Active Sulfur Recycling in Billion-year-old Water from Canadian Shield Rocks

Life at Earth’s surface relies on the sun to provide energy. Plants and other photosynthetic organisms harvest the energy in sunlight to produce sugar as a first step in powering our planet’s complex ecosystems. Underground, the sun’s energy is preserved as coal, oil, and natural gas, and many organisms rely on such fossilized energy to survive.

However, some places on Earth have not felt the sun’s influence for millions, or even billions, of years. Any life surviving in these deep ecosystems, therefore, requires alternative sources of energy. In water samples collected from deep below Scandinavia and South Africa, researchers found that microbes in these ecosystems survive in fracture waters isolated for millions of years by coupling molecular hydrogen oxidation to sulfate reduction [1,2]. The hydrogen is produced by geochemical reactions such as serpentinization and radiolysis [3], but the source and sustainability of sulfate are still unknown.

In a new paper published recently in Nature Communications, DCO’s Long Li (University of Alberta, Canada), Barbara Sherwood Lollar (University of Toronto, Canada), and colleagues examined samples of billion-year-old fracture fluids deep in the Canadian Shield to trace the source and production mechanism for the dissolved sulfate. This allowed them to assess the sustainability of sulfate to support a deep ecosystem [4].

Funded by Canada’s Natural Sciences and Engineering Research Council as well as the Deep Carbon Observatory, the samples used in this study came from the Kidd Creek copper-zinc-silver mine in Ontario, Canada, which is currently the deepest base metal mine in North America. The water trapped in fractures ~2.4km below the surface has been there for more than a billion years and contains molecular hydrogen and methane [5]. Thus, some of the key molecules needed to power life in the deep are present in these ancient fluids.

Li et al set out to address the other half of the question; do these fluids also contain sulfate? If they do, where does the sulfate come from, and how sustainable is it?

The authors did find sulfate in the fluids from Kidd Creek mine. They analyzed the isotopic composition of the sulfur, which showed that the dissolved sulfate was characterized by a sulfur isotope mass-independent fractionation. This fractionation has only been observed in minerals formed before 2.4 billion years ago. By comparing multiple isotope compositions of the dissolved sulfate and the sulfide minerals in the 2.7 billion-year-old ore rocks, they demonstrate that this dissolved sulfate originated from sulfide minerals in the ore rocks. The sulfur in sulfide was transformed into sulfate as a result of reactions with radiolysis products. This suggests that geochemical fluid-rock interactions can provide, steadily over geological time scales, all the components necessary to power a deep biosphere.

Through modeling the concentrations of sulfate and hydrogen in the fluids, they suggest that these fracture fluids could support 100-3,000 cells per liter, similar to the biomass in fracture fluids from South African gold mines.

This has important implications for the extent of the current deep biosphere on Eatth, and the potential for life on other planets. For example, rocks of similar age and mineralogy to those of the Canadian Shield are also found on Mars. While potentially inhospitable to life today on the surface, if these rocks contain liquid groundwater at certain depth, they could also have enough energy to power life.

Image: Sampling ancient water in deep mines. Credit: Gaetan Borgonie and Barbara Sherwood Lollar

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