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 post docs have worked on include:
Experimental investigation of the preservation of microbial mats in clay-rich sediments during early diagenesis
Experimental taphonomy can be used to investigate the effects of early diagenesis on the preservation of microbial mats in clay-rich environments. This work is critical to our understanding of fossilization processes on early Earth and also has implications for the preservation of organic material in clay-rich environments on Mars. Photosynthetic microbial mats were incubated in the presence of clay substrates (kaolinite, illite, montmorillonite) and artificial seawater containing high concentrations of dissolved silica (0.1 mM) and low concentrations of sulfate (5 mM) relative to modern oceanic values. Centimeter-thick pieces of microbial mat and 20 microliters of wet clay were removed from the growth experiments after two months. These were heated to 250˚C (at 1,000 bar) in cold seal pressure vessels for up to three days to simulate early diagenetic conditions. Microbial mats lost cohesion as a result of heating. This likely resulted from the volatilization of the microbial sheaths and exopolymeric substances, which had previously held the microbial mats together. Comparison of organics before and after heating by scanning electron microscopy (SEM) revealed an increase in the encrustation of microbes by clay minerals during the three-day experiment. Taken together, this suggests that some diagenetic effects may inhibit microbial fossilization (i.e., loss of microbial mat cohesion), while others may facilitate preservation (i.e., encrustation of microbes by minerals).
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.
Phosphorite formation, diagenesis, and microbial taphonomy in Ediacaran seaways of the São Francisco Craton
The Una and Bambuí Groups of Northeastern and Central Brazil preserve cap carbonate sequences on the São Francisco Craton during major ecological transitions of the Late Neoproterozoic, including a globally widespread phosphogenetic event. The basal units of these stratigraphic packages in particular contain early phosphatic cements and phosphatic intraclasts in association with distinctive sedimentary structures: elongate digitate stromatolites in the more northern Irecê and Salitre Basins (Salitre Formation), and microbial laminites and aragonite crystal fan pseudomorphs in the more southern São Francisco Basin (Sete Lagoas Formation). The formation, diagenesis, and distribution of phosphorite deposits near the Precambrian-Cambrian boundary are the subject of much debate, particularly with respect to the paleoecological constraints and taphonomic potential phosphorite itself may represent. Previous workers have suggested phosphogenesis in Ediacaran seas was the result of Fe-Mn-cycling in redox-stratified sediments, microbial sulfate reduction, or just simple oxygenic photosynthesis on a shallow carbonate platform, but the textural data for these models are limited and have not been compared between correlated basins. Here, we attempt a stratigraphic correlation of the São Francisco Craton’s Irecê and São Francisco Basin deposits and present a preliminary analysis of depositional and early diagenetic fabrics in their phosphatic members. By chemical, isotopic, and mineralogical analyses, we examine the likely roles of (1) Fe- and Mn-redox-cycling in the accumulation of porewater phosphate, (2) ionic inhibitors to carbonate micrite formation including Fe, Mn, and Mg, and (3) microbial metabolic interactions that might have influenced porewater alkalinity and phosphate concentration, in the formation of primary phosphate minerals in a Marinoan cap carbonate. Preliminary results from compositional mapping via X-ray fluorescence of early phosphatic cements in Salitre Formation phosphorites suggest that Fe and Mn abundance is negatively correlated with phosphate mineralization. This would not be expected if phosphogenesis was the result of Fe- and Mn-redox-cycling and cyclic adsorption and desorption of phosphate ions. Furthermore, we find a distinct lack of reduced sulfur species, re: pyrite derived from biogenic sulfide, and no evidence for heightened Mg, SO42-, or other ionic inhibitors of carbonate micrite growth in phosphatic facies. With Raman scattering and optical transmission microscopy, we demonstrate the exclusive association of carbonate fluorapatite with stromatolite laminae and absence from sediments draping or interfingering stromatolites. These data suggest tentatively a yet-unexplored path to phosphogenesis in the Salitre Formation, unrelated to Fe-Mn-redox-cycling and sulfur metabolisms. However, a different mechanism entirely may be responsible for phosphogenesis in the Sete Lagoas Formation. We use micro- and macro-scale textures and facies associations in both the Salitre and Sete Lagoas Formations to explore the possible restriction of the Ireê and São Francisco Basins from global oceans and the dominant role of local biogeochemistry in phosphogenesis on the Ediacaran São Francisco Craton.
Context, Biogeochemistry, and Morphology of Diverse Microbial Mats, Little Ambergris Cay, Turks and Caicos
Little Ambergris Cay (21.3 N°, 71.7° W) was the site of an integrated geobiological study conducted during July of 2016. The cay (~6 km long x ~1.6 km wide) is developed on a broad bank marked by converging ooid shoals, influenced by strong westerly trade winds (avg. 7.5 m/s). Lithified upper shoreface to eolian ooid grainstones form a ~2 m high bedrock rim that protects an extensive interior tidal marsh with well-developed microbial mats. The rim is locally breached to allow tidal flows to inundate interior bays floored by microbial mats. Three mat types are observed based on texture: dark toned “blister mat”, which flanks the bays where they intersect with the bedrock rim; light-toned “polygonal mat” which covers broad tracts of the bay and is exposed at low tide; and lighter-toned “eps mat” which is generally submerged even at low tide. A suite of remote and in-situ datasets were gathered to evaluate the role of environmental and biological controls on the morphology of the mats. 30 different mat locations were studied and sampled for groundwater salinity, pH, DNA content, photosynthetic efficiency, C and S isotope composition, lipid biomarkers, and taphonomic state. The island was mapped using multispectral Landsat images (m-scale resolution), Worldview satellite images (50 cm-scale resolution), and photogrammetry from two UAVs. The UAVs captured more than 1500 nadir images from a ~350 m standoff distance and were processed to generate a 3-band visible light mosaic map of the island at <15 cm/pixel. Topography and nine sedimentologic facies were mapped at cm-scale resolution based on 910 differential GPS data points and combined with UAV orthophotos to map the island. Sub-cm resolution drone-based orthophotos of microbial mats were used to quantify polygonal mat size and textures. The mapping results highlight that sedimentary and bio-facies (including mat morphology and fabrics) correlate strongly with elevation. Notably, mat morphology was observed to be highly sensitive to cm-scale variations in topography and water depth.