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 former students have worked on include:
“Stratigraphy and structure of the Naukluft Nappe Complex”
The Naukluft Nappe Complex is an outlier of the 650-540 Ma Damara orogen consisting of carbonate and clastic strata organized into several tectonic packages and thrust southwestward atop the Nama group, a carbonate foreland basin sequence that captures the Precambrian-Cambrian boundary. Several of the upper nappes are similar to Marinoan cap carbonates exposed elsewhere in Namibia, but the Zebra Nappe, the lowest of the allocthonous units, has an uncertain age and affinity. We use regional structural mapping, detailed stratigraphic columns, and chemostratigraphy to understand the formation environment of the Zebra Nappe, its internal divisons, tectonic context, and potential correlations with known intervals such as the basal Nama.
The Zebra Nappe contains ~500 m of carbonate and clastic sediments with sedimentary structures consistent with terrestrial, intertidal, and shallow-marine environments. The nappe can be divided into several formations along the lines proposed by Hartnady (1978). We evaluate and refine these divisions through detailed mapping and stratigraphic studies. The lower strata of the nappe shows evidence of subaerial deposition, while the upper sequence contains hummocky cross-bedded grainy limestones, thick shales, and platform-facies dolomites. Stromatolites are found at various levels of the section, but no Ediacaran or calcified fossils are observed. The chemostratigraphic profile of the Zebra Nappe is unlikely to correlate with the basal Nama or an outboard extension of the foreland basin, but it may link with the older Witvlei group, or a missing pre-Damara passive margin of the Kalahari craton.
Deformation within the Zebra Nappe is dominantly southwest-vergent, in contrast to the southward-verging structural grain of the suprajacent Pavian and Kudu nappes. The nappe exhibits significant internal shortening accommodated by shearing and imbrication at multiple scales. Dolomites at the base of the nappe are imbricated at ~30 m scale, while the middle strata form z-geometry folds and imbrications with a larger amplitude. The upper strata, dominantly platform-facies dolomites, fold gently over a ~5 km baseline. Shale packages separating these layer-bound domains of deformation are heavily strained. The layer-bound modes of deformation within the Zebra Nappe are consistent with varying rheological properties of the strata. This interpretation significantly differs from that of Hartnady, 1978, which envisions the heavily deformed lower and middle Zebra Nappe as being deposited syntectonically against the advancing toe of the upper Zebra Nappe. This new picture of internal deformation of the Zebra Nappe, along with clarified tectonic elements within the adjacent nappes and para-autochthonous Nama group, will enhance understanding of the emplacement history and possible source of the lower Naukluft Nappe Complex.
Context, Biogeochemistry, and Morphology of Diverse Microbial Mats, Little Ambergris Cay, Turks and Caicos
Nathan Stein, Lizzy Trower
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.
Constraints on Ediacaran carbon cycle dynamics from the Shuram Excursion of Oman
Neoproterozoic large negative carbon isotope excursions remain an enigmatic feature of the long-term carbon isotopic record yet their temporal distribution pre-dating the appearance and diversification of complex animals in the fossil record demands an improved understanding of them. There is considerable uncertainty on whether these extreme excursions reflect primary perturbations in marine DIC, or whether they are post-depositional features.
We examine problematic aspects of the largest negative excursion on record – the Shuram Excursion in the Sultanate of Oman including: (1) co-variation in δ13C and δ18O carbonate, (2) elevated trace metal signatures, and (3) isotopic variability between the different carbonate phases. We combine bulk and micro-scale analysis of isotopic composition using carbonate clumped isotope thermometry and SIMS, and trace metal enrichments using ICP-OES, XANES and electron microprobe measurements.
Insights into the paleoecology and ocean chemistry of the Athel Basin, Oman at the Precambrian-Cambrian boundary
The project goal is to study samples from the Athel Basin, Oman using organic and inorganic geochemical proxies in order to gain insight into the environmental conditions of early animal habitats. The Athel Basin formed, within error, at the time of the Precambrian-Cambrian boundary and contains large quantities of indigenous organic matter. It is thus well suited for an in-depth study of the geobiology and paleoecology of a sedimentary basin at this critical time in Earth’s history.
We conducted organic geochemical measurements on 15 samples from drilled cores. Organic biomolecules (biomarkers) were extracted at Caltech and identified via gas chromatography coupled to mass spectrometry at both Caltech and the University of California Riverside (UCR). Biomarkers demonstrate that the Silicilyte Formation within the Athel Basin represents an environment and reservoir distinct from the bounding shales. The silicilyte was more reducing and had a distinct biology compared to the shales. Finally, biomarkers for sponges (the earlier type of animals) are found in abundance in these sections, as previously observed, potentially indicating nominally aerobic animals living in a low-oxygen environment. Multiple replicate measurements were conducted to ensure the validity of the measurements.
We measured the speciation of iron in all samples extracted for biomarkers. Iron content and quantitative relative abundances of iron phases were measured at UCR. These measurements are used as a proxy for the oxidative state of the water column. Iron speciation results are consistent with the biomarkers demonstrating a reducing, potentially sulfidic water column at the time of deposition despite the presence of spongal biomarkers.
Oxygen isotopes of quart were measured at Caltech on thin sections from three core samples via secondary ion mass spectrometry. Values found are consistent with a diagenetic origin of the quartz occurring over a large depth gradient (1-2 km) in the sediment.
Silicon Isotopes were measured at UCLA via inductively coupled plasma mass spectrometry. These measurements were performed to test if sponges could have produced the silica present in the Silicilyte Formation. Isotopic values were inconsistent with a spongal origin.
Thin sections were cut and examined via standard light microscopy. Additionally SEM work was conducted to map out the relationship between organic matter and the silica via energy dispersive X-ray spectroscopy.