consistent with field-based interpretations that these biotic communities were associated with microbial matgrounds and brine pools. Emu Bay appears to have been deposited under oxygenated conditions as well, but lacked a consistently stable substrate.