Loyd Lab Research

 

Paragenetic evolution and precipitation mechanisms of salt dome cap rocks

Salt domes and diapirs underly terrestrial and marine sediments and sedimentary rocks of the Gulf Coast region. These structures generate structural traps for crude oil and gaseous hydrocarbons. In concert with hydrological interaction with the salt, microbial degradation of hydrocarbons can yield massive deposits of calcium carbonate, elemental sulfur and metal sulfides. We use carbonate geochemistry to specify microbial reaction pathways.

 

Constraining clumped isotope compositions of cold seep carbonates and concretions

Cold seeps represent one of the best examples of the interaction between biologic activity and mineral formation. These systems occur at multiple locations on the seafloor and are typified by massive buildups of authigenic carbonate. The dominant geochemical 'engine' that fuels the associated orgnanism communities and mineral formation involves the anaerobic oxidation of methane primarily coupled with sulfate reduction. Similar reactions promote carbonte mineralization in the form of subsurface carbonate concretions. Some of these processes are actively driving modern mineral formation and thus we are provided with a unique opportunity to explore carbonate authigenesis in a somewhat well-constrained context. As a result, we examine the clumped isotope compositions (primarily used as a temperature proxy) of cold seep carbonates and concretions in order to 1) comment on the spatial/temporal evolution of these precipitates and 2) explore potential non-equilibrium infuences on clumped isotope signatures recorded in carbonates.

 

Identification of microbial processes that mediate mineralization of carbonate concretions and their modern analogs in marine sediments

Highly cemented regions in sediments and sedimentary rocks, called concretions , have been identified throughout the geologic record and their probable analogs exist in "still-soft" marine sediments. Cementation occurs in the sediment column by the precipitation of mineral phases (mainly carbonate minerals) within the open pore space. Ultimately this mineral growth fills-in and reduces porosity yielding a zone with a decreased fluid flow potential. The exact mechanism by which mineral precipitates form is poorly understood. Biologic and abiologic degradation of sedimentary organic matter represents a possible source of carbonate necessary for cement precipitation. The chemical constituents (both trace and major elements) of a given cementing mineral can provide information about its source. To this end we use geochemical methods including trace sulfate concentration and sulfur and carbon isotopic signatures to elucidate driving forces of concretion formation as well as assess the relative contribution from biologic/abiologic processes. This study is directly related to economic interests as the porosity reduction associated with cementation may effect petroleum migration in host and reservoir rock.

 

Evaluation of the chemistry of the Neoproterozoic and Paleozoic oceans

This project involves investigation of the global chemistry of Earth’s marine environment during the terminal Neoproterozoic and Paleozoic. This period in Earth history is characterized by the first occurrence of large multicellular life forms as well as severe perturbations in oceanic chemistry.   An understanding of the chemical environment of this evolutionary threshold is imperative if we are to understand the conditions necessary for multicellular life to arise. Here we employ coupled carbon and sulfur isotopic variation to determine the characteristics of these two closely biologically-linked elements in the early ocean, explore a potential marine oxygenation event at this time and to characterize the state of sulfate in Earth's ocean. Intriguingly, high stratigraphic, close spatial and regional sulfur isotope variability suggest low sulfate concentrations well into the Cambrian. These low sulfate conditions probably persisted even further into the Paleozoic, reflecting a delayed buildup of marine sulfate.

 

Drivers of hot spring and alkaline lake carbonate mineral formation

Authigenic carbonates can form from a multitude of processes. Oftentimes, deciphering among abiotic and biotic mineralization pathways is difficult. Although commonly colonized by microbes, carbonates associated with hot springs and alkaline lakes lack indicators of microbial involvement. We explore carbonate mineralization from alkaline lake and hot spring settings, including Big Soda Lake (NV) and Travertine Hot Spring (CA).