[M-IS16] Paleoclimatology and paleoceanography

[MIS16-17] Reconstruction of the Mediterranean sulfur cycle during the Messinian salinity crisis using evaporites

*Takeru Kochi¹, Junichiro Kuroda¹, Nanako O. Ogawa², Toshihiro Yoshimura², Takashi Miyazaki², Bogdan Stefanov Vaglarov², Naohiko Ohkouchi² (1.the University of Tokyo, 2.Japan Agency for Marine–Earth Science and Technology)

Keywords:sulfur isotope, evaporite, the Messinian Salinity Crisis

Evaporite is a group of rocks composed of minerals precipitated from a saturated brine through solar evaporation. Halite (NaCl) and gypsum (CaSO₄·2H₂O) are representative evaporite–forming minerals in the natural environment. During the Phanerozoic, voluminous evaporite bodies, so–called “salt giant”, have been formed repeatedly. The Messinian Salinity Crisis (MSC: 5.97–5.33 Ma) occurred in the Mediterranean Sea during the later stage of Messinian (7.246–5.333 Ma) represents one of the largest and the youngest salt giants in the Phanerozoic. This event is subdivided into three stages (Stages 1 to 3) based on the lithological features of evaporites. It has been proposed that the MSC was caused by tectonic uplift around the Atlantic spillway and global sea level drop, both of which could have limited the connection between the Mediterranean Sea and the Atlantic Ocean. During the MSC, a huge amount of gypsum was deposited in marginal and central deep basins of the Mediterranean Sea. The sulfate removed from seawater could have affected the sulfur isotopic composition (δ³⁴S) in the Mediterranean Sea. Previous studies have already demonstrated that, gypsum samples deposited during the MSC have sulfur isotopic compositions slightly heavier than that of the contemporaneous global seawater sulfate. However, there are few studies on the detailed spatiotemporal variation of sulfur isotopic composition within the Mediterranean basins through the MSC, which is necessary to quantitatively reconstruct the sulfur cycles in the Mediterranean Sea during the MSC.
In this study, we assessed how sulfur cycles in the Mediterranean Sea changed during the MSC based on sulfur isotopic composition of sulfate minerals deposited on six sites all around the Mediterranean Sea (three on land sites and three offshore borehole sites). The sulfur isotopic compositions were measured using an Elemental Analyzer–Isotope Ratio Mass Spectrometer (EA–IRMS) at Japan Agency for Marine–Earth Science and Technology (JAMSTEC). Sulfur isotopic compositions of evaporite samples deposited during the MSC are summarized as follows: gypsum deposited during Stage 1 ranges from 22.37 to 23.75 ‰, gypsum and halite deposited during Stage 2 ranges from 22.61 to 23.60 ‰, and gypsum deposited during Stage 3 is from 21.20 to 22.82 ‰. Based on these values, we reconstructed the sulfur isotopic composition of the brine water of the Mediterranean Sea with applying sulfur isotopic fractionation associated with the precipitation of evaporitic minerals from brine water estimated by our evaporation experiments (i.e., ε³⁴S = δ³⁴S_mineral - δ³⁴S_brine = 1.4 ‰ for gypsum and ε³⁴S = 0 ‰ for halite). This reconstruction showed that the sulfur isotopic compositions were constant during Stage 1 throughout the Mediterranean Sea, but they decreased by 1–1.5 ‰ from Stage 2 to Stage 3. To explain the decreasing sulfur isotopic composition, we estimated seawater exchange rate and gypsum deposition volume using a box model. The calculation results suggest that the variation of sulfur isotopic compositions could be explained by combined effects of (1) a decrease in the relative contribution of seawater influx from the Atlantic Ocean (or increased riverine flux), (2) voluminous deposition of gypsum with ³⁴S–enriched sulfur, and (3) additional local effects by biological sulfate reduction that may account for the local heterogeneity in sulfur isotopic compositions.<gdiv></gdiv>