S and Sr Isotope Compositions and Trace Element Compositions of the Middle Jurassic Evaporites in Eastern Tibet: Provenance and Palaeogeographic Implications

Abstract

The origin of Middle Jurassic evaporites in the Qamdo Basin is still controversial because palaeontological studies have reported that they have both marine and continental characteristics. The 87Sr/86Sr ratios of the gypsum in the Middle Jurassic Dongdaqiao Formation in the Qamdo Basin range from 0.707602 to 0.708163, which are higher than that of contemporaneous seawater. Model calculations suggest that continental water prevailed over seawater during the precipitation of these evaporites. However, the majority of the gypsum samples have δ34S values of 15.3‰ to 16.3‰, which are consistent with that of contemporaneous seawater. This range of values (15.3‰ vs. 16.3‰) was likely caused by S isotope fractionation during evaporation because the δ34S values and Sr contents are negatively correlated. The δ34S values of the other three gypsum samples are 20.0‰, 20.5‰, and 20.8‰, which are significantly higher than that of Middle Jurassic seawater. The trace element compositions and scanning electron microscopy (SEM) observations indicate that these elevated δ34S values were caused by bacterial sulphate reduction (BSR). The Sr and S isotope systematics of the gypsums from the Dongdaqiao Formation demonstrate that the parent brines from which the evaporites precipitated were marine based with a large quantity of continental input. A comparison of the lithologies and Sr isotope compositions of the Middle Jurassic sequences in the Qamdo and Qiangtang Basins revealed that the Qiangtang Basin was mainly recharged by Jurassic seawater, while the Qamdo Basin was primarily recharged by continental water with some seawater-derived overflow from the Qiangtang Basin.

Geochemistry and Sr, S, and O stable isotopes of Miocene Abu Dhabi evaporites, United Arab Emirates

“…A large amount of information regarding paleosalinity, initial brine composition, depositional basin water depth, paleotemperature, evaporite origins, and diagenetic processes have been based on the geochemistry of major and trace elements [ [5] , [6] , [7] ]. Moreover, a significant contribution for determining the origin, marine vs non-marine, of ancient evaporites and the influence of different water types on the depositional basin may be acquired from the use of strontium ( 87 Sr/ 86 Sr), oxygen (δ 18 O sulfate ), and sulfur (δ 34 S sulfate ) isotope data [ [8] , [9] , [10] , [11] ]. Karakaya et al [ 9 ] employed 34 S and 18 O values to deduce the contribution of the recycling-dissolution of the predeposited marine sulfates (Oligocene-Eocene), arid conditions and bacterial sulfate reduction processes on the compositions of the Miocene evaporites in the Tuz Gölü basin, Turkey.…”

“…Karakaya et al [ 9 ] employed 34 S and 18 O values to deduce the contribution of the recycling-dissolution of the predeposited marine sulfates (Oligocene-Eocene), arid conditions and bacterial sulfate reduction processes on the compositions of the Miocene evaporites in the Tuz Gölü basin, Turkey. The lithologies and Sr isotope compositions of Middle Jurassic evaporites in the Qamdo and Qiangtang Basins of Eastern Tibet were compared by Fei et al [ 10 ]. Their findings suggested that the Qiangtang Basin was mostly recharged by Jurassic seawater, whereas the Qamdo Basin was primarily recharged by continental water, with some Qiangtang Basin-derived overflow.…”

The Sulfur Isotopic Characteristics of Evaporites in the Yarkand Basin of Xinjiang Province in the Paleocene and Its Paleoenvironmental Evolution

“…Sulfur isotopic techniques have been widely used to study evaporites in saline lakes in recent years [5], as they allow for regional and even global comparison of marine sedimentary layers due to their highly regular distribution in sulfate. Especially under complex conditions, analyzing the distribution of sulfur isotopes provides substantial evidence for reconstructing paleogeographic sedimentary environments and understanding the genesis of evaporites [6][7][8][9]. The sulfur isotope composition of marine evaporites records the evolutionary history of seawater sulfates in different geological periods.…”