ANCIENT TO MODERN GEOLOGIC AND HYDROLOGIC FORCINGS DRIVE DEEP BIOSPHERE ACROSS COLORADO PLATEAU

Our understanding of how deep subsurface (>500 m) microbe, rock, and fluid systems are coupled and have dynamically evolved over geologic time is limited and precludes effective subsurface resource and waste management. Specifically, it’s relatively unknown how shifts in geologic (e.g., burial/erosion, deformation) and hydrologic (e.g., topographic evolution, meteoric influx) forcings over millennial timescales impacted the deep biosphere. What hydraulic gradients, permeabilities and fluxes are required to maximize the potential for subsurface microbial life? And, what evidence of microbial activity and associated fluid flow and fluid-rock reactions is imprinted in the rock record?

To address these questions, we are integrating multi-disciplinary observations, analyses, and modeling of modern processes and dynamics with the geologic record. We have focused on modern to ancient Fe-, S-, and C-cycling systems in the Colorado Plateau from near-surface (e.g., fresh to brackish aquifers and brine seeps) to over 3 km depth (basal aquifer system). We discovered that migration of hydrocarbons during maximum burial of basin sediments (~100-25 Ma; when temperatures were relatively hot based on clumped isotopes of calcites) into shallow reservoirs set the stage for later microbial activity by bringing organic carbon closer to the surface. Recent denudation (<3-4 Ma), related to rapid incision of the Colorado River, decreased in-situ temperatures and dramatically increased hydraulic gradients, activating deep meteoric circulation, flushing of saline fluids, and possibly transport of microorganisms. Microbial communities capable of degrading oil and thermogenic gas, reducing sulfate, and generating microbial methane are present in hydrocarbon reservoirs from near-surface to the basal aquifer, consistent with natural gas conventional and clumped isotopic signatures. Carbon and oxygen isotopes of individual calcite growth bands in fractures record shifts in fluid sources (from basinal brines to meteoric waters) that stimulated bacterial sulfate reduction and microbial methanogenesis, in the presence of hydrocarbons, within the last ~4 Ma, based on U-Pb dating. These results illuminate specific time-periods in the geologic past and locations within the earth’s crust where hydraulic drivers and pathways were optimal for subsurface microbial activity.