Signs of photosynthesis on a Neoproterozoic snowball earth .

摘要

Laterally extensive black shales were deposited on the Sao Francisco craton in southeastern Brazil during one of the low-latitude Neoproterozoic glaciations. The effect these glaciations had on the biosphere has long been controversial, but there has not been sufficient data to assess the impact. This study, the first organic geochemical investigation of synglacial sediments, synthesizes geological observations and biomarkers information from subsurface cores drilled through the glacial sediments. These rocks contain up to 3.0 wt.% organic carbon and pseudomorphs of the hydrated carbonate mineral ikaite, which are interpreted as representing the preserved record of abundant marine primary productivity from glacial times. Extractable biomarkers reflect a complex and productive microbial ecosystem, including both phototrophic bacteria and eukaryotes, living in a stratified ocean with thin or absent sea ice, oxic surface waters, and euxinic conditions within the photic zone. Furthermore, this environment is inconsistent with parts of the "Snowball Earth" hypothesis.;Additionally, the results of two other investigations into the Proterozoic biosphere are presented. The first is a re-analysis of stromatolite "diversity." Stromatolites are thought to be the preserved record of microbial mats, and their distribution through time is often interpreted as a proxy for phenomena as disparate as tectonic basin evolution, the carbonate saturation state of the Proterozoic ocean, and the evolution of metazoa. However, this reanalysis indicates that many of the previously interpreted "diversity" patterns are instead the result of data artifacts, and the true distribution of stromatolites through the Proterozoic may be less variable than previously thought.;The final project is the development of a method to measure the isotopic value of individual acritarchs, organic-walled microfossils with a robust record through the Proterozoic. The hope is that the 13C/ 12C ratio of individual acritarchs of different morphology can be used as a tracer of paleooceanic processes, much the way the 13C/ 12C ratio of benthic and pelagic foraminifera are used to study the Phanerozoic ocean.