Trace Fossil Evidence for Fluctuating Oxygen Levels in the Spence Shale, a Middle Cambrian Konservat-Lagerstätte from Utah

Burgess Shale-type preservation provides a unique glimpse into the diversification of metazoan life during the Cambrian. Although anoxia has long been thought to be a prerequisite for this soft bodied preservation, the paleoenvironmental conditions controlling this exceptional taphonomic window have not been fully constrained. This study takes a microstratigraphic approach to examine bottom water oxygen fluctuation in the Spence Shale of Utah using sedimentary fabric and trace fossils as a proxy for relative oxygen in strata containing Burgess Shale-type preservation.

Results indicate that these mudstones alternate between laminated and bioturbated on a mm scale with burrows generally small with burrow depths and widths both in the sub-cm scale. This suggests a rapidly fluctuating bottom water oxygenation with background oxygen levels not high enough to support infaunal communities punctuated by oxygenation events. Burgess Shale-type preservation within the Spence Shale (mostly of cyanobacteria) is largely confined to laminated sediments consistent with anoxia. Within the same strata as soft bodied fossils there is also a diverse skeletonized benthic fauna including various polymerid trilobites, hyolithids, lingulid brachiopods and eocrinoids suggesting that during times when oxygen was present a complex dysoxic benthic community was established.