Deep Subsurface Basalts Targeted Locus (Loci)

Sampling

Approximately 42% of the continental crust is composed of mafic magmatic rocks, such as basalts. These rocks often manifest themselves as massive areas known as large igneous provinces (LIPs). LIPs cover thousands to millions of square kilometers and thus comprise major portions of the subsurface and the biosphere. LIPs, and more specifically basalts, are known to be chemically reactive and favorable for microbial life due to the abundance of reduced compounds in the rock. LIPs exist both in marine systems and terrestrial systems. In marine systems, microorganisms are known to extensively interact with minerals in basalt and may play a large role in the global biogeochemical cycles of C, Fe, and S. Sources of energy available to lithoautotrophic microorganisms below the seafloor include minerals containing reduced Fe and S and also H2 generated from water-rock reactions. Hydrogen is of particular interest in terrestrial subsurface basalt systems, where the subsurface lithoautotrophic microbial ecosystem (SLiME) hypothesis originated. SLiMEs are microbial communities that subsist in deep oligotrophic environments without utilizing energy rich reduced organic compounds that originated from photosynthesis. SLiMEs were first hypothesized to exist in the Columbia River Basalt Group (CRBG). Basalt systems also host diverse communities apart from SLiMEs, such as microbial communities consisting of iron reducers, sulfate reducers, acetogens, and methanogens. In the basaltic Snake River Plain Aquifer (SRPA) in Idaho, microbial communities cycle carbon by forming and oxidizing methane. Today, LIPs are being examined as potential geological storage sites for carbon dioxide in an attempt to sequester CO2 away from the atmosphere to alleviate temperature increases due to climate changes. The Wallula pilot well being used for a geologic carbon sequestration project in eastern Washington State and located in the CRBG provides a window to the subsurface where the microbial diversity of these geologically important regions can be explored. In addition, the well will provide insight into the microbial communities present in the basalts that could play a role in carbon cycling in the deep subsurface where supercritical CO2 (scCO2) is injected. Analyzing samples from the CRBG using deep DNA sequencing technology will further the understanding of the unique microbial diversity of the subsurface, especially with respect to community composition of LIPS and different members contained in different formations. Pyrosequencing will also establish an important baseline for understanding the microbial communities in the aquifers of the Wallula pilot well prior to the injection of scCO2. These communities will certainly change following the injection and pyrosequencing could play a critical role in the analysis of the samples obtained after the injection of scCO2 into the system. The high resolution associated with deep sequencing technology would allow the detection of shifts in the diversity of the microbial communities. All samples presented here originated from the Wallula pilot well in eastern Washington State. The well penetrates through three Columbia River Basalt formations, the deepest of which is targeted for carbon sequestration. This study aimed to characterize the microbial community prior to the injection of carbon dioxide to provide a baseline for comparison after carbon has been stored in the basalt. Samples Wal213, Wal220, Wal31, Wal39, and Wal413 were collected from pristine formations as the well was drilled using a progressive drill-and-test technique that provides samples more representative of the formation of origin. Quality of the samples was also ensured due to the use of an underbalanced drilling technique in which water from the formation acted as drilling fluid, as well as the collection of samples after extensive pumping of the well. Sample Wal34 was collected 2 years after the completion of the well. The microbial diversity of other locations in the Colombia River Basalt Group has been investigated previously (Stevens et al., 1993; Stevens and McKinley, 1995; Fry et al., 1997), but not at the location of the Wallula pilot well and not using sequencing technology which provides a more complete picture of the microbial community. A more detailed description of the Wallula pilot well can be found in the 2009 report Preliminary Hydrogeologic Characterization Results from the Wallula Pilot Study by McGrail et al. (report number PNWD-4129).

Method steps

1. Pipeline used: https://www.ebi.ac.uk/metagenomics/pipelines/4.1