Ancient microbial activity recorded in fracture fillings from granitic rocks (Äspö Hard Rock Laboratory, Sweden)

Fracture minerals within the 1.8‐Ga‐old Äspö Diorite (Sweden) were investigated for fossil traces of subterranean microbial activity. To track the potential organic and inorganic biosignatures, an approach combining complementary analytical techniques of high lateral resolution was applied to drill core material obtained at ?450 m depth in the Äspö Hard Rock Laboratory. This approach included polarization microscopy, time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS), confocal Raman microscopy, electron microprobe (EMP) and laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). The fracture mineral succession, consisting of fluorite and low‐temperature calcite, showed a thin (20–100 μm), dark amorphous layer lining the boundary between the two phases. Microscopic investigations of the amorphous layer revealed corrosion marks and, in places, branched tubular structures within the fluorite. Geochemical analysis showed significant accumulations of Si, Al, Mg, Fe and the light rare earth elements (REE) in the amorphous layer. In the same area, ToF‐SIMS imaging revealed abundant, partly functionalized organic moieties, for example, C_xH_y⁺, C_xH_yN⁺, C_xH_yO⁺. The presence of such functionalized organic compounds was corroborated by Raman imaging showing bands characteristic of C‐C, C‐N and C‐O bonds. According to its organic nature and the abundance of relatively unstable N‐ and O‐ heterocompounds, the organic‐rich amorphous layer is interpreted to represent the remains of a microbial biofilm that established much later than the initial cooling of the Precambrian host rock. Indeed, δ¹³C, δ¹⁸O and ⁸⁷Sr/⁸⁶Sr isotope data of the fracture minerals and the host rock point to an association with a fracture reactivation event in the most recent geological past.     

Abundance and Diversity of Biofilms in Natural and Artificial Aquifers of the Äspö Hard Rock Laboratory,Sweden

Six cores were drilled and retrieved from 186-m depth in the Äspö Hard Rock Laboratory (HRL) tunnel to investigate whether indigenous biofilms develop on fracture surfaces in groundwater-conducting aquifers in granitic rock. A clone library was constructed from fracture surface material (FSM), for community composition analysis. Quantitative polymerase chain reaction (qPCR) was applied to quantify gene copies using the 16S rRNA gene for domain Bacteria and the adenosine-phosphosulfate reductase gene (apsA) for sulfate-reducing bacteria (SRB). Results were compared with three groundwater systems with biofilms in laminar flow reactors (LFRs) at 450-m depth in the Äspö HRL. The total number of cells, counted microscopically, was approximately 2?×?10⁵ cells cm^–2 in the LFR systems, consistent with the obtained qPCR 16S rRNA gene copies. qPCR analysis reported ～1?×?10² up to ～1?×?10⁴ gene copies cm^–2 on the FSM from the drill cores. In the FSM biofilms, 33% of the sequenced clones were related to the iron-reducing bacterium Stenotrophomonas maltophilia, while in the LFR biofilms, 41% of the sequenced clones were affiliated with the genera Desulfovibrio, Desulforhopalus, Desulfomicrobium, and Desulfobulbus. The community composition of the FSM biofilms differed from the drill water community, excluding drill water contamination. This work reports significant numbers of microorganisms on natural hard rock aquifer fracture surfaces with site-specific community compositions. The probability that biofilms are generally present in groundwater-conducting aquifers in deep granitic rock is consequently great.     

Long-term history of land-use and vegetation at Femtingagölen — a small lake in the Småland Uplands, southern Sweden

Pollen analysis was carried out on gyttja from the small lake Femtingagölen in the Småland Uplands, southern Sweden. The interpretation of the pollen diagram focused on land-use history and comparisons were made to archaeological and historical information from the area. An absolute chronology, based on AMS dates from terrestrial plant macrofossils, was complemented by inferred dates. The pollen analytical data suggest interference with the woodland cover from ca. 1700 B.C. onwards. Intensified grazing and forest clearances resulted in semi-open pastures between ca. A.D. 400–600 which was followed by forest regeneration (chronology based on AMS ¹⁴C dates and cross-correlation with other well dated profiles). The landscape became more open again between A.D. 800 and 1400. Animal husbandry was complemented by small-scale shifting cultivation during the Iron Age. Permanent arable fields were probably not introduced until the Late Iron Age or the Middle Ages. Hordeum and Triticum were grown during the Iron Age, Hordeum, Triticum, Secale and Cannabis sativa during the Middle Ages and early Modern time, and Hordeum and Avena in the recent past. Sandy and silty soils, where stone clearance was not necessary, have probably been used for cereal growing in prehistoric and historic time.