Is the Proximal Gulf of Mexico a Modern Analog for the Middle Cambrian House Embayment?
for the Middle Cambrian House Embayment?
Marty Erwin School of Life Sciences University of Nevada Las Vegas
Steve Rowland Department of Geosciences University of Nevada Las Vegas
Species richness is a gauge of biodiversity in an ecological context. In contemporary settings, various forms of disturbance are known to be responsible for significant changes in community-level species richness. Some types of disturbance, such as the deposition of turbidites, or the deposition of a volcanic ash beds, are readily detected in the stratigraphic record. In the absence of such clear stratigraphic signals of disturbance, and allowing for possible taphonomic variability, significant changes in species richness may be hypothesized to be responses to environmental disturbances.
The Middle Cambrian Wheeler Shale of western Utah exhibits significant variability in the species richness of benthic trilobites, both geographically and stratigraphically. Gaines and Droser (2003) showed anoxic to dysoxic conditions cyclically occupied portions of the House Embayment. Here we explore the hypothesis that anoxia and dysoxia were caused by the episodic influx of nutrient-rich waters, which triggered blooms of phytoplankton. A possible modern analog for dysoxia in the House Embayment is the broad swath of the Gulf of Mexico affected by the influx of nutrient-rich waters from the Mississippi River. The House Embayment appears to have been relatively isolated from oceanic circulation, and taphonomic conditions during Wheeler Shale deposition appear to have been relatively constant. Thus taphonomic bias does not adequately account for the significant differences in diversity of benthic species between localities. Species richness in the vicinity of the dysoxic dead zone of the Gulf of Mexico fluctuates in response to the intensity and location of the zone of dysoxia. Variation in species richness within the Wheeler Shale may record similar phenomena in the House Embayment.