Dataset: Sulfate triple oxygen isotope signatures in Messinian evaporite and barite from cores collected

Evaporite samples

Evaporite samples from the Rio de Aguas section (37°05′23.2″N 002°06′54.2″W) of the Sorbas Basin in southeastern Spain, were collected in April 2013 and March 2014. Additional evaporite samples were collected from the Monte Grotticelle Formation in the Caltanissetta Basin, Sicily. Samples were drilled from each outcrop, and care was taken to remove weathered surface material (Evans et al., 2015). Evaporite samples from the Tyrrhenian Basin come from Ocean Drilling Project (DSDP) Leg 107, Site 654, Hole A, were collected in February 1986 (Site 654, 1987).

For each sample, ∼100 mg of powdered, homogenized evaporite was dissolved in weak HCl (0.1 mM, 125 mL) on a shaker table at 60 °C overnight. The dissolved sulfate was isolated via chromatography (Le Gendre et al., 2017). Following this method, empty polypropylene SPE tubes with 20 mL volume were packed with 5 g of AG1-X8 anion exchange resin and preconditioned with 3x20 mL of 3 M HCl followed by 3x20 mL of deionized (DI) water. Dissolved SO₄ solutions were loaded onto columns at a rate of 1 mL/min, and subsequently eluted with 44 mL of 0.4 M HCl. To quantitatively precipitate BaSO₄, 1-2 mL of 1 M BaCl₂ solution was added to the collected fraction. Samples were then centrifuged, rinsed 3 times with DI, and dried in a 60 °C oven.

Barite samples

Marine sediments from the Ocean Drilling Project (DSDP) Leg 138, Site 849, Hole D, were collected in June 1991 (Site 849, 1992). These sediments were requested from the IODP repository. Sediments were treated with a sequential leaching procedure to purify barite minerals, which is outlined in previous work (Paytan et al., 1993; Markovic et al., 2016). Following extraction of barite from sediments, the barite was collected onto filter paper and heated at 750 °C in a furnace for 1 h to oxidize highly refractory organic matter. 

Subsequently, the barite was further purified following a sodium carbonate dissolution method (Breit et al., 1985; Von Allmen et al., 2010; Markovic et al., 2016). Afterwards, samples were weighed and added to PTFE vials with a 0.5 M Na₂CO₃ solution in a ratio of 10 mg BaSO₄ to 2 mL of Na₂CO₃ solution. The closed vials containing sample mixtures were sonicated at room temperature for 60 min and then placed in an 80 °C oven for 16 h. After heating, the solution was transferred to a 15 mL falcon tube, and additional 0.5 M Na₂CO₃ solution was added to the vials to react with the precipitates. The sodium carbonate dissolution step was performed three times, and the supernatant was collected in the same 15 mL falcon tube after each heating step. After the third collection, barium chloride was added (10% BaCl₂ in 2 M HCl) to the 15 mL falcon tube until samples reached pH<2, to precipitate BaSO₄. The tubes were centrifuged to collect the BaSO₄ precipitate, and BaSO₄ was rinsed 2 times in 2 N HCl, then 3 times in DI, and then dried at 60 °C.

delta18O

The delta18O composition of purified barite was measured using a high-temperature conversion elemental analyzer (TC/EA) coupled with a Thermo Scientific Delta V Plus isotope ratio mass spectrometer (IRMS), configured in a continuous flow mode.  In short, ∼250±50 μg of clean, dry barite was weighed in triplicate into silver capsules (Elemental Microanalysis; 4x3.2 mm) with AgCl and glassy C additive in an approximately 2:1 mass ratio. Before measurement, weighed sample capsules were dried at 60 °C in a vacuum oven overnight.  The delta18O values were corrected Vienna Standard Mean Ocean Water (VSMOW) scale using the accepted delta18O values for three International Atomic Energy Agency (IAEA) standards included in each run: IAEA-SO5, IAEA-SO6, and NBS-127 (Brand et al., 2009). Samples from each run were corrected for the amount of additive, drift over the course of the analysis, and scale compression.

Delta'17O

The Delta'17O composition of purified barite was measured using a custom-built laser fluorination line, coupled with a Thermo Scientific MAT 253 isotope ratio mass spectrometer (IRMS), as previously described (Cowie and Johnston, 2016). Approximately 5 mg of purified barite was reacted in a pure F2 atmosphere by heating with a 50 W CO2-laser, which liberates O2 along with other fluorinated byproducts. Sample gas was passed through multiple cryofocus steps and an in-line gas chromatograph (GC) before being introduced as pure O2 to a Thermo Scientific MAT 253 gas source isotope ratio mass spectrometer configured in dual-inlet mode. Each delta18O and delta17O were taken as the mean of 4 acquisitions of 10 cycles with a target of 3000-5000 mV on the m/z 34-cup. Delta'17O was calculated from the measured delta18O and delta17O of the O2 gas.