Sulfur cycling in seafloor methane seeps
Seafloor methane seeps are found along the edges of continental shelves at both passive and active plate margins and represent an estimated 8% of global methane production. A significant portion of the microbial population found in these environments are anaerobic methane oxidizing consortia that consume methane prior to venting on the seafloor and are also intricately tied to the sulfur cycle in the subsurface. Using a high-resolution sulfide film technique to analyze δ34S on the cm to micron scale captures rapid fluctuations in subsurface microbial activity and can be used to compare activity at methane seeps in different tectonic environments. Analyzing the mineral record paired with porewater profiles using a variety of techniques on different spatial scales will help determine how ephemeral microbial processes might ultimately become recorded in the rock record. High school students participating in the STARS program will conduct part of this research.
- Paleozoic Carbon Isotope Chemostratigraphy
Stable carbon isotopes isotopes (δ13Ccarb) can be used as a chronostratigraphic tool as the shallow ocean homogenizes with atmospheric values on a 1,000 year time-scale. Therefore, C isotopes can indicate syndepositional units and can provide better correlative resolution that lithostratigraphic methods. We are using C isotopes to understand the depositional and erosional history of Late Ordovician units deposited during the Guttenburg inorganic Carbon isotope excursion (GICE). These correlations can then be used to understand numerous geologic and paleoenvironmental characteristics relevant to Earth historians and stratigraphers.
Millbrig K-bentonite (altered volcanic ash bed in the Castlewood limestone (Decorah Fm.) near Barnhart, MO. Numerous widespread ash beds occur during GICE time and are used as chronostratigraphic markers for spatially disparate sections.
- Ordovician Hirnantian glaciation and mass extinction
The two-stage end Ordovician extinction is one of the most dramatic mass extinctions in the rock record, and one of just three extinctions caused exclusively by increased extinction rates as opposed to decreased origination rates. The extinctions are associated with the onset and end of the Hirnantian δ13Ccarb excursion, the largest positive isotope excursion in the Phanerozoic rock record. This interval is also marked by major glaciation and a global sealevel lowstand. A parallel positive excursion in δ18Ocarb accompanies the Hirnantian δ13Ccarb excursion, suggesting a linkage between glaciation and restructuring in the carbon biogeochemical cycle. By synthesizing fossil abundance and diversity, chemostratigraphic, and sequence stratigraphic data, we are continuing to address the causal relationships between glacio-eustasy, ocean circulation, carbon and sulfur cycling, and this mass extinction.
Dr. Davey Jones putting his finger on the beginning of the Hirnantian δ13Ccarb excursion, Anticosti Island, Canada.
- Micron-scale analysis of microbial systems
The metabolic activities of microbial mats have likely dominated biogeochemical cycling and ocean/atmospheric redox over most of Earth history. However, the fine-scale (micron) spatial distribution of these microbes within sedimentary environments and the exact relationship between metabolic activity and the establishment of geochemical gradients remains poorly constrained. Working with Prof. Victoria Orphan (Caltech) and Prof. Greg Druschel (IUPUI), we are pursuing a parallel microgeochemical and microbiological study of um-scale biogeochemical cycling within microbial mats to help shed light on these topics. The parallel application of the methodologies developed here provides a new toolset that can shed light on metabolic activity at an unprecedented scale within microbially dominated sedimentary environments.
- Micron-scale isotopic analysis of Archaen pyrites: constraining the rise of Oxygen
Sulfur isotopes have proved key to our understanding of the Archean-Proterozoic transition and the initial accumulation of oxygen in the atmosphere. Traditional analyses have been based primarily on bulk hand samples. Here, we are investigating the absolute value and spatial variability of sulfur isotope signatures (δ34S and Δ33S) within individual pyrite grains allowing us to identify discrete populations of these sulfides and to better understand the environmental/diagenetic conditions associated with their formation.
- High-precision SIMS analysis of carbonate-associated sulfate (δ34SCAS)
Traditional analysis of carbonate-associated sulfate (CAS) relies on large hand samples that often have contain many different generations of carbonate and are often associated with complicated diagenetic histories. Here, we are working to analyze CAS at a much finer scale (~10 microns) that will allow us to analyze individual grains or fossils, thereby eliminating much of the uncertainty in complex multi-component sample.
- Identifying the source(s) of gypsum in Mammoth Cave, KY
The Mammoth Cave system is the longest navigable cave system in the world at >390 miles mapped. Numerous passages are coated in gypsum with morphologies ranging from “snowballs” to spectacular blades. However, the source of gypsum has been debated for over a century. Our group, in collaboration with veteran cave mappers Bob Osburn and Aaron Addison (both WashU), is using stable S isotopes to identify the possible sources of S in S-containing phases of the surrounding bedrock and water.