Table of Contents

• Coverage
• Dataset Description
  • Methods & Sampling
  • Data Processing Description
  • BCO-DMO Processing Description
  • Problem Description
• Data Files
• Supplemental Files
• Related Publications
• Parameters
• Instruments
• Deployments
• Project Information
• Funding

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 February 3-8, 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.

The measured delta18O and Delta'17O values were corrected to VSMOW/SLAP scale using a least squares regression through the accepted values for the triple oxygen isotope composition of air O₂ and the IAEA silicate standards UWG-2 and NBS-28 (Wostbrock et al., 2020).

- Imported "Messinian marine sulfate oxygen isotope data.xlsx" into the BCO-DMO system
- Renamed parameter names to remove special characters to comply with BCO-DMO naming conventions
- Replaced all lat and lon values (appeared in multiple formats) with corresponding values in decimal degrees format
- Submitter provided a list of edits for "Identifier_1" and "Estimated_Height"; edits were made on a cell by cell basis
- Exported file as "960575_v1_messinian_sulfate.csv"

NSF Award Abstract:

Marine sediments contain a record of climate evolution and how Earth's surface has changed over the last 125 million years. Part of this story is locked up in marine barite (BaSO4), a mineral that precipitates from seawater and contains sulfur and oxygen. Through studies of the stable isotopes in seawater sulfate (SO4), changes in biogeochemical processes and variations in other chemical cycles that are important for life on Earth have been determined. However, due to the properties of this sulfate and its ability to incorporate information from processes as diverse as climate, microbial metabolism and weathering, interpretation of traditional oxygen isotope (18O/16O) and sulfur isotope (34S/32S) measurements commonly result in interpretations that are not unique. This research explores a new, independent means for unraveling the information stored in marine barite via the addition of a previously difficult-to-measure, and hence generally overlooked, isotope of oxygen: 17O. Only recently, with the development of increasingly sensitive mass spectrometers and new laboratory methodologies, have these measurements become possible. The 17O signal locked inside marine barite has the potential to identify the oxygen/carbon dioxide ratio of the atmosphere as well as the intensity of biospheric activity and how these parameters have changed over time, important knowledge for understanding present atmospheric compositions and processes related to global warming. This pilot study analyzes barite in the core tops of ten marine cores to quantitatively evaluate whether barite is a faithful recorder of marine sulfate. To further validate whether 17O can provide a reliable proxy for unraveling the influences of various environmental processes, analyses of samples from additional cores and down core of the those analyzed in the core top study will be carried out.

This research further develops and explores the potential of using marine barite to construct a reliable record of the 17O fingerprint of seawater sulfate over the last 125 million years (i.e., from the Cretaceous through the Cenozoic). If successful, the work, in combination with the results of more commonly measured oxygen (18O/16O) and sulfur (33S/32S and 34S/36S) isotope ratios, has the potential to provide insights into the O2/CO2 composition of the atmosphere over time and activity of Earth's biosphere. Although previously 17O measurements of marine barite were difficult to make and had high uncertainties, this limitation has now been overcome by advances in mass spectrometry and the development of laboratory procedures that are tuned to increase the analytical precision of 17O. Goals of the research are to measure 17O in carefully selected barite samples from sediment cores collected from the floor of the Pacific Ocean and quantify the offset, if any, between modern core-top barite and contemporaneous water column sulfate. It will also measure down core variations of 17O to examine variations in the isotope signature with time and compare these with the results from co-existing pore waters to examine possible diagenetic effects. The result will be validation of this new proxy in the marine record. Samples of barite will be extracted from sediment from the tops of ten cores, using acid leaching and other separation techniques to remove all oxygen-bearing phases but barite. The resulting barite will be checked for purity using techniques including scanning and analytical electron microscopy. Preparation of samples for isotope work will include fluorination and analysis on a high resolution isotope ratio mass spectrometer.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.