An X‐ray spectroscopic perspective on Messinian evaporite from Sicily: Sedimentary fabrics, element distributions, and chemical environments of S and Mg

Abstract

The Messinian salinity crisis is a dramatic hydrological and biological crisis that occurred in the Mediterranean basin at 5.97–5.33 Ma. The interpretation of the facies and stratigraphic associations of the Messinian salt deposits is still the object of active research because of the absence of modern depositional analogues of comparable scale. In this study, the spatial distributions of Na, Mg, S, O, Si, and Al in a potassic‐magnesian salt and a halite layers of Messinian evaporites from the Realmonte mine on Sicily were determined using synchrotron based micro‐X‐ray fluorescence. The dominant molecular host site of Mg and S obtained by X‐ray absorption near edge structure (XANES) is applied to specify the hydrochemistry of hypersaline brines and the presence of diagenetic minerals, thus shedding light on evaporative concentration processes in the Caltanissetta Basin of Sicily. Mg and S K‐edge XANES spectra revealed the presence of highly soluble Mg‐bearing sulfates. The massive halite layer “unit C,” contains less soluble minerals, thus did not exceed the stage of halite crystallization. We infer that as evaporative concentration increased, the density of the brine at the shallow margin of the basin increased as salinity increased to concentrations over 70 times the starting values, creating brines that were oversaturated with Mg‐sulfate. Density stratification of the deep basin caused heavy brines to sink to the bottom and become overlain by more dilute brines. We propose lateral advection of dense Mg‐sulfate brines that certainly affected marine biota.

Biomarker records and mineral compositions of the Messinian halite and K–Mg salts from Sicily

“…The S μ-XANES spectra obtained from the measurement points 1-1, 1-2, 2-2, and 3-4 exhibited a pronounced peak appearing at 2482 eV and some resonances at around 2486.5, 2491, 2494.5, and 2499 eV in the post-edge regions (Fig. 4), which is identical with the spectrum of kainite reported previously (Yoshimura et al 2016). The XRD analysis also confirms this mineral as kainite (Additional file 1: Fig.…”

“…The contacts between Unit B and C are characterized by the localized dissolution pits filled with mud, features that suggest desiccation of the basin and the formation of large expansion-contraction salt polygons (Lugli et al 1999). Unit C, deposited after the basin had undergone desiccation, is composed of more than 120 lithological cycles, each one represented by the superposition of three main facies: (1) a millimeter-thick shale layer, (2) a millimeter-centimeter-thick anhydrite (CaSO 4 ) or polyhalite (K 2 MgCa 2 (SO 4 ) 4 •2H 2 O) layer, and (3) a decimeter-thick halite layer with white patchy polyhalite grains (Lugli et al 1999;Yoshimura et al 2016). The highly pure halite cumulates of skeletal hoppers with chevron overgrowths indicate that the precipitation occurred in a relatively shallow, non-stratified water body during the dry season of the year, whereas the shale layers are considered to have deposited under a stratified water body during the wet season (Lugli et al 1999;Manzi et al 2012).…”

“…The microscale elemental mapping of geological material is a commonly employed approach for identifying the depositional environments (e.g., Yoshimura et al 2016). The high-sensitivity scanning X-ray fluorescence (XRF) that utilizes synchrotron radiation sources with X-ray focusing optics (i.e., μ-XRF) has the advantage of generating trace element distributions with a micrometer-scale spatial resolution (e.g., Sutton et al 2002;Tamenori et al 2014).…”

“…Previous studies have explored the relationships among the climate, the hydrological conditions, and the precipitating evaporites during the MSC peak, based on the distributions of mineral phases and elements in the evaporites (Garcia-Veigas et al 1995;Lugli et al 1999;Yoshimura et al 2016), rhythmical alternation of the evaporitic sequences (Manzi et al 2012), chemistry of the fluid inclusions in halite crystals (Rigaudier et al 2011), and Os isotopic records (Kuroda et al 2016). By contrast, much less is known on the responses of the biological communities, partly because evaporites generally do not contain microfossils commonly used in biogeochemical studies (Bertini et al 1998).…”

Diazotrophy Drives Primary Production in the Organic-Rich Shales Deposited Under a Stratified Environment During the Messinian Salinity Crisis (Vena del Gesso, Italy)

“…Relatively high δ 13 C of chlorophyll c-producing algae in part explains the 13 C-enrichment in TOC of the PLG shales compared to other stratified basins (e.g., van Breugel et al, 2005). In addition, considering that other MSC deposits also exhibit elevated δ 13 C TOC (Schouten et al, 2001;Yoshimura et al, 2016), a physicochemical process specific to hypersaline condition, such as the degassing of 13 C-depleted CO 2 (aq) from the brine induced by a decrease in gas solubility during evaporation (Stiller et al, 1985;Isaji et al, 2017), may also have contributed to δ 13 C TOC increase. It is noteworthy that, compared to other modern and ancient stratified basins, the δ 13 C values of the CO 2 at the chemocline during the deposition of PLG 4 shale are less depleted in 13 C with respect to CO 2 (atm) (van Breugel et al, 2005).…”