My research addresses the chemical development of the early oceans and atmosphere, and the environmental context of early evolution. Field mapping studies are the starting point for more topical laboratory-based studies involving geochemical, paleontological, and geochronological techniques. My research is focused on the reconstruction of environmental conditions associated with the Cambrian radiation of animals in Oman, Namibia and Siberia.
Some projects my current students and postdocs are working on include:
Facies Distribution in Cryogenian Cap Carbonate Sequence, Naukluft Mountains, Namibia
The termination of the Marinoan "Snowball Earth" event constitutes a significant period of climate change in Earth's history. This field study seeks to investigate this termination by characterizing the cap carbonate sequence post-dating the Marinoan glacial deposits exposed in the Naukluft Mountains of Namibia by using structural, sedimentologic, and stratigraphic field observations, in addition to chemostratigraphic sampling, optical petrography, and quantitative chemical textural imaging. Multiple stratigraphic sections and bed/unit tracing allows delineation of stratigraphic stacking patterns and regional facies changes. Immediately overlying the Marinoan glacial diamictite unit are well laminated, fine-grained dolostones. In the more proximal regions of the cap, the laminated dolostone grades vertically into cm-scale stromatolite domes that form decameter-scale buildups, that become strongly elongate upward. More distally the stromatolite facies passes laterally into laminated, fine-grained dolostones. Most distally the cap carbonate appears to pinch out. The cap carbonate facies assemblage grades vertically into laminated dolostone with increasing quantities of siliciclastic mudstone that shows lenses of imbricated, edgewise, intraclast conglomerates associated with cross-stratified fine quartz sandstone. In one of the more distal outcrops, the cap instead passes abruptly into a succession of matrix and clast supported breccias with clasts of reworked cap dolostone in a quartz sandstone matrix, and interstratified with siliciclastic mudstone. More proximally, cap carbonates are overlain by massive mudstone and intraclast conglomerate limestone; regionally the cap facies pass vertically into thick laminated dolostones and intraclast conglomerate dolostone variably mixed with quartz sands. This cap carbonate facies distribution indicates that the more proximal stromatolite-bearing strata were deposited in shallower marine environments, which pass downdip into more distal laminated dolostones, consistent with Precambrian carbonate facies of other ages. Vertical facies trends imply shallowing of depositional environments, coincident with an influx of shallow marine siliciclastics. Together, these conclusions suggest that the Marinoan cap carbonate in this region represents overall marine regression, rather than the glacioeustatic transgression that has been suggested by some studies of Marinoan cap carbonates elsewhere. Transgression is still likely indicated by the onlap of glaciogenic diamictite facies by finely laminated dolostones, but the internal architecture of the cap carbonate itself suggests a regressive history developed on a carbonate ramp.
Distinct organic biosignatures may survive low-temperature and low-pressure diagenetic processes on geological timescales even when identifying cell morphology in fossils has been lost. Dynamic interactions between organic matter and aluminosilicate clay surfaces are increasingly appreciated as a significant driver in organic preservation and microbial fossilization. In hyperalkaline and hypersaline environments, cation bridging can dominate clay-mineral interactions over ligand exchange and promote preservation. Because we do not know the extent to which these interactions impact biological degradation or are sensitive to pressure and temperature, we do not have a clear idea of which environments are most likely to preserve biomarkers or cell morphology. Here we interrogate the impacts of sulfide, salinity, alkalinity, and mineral associations on the preservation of organic-rich sediments from Mono Lake, CA, and of cells in artificial sediments via targeted incubation experiments and hydrous pyrolysis experiments. Preliminary results of SEM/EDX analyses on modern sediment incubations reveal spatial association between organic matter and authigenic Mg-smectite clays such as stevensite, hinting at a potential physical control for sequestration. To observe the long-term preservation potential of cell morphology and biomarkers, we perform hydrous pyrolysis experiments with natural sediments and with single-strain cyanobacterial biomass or isolated organic compounds incubated with artificial soda lake water and Mg/Fe-bearing aluminosilicate clays. Samples are sealed into 1.5cm-long gold capsules then heated within cold-seal pressure vessels to 250-300°C and 250 bars for up to three days. Raman spectroscopy and the evolution of N:C by EA-IRMS are used to evaluate decomposition of organic matter content. SEM/EDX analysis on chemically-dried samples enables visualization of cell morphology and the placement of organic materials with elemental mapping. Our findings may help elucidate geochemical conditions that foster preservation in Earth's history or on other planets, and the extent to which those biosignatures survive early diagenesis.