Characteristics of the bromide profiles

The results of this comparative investigation between locations with different deformation backgrounds reveal a similar general shape to the bromide profile in all investigated locations.

This bromide profile is characterized by low bromide values in the lower half of the Hauptsalz and more or less steeply increasing bromide values in the upper half. However, in detail, differences can be observed between the locations, concerning the dispersion of the bromide values along the running average curve, the average bromide content, and partially the inclination of the curve in the upper part of the profile.

Minimum bromide contents and variance

In all studied locations, the bromide values in the lower half of the Hauptsalz predominantly range between 40 ppm and 60 ppm (Figs 3.7 and 3.9) and thus lie considerably below the value of 75 ppm expected for the first halite to precipitate from evaporating sea water. Such low bromide contents were also reported from basal halite in other Stassfurt rock salt deposits (Schulze 1958, 1960a; Kühn 1968). The most plausible explanation for such discrepancy between theoretical and observed values is that the bromide content of the brine at the beginning of halite precipitation was lower than the bromide content of concentrated normal sea water. Such decreased bromide content was probably the result of recycling processes of already precipitated halite due to an influx of either sea water or meteoric water, as, for example, suggested by Wardlaw & Schwerdtner (1966) or Hovorka et al. (1993). Meteoric water influence is not very likely for the Hauptsalz of the studied locations, as in that case bromide values would be far below the observed 40 to 60 ppm (Hovorka et al. 1993). During recycling, sea water partly reaches saturation by the dissolution of already precipitated halite and, consequently, the bromide content of the first halite that crystallizes from such saturated brine will be lower than that of concentrated normal sea water. Such dissolution processes would preferentially occur at a stage of evaporation in which sea water concentration fluctuated about the point of saturation of sodium chloride (Wardlaw & Schwerdtner 1966).

These fluctuations are in line with the rhythmic stratification of rock salt sequences and

anhydrite layers characteristic of the Hauptsalz of the Stassfurt Formation (Z2). Another possibility would be that the bromide content of the Permian sea water was lower than that of modern sea water. However, due to its high residence time of about 100 Myr in the world oceanic waters (Holland 1978; Chester 2000), the bromide concentrations are assumed to have not changed significantly during the Phanerozoic. A decrease of the partition coefficient due to reduced crystal growth rates as discussed, for example, by Herrmann et al. (1973) and Herrmann (2000) seems unlikely, because recent evaporation experiments revealed that the partition coefficient more strongly depends on the bromide concentration or the major element composition of the brine than on the evaporation rate (Siemann & Schramm 2002).

The comparatively high dispersion of the bromide contents in the bedded rock salt of Teutschenthal (Figs 3.7A and 3.9a) can be most satisfactorily explained with rhythmic changes in sea water concentrations. Such changes result from evaporation and dilution that alternate due to periodic influxes of sea water or meteoric water into the basin and indicate that the basin was connected to the open sea at that time. Similar observations have been made by Kühn (1955) and Wardlaw & Schwerdtner (1966), who generated detailed bromide profiles through halite-anhydrite seasonal layers. In comparison, the bromide profile of the Hauptsalz of Morsleben and Gorleben is more regular, especially in the lower half (Figs 3.7B and c, 3.9a and b; Table 1). Given that the Hauptsalz of the studied locations formed under similar conditions, the smoother curve in Morsleben and Gorleben, combined with the absence of the Kristallbrocken halite type in these locations, suggests an influence of post-depositional, presumably deformation-related processes (discussed further below).

Average bromide content

In Morsleben and Gorleben, the average bromide contents in the upper half of the Hauptsalz are higher than that of the same stratigraphic intervals in Teutschenthal (Fig. 3.7). The comparatively higher bromide values can especially be observed in the Kristallbrocken. As this halite type has not yet disintegrated during deformation, their high bromide concentration has been present since their formation, i.e. post-depositional alteration of the bromide contents by secondary, bromide-rich brines cannot account for the comparatively increased bromide values. The varying amounts of bromide in the studied locations can be better explained by their different paleogeographic positions in the Zechstein basin (Fig. 3.2), as different parameters like water depth, the proximity to the influx of the basin, or the proximity to the basin edge influence the bromide content of precipitating halite (Fisher & Hovorka 1987; Hovorka et al. 1993; Raup & Hite 1996). Similar observations were made in several earlier studies showing that the bromide content of stratigraphically equivalent rock salt beds

increases generally from deposits close to the basin margin to those near the basin centre (Schulze 1958; Haltenhof & Hofrichter 1972; Raup & Hite 1996). The reason for this lateral bromide gradient may be a stratified brine column representing a vertical salinity gradient (Raup & Hite 1996). At the deepest point of the basin, high-density brines would be near the sediment surface, whereas in more upslope positions like the basin edge, less dense brines would be close to the sediment surface.

Inclination of the profile in its upper half

The bromide profile of the Hauptsalz is generally inclined in its upper part indicating that the basin conditions changed towards increasing salinity (Raup & Hite 1996). In Teutschenthal, the profile has a gentle increase from the middle, and shows rapidly increasing bromide changes in sea water concentrations, induced by relative positions in the evaporation basin.

On the other hand, it has to be considered that processes related to salt migration may also cause deviations from the original bromide profile trend. For example, during salt migration, rock salt beds are subjected to intense folding, which results in thickening or thinning of salt beds. Even though the Hauptsalz unit appears to be completely preserved between the overlying and underlying stratigraphic sections, parts of the Hauptsalz may have been thinned, thickened, or doubled by salt migration. In case of Morsleben and Gorleben, however, this would not be visible in the cores due to the homogeneous fabric of the rock salt.

For example, as the studied rock salt of Morsleben was obtained from a well drilled into the flank of an anticline (Fig. 3.3B), it is very likely that the rock salt sequence has been thinned out during salt migration, altering the original thickness. The abrupt rise of the bromide profile from the middle part upwards might be a consequence of such a process. In addition, the slight fluctuations in the trend of the bromide profile that can be observed to some extent in the lower third of the Hauptsalz of Gorleben may be caused by sampling of folded areas.

Therefore, when comparing the bromide content of various rock salt profiles of diapirs, changes in the original profile thickness (multiplication or reduction) due to halokinesis should be considered. These changes in thickness influence the trend or slope of bromide profiles and thus may lead to genetic misinterpretations when comparing them with each other (Fig. 3.9). Accordingly, variations in the trend of bromide profiles from the same

stratigraphic intervals, as observed in Gorleben (Bornemann et al. 2008), may also partially be due to comparing different thicknesses of otherwise equivalent stratigraphic intervals.