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Project Progress

During Year 5 of NAI membership, we completed a study of the fossilization processes associated with oncoids (spherical stromatolites) of the Rio Mesquites in the Cuatro Cienegas Basin, Central Mexico. This was done in collaboration James Elser’s group, who have been studying the ecological aspects of the same system. We determined that surface biofilms have a bilayered community structure, with the upper zone being dominated by several groups of larger, mostly erect-growing filamentous cyanobacteria and many species of diatoms. Microelectrode studies show that these surface species control precipitation of calcium carbonate through photosynthesis. However, at a depth of 1-2 mm below the biofilm surface, a second community is dominated by smaller filamentous species. This community is associated with pervasive carbonate precipitation that entombs the deeper community, preserving numerous cellular remains. At this depth in the bi-layered biofilm, the surface community is largely excluded, creating a strong taphonomic bias that favors preservation of the subsurface community. Larger filamentous forms of the surface community are rarely preserved in the oncoid interior and upon death, diatom remains (siliceous frustrules) appear to undergo rapid dissolution under the alkaline conditions that prevail in the surface biofilm environment. The basic results of this study were presented as a poster at the NAI General meeting and are presently being written up for publication.

A second study, started during Year 5, involves the characterization of biofilms and microbial fossil assemblages associated with low temperature carbonate-precipitating springs on the floor of Mono Lake, an alkaline-saline lake located in eastern California. The goal of the study is to trace the fate of biosignatures during carbonate precipitation and early diagenesis. The results of the work in Mono Lake will be compared with ancient carbonate spring deposits elsewhere in the Mono Basin that were formed during the last glacial period when the lake was much deeper and colder. The work in the Mono Basin involves collaboration with the Woods Hole team, who will be focusing on the eukaryotic biodiversity of the lake biota.