Anomalous ¹⁷O compositions in massive sulphate deposits on the Earth

Abstract

The variation of δ¹⁸O that results from nearly all physical, biological and chemical processes on the Earth is approximately twice as large as the variation of δ¹⁷O. This so-called ‘mass-dependent’ fractionation is well documented in terrestrial minerals^1,2. Evidence for ‘mass-independent’ fractionation (Δ¹⁷O = δ¹⁷O - 0.52δ¹⁸O), where deviation from this tight relationship occurs, has so far been found only in meteoritic material and a few terrestrial atmospheric substances³. In the rock record it is thought that oxygen isotopes have followed a mass-dependent relationship for at least the past 3.7 billion years (ref. 4), and no exception to this has been encountered for terrestrial solids⁵. Here, however, we report oxygen-isotope values of two massive sulphate mineral deposits, which formed in surface environments on the Earth but show large isotopic anomalies (Δ¹⁷O up to 4.6‰). These massive sulphate deposits are gypcretes from the central Namib Desert and the sulphate-bearing Miocene volcanic ash-beds in North America. The source of this isotope anomaly might be related to sulphur oxidation reactions in the atmosphere and therefore enable tracing of such oxidation. These findings also support the possibility of a chemical origin of variable isotope anomalies on other planets, such as Mars⁶.

Main

We have analysed sulphates from laboratory experiments and natural sources (Fig. 1). The δ¹⁸O and δ¹⁷O of seawater sulphate, evaporites, sulphate from microbial sulphate reduction experiments, and sulphates formed from mineral-sulphide oxidation in air or soils, all fall on the mass-dependent terrestrial fractionation line, given by the relationship δ¹⁷O = 0.52δ¹⁸O. The deviation from this relationship, defined as Δ¹⁷O = δ¹⁷O - 0.52δ¹⁸O, is approximately zero ( -0.04 ± 0.05‰, n = 36). The central Namib Desert gypcretes and the Miocene volcanic ash-falls in the western United States, however, possess sulphate δ¹⁸O and δ¹⁷O values that deviate from the terrestrial fractionation line. The sulphate Δ¹⁷O of Namib gypcretes ranges from 0.20‰ to 0.51‰ (Table 1), well outside the experimental error of ±0.05‰. Gypsum and other water-soluble sulphate minerals from Miocene volcanic ash deposits in Nebraska and South Dakota have strikingly large Δ¹⁷O values, up to 4.59‰, in contrast to the zero or slightly positive Δ¹⁷O found in adjacent soil and fluvial horizons (Table 2).

Figure 1: δ¹⁷O versus δ¹⁸O for various sulphates.

a, A fractionation line defined by major sulphate-involving processes on Earth. The seawater sulphate consists of sea water from La Jolla, California, and seawater sulphate adsorbed on the surface of amorphous and poorly crystalline ferric oxides from Red Seamount, Pacific Ocean. Sulphate from the oxidation of mineral-sulphides includes several non-evaporative gypsum minerals from glacial-till deposits in Akron, Ohio, and water-soluble sulphate from surface oxidation of marcasite (FeS₂) exposed to air. The sulphate reduction experiment was conducted by adding organic-rich soil into a sealed tank of La Jolla sea water and allowing time for fractionation of oxygen isotopes by microbial sulphate reduction. Gypsum, other sulphate-bearing evaporitic minerals, and sulphate from final residual solution were collected from two seawater evaporation experiments conducted at 22 °C and 55 °C, respectively. b, Gypcrete and gypsum-bearing soils were collected from different soil horizons and different localities in the central Namib Desert. c, Volcanic ash and associated deposits were from the Miocene Arikaree Group, Nebraska and South Dakota (see Table 1 and Table 2 for details). The straight line in b and c is the terrestrial fractionation line (δ¹⁷O = 0.52δ¹⁸O). The error bar (about ±0.7 for δ¹⁸O) was not plotted on the diagrams. Due to the high correlation between δ¹⁷O and δ¹⁸O during mass spectrometry analysis, the error for Δ¹⁷O is very small (less than ±0.05). Thus, a thin rod with slope of 0.52 would best represent the actual point on the diagram. Delta values are given in SMOW.

