Mat 253

Half of each quartz fiber filter was cut into pieces for δ³⁴S analysis. They were extracted into 50 mL of ultrapure water and sonicated for 15 min, and then repeated three times. The solution was then filtered with 0.45 μm syringe filters. The filtrate was acidified to a pH < 2 by adding HCl solution and then trapped as barium sulfate (BaSO₄) by adding droplets of supersaturated BaCl₂ solution. The BaSO₄ precipitation was filtered using 0.22 μm acetate filters 24 h later. After heating in an oven with step-wise heating at 200 °C for 2 h, 400 °C for 2 h, and 850 °C for 4 h, 340–450 μg BaSO₄ were weighted and wrapped in tin capsules. The tin cups were crushed into a small size and placed into the isotope ratio mass spectrometer (MAT253, Thermo, Waltham, MA, USA). The δ³⁴S of BaSO₄ was standardized with laboratory barite standards (referenced to NBS-127 and IAEA SO-5), and reported to the international sulfur isotope standard V-CDT [22 (link)].
The δ³⁴S of SO₄²⁻ was calculated by Equation (1):
where the isotopic ratio of (³⁴S/³²S)_standard = 0.044. Overall uncertainty for δ³⁴S_sulfate values reported here were ±0.2‰ [22 (link)].

All sulfur species - both reduced and oxidized moities - were converted to silver sulfide (Ag₂S) (see Johnston et al., 2007 (link); Leavitt et al., 2014 (link)). Following this, samples were fluorinated under 10X excess of F₂ headspace at 250°C, generating analyte sulfur hexafluoride (SF₆). SF₆ was then purified cryogenically via distillation at −117°C and chromatographically on a 6-foot molecular sieve 5 Å column coupled to a 6-foot HayeSep Q 1/8-inch column, with a TCD for detection and quantification. Purified SF₆ was measured as ${\text{SF}}_5^{+}$ (m/z of 127, 128, 129, and 131) on a Thermo Scientific MAT 253 (1σ: δ³⁴S ± 0.2 ‰, △³³S ± 0.006 ‰, △³⁶S ± 0.2 ‰).

Soil WHC was measured by repeatedly saturating soils (20 g fresh soil) with deionized water for 2 h, draining in a funnel with an ash-free cellulose filter paper for 8 h, and then drying in 105°C for 24 h. Soil pH was measured at a 1:2.5 ratio of air-dried soil to deionized water by a pH electrode (Leici, Shanghai, China). Soil organic carbon (SOC) and total nitrogen (TN) were measured using an elemental analyzer (MAT-253, Thermo Fisher Scientific, USA) with ball-milled dry soil. Soil clay content (Clay) was measured by the hydrometer method [37 (link)].
We used the chloroform fumigation extraction (CFE) method to estimate the soil microbial biomass [38 (link)]. Briefly, 20 g of fresh samples were weighed in a 100-ml beaker, placed in a desiccator, and fumigated in the dark with alcohol-free chloroform for 48 h. Dissolved organic carbon (DOC) and nitrogen (DON) were extracted using 0.5 M K₂SO₄ from fumigated and nonfumigated soils. The difference in DOC between the fumigated and nonfumigated soils was considered as the microbial biomass carbon with a conversion coefficient of 0.45.

For δ¹³C analysis, approximately 150 μg samples were reacted with ~103% phosphoric acid at 70 °C in a Kiel IV carbonate device connected to a Thermo Scientific MAT 253 mass spectrometer. The carbon isotopic compositions are reported in the standard delta (δ) notation as permil (‰) deviations from Vienna PeeDee Belemnite (V-PDB), with external precision of ~0.05‰ (1σ) based on duplicate analyses of an internal standard. For analysis of ⁸⁷Sr/⁸⁶Sr ratio, carbonate powders of ~120 mg were dissolved in 30% acetic acid at room temperature to avoid dissolution of the non-carbonates. The solution then was centrifuged, evaporated and re-dissolved in 2.5 N HCl, standard cation-exchange procedures were performed to purify Sr from matrix ions and ⁸⁷Sr/⁸⁶Sr ratios were analyzed using a Thermo Scientific Triton thermal ionization mass spectrometer, following methods outlined in Lin et al.^{45 (link)}. The reported ⁸⁷Sr/⁸⁶Sr ratios were corrected for instrumental mass fractionation using a ratio of ⁸⁶Sr/⁸⁸Sr = 0.1194. Trace metal concentrations (Mn, Sr) were measured using an ICP AES instrument with a reproducibility of ±10% (2σ).

The stable isotopic composition of total organic carbon (δ¹³C) and total nitrogen (δ¹⁵N) in soil, bedrock, and loose exposed rock samples was measured in triplicates by isotope ratio mass spectrometry (IRMS). A MAT 253 (Thermo Fisher Scientific) IRMS equipment was used, following the analytical methods of the US Geological Survey (Révész et al., 2012 ). About 0.5 g of dried sample was grounded and homogenized using autoclave-sterile mortar and pestle, and carbonates were removed with HCl (3 M). After 24 h of equilibration, pH was adjusted to neutral values with ultrapure water, and the sample then dried in an oven (50°C) until stable weight (~48 h). The δ¹³C and δ¹⁵N ratios were reported in the standard per mil notation (‰) using three certified standards (USGS41, IAEA-600, and USGS40), with an analytical precision of 0.1‰. The elemental content of both TOC and TN was measured as percentage of dry weight (%) during stable isotope measurements, using Flash HT Elemental Analyzer (Thermo Fisher Scientific).

Dissolved gas samples were collected with a bladder pump. Evacuated crimp-top glass serum bottles with butyl septa and preserved with mercuric chloride were filled by piercing the septa with a needle. Gas composition was determined in the Applied Geochemistry Laboratory at the University of Calgary by gas chromatography. The gas dryness parameter is defined as the ratio between the concentrations of methane and those of higher n-alkanes. The isotopic composition of methane and carbon dioxide was analyzed in the Isotope Science Laboratory at the University of Calgary on a ThermoFischer MAT 253 isotope ratio mass spectrometer (IRMS) coupled with a Trace GC Ultra and GC Isolink (ThermoFisher) and reported relative to V-PDB for δ¹³C and V-SMOW for δ²H. The precision for carbon isotope analyses was higher than ±0.5‰ for hydrocarbons, ±0.2‰ for carbon dioxide, and ±2‰ for δ²H of methane.

Stable isotopic composition of total nitrogen (δ¹⁵N) and organic carbon (δ¹³C) was measured by Isotope-Ratio Mass Spectrometry (IRMS), using a MAT 253 (Thermo Fisher Scientific) and applying the USGS methods⁵² , as described elsewhere^{53 (link)}. Briefly, the homogenized, grounded samples were decarbonated with HCl and then dried (50 °C) after adjustment to neutral pH until constant weight. An analytical precision of 0.1‰ was determined by using three certified standards for carbon/nitrogen (USGS41, IAEA-600 and USGS40). The content of total nitrogen (TN) and organic carbon (TOC) was measured with an elemental analyzer (Flash HT, Thermo Fisher Scientific) during the stable isotope measurements.