Alan J. Kaufman, University of Maryland Shuhai Xiao, Virginia Polytechnic Institute and State University Dima Grazhdankin, Russian Academy of Sciences
The PIs will undertake a basin-wide stratigraphic, geochemical, and paleobiological study of the carbonate-dominated Khatyspyt Formation in remote northern Siberia. Ediacaran fossils in this unit are remarkably preserved in fine-grained carbonates, promising a much enhanced anatomical and paleoecological view of Earths earliest animals. Insofar as the Ediacaran biota may have required abundant free oxygen to sustain metabolic activities, we hypothesize that their distribution may track environmental conditions, and propose tests of this linkage through high resolution time-series analyses of carbon and sulfur isotopes in multiple equivalent sections across the Khatyspyt depositional basin as well as other biological, elemental and isotopic indicators of redox conditions. The Khatyspyt Formation contains a wide range of unique carbonate-hosted Ediacaran organisms, as well as carbonaceous macrofossils, small shelly fossils, trace fossils and planktonic microfossils all in a single, 450-m-thick continuous succession cropping out along the Olenek, Khorbosuonka and Lena rivers. Results of the study should provide insights to the phenomenon of soft tissue preservation, increase the ecological resolution of the Ediacaran-Cambrian transition, and provide important constraints on the history of Earths earliest metazoans and the oceans in which they originated, diversified, and ultimately perished. The primary aim of this field and laboratory based proposal is to test whether the geologically brief Ediacaran experiment in complex multicellular life was controlled by the oxidation state of shallow marine environments. The proposed research will establish a new international collaboration with colleagues in Russia, and require two month-long field seasons above the Arctic Circle in northern Siberia. Graduate students from both U.S. institutions will accompany the PIs in the field, providing them with an enhanced learning experience that will promote both scientific and cultural exchange.
Our field research documented that the succession in the Olenek Uplift of arctic Siberia was stratigraphically truncated by a thick boulder-bearing and poorly-sorted diamictite of mixed glacial and volcanic origin near the Ediacaran-Cambrian boundary (around 542 million years ago). High resolution time-series carbon and sulfur isotopic measurements of carbonate rocks below this critical transition, which contain a unique fossil assemblage of Earth’s earliest complex multicellular animals, reflect significant fluctuations in environmental conditions through the end of the Ediacaran Period. These include strong surface-to-deep gradients in the abundance of oxygen and of sulfate – an important oxidant used by a class of micro-organisms that consume nearly half of the organic matter in Modern ocean sediments – in the depositional basin. The chemical gradients in seawater were likely harnessed by other micro-organisms, including photoautotrophs that utilized hydrogen sulfide rather than water for a source of electrons to reduce carbon dioxide to organic matter, and heterotrophic bacteria that consumed methane or oxidized toxic hydrogen sulfide to run their metabolic activities. Insofar as the enigmatic soft-bodied animals that populated this shallow marine environment were able to survive in oxygen-deficient and potentially hydrogen sulfide-rich seawater, some may have harbored bacteria within their bodies in a symbiotic relationship, similar to Modern day tube worms living near hydrothermal vents. The Ediacara biota in the Siberian strata disappeared abruptly in strata below the diamictite, which we associate with the glaciation and a profound disturbance in the carbon cycle. On the other hand, above the diamictite the Siberian sedimentary rocks we studied preserve fossil evidence of the ‘Cambrian Explosion’ of animals – the geologically sudden appearance of the basic biological patterns and body plans that would form the basis of Modern animals. A volcanic ash horizon near the top of the succession was discovered during our stratigraphic investigations. Zircons extracted from the ash were characterized and age dated using two different but complementary U-Pb techniques. Both resulted in ages around 530 million years for the volcanic ash. The new age constraint coupled with biostratigraphic and isotopic data from the same succession indicate that the fuse for the Cambrian was several million years shorter than previously predicted, burning for perhaps 9 to 10 million years after ignition following the potential glacial episode near the Ediacaran-Cambrian boundary. Comparative paleontological and isotopic analysis of time-equivalent terminal Ediacaran Period strata in South China and in Early Cambrian aged strata in Newfoundland were conducted as part of this project. For the Ediacaran, we find important contrasts in the time series geochemistry of the sedimentary archives in Siberia and South China that suggest chemical heterogeneity between ocean basins. The inter-basinal contrasts complicate the correlation of sedimentary horizons and the reconstruction of the general oxidation state of late Ediacaran ocean masses. For the Early Cambrian, our paleontological, sedimentological, and isotopic evidence from Newfoundland suggests that Early Cambrian environmental conditions may have been progressively stabilized by animals that were the first to stir up and ventilate the sediments in the search for food and shelter.
