Really, any signal would do; just a single fluorescent molecule was all anyone was asking for.
As he looked over data from a collaborator at a recent meeting, Andy Fisher knew that even a tiny blip would signify a critical discovery in the ever-changing landscape of deep subsurface biosphere science, but he wasn't allowing himself to get his hopes up.
Fisher is a Professor of Hydrology at the University of California Santa Cruz and a member of the executive committee of the Center for Dark Energy Biosphere Investigations (C-DEBI). C-DEBI is a National Science Foundation-funded consortium of scientists committed to the exploration of life beneath the seafloor, a new biological frontier. The existence of subsurface microbial life has thrown models of the planet’s biosphere into chaos – estimates of intra-terrestrial life range widely from 0.6% to 30% of the planet’s total biomass.
Whatever the ultimate proportion of deep life turns out to be, the very presence of such hardy organisms is remarkable: habitats within the Earth’s crust experience crushing pressures and a torturous range of temperatures and chemical cocktails. But as with other extreme corners of the planet, whenever there are nutrients and water, life finds a way. The delivery of these critical ingredients, however, is a mystery: are subsurface environments hydrologically connected? How might nutrients – and microbes – move between different niches?
Fisher has spent the last 16 years working to find out. But when your field site is about 2.5 kilometers beneath the surface of the ocean, performing detailed hydrological measurements isn’t so easy.
On land, scientists hoping to learn about the subsurface exchange of water drill holes in a grid network over a few dozen square meters. They inject tracer molecules (inert gases or fluorescent particles, for example) into the groundwater at one hole and watch as they appear at the others, allowing for the reconstruction of the subsurface flow regime.
In replicating this sort of experiment within deep-sea volcanic rock, Fisher was embarking on an ambitious experimental program. Over the course of several oceanographic cruises, he and a team of scientists and engineers drilled six holes in the oceanic crust near the Juan de Fuca Ridge in the eastern Pacific Ocean. One of these holes would be the pumping station, into which 500,000 liters of seawater would be flushed each day. The other five were monitoring stations, positioned up to 2.2 kilometers away, and were programmed to suck up ambient seawater periodically to see if chemical tracers were present. It was a far cry from the land-based experiments hydrologists are used to, but it was the team’s best shot at landing a big result.
A subsurface water flow experiment is only as good as your tracers, and Fisher wasn’t leaving anything to chance, using three types of molecules that, if all went according to plan, wouldn’t just reveal water flux, but would also begin to characterize water-rock interactions.
Sulfur hexafluoride was the ace in the hole – an non-reactive molecule that can be detected at a dilution factor of up to 1:100000000. Given the huge distances between monitoring stations, with water spreading in three dimensions over hundreds or thousands of meters, such a low detection limit was a promising feature. Fisher also used metallic salts – erbium chloride and cesium chloride – that could react with rock surfaces during their subsurface journey. But what he lost in potential signal, Fisher gained in environmental characterization, since varying levels of salt detection could point to preferential reactions and mineralogical information. Finally, there were the fluorescent microspheres that would glow under the right type of light, providing a sense of how particles of different sizes move through the rock.
“This was the first step in learning about the hydrologic regimes of the oceanic subsurface,” Fisher recalls thinking. “This is the first time anyone’s ever done this, and it was never going to be easy.”
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When Fisher ultimately did see a signal of water transfer between different drill holes, he immediately appreciated the significance. “This is no small thing,” he says, “and we showed that these two parts of the crust are connected. We certainly thought this would be the case, but to actually show that it’s true is really important.”
The subsurface connectivity could have wide-reaching implications: underground environments are astrobiologists’ best hope for extraterrestrial life, and the fluid infused rock habitat could be a window into Mars, Europa, or the early Earth.
There’s still plenty of work to be done – monitoring stations must be recovered from the seafloor to analyze the water samples that have been collected – but the nutrient fluxes point to a vast and dynamic biosphere, a hidden world that Fisher and his C-DEBI colleagues are eager to explore.
