Agouron Postdoctoral Fellow
Department of Earth, Atmospheric,
and Planetary Sciences
Massachusetts Institute of Technology
I work in modern microbial ecosystems and their ancient sedimentary record to explore the history of life on Earth. My research combines field observations with detailed sedimentary and geochemical analyses to assess records of early animal and microbial activity preserved in carbonates. Current research projects focus on habitats of the cryosphere, including both modern polar settings and ancient periods when cold conditions could have had significant impacts on evolutionary pressures. I am currently an Agouron Postdoctoral Fellow in the Massachusetts Institute of Technology Department of Earth, Atmospheric and Planetary Sciences.
The expansion of complex multicellular life like animals marks a pivotal point in Earth history, with the origin of novel behaviors that transformed the evolutionary and biogeochemical landscape. Evidence for the first animals comes in the midst of the major climate perturbations of the Cryogenian (635-850 Ma), but the habitats that would have been available for these early animals are poorly defined.
I work with Cryogenian sedimentary successions from Svalbard, where transitions between glacial Snowball Earth and interglacial conditions are preserved in mixed carbonate and siliciclastic sections. Within these varied depositional environments, I explore how changes in climate affect the habitats that would have been available for early animal evolution. This work combines sedimentary and petrographic observations with analysis of clumped isotopes, synchrotron-based characterization of Mn and Fe redox state, and biomarkers across Sturtian and Marinoan glacial deposits in Svalbard. By combining these data sets I assess (a) diagenetic conditions that could have modified environmental signatures, (b) the range of physical environments and oxygenation of habitats available during glacial episodes and (c) the biological significance of the habitats.
Morphological indications of microbial activity
Microbial mats and calcified proto-stromatolites are abundant in lakes of the McMurdo Dry Valleys, Antarctica, and their growth morphologies preserve a record of interactions between microbial activity and the depositional environment. My work combines field observations of microbial mat morphologies growing in specific depositional environments with carbonate petrography, X-ray computed tomography, and 3D reconstructions. Sampling of delicate microbial mats with detailed environmental context is made possible by diving.
Significant findings include:
1. Mud sedimentation simplifies stromatolite morphology. I demonstrated that distinctive mat morphologies commonly taken as evidence for photosynthetic activity can be dampened by even small influxes of mud, and this may bias our record of microbial evolution through time (Mackey et al., 2017).
2. Stromatolite branching can result from changes in microbial community. Changes in stromatolite branching are traditionally interpreted as a product of changing sedimentation or wave energy, but in the low energy depositional environment of Lake Joyce, I found that these patterns could also emerge from changes in the microbial community composition (Mackey et al., 2015).
3. Mat morphology across sedimentary and irradiance microenvironments can provide evidence for photosynthetic activity. By comparing mat morphology with 3D reconstructions of the lake bottom, modeled irradiance, and sedimentary microenvironments in Lake Vanda, I demonstrated that phototactic activity of the microbial communities in a directional light environment modified the patterns of pinnacle growth more strongly than changes in total irradiance or sedimentation (Mackey et al., in review).
Chemical indications of microbial activity
Carbonates that precipitate alongside active microbial communities can leave signatures of this activity in carbon and oxygen isotopes. By comparing the petrography of Lake Joyce stromatolites and the d13C and d18O values of microdrilled calcite, I documented how rising lake levels from summer melt reduced photosynthetic rates and demonstrated that the ice covered conditions modulated isotopic signatures of photosynthesis (Mackey et al., in review).
Pavilion Lake Research Project
Pavilion and Kelly lakes, British Columbia, Canada
Research on modern microbialites and logistics for exploration with PLRP NASA Analog Mission (e.g. Harwood-Theisen et al., 2015).
Effects of ice cementation on beach deposits
Observations of modern sedimentation associated with ice-cementation of beaches during winter storms on Lake Superior and application to similar features from Paleozoic beach deposits of SE Minnesota (Runkel et al., 2010).
tjmackey <at> mit.edu
Department of Earth, Atmospheric, and Planetary Sciences
Massachusetts Institute of Technology, 54-1025
Cambridge, MA 02139