Timing and origin of natural gas accumulation in the Siljan impact structure, Sweden

Fractured rocks of impact craters may be suitable hosts for deep microbial communities on Earth and potentially other terrestrial planets, yet direct evidence remains elusive. Here, we present a study of the largest crater of Europe, the Devonian Siljan structure, showing that impact structures can be important unexplored hosts for long-term deep microbial activity. Secondary carbonate minerals dated to 80 ± 5 to 22 ± 3 million years, and thus postdating the impact by more than 300 million years, have isotopic signatures revealing both microbial methanogenesis and anaerobic oxidation of methane in the bedrock. Hydrocarbons mobilized from matured shale source rocks were utilized by subsurface microorganisms, leading to accumulation of microbial methane mixed with a thermogenic and possibly a minor abiotic gas fraction beneath a sedimentary cap rock at the crater rim. These new insights into crater hosted gas accumulation and microbial activity have implications for understanding the astrobiological consequences of impacts.


Additional biodegradation signatures interpreted from gas compositions
The lack of 13 C-enrichment in C4 compared to C3 can be due to that propane is biodegraded most rapidly of the hydrocarbons, already at slight levels of biodegradation 1 . n-alkanes are preferentially utilized during biodegradation of oil and gas 2 . The high i-C4/n-C4 in the Siljan gas (Supplementary Data 8, as well as higher i-C5 than n-C5 [below detection]) also suggests significant microbial utilization of primary thermogenic gas, and analogously does high neo-C5/i-C5, because neo-C5 is relatively resistant to biodegradation 3 .

Biomarkers in calcite, bitumen and oils
Bimodal n-alkane distribution indicates that the Solberga bitumen formed by mixing of more than one charge of oil at various degrees of degradation 4 suggesting mobilization and degradation of hydrocarbons at several events. Furthermore, presence of a large hump of unresolved complex mixture (UCM) of hydrocarbons in seep oil and bitumen and preferential removal of almost all the n-alkanes and alkylcyclohexanes in seep oil are indicative of moderate to severe biodegradation of these materials in the limestone 4 . The irregular n-alkane distribution in combination with humps of UCM in the granite fractures (CC1:539; 608) indicate more severe biodegradation of organic matter (Fig. 7). Poor straight chain carbons and S&R-hopanoid isomers in hydrocarbons of the 13 C-rich calcite coatings in granite indicate moderate to severe biodegradation. However, the lack of other carbon sources in the granite fractures lead to higher degree of microbial utilization of bitumen and oil stains in the granite fractures (Fig. 7). This is in line with calculated carbon preference index values indicate influence for migrated seep oil/bitumen similar to Bitumen 1 [ref 4 ], also deep within the granite (Fig. 7, Supplementary Data 7). These levels of degradation are corroborated by the very small amounts of C29 25-norhopane in the seep oils and bitumen 4 .

Supplementary note 2. Summary and interpretation of previous gas data from deep Siljan wells.
Previous investigations in deep boreholes within the central granite dome reported isotopically heavier methane (δ 13 CCH4: -35 to -15‰ in the superdeep Gravberg-1 well) interpreted as abiotic or thermogenic 5 . Significantly isotopically lighter methane dissolved in groundwater (δ 13 CCH4 values of -77.9 and -77.6‰) at shallower depth (173-184 m and 443 m), were observed in two other wells in the central dome 6 . 13 C-depletion of such magnitude points to microbial methanogenesis. Methane dissolved in groundwater of mixed origin in a 500 m borehole at Solberga that penetrated both sediments and deeper granitic basement had two distinct δ 13 CCH4 populations: -79‰ and -35‰, interpreted as microbial and thermogenic 6 , respectively, marking contribution of gases of different origin to the aquifer.

Supplementary note 3. Potential microbial processes linked to the preserved fatty acids
The preserved fatty acids n-C12 to n-C18, particularly the odd chain and branched iC15, aiC15, n-C15, 12Me-C16, aiC17, and 12OH-C18 as well as the n-alcohols and the 1-o-nhexadecylglycerol preserved within 13 C-rich, methanogenesis-related, calcite coatings can be tied to fermentation 7 and/or sulfate reduction by bacteria 8 , in line with the S isotope record in pyrite in the fractures. Even though methanogenesis is commonly attributed to archaea, which do not produce phospholipid fatty acids, recent studies highlight bacterial methane production through N-fixation together with CO2 9 , which also is a plausible process in the fractures. Furthermore, soil-derived fungi are able to produce methane through biodegradation in relationship with methanogens 10 , but also without 11 , and high diversity of fungi have been detected in the continental crust 12,13 . Fungi produce phospholipids and other FA than bacteria, but potential presence of fungi at Siljan is yet unexplored. The particular FA detected cannot be used as diagnostic markers for methanogens, in contrast to the heavy δ 13 Ccalcite-values.

