New Mexico Geological Society Annual Spring Meeting — Abstracts

The Cerro Negro Microbial Origins Project is part of the Deep Microbiology Subprogram of the DOE's Subsurface Science Program, which is ultimately aimed at improving bioremediation teclmiques for contaminant clean up and waste disposal. A large scale, multi-disciplinary research effort at Cerro Negro; NM is investigating the origins of microorganisms in the deep subsurface and their behavior, in situ survival potential and long distance transport in subsoils and groundwater over thousands of years. Cerro Negro, a volcanic neck in the southeastern San Juan Basin, intruded approximately 3.39 million years ago, presumably producing a sterile zone, free from microorganisms, due to the associated heat of volcanism. Recent drilling and sampling in Cretaceous sediments near Cerro Negro address the questions of whether or not microorganisms have recolonized the previously sterilized area and if so, by what means.

Microorganisms and evidence for microorganisms have been detected at depth within the sterilized zone. Preliminary results suggest that both groundwater transport and in situ survival are responsible for present-day microbial populations within the thermal aureole of Cerro Negro. Groundwater transport is probably the predominant mode of microbial origin.

The focus of this study is to describe the groundwater flow pattern and flow rates from the recharge area, Mount Taylor region, to Cerro Negro in order to constrain microbial transport rates and recolonization potential. Horizontal and vertical flow rates were estimated through geochemical modelling and radiocarbon dating along the flow path. Carbon-14 analyses indicate that groundwater samples near Cerro Negro are > 38,000 years old. Regional flow rates based on radiocarbon ages yield a conservative groundwater travel time of 91,000 years from Mount Taylor to Cerro Negro at 0.11 m/yr. These rates suggest that relatively rapid microbial recolonization of the thermal aureole by transport was possible.

A northeast-southwest cross-sectional model, extending 10 km from the recharge area, Mount Taylor region to Cerro Negro integrates horizontal and vertical flow rates determined in the isotopic study, available water level data and subsurface geology of the Mesozoic sediments. Particle tracking is used to quantitatively illustrate the potential for microbes to be transported in groundwater in the past 3.39 million years to re-establish themselves within the previously sterilized zone. The model supports the microbial transport theory and constrains the timing of recolonization subsequent to the intrusion.