Sulphate extraction for isotopic analysis
Sulphate was extracted from bedrock samples following the method of Wynnet al. (2008). 300 mg aliquots of bedrock sample were dissolved in 1.5 ml of 4M HCl to yield at least 350 μg of BaSO4for both δ34S-SO4 and δ18O-SO4. The solution was filtered using mixed cellulose 0.45 μm filters into 2 ml Eppendorf polypropylene tubes.
Rainfall and dripwater samples were prepared for analyses following the method in (Wynn et al. , 2015). Resin-filled syringes were prepared by mixing a dry resin component with MilliQ water. This uninitialized resin was poured into 5 ml polypropylene syringe tubes. The resin was then initialised by dripping through 10 ml of 1M HCl and then rinsed using 40 ml of MilliQ water. Aliquots of dripwater and rainfall were taken with sufficient to yield 350 μg of BaSO4 for both δ34S-SO4 and δ18O-SO4. Each aliquot was then passed through a syringe tube filled with 50 W-X8 resin to remove the cations, and through a syringe tube filled with AG2-X8 to load anions from the solution onto the resin. The anion resin was then eluted with three sequential 0.5 ml aliquots of 1M HCl, and the resulting solution collected in 2 ml Eppendorf tubes.
For both bedrock and water samples, 0.2 ml BaCl2 was added to each tube and thoroughly mixed. Solutions were refrigerated for ≥ 48 hours to allow the BaSO4 to precipitate. The BaSO4 precipitate was separated from the eluant by centrifuging for 20 minutes at 3500 rpm and rinsed three times with MilliQ water. Once the solution pH was neutral (tested with pH paper), the samples were dried at 40°C for ≥ 48 hours to evaporate the remaining water, leaving behind the dried BaSO4 pellet.