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The Coast Range Ophiolite (CRO) is an actively serpentinizing Jurassic age ophiolite (ocean crust tectonically emplaced on land) in Northern California (Figure 1). In 2011, RPL Co-I’s Cardace, Hoehler, McCollom, and Schrenk acquired NAI Director’s Discretionary funds to establish the Coast Range Ophiolite Microbial Observatory. There exists a legacy of subsurface wells from when the CRO was home to the former Homestake mine and 8 newly-drilled boreholes that tap varying depths within the actively serpentinizing rocks. The fluids at CROMO have pH values as high as 12.7 but H₂ concentrations consistently lower than typical serpentinized fluids (e.g. 5-6 orders of magnitude lower than those at Lost City hydrothermal vent field). Microorganisms at CROMO may have significantly diminished metabolic energy availability and thereby a greater challenge coping with extreme alkalinity and oxidant availability. As such, the range of conditions at CROMO may approach or cross the “biotic fringe”. Alternatively, the low H₂ concentrations may reflect use of H₂ in microbial metabolisms, which our team will directly test through in-field measurements of process rates.
 

Coast Range Ophiolite Microbiological Observatory Sites

 

Coast Range Ophiolite Microbiological Observatory Site 1 (CROMO I)

This borehole was drilled in the Core Shed area of the McLaughlin Natural Reserve. This area is a grassy flat with low shrubs and a few introduced trees, and the Homestake Core Shed houses archived cores from initial mining surveys of the land. Upslope of the drilling area are reddish altered peridotite blocks, and green-blue scrapes showing fresher (i.e., less oxidized by surface weathering) serpentinite where the soil cover has eroded. A few meters of soil was underlain by bedrock. Bottom of hole was at a depth of 45 m. Post-drilling measurement of the uncased borehole gave a pH of 11.4. Core was recovered only from primary hole at this site; four additional water monitoring wells (2-inch diameter) were drilled within meters of this site to add lateral and vertical context for the central hole.

 

Coast Range Ophiolite Microbiological Observatory Site 2 (CROMO II)

This borehole was drilled in the Quarry Valley area of the McLaughlin Natural Reserve. This area is characterized by serpentine meadow flats, fairly steep slopes with reddish altered peridotite outcrops or blocks (visible even in aerial photos). A few meters of soil graded into lesser altered material and finally bedrock. Bottom of hole was at a depth of 31 m. Post-drilling measurement of pH in the uncased borehole gave a pH of 12.5. Core was recovered only from primary hole at this site; two additional water monitoring wells (2-inch diameter) were drilled within meters of this site to add lateral and vertical context for the central hole.