A SHORTER FUSE FOR THE CAMBRIAN EXPLOSION?

Well-preserved strata of the Khorbusuonka Group in the Olenek Uplift, arctic Siberia provide important sedimentologic, chemostratigraphic, and radiometric constraints on the demise of the Earth’s earliest metazoans and the subsequent Cambrian Explosion. Ediacara fossils in bituminous limestone of the Khatyspyt Formation occur as three dimensional casts and as carbonaceous compressions, whereas the earliest small shelly fossil of Cambrian aspect (Anabarites trisulcatus Zone) are preserved in dolomite of the overlying Turkut Formation. The upper reaches of Turkut Formation are variably truncated and overlain by the Kessyusa Group, a new stratigraphic subdivision including widespread diamictite in the Tas-Yurakh Formation, which is interbedded with tuff breccia dated at ca. 543 Ma, as well as mixed siliciclastic and carbonate of the Syhargalakh and Mattia formations. Small shelly fossils in the lower reaches of the Mattia Formation include elements of the Purella antiqua Zone, which is overlain with fossils of the basal Tommotian Nochoriocyathus sunnaginicus Zone only a few meters higher. High resolution sampling and carbon isotope analysis of Mattia Formation carbonates define a positive excursion up to +5‰ associated with ⁸⁷Sr/⁸⁶Sr compositions as low as 0.70818. A white clay layer collected from near the top of the Mattia Formation interpreted as a volcanic ash yielded abundant euhedral zircons, which were measured by both LA-ICP-MS and TIMS techniques. U-Pb dates determined by both methods suggest an age around 529-530 Ma for the sample. These results suggests that the base of the Tommotian Stage of the Cambrian Period is some 4-5 million years older than previously estimated. Furthermore, the new age constraint coupled with biostratigraphic and isotopic data from the same succession indicate that the fuse for the Cambrian Explosion may have been several million years shorter, burning for perhaps 9 to 10 million years after ignition following a potential glacial episode near the Ediacaran-Cambrian boundary.