Abstract
Recent studies reveal that organisms from all three domains of life—Archaea, Bacteria, and even Eukarya—can thrive under energy-poor, dark, and anoxic conditions at large depths in the fractured crystalline continental crust. There is a need for an increased understanding of the processes and lifeforms in this vast realm, for example, regarding the spatiotemporal extent and variability of the different processes in the crust. Here, we present a study that set out to detect signs of ancient microbial life in the Forsmark area—the target area for deep geological nuclear waste disposal in Sweden. Stable isotope compositions were determined with high spatial resolution analyses within mineral coatings, and mineralized remains of putative microorganisms were studied in several deep water-conducting fracture zones (down to 663 m depth), from which hydrochemical and gas data exist. Large isotopic variabilities of δ13Ccalcite (−36.2 to +20.2‰ V-PDB) and δ34Spyrite (−11.7 to +37.8‰ V-CDT) disclose discrete periods of methanogenesis, and potentially, anaerobic oxidation of methane and related microbial sulfate reduction at several depth intervals. Dominant calcite–water disequilibrium of δ18O and 87Sr/86Sr precludes abundant recent precipitation. Instead, the mineral coatings largely reflect an ancient archive of episodic microbial processes in the fracture system, which, according to our microscale Rb–Sr dating of co-genetic adularia and calcite, date back to the mid-Paleozoic. Potential Quaternary precipitation exists mainly at ~400 m depth in one of the boreholes, where mineral–water compositions corresponded.
Highlights
The deep continental biosphere is a potentially vast microbial habitat, and it has been estimated that 2–19% of the global biomass may be accumulated within it [1]
The KFM01D samples and the shallowest KFM24 samples featured fine-grained subhedral calcite aggregates, the KFM02A calcite occurred as thin slickenfibres, and the KFM08D calcite as equant crystals on euhedral quartz
KFM24 samples from 379 to 414 m, euhedral calcite crystals and cubic, octahedral, and pyritohedral pyrite were observed on top, and intergrown with euhedral quartz, and occasionally, euhedral adularia
Summary
The deep continental biosphere is a potentially vast microbial habitat, and it has been estimated that 2–19% of the global biomass may be accumulated within it [1]. Observations have, been limited to a few sites with deep boreholes or to underground constructions, such as research tunnel facilities and deep mines. Studies of modern microbial activity in the continental deep biosphere include, but are not limited to, works in South. African mines [2,3,4], Canadian mines [5], deep research boreholes, and underground facilities in Sweden [6,7,8,9,10], Finland [11,12,13,14,15], and Japan [16,17], showing a high degree of site-specificity on the microbial community profiles. The metabolisms tend to shift from dominantly heterotrophic, at shallow depth in the crust, to autotrophic at greater depth [18,21,22]
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