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Project Progress

Geochemical Signatures of Multicellular Life (Summons, Pearson; Project Leads)

The Ediacaran period was a time of critical changes to the biosphere that may have been intimately linked to concurrent changes in global biogeochemistry. While fossil deposits of this age are rare, a rich stratigraphic record of environmental change is preserved.

One such record is the Huqf Supergroup of Oman being investigated by Magdelena Osburn and John Grotzinger. Dated between 635Ma and the Cambrian boundary, these rocks provided a detailed record of marine chemistry through this time including major isotopic excursions in both carbon and sulfur. The Shuram carbon isotopic excursion – one of the largest deviations in Earth history- was first discovered in Oman before being found in deposits across the planet. While the causes of this isotopic excursion are as yet unknown, one hypothesis implicates major changes in Ediacaran carbon cycling. The Shuram Excursion begins in the uppermost Khufai Formation, reaches a nadir of -12‰ in the Shuram Formation and recovers in the overlaying Buah Formation. The goal of this research is to better characterize the Khufai formation and the initial isotopic decline. Their approach is multidisciplinary, drawing from the disciplines of stratigraphy, isotope geochemistry, and organic geochemistry. The first part of this project is to characterize the sedimentology and stratigraphy of the unit by creating detailed lithologic/facies logs and interpreting paleoenvironments within the context of sequence stratigraphy. This creates a framework in which geochemical observations can be placed. They are employing a number of chemostratigraphic tools including carbon and sulfur isotopes and abundances to try to understand the dynamics of these geochemical cycles during this time period. Organic analysis of biomarker compounds allows for more direct questions about the biosphere by probing they type of microbial communities and the environmental conditions at the time of deposition.

The Pearson group has been involved in several different lipid biomarker projects. Post-doc Lindsay Hays has initiated work on the Tayshir (and/or Trezona) anomalies that occur between the Sturtian and Marinoan glaciations of the Cryogenian; there are no publications yet from this project. Post-doc Dan Rogers is working on a comprehensive analysis of hopanoid synthesis genes from a suite of 28 environmental samples spanning a range of environmental gradients; a publication is in preparation. Graduate student Hilary Close has completed a modeling paper that addresses the “Logan hypothesis” for carbon isotopic enrichment and the anomalous isotopic ordering of n-alkanes and isoprenoid hydrocarbons in Proterozoic bitumens. Her work (Close et al., 2010) shows that although the intense heterotrophy proposed by Logan and colleagues (Terminal Proterozoic reorganisation of biogeochemical cycles, Nature 1995, 376, 53-56) is a feasible way to generate the observed isotopic signatures, a more plausible route would be through including both a heterotrophic community and a community of isotopically-enriched photosynthetic bacteria (probably Cyanobacteria). This suggests that the disappearance of isotopically-enriched bitumens at the Cambrian boundary is a direct consequence of the rise to dominance of eukaryotic phytoplankton.

Complementing this work, former MIT graduate student Amy Kelly is finalizing a paper on the C-isotope ordering anomaly in a suite of samples from a globally distributed suite of sedimentary basins spanning Cryogenian to Cambrian times. Samples predating the Cryogenian animal radiation are ‘anomalous’ in that the fatty acid-derived C₁₇ and C₁₈ n-alkanes are enriched in ¹³C compared to the chlorophyll-derived isoprenoids pristane and phytane confirming the observations of Logan et al. (1995). Sediments and oil samples from the late Ediacaran and Cambrian all show ‘normal’ ordering, that is, patterns that one expects based on the different biosynthetic pathways leading to acetogenic lipids (i.e. n-alkanes) and isoprenoidal lipids (pristine and phytane). The distributions of hydrocarbon biomarkers in the late Ediacaran to Cambrian samples is consistent with the Close et al. (2010) hypothesis, that is, the isotopic patterns reflect the rise eukaryotic phytoplankton but not to the exclusion of other explanations.

On-going research in the Summons laboratory is focused on understanding some of the more unusual aspects of steroid distributions in Neoproterozoic sediments and oils. A new collaboration with Professors Jürgen Rullkötter and Jens Christoffers of uses synthetic organic chemistry and mass spectrometric approaches to identify novel C₁₉ steranes seemingly present only in Ediacaran to Early Cambrian oils and sediments. Graduate student Jon Grabenstatter is studying the sterol compositions of a range of cultured protists that have phylogenetic connections to the basal animal lineage of sponges.