Deep Life by Onstott Tullis C.;

Author:Onstott, Tullis C.;
Language: eng
Format: epub
Publisher: Princeton University Press
Published: 2016-09-24T16:00:00+00:00

CORING THE BLACK DYKE: JULY 30, 2003, NO. 7 SHAFT, KLOOF GOLD MINE, SOUTH AFRICA

Before we left, however, we had to finish one last project. I had been working with Arnand van Heerden at Kloof Mine. Arnand had been the geologist who helped us at the no. 4 shaft with the David Suzuki film and he and his wife, Renee, had become good friends when our field teams were living at the Glenharvie house. Over the past two years he had guided us to some of the best (and most arduous) fracture-water intersections we sampled. The data recovered from Kloof Mine were fairly representative for the three-plus-kilometer-deep fracture-water intersections of that region.9 At these depths, where the water temperatures were about 60°C, sulfate-reducing, thermophilic bacteria appeared to dominate the fracture’s microbial population, but there were no methanogens. Noble gas and cosmogenic isotopic analyses yielded subsurface residence ages ranging from 30 to about 100 million years.10 According to radiolytic theory, O2 and H2O2 were produced, yet we never detected either in the fracture water. One possible explanation was that these oxidants converted the sulfide in the rock matrix to sulfate that diffused into the fracture zone, where it was consumed by the SRBs. But to test this hypothesis we needed core from rock adjacent to fluid-filled fracture. Another question that such a core could answer was, where were all the microorganisms living? Were they concentrated in the fracture water, on the fracture surface, or within the tiny pores of the rock matrix? But no sane mine geologist intentionally plans to core into a high-pressure fracture-water zone, because of its potential danger. Despite Arnand’s best efforts, he had failed to convince his boss at the no. 4 shaft to undertake the drilling operation.

That May, Arnand had been promoted to chief geologist at the no. 7 shaft, the mine overlooking the Crocodilian Estates. He began to deftly manage his drill rigs so that one could be used to collect a core for us. Arnand told me that the chances were 95% of getting core from a high-pressure fracture zone near the Black Dyke in the no. 7 shaft near the end of July. We knew the risks involved in trying to predict when one would get a core in a working mine, but we had been there before. Our oldest water samples with the highest H2 concentrations had been collected nearby two years earlier from the same fracture zone, and if the cores from the same fracture zone contained microbial life, then they would have been at least 100 million years old. The locations of the fractures were known and the distance to the fracture small, only twenty yards. Being the chief geologist, Arnand had control of the coring rig and could set the drilling pace. The miners had already penetrated the fracture zone, cemented, and tunneled through it two years earlier, but Arnand said he could back up the rig into a cubbyhole away from the tunnel and drill at an angle to intersect the fault far enough away from the grouting to guarantee no contamination.

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