Table 1 Isotopic compositions and occurrences of the Namib gypcretes

Table 2 Description of the Miocene volcanic ash-bed deposits

Sulphate minerals with Δ¹⁷O ≈ 0‰ are expected because thermodynamic and kinetic (including biological) processes such as evaporation, mineral-sulphide oxidation (by Fe³⁺ or air O₂), and sulphate reduction generate mass-dependent compositions (Fig. 1). Therefore, sulphate minerals with positive Δ¹⁷O values, such as those found in the central Namib Desert gypcretes and the Miocene volcanic ash-falls in the western United States require a different process. The only documented terrestrial reservoirs with positive Δ¹⁷O are from the atmosphere. Oxidants such as O₃ and H₂O₂ are known to have positive Δ¹⁷O values that range from 1 to more than 25‰ (refs 7,8,9,10). Others, like OH and NO_x, remain to be measured.

Tropospheric O₃ and H₂O₂ in rainwater may transfer their positive Δ¹⁷O values to the product sulphate by in situ oxidation of surface minerals (for example, sulphides). Our measurement of the sulphate produced by marcasite (FeS₂) oxidation in the air yielded Δ¹⁷O ≈ 0‰, indicating that the in situ pathway is not the major source of positive Δ¹⁷O. A more likely source is the wet and dry atmospheric deposition of sulphate produced by atmospheric oxidation of reduced gaseous sulphur compounds. The anomaly can come from atmospheric oxidants such as O₃, H₂O₂ or OH radicals as a result of aqueous or gas-phase S( IV) oxidation. A similar explanation was invoked to interpret positive Δ¹⁷O values (0.20‰ to 1.80‰) observed in aerosol and rainwater sulphate^11,12. Although the mechanisms of these atmospheric processes are still subjects of intensive study, the connection between our Δ¹⁷O-positive sulphate minerals and atmospheric oxidation processes is unequivocal, as shown by the close association with a high flux of atmospheric reduced sulphur compounds in our two reported cases.

Gypcrete soils in the central Namib Desert occur extensively near the coast and gradually thin off and disappear at about 50–70 km from the coast, constituting one of the most extensive gypsum accumulations in Africa. On the basis of δ³⁴S, meteorological, hydrological, and geological information, Eckardt and Spiro¹³ suggest that sulphate in the Namib Desert originates mostly from biologically produced marine sulphur (that is, not derived from sea salt), particularly the oxidation of marine dimethyl sulphide (DMS). This conclusion is also supported by the lack of correlation between gypcrete accumulation and bedrock in this location and the positive correlation between gypcrete accumulation and the proximity to the ocean, a source of DMS. The Benguela Current, which originated in the early Late Miocene (about 10 million years ago) off the west coast of South Africa and Namibia, provides upwelled waters extremely rich in phosphate, nitrate and silica, which support a rich population of phytoplankton¹⁴ and a high DMS concentration in the local atmosphere. According to this interpretation, the anomalous Δ¹⁷O of the Namib gypcretes reflects chemistry associated with atmospheric oxidation of DMS.

Volcanic activity reached its maximum during the Oligocene in western North America¹⁵. Layers of volcanic-associated deposits are exposed throughout the Oligocene/Miocene continental deposits in eastern Wyoming, South Dakota, North Dakota and Nebraska^16,17. The current water-soluble sulphate content is about 0.02% in the Miocene Rockyford ash-bed (sample BNP-SP). It is well known that volcanic ash contains a significant amount of sulphate, occurring as sulphuric acid droplets that react to form gypsum or other sulphate minerals¹⁸. The occurrence of gypsum pseudomorph in the sandy Gering Formation may suggest high sulphate content in overlaid ash-beds. A volcanic origin for the sulphate sulphur is supported by its juvenile δ³⁴S values (Table 2). The large sulphate Δ¹⁷O signatures from these widespread Miocene ash-beds indicate a link between the δ¹⁷O-anomaly and volcanic processes. Some of the sulphate Δ¹⁷O values are more positive than has been observed in sulphate aerosol, rainwater sulphate, and desert gypcrete. We expect the sulphate Δ¹⁷O value in volcanic ash will vary depending on distance from source and the nature of eruption.

