Exobiology and Evolutionary Biology

Geochronological and Paleomagnetic Constraints on the Origin of Animals

PI: Alan Kaufman

Here we request funds for geochronological and paleomagnetic research on Ediacaran and Cambrian sediments on the Siberian craton. The succession is richly fossiliferous; it contains evidence for abundant complex Ediacaran metazoans, and of Early Cambrian body and trace fossils. We discovered multiple levels of volcanic ash, as well as several unusual clay horizons interpreted as volcanic in origin. Only one age is available for the succession; this comes from a basal Kessyusa Fm. volcanic breccia dated at ca. 544 Ma. Otherwise the Siberian strata are temporally constrained by comparison of carbon isotope trends with those from Namibia (between ca. 550 and 542 Ma). If authigenic zircons are abundant in the ash horizons, we may better constrain the timing of both evolutionary and biogeochemical events. Complex Ediacaran organisms lived in a narrow window between two profound biogeochemical anomalies. During these events carbon isotope values take a stratigraphic plunge to -10 permil or more. While these perturbations are related to significant sea level fall, the relationship with Neoproterozoic glaciation (i.e. Snowball Earth) remains unclear. No one has yet suggested that the boundary excursion was related to glaciation, although there is geochemical evidence for ocean overturn.

We discovered a poorly sorted diamictite sandwiched between Turkut carbonates, which contain the boundary excursion, and overlying Kessyusa siltstones, which have abundant Early Cambrian fossils. The diamictite, interpreted to be glacial in origin, has a green-grey sandy to clayey carbonaceous matrix with abundant ice-rafted debris. Randomly-oriented clasts range from granules to boulders of Turkut and underlying Khatyspyt carbonate and shale, but also a few green igneous rocks. Sub-rounded carbonate clasts are often faceted although no striations were observed. Angular clasts of shale and platy carbonate imply that sediments were not transported by water through long distances; the thickness of the unit argues against a mudflow origin. Lacking tectonic evidence for exposure of underlying strata, eustatic sea level must have fallen by more than 100 meters to expose Khatyspyt strata during the ice age. The diamictite is abruptly truncated by ferruginous shale deposited during post-glacial transgression. The glacial interpretation provides an explanation for the boundary anomaly and the demise of the Ediacaran metazoans through drawdown of sea level and enhanced upwelling of H2S and HCO3- charged waters. Furthermore, the ecospace vacated by the Ediacaran biota may have been re-populated with Cambrian faunas in the glacial aftermath. Insofar as the boundary ice age dissected a carbonate platform, it is possible that Snowball Earth-like conditions continued to the end of the Neoproterozoic Eon. The Ediacaran paleolatitude of the Siberian Craton is poorly constrained, but may lie between 5 and 30 degrees of the equator. An igneous sill cuts through the Turkut Formation in our study area, and we propose to sample this unit for geochronometric and paleomagnetic analysis in order to test the low latitude glacial hypothesis. Zircon separations will be done in the mineral separation lab at UM, which is specifically designed for this purpose. Elemental maps of the zircons will be done by electron microprobe at UM, and these will then be dated at the University of Arizona geochron laboratory. Paleomagnetic sampling will also include oriented block samples and drill cores of sediments distant from the sill. The samples will be analyzed for magnetic susceptibility and paleolatitude with instrumentation at the California Institute of Technology. This proposal fits with the specific Exobiology objective to investigate the Evolution of Advanced Life, and with the general focus on the co-evolution of life and environment.