Research

RESEARCH OVERVIEW

The overarching theme of my research is how environmental changes impact ecosystems, and how these interactions between organisms and their environment are preserved in sediments and the rock record. My dissertation research focused on microbial ecosystems, investigating this question across a range of time and spatial scales in settings from the Archean to modern sediments and using a variety of biological, chemical, and geological tools.

MICROBES, CARBON, AND CLIMATE IN AN ANOXIC LAKE HURON SINKHOLE

I studied how an Middle Island Sinkhole, an anoxic sinkhole ecosystem in Lake Huron is responding to changing climate. I worked with sediment carbon and nitrogen data alongside weather station and satellite-derived climate and environmental data to understand how changing climate, particularly ice cover and precipitation, drives sediment geochemistry. We found that more carbon from phytoplankton was present in sediments in years with low ice cover.

Microbial mats in Middle Island Sinkhole. Photo credit: NOAA TBNMS

Shipboard processing of sediment cores from MIS. Photo credit: NOAA TBNMS

I also investigated how the microbial community at MIS is changing year to year using 16S community sequencing. I will be integrating 16S data from 5+ years of sampling with climate data to understand drivers of change in the sediment microbial communities.
For projects at MIS, we worked closely with researchers at the NOAA Thunder Bay National Marine Sanctuary, who provide logistical support and resources that are integral to our work.

USING 3D IMAGERY TO INVESTIGATE ARCHEAN MICROBIALITES

Images from μCT scans showing fossil mat laminae with entrapped grains and tufts (Fig. 5 in Howard et al., 2024).

We used microCT scanning to reconstruct morphologies of Archean (3.48 Ga) microbially induced sedimentary structures (MISS) from the Dresser Formation. This study demonstrated the capability of microCT scanning to visualize and make 3D measurements in order to understand biotic-environmental interactions. We identified fossilized mat laminae with trapped grains and tufts (see figure to left), as well as making measurements of fossilized mat chips that were consistent with sporadic influence from high velocity flow such as extreme tidal events.

MICROBIALITES AS RECORDS OF ENVIRONMENTAL CHANGE IN THE EOCENE GREEN RIVER BASIN

Large microbialites nucleated on wood in the Green River Basin, WY.

I am working with microbialites from the Eocene Green River Basin of Wyoming to study how the dramatic changes to the environment in which these microbialites were deposited are recorded. These microbialites formed in a paleolake during the Early Eocene Climatic Optimum, a period of extraordinarily high atmospheric carbon dioxide. This lake also varied dramatically in size and chemistry while microbialites were forming. I am using microCT scanning alongside an array of geochemical techniques, including electron microprobe analysis and clumped isotope paleothermometry, in order to link morphological changes in microbialites to chemical and environmental shifts such as lake depth, temperature, and water chemistry.

MICROBIALITE DEPOSITIONAL ENVIRONMENTS

I use data from the literature to compile occurrences of microbialites and early terrestrial life in the Archean (Sheldon et al., 2025) and Paleoproterozoic (Howard and Sheldon, 2025) in order to understand how (1) the advent of a terrestrial biosphere may have impacted nutrient cycling and delivery and (2) the depositional environments in which early life was preserved changed across major boundaries. My work on the Archean has shown the ubiquity of life in tidal and terrestrial ecosystems, emphasizing the importance of considering the influence of emergent land on the development of these early microbial communities. I expanded this work into the Paleoproterozoic to investigate how microbialite distribution changed following major changes such as the Great Oxidation Event, the Huronian Glaciations, and the advent of modern plate tectonics (Howard and Sheldon, 2025).