Sulphate is one of a few non-labile oxygen-bearing ions (aqua-metal ions or oxo-anions)¹⁹ that, once produced, are resistant to oxygen exchange with other ambient oxygen-bearing compounds (for example, water) under most surface environments. This chemical feature is essential for the preservation of the anomalous Δ¹⁷O signature in sulphate minerals.

The discovery of positive Δ¹⁷O values in minerals formed on Earth has important implications in Earth science. First, this is the first, to our knowledge, demonstration of a transfer of δ¹⁷O anomaly from atmosphere to crustal minerals. At this time, only a few approaches are available for probing ancient atmospheric conditions^20,21,22. Sulphate Δ¹⁷O has the potential to be a tracer for ancient atmospheric ozone activity, chemistry in volcanic plumes, and the sulphur biogeochemical cycle in desert environments, where atmospheric sulphate is a major sulphur component. Second, it is interesting to evaluate recent findings of anomalous Δ¹⁷O for water, sulphate, and carbonate in SNC meteorites relative to coexisting igneous silicate minerals^6,23,24. We document here a larger-magnitude Δ¹⁷O signature in terrestrial sulphate minerals than have been observed in SNC meteorites. Our observations are consistent with an atmospheric chemical origin for the SNC oxygen-isotope systematics. To establish the full implications of the δ¹⁷O anomalies found in terrestrial minerals, however, requires a fundamental understanding of the sulphur oxidation processes occurring in the atmosphere. The measurements of isotopic fractionation factors for the relevant oxidation reactions will permit quantification of the observations, as has been done for many other species³.

Methods

Barite is precipitated from filtered and acidified solutions, which contain water-soluble sulphate from crushed gypcretes, sand crystals, volcanic ash-beds, and fluvial deposits. Molecular oxygen is extracted directly from barite following a newly developed method²⁵, which employs a 30 W CO₂-laser fluorination technique. Fine-grained barite was heated in a BrF₅ atmosphere, and more than 90% of the samples were analysed in duplicate. Samples are often loaded and analysed together with other δ¹⁷O-normal barites in one set. O₂ was run on a Finnigan MAT 251. We found no evidence for mass interference by NF₃ fragments or S compounds in our analyses. The samples with large positive Δ¹⁷O values were purified and reanalysed using a method²⁶ that quantitatively removes these compounds, after initial mass spectrometric analysis. No differences in Δ¹⁷O were observed. The results are highly reproducible and the difference between duplicate analyses is better than ±0.7‰ and ±0.05‰ for δ¹⁸O and Δ¹⁷O, respectively. Most samples give 28–33% O₂ yield during laser fluorination. We found by analysing NBS127 (NIST sulphate standard), seawater sulphate, and a low-δ¹⁸O in-house standard that the raw δ¹⁸O values are consistently 9.4 lower than the reported ones, owing to the incomplete O₂ generation. The correction factor is also verified by more than a dozen natural sulphate samples—that have been analysed using both the laser-fluorination method and the graphite reduction/CO₂-fluorination method²⁷—that normally reach a 85% to 100% O₂ yield in our laboratory. This comparison also verifies that the incomplete O₂-generation using a CO₂-laser does not deviate from the slope of 0.52. Thus, the reported δ¹⁷O value is increased by 4.89‰. δ³⁴S was analysed using the SF₆ method²⁸. In this report, all Δ¹⁷O values are calculated on the basis of raw δ¹⁷O and δ¹⁸O values. We use the linear equation to calculate Δ¹⁷O because all the raw δ¹⁷O and δ¹⁸O values are close to the vicinity of the origin and the range of δ¹⁷O and δ¹⁸O values among different samples are small.