Supplementary note 4: Influence of limestone derived DIC and micro-scale isotope distillation
The limestone in VM-1 and Solberga-1 has δ 13 C values of 0 to+2‰ 14 and shallow wells in the aquifer contain HCO3concentrations of up to 400 mg L -1 , compared to 10-80 mg L -1 in the granitic rock aquifer 6 . In the limestone aquifer, it is more likely that equilibrium between DIC and the wall rock dominates. Methanogenesis through carbonate reduction of limestone derived DIC would require smaller 13 C enrichment than utilization of 13 C-poor DIC formed by oxidation of organic matter to reach the heavy δ 13 Ccalcite values observed. This means that microbial carbonate reduction may to some degree have utilized limestone derived DIC, at least in the limestone aquifer. However, a system with abundant DIC would likely dilute and mask methanogenesis-related δ 13 C signatures in the produced carbonates. Nevertheless, there is evidently substantial 13 Ccalcite and 13 CCO2 enrichment in the sedimentary aquifer. It has been shown in deep energy-poor fracture systems that isotopic fractionation and distillation can occur in microscale in biofilms resulting in isotopic compositions of produced minerals that are very different from the bulk groundwater. At Äspö in Sweden, pyrite precipitated over a 17yr period from a deep sulfate-rich water with relatively constant δ 34 Ssulfate of 20-30‰ had δ 34 S values of -47.3 to +53.3‰ 15 and at nearby sites, calcite had extreme δ 13 C variation (-125 to +37‰) compared to the corresponding deep groundwater δ 13 CDIC (-17±3‰) 16 . We propose that similar kinetic microbial processes have locally influenced the DIC signature in the Siljan aquifer, particularly in pore space infiltrated by gases, bitumen and seep oils, as shown by spatial relation of these features to significantly 13 C-rich calcite (Fig. 3).

Moderately 13 C-depleted calcite
The δ 13 Ccalcite values of c. -39 to -35‰ at 175 m depth in the crystalline bedrock is proposed to reflect AOM. These δ 13 Ccalcite values overlap with methane of abiotic, thermogenic and microbial origin [17][18][19] and dilution by other C sources may have occurred. Moderately depleted δ 13 Ccalcite values of around -30 to -20‰ are overlapping with values of abiotic methane, but can, nevertheless, not be fully distinguished from bicarbonate formed following microbial utilization of seep oils with δ 13 C of -30‰ 20 .

S-enriched pyrite
During a closed system Rayleigh fractionation cycle the δ 34 SSO4 values will increase gradually as the sulfate pool is exhausted and consequently pyrite will show a large span in δ 34 Spyrite values and increase from core to rim of the crystals 21 . The large δ 34 Spyrite variability spanning as much as almost 120‰ overall in the granite fractures (from -41.9‰ to +78.0‰, Supplementary Figure 1d) marks these MSR-related reservoir effects in the fracture system (Supplementary Figure 5b).

Supplementary note 6. Tectonic context of the fracture reactivation events.
The significantly higher 87 Sr/ 86 Sr values of the Late Cretaceous to Neogene calcite compared to the Late Neoproterozoic-Early Paleozoic calcite (Fig. 2) are proposed to result from increased 87 Sr with time due to beta decay of 87 Rb in the rocks, and prolonged water rock interaction. This increase is, as expected, of largest magnitude in calcite lining Rb-rich granitic wall rock. The higher 87 Sr/ 86 Sr of the calcite overgrowths is a relative timing indicator that is in agreement with the U-Pb dating of two temporally separated calcite populations. Although there are a few examples of thin slickenfibre calcite that may be formed during strike-slip movement (e.g. Fig. 2i), the euhedral overgrowths of methane-related calcite on Late Neoproterozoic-Early Paleozoic precursors in open fractures suggest predominant formation in an extensional stress regime. Whether the re-activation of the fracture system at Siljan can be related to tectonic events in the far-field requires temporal linkage to such events. Far-field candidates include: Alpine/Pyrenean crustal shortening documented from calcite formation in faults on the British Isles (55 to 25 Ma 22 ), in the Helvetic Alps (35-20 Ma) 23 and in the Early Ordovician platformal limestone at Öland, Sweden (65-55 Ma 24 ); northern Europe graben system formation and major fluid circulations (35-30 Ma 25 ); opening of the North Atlantic 26 ; and Late Cretaceous-Sybhercynian tectonics/faulting in NW Europe 27 . After the North-East Atlantic breakup at the Paleocene-Eocene transition, three phases of uplift by erosion of Upper Cretaceous to Oligocene sediments affected the western Fennoscandian shield 28 , that may have induced extensional fracture reactivation in the basement. In summary, there are tectonic events in the far-field and uplift events that temporally coincide with the ages of the methane related calcite at Siljan. Methane cycling can thus be related to these fracture reactivation events which are more than 300 Myr younger than the impact, for which 40 Ar/ 39 Ar laser dating of melt breccia discloses a late Devonian age (380.9±4.6 Ma 29 ).  and one peak marked with square is assigned to an inorganic ion at m/z 112.92 (Ca2O2H). Negative ToF-SIMS spectra of (e) C16:0 fatty acid (m/z 255.2), (f) C18:1 fatty acid (m/z 281.2) and (g) C18:0 fatty acid (m/z 283.2). A second type of PAH detected in the sample, of probable modern origin, is shown in Supplementary Figure 4. Circulation of hydrocarbons of sedimentary origin in the deep granitic fracture system is thus also supported by the detected PAH (Fig. 7) which resemble PAH from seep oil in the sediments cropping out at Solberga 30 . Negative ToF-SIMS ion image overlay of CxH peaks (red; added C2H, C4H, C6H, C8H, C10H and C12H ) and alkyl benzene sulfonates (ABS green; added C27H47SO3, C28H49SO3, C29H51SO3, C30H53SO3 and C31H55SO3). The latter can be assigned to alkylbenzene sulfonates which are probably derived from drilling additives. The ion images of PAHs, CxHs fragments (presumably produced by PAHs) and alkylbenzene sulfonates show that these compounds have different spatial distribution indicating two different sources for the PAHs and sulfonates (Supplementary Figure 3). Additionally, the PAHs have more typical geological peak pattern in the spectra with decreasing intensity with increasing mass compared with the alkyl benzene sulfonates. As water and not oil was used during drilling no additional PAHs should have been added to the system.