Geology Reference
In-Depth Information
•
Ostracods
are most valuable indicators of paleo-
salinities (Pl. 48/2; Pl. 93/1; Pl. 130/2). They occur with
facies-diagnostic species and assemblages in freshwa-
ter, brackish water and marine environments.
•
Charophycean algae
(Pl. 65/1, 3; Pl. 130/4) are used
as indicators of freshwater and brackish water envi-
ronments (Sect. 10.2.1.8). Alternations of limestones
with charophyta, ostracod limestones and intercalations
of evaporites may reflect seasonally varying salinities.
•
Faunal diversity
(species richness and evenness) can
reflect high-stress conditions. A diversity minimum
exists around a salinity of 5‰. Diversity decreases away
from the species minimum toward both the freshwater
end and the marine end of the salinity range (Remane
1958). Similarly, the number of individuals often de-
creases from normal marine environments to both
brackish or hypersaline restricted environments. See
Croghan (1983) for critical remarks.
•
Shell size and thickness:
The thickness and size of
freshwater and brackish shells is often smaller than that
of marine shells. Decreasing and increasing salinities
are often related to a decrease in the maximum size.
•
Taphonomic features.
Burrowers and grazers nor-
mally destroy microbial mats in marine waters of nor-
mal salinity. These organisms are absent in high salin-
ity areas. Microbial mats formed in hypersaline envi-
ronments therefore have a greater chance for preserva-
tion.
Stable isotopes
Differences in the stable carbon and oxygen com-
position from either fossil or rock samples between ma-
rine and nonmarine carbonates (Keith and Weber 1964)
can be used to reconstruct paleosalinities (Mook 1971;
Dodd and Stanton 1990; Yin 1991; Joachimski 1991).
The results, usually based on δ
13
C data, should how-
ever be checked by paleontological criteria (Yin et al.
1995, Mátyás et al. 1996).
Trace elements
Trace elements in skeletons, both of fossils and in
carbonate rocks provide a possibility for estimating
paleosalinities:
The favorite fossils used in paleosalinity interpreta-
tions are
ostracods
because these organisms occur in
continental, estuarine, marine and hypersaline waters,
because their Low-Mg calcite valves are relatively re-
sistant to diagenetic changes influencing the geochemi-
cal data, and because the relationships between the
uptake of elements and chemical conditions of the
aquatic environment are well studied (Bodergat 1983).
Contrary to mollusk shells, which represent a whole
life growth pattern, ostracod tests - corresponding to
molts - may record water chemistry during just a very
short interval of a few days only.
In tropical seawater, the concentrations of
Mg, Sr
and Na
increase with increasing salinity. These elements
and their ratios are commonly studied in the context of
paleosalinity (e.g. Dodd 1967; Chivas et al. 1986). So-
dium in carbonate rocks has been used as a possible
index to the salinity of diagenetic solutions (Land and
Hoops 1973) as well as a paleosalinity indicator (Veizer
et al. 1977; Ogorolec and Rothe 1979; Sass and Bein
1988).
Changes in salinity during the deposition of subtidal
to supratidal limestone/dolomite cycles are reflected by
decreasing Sr and increasing Na, Fe and Mn contents
(Buggisch et al. 1994).
Since rare element concentrations and the Mg/Ca,
Sr/Mg and the Sr/Ca ratios of fossil shells and carbon-
ate rocks depend on many other factors than salinity,
their implication for paleosalinity interpretations is
much debated (Dodd and Stanton 1990; Rosenberg
1990; Carpenter and Lohmann 1992; Holmes et al.
1992; Rao 1996).
Microfacies data
reflecting paleosalinity deviating
from that of the normal marine seawater include spe-
cific grain types:
• Radial-fibrous ooids are common, but not restricted
to freshwater, brackish and hypersaline environments
(Sect. 4.2.5; Fig. 4.20; Fig. 4.26).
• Distorted ooids and half-moon ooids (Fig. 4.23).
• Pisoids/vadoids (Sect. 4.2.6; Box 4.19; Pl. 14).
• Microbial peloids are abundant in hypersaline envi-
ronments.
• Freshwater- and brackish oncoids (Sect. 4.2.4.1) dif-
fer from marine oncoids by specific nuclei and lamina-
tion, and by better preservation (Box 4.10; Pl. 12/7; Pl.
131/5).
The case study displayed in Pl. 104 demonstrates
the controls of alkalinity and salinity on carbonate for-
mation and lithification.
12.1.7.2 Geochemical Proxies of Paleosalinity
Boron:
Another tool studied in determining paleo-
salinities is the boron content of argillaceous sediments
(Harder 1970; Perry 1972).
Spivack et al. (1987) have
shown that the bulk of boron in clay minerals is not in
isotopic equilibrium with seawater and can, therefore
Common criteria used in recognizing differences in the
salinity of waters and diagenetic fluids are stable iso-
topes and trace elements.
