A chilly Late Cambrian

Application of the Uniformitarian Principle by geologists from Minnesota, USA (Runkel, A.C. et al. 2010. Tropical shoreline ice in the late Cambrian: implications for Earth’s climate between the Cambrian Explosion and the Great Ordovician Biodiversification Event. GSA Today v.20 (November 2010), p. 4-10) may have shown that around the end of the Cambrian period (500 to 488 Ma) global climate was sufficiently cold for sea ice to have formed in the tropics of the time. The evidence comes from curious metre-scale clasts of cemented sands in Late Cambrian beach deposits of the northern USA, some of which show imbrication as if the bodies were shoved together. Others seem to have been extended into boudin-like plates without any sign of tectonic activity, so that isolated clasts occur in offshore deposits. Yet more have been bent to drape over irregularities in the surface beneath them. Somehow individual sand beds must have become cemented quickly so that water action could fracture them in a brittle fashion and then they became softer to experience ductile deformation and even boring by worm-like animals. Almost exact replicas of such structures form on the shores of the American Great Lakes in winter when water in shoreline sands freezes to cement the grains. Breaking waves and melting explain the peculiar structures in these intraclasts. Examples of ice-cemented sediments abound in glaciogenic deposits, but the Late Cambrian world is widely considered to have experienced greenhouse conditions.

Land distribution during late Cambrian.
Image via Wikipedia

Apparently not as the North American crust was definitely close to the Equator at that time. The intraclasts occur only in one stratigraphic Formation of the Minnesotan Cambrian, because it preserves littoral facies. There are no other reports from elsewhere, but that may well be because few geologists were able to combine the experience of modern frigid shore conditions with that of Cambrian stratigraphy as those from Minnesota surely do.

The Middle to Late Cambrian was a period of faunal hiccups, diversification after the Cambrian Explosion failing to get underway because of repeated minor extinctions spread across the known occurrences of rocks of that age (see Linking oxygen levels to great animal radiations, this issue) . The Minnesotan evidence could indicate that the global climate was extremely unstable at that time in the manner of Neoproterozoic ‘Snowball Earth’ conditions, but not so severe. The widespread occurrence of microbial carbonate facies of this age range has long been used as evidence of a warm Earth, but such carbonated form today over a wide range of latitudes: witness the huge coccolithophore blooms so common at high latitudes nowadays. Shoreline sandy sediments of Cambrian age are not uncommon, occurring throughout the English Midlands and in NW Scotland, for instance. So it might be interesting to re-examine easily-reached occurrences such as these to see if similar structures turn-up.

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