Site Description:

In northern California, the Coast Ranges stretch north ~600 km from the Golden Gate in San Francisco, in an area bounded loosely by the Pacific Ocean to the west, the Coast Range of Oregon and Washington to the north, the Klamath Mountains to the NE, the Great Valley of California to the SE, and the drainage of the Sacramento and San Joaquin rivers to the south (Alexander et al. 2007). Serpentine-associated soils occur throughout this region, with distinctive vegetation and ecology, all dependent on the subaerial weathering of ultramafic bedrock (black features in left panel of Figure 1 show ultramafics). Blocks of the CRO, equivalent to Jurassic ocean crust tectonically emplaced on land, are weathering/reacting with variably diluted trapped Cretaceous seawater in this region (Peters, 1993; Shervais et al., 2005). CRO formation fluids are accessible via historic 1980s-era groundwater wells at the McLaughlin Natural Reserve (located at green star, Figure 1), a UC-Davis-administered research preserve, south-east of Lower Lake, CA.

Preliminary investigations of 1980s-era wells sunk in CRO rocks at the McLaughlin Natural Reserve revealed ongoing serpentinization, indicated by the strongly alkaline pH (>10) and abundant H₂ and CH₄. Furthermore, historic data for cores obtained in the vicinity show serpentinization in process: olivines, pyroxenes, and amphiboles (original parent rock minerals) are evident, with high and low temperature serpentine mineral phases attesting to different generations of serpentinization and associated chlorite, magnetite, hydroxides, and talc.

Preliminary Findings Reported at Past AGU Meetings

• Cardace has been working to put CROMO waters (see Table 1 for one round of field data) in a global context, with geochemical modeling results showing a consistent bioenergetics story for deeply sourced fluids from ultramafic rocks. Metabolisms that exploit metal redox are quite feasible in this subsurface environment, however the carbon source for microbes remains cryptic. Carbon uptake is confounded by the very low activity of CO₂ and organic acids in high pH waters. Furthermore, the nitrogen cycle seems to have missing links: not all steps in the N cycle can ‘go’ in this setting.
• Tori Hoehler and colleagues summarized preliminary findings on the carbon source constraint on habitability in the serpentinizing subsurface. Following detection of CO in our sampled waters, he modeled its utility to extremophiles.  He found that methanogenesis from CO is a feasible metabolism for this environment.
• Matt Schrenk and colleagues have compelling initial results that suggest  organotrophy, hydrogen oxidation, and iron reduction are coded in the genome of serpentinizing fluid samples, with potential for microbial activity fueled by H₂ and acetate evidenced by preliminary microcosm experiments. 

Table 1. Aqueous geochemistry for March 2012 samples from monitoring wells and boreholes. Note generally elevated pH > 10 except for groundwater control sites CSW1,3 and QV1,2. Nanomolar dissolved H₂ and millimolar CH₄ levels are under monitoring. (Cardace et al., 2012, AGU presentation)

 Drilling Logistics and Operations

In our proposal to the NASA Astrobiology Institute Director’s Discretionary Fund, we described our aim to establish a subsurface “observatory” by drilling into an actively serpentinizing peridotite body, characterizing cored rocks, and outfitting the boreholes for a program of long-term observation and experimentation to resolve the serpentinite-hosted subsurface biosphere. We completed this work in late summer 2011, drilling two priority boreholes and six additional monitoring wells in the Coast Range Ophiolite locality in the UC-Davis McLaughlin Natural Reserve, Lower Lake, CA.

Drilling into serpentinite at the McLaughlin Natural Reserve was ideal for a number of reasons: continental setting, research status of reserve, existing physical plant, and proximity to NASA ARC. Access to subsurface serpentinite on land allows deployment of a broader array of instrumentation and experiments, affording time series sample collection and analysis also, with no limit to the time spent on site. McLaughlin is already a designated research reserve, part of the UC Davis Natural Reserve System, dedicated to providing “secure sites for research, instruction, and public outreach” (http://nrs.ucdavis.edu/index.html), and there is avowed interest on the part of reserve directors in our project. The physical plant at McLaughlin is outstanding, with a former county road running near the proposed drill site, easily traveled by the drilling contractor. The field station has lab/lodging/communal cooking spaces, and would provide a base of operations for the field work. The site is also just ~3 hours’ drive north of the San Francisco Bay Area.