Anthony C. Runkel¹*, Tyler J. Mackey²**, Clinton A. Cowan², David L. Fox³

1 Minnesota Geological Survey, Univ. of Minnesota, 2642 University Ave. W, St. Paul, Minnesota 55114-1057, USA
2 Geology Dept., Carleton College, 1 North College St., Northfield, Minnesota 55057-4001, USA
3 Dept. of Geology and Geophysics, Univ. of Minnesota, 310 Pillsbury Dr. SE, Minneapolis, Minnesota 55455-0129, USA

Abstract

Middle to late Cambrian time (ca. 513 to 488 Ma) is characterized by an unstable plateau in biodiversity, when depauperate shelf faunas suffered repeated extinctions. This poorly understood interval separates the Cambrian Explosion from the Great Ordovician Biodiversification Event and is generally regarded as a time of sustained greenhouse conditions. We present evidence that suggests a drastically different climate during this enigmatic interval: Features indicative of meteoric ice are well preserved in late Cambrian equatorial beach deposits that correspond to one of the shelf extinction events. Thus, the middle to late Cambrian Earth was at least episodically cold and might best be considered a muted analogue to the environmental extremes that characterized the Proterozoic, even though cooling in the two periods may have occurred in response to different triggers. Such later Cambrian conditions may have significantly impacted evolution preceding the Ordovician radiation.

*E-mail:

**Now at Geology Dept., Univ. of California, One Shields Ave., Davis, California 95616-8605, USA

Manuscript received 8 Oct. 2009; accepted 22 Feb. 2010

DOI: 10.1130/GSATG84A.1