Isotopic composition of gypsum hydration water in selected landforms from central Iran

Abstract

During its formation, hydration water of gypsum may be preserved, due to lack of sufficient contact with soil moisture in arid environments. If this hypothesis is valid, the isotopic composition of the hydration water of gypsum may provide very useful information, particularly about the paleo-hydrological conditions in soils and sediments. Little information is available on the stable isotope geochemistry of gypsum hydration water in arid regions. This study was initiated to measure the δD and δ¹⁸O values of the gypsum crystallization water in different landforms to determine their relationship with meteoric water and to identify the possible mechanism(s) for the large gypsum accumulation in the Isfahan region, central Iran. Samples from different horizons of twelve pedons covering two adjacent soil landscapes were analyzed. The δD and δ¹⁸O values of eight rainwater samples from different seasons and water samples from the Zayandehrud river in the area were also determined. The mean and standard deviation of δD and δ¹⁸O values of gypsum hydration water were −68.75±5.51‰ and +1.11±1.68‰ for colluvial fans and −56.5±7.28‰ and +4.47±2.07‰ for plateaus. The slope of ΔδD/Δδ¹⁸O=+3 and the significant positive correlation (P<0.01) between the isotopic composition of the gypsum crystallization water (both δD and δ¹⁸O) and the amount of gypsum in the soils indicated that evaporation is the major process for the accumulation of gypsum. The δD and δ¹⁸O values confirmed that gypsum was formed likely before the Quaternary era and paleowater, from a slightly more moist and/or colder climate, or different climatic pattern in the past, is still preserved (partly or entirely) in the gypsum hydration water. Differences found in the isotopic composition of gypsum hydration water, with respect to landscape position, may be useful to establish, more accurately, segments of a landscape in arid environment.

Introduction

The isotopic composition of gypsum hydration water can provide valuable information with respect to the mechanisms and conditions of gypsum accumulation in nature (Stewart, 1974; Sofer, 1978).

Because the isotopic composition of hydration water of gypsum reflects that of water in the soil environment, Dowuouna et al. (1992)suggested that this information may be useful as a proxy for the isotopic composition of soil moisture at depth where gypsum accumulates. Considering the limitations in the methods used to extract soil moisture for isotopic analyses (Walker et al., 1994), this technique appears to be reasonable.

Matsubaya and Sakai (1973)measured the isotopic composition of crystallization water in Japan and concluded that the gypsum ores were in isotopic equilibrium with today's meteoric waters rather than seawater. Whereas the sulfur and oxygen of SO₄ in mirabilite from Victoria Land, Antarctica, were isotopically close to seawater sulfate, analysis of the water of crystallization of this salt showed that it was similar to today's meteoric water (Bowser et al., 1970). On the other hand, studies of the gypsum hydration water geochemistry by Lyon (1978)from the Miers Valley, Southern Victoria Land, Antarctica, showed that the salt precipitated from water similar to glacier ice and isotopically lighter than the present Lake Miers water. These and other findings suggest that hydration water can be partly or entirely retained, if gypsum is formed from water isotopically different from the current meteoric water.

Halas and Krouse (1982)measured the isotopic abundances of water of crystallization of gypsum from Miocene evaporite in the Carpathian Foredeep, Poland. They concluded that the original gypsum hydration water was equilibrated with marine water. Subsequently, it re-equilibrated towards very isotopically light water during a glacial or postglacial period and is now approaching a new equilibrium with current waters circulating through the deposits.

Using isotopic composition of the hydration water in gypsum, Sofer (1978)could differentiate the mechanism of formation of different gypsum samples from the Namib Desert (South Africa), west Germany, and Israel, formed through evaporation, hydration of anhydrite, and oxidation of pyrite or recrystallization processes, respectively. Yonge and Krouse (1987)examined the mechanisms of sulfate precipitation in Castleguard Cave by stable isotope geochemistry of gypsum hydration water. They noted that gypsum was precipitated partly by evaporation and partly by expulsion of CO₂ from sulfate-bearing solutions.

Extremely gypsiferous soils (>50% gypsum in some gypsic horizons) commonly occur in different landforms in central Iran. They provide ideal conditions to study the geochemistry of gypsum formation in arid environments. Limited information is available on the use of isotopic techniques in landscape investigation, specifically in arid regions. This study was conducted to determine the δD and δ¹⁸O values of the gypsum crystallization water in different landforms to understand how and when gypsum accumulated in the gypsiferous soils of central Iran.