Are fluid inclusions in gypsum reliable paleoenvironmental indicators? An assessment of the evidence from the Messinian evaporites

D Bigi,M Roveri,S Lugli,V Manzi

doi:10.1130/g49475.1

Abstract

Abstract The paleosalinity of water from which the gypsum precipitated during the Messinian salinity crisis is a controversial issue. Recent microthermometry studies on primary fluid inclusions in gypsum provided very low salinity values not compatible with precipitation from seawater, and suggested strong mixing between seawater and nonmarine waters enriched in calcium sulfate. We applied a new microthermometric protocol on gypsum crystals from nine Mediterranean sections that were experimentally stretched to measure a larger population of fluid inclusions. The results show salinities ranging from 9 to 238 wt‰ NaCl equivalent, largely falling within the evaporation path of normal seawater. The data from previous studies were obtained mostly from those fluid inclusions capable of nucleating a stable bubble after a weak stretching, which probably correspond to those having a lower salinity acquired through post-depositional crack-and-seal processes. Our data suggest instead that the primary gypsum precipitated from a marine brine, later modified by post-trapping processes during tectonics and exhumation.

Highlights

Fluid inclusions (FIs) in marine minerals such as calcite, gypsum, and halite are a very useful tool for the understanding of the depositional environment because they represent microsamples of ancient seawater from which the minerals precipitated
The salinity values obtained are shown in Figure 2 and in Tables S2 and S3
The large differences in salinity for FIs lying along the same growth band could only be interpreted via variable postdepositional modification of the trapped primary fluid

Summary

Introduction

Fluid inclusions (FIs) in marine minerals such as calcite, gypsum, and halite are a very useful tool for the understanding of the depositional environment because they represent microsamples of ancient seawater from which the minerals precipitated. This information can be obtained by microthermometric analyses unless FIs were modified after trapping. This approach has been applied to the primary bottom-grown selenite gypsum accumulated in the Mediterranean Sea during stages 1 and 3 of the Messinian salinity crisis (MSC; CIESM, 2008; Roveri et al, 2014a; Fig. S1 in the Supplemental M aterial). At stage 3 of the MSC is a signal clearly distinct from the global ocean recorded by evaporites, limestone, molluscs, and ostracods (Fig. S2)

Methods

Results

Discussion

Conclusion