The Sulfur Isotopic Composition of Seawater as Deduced From Trace Sulfate in Carbonates

Harald Strauss Institut für Geologie, Ruhr-Universität Bochum, D-44780 Bochum, Germany



Our knowledge of the sulfur isotopic composition of seawater sulfate and its evolution through time is based
on studies of ancient marine evaporitic sulfate deposits (anhydrite, gypsum). The sulfur isotope values for these sulfates have been determined under the assumption that the isotopic composition of seawater is homogenous and that the isotope values recorded for ancient evaporitic sulfates are representative for the seawater from which they precipitated. The most severe disadvantage of the currently available sulfur isotope record for ancient seawater is, however, the unequal distribution of evaporite deposits through time as well as their often only poorly constrained age assignments (Strauss, 1996). This is in strong contrast to available isotope records for carbon, oxygen and strontium.

Conceptual Approach

Addressing this unsatisfactory time resolution, an alternative approach to marine sedimentary sulfate deposits acknowledges sulfate as important trace constituent in all marine calcites, present as structurally-bound sulfate ion at the carbonate lattice (Takano, 1985; Staudt et al., 1994). Abundances range from a few tens of ppm present in inorganic precipitates to several thousand ppm in various biogenic carbonates (Busenberg and Plummer, 1985). Trace sulfate, thus, offers the potential for determining its sulfur isotopic composition, as has been demonstrated in an initial study on Neogene foraminifera by Burdett et al. (1989). Therefore, trace sulfate represents a valuable source of information, particularly in times with no evaporite deposits. In addition, time resolution for a resulting sulfur isotope record can be bound to biostratigraphy, finally offering a dataset comparable to the other marine isotope records.


Structurally-bound sulfate has been extracted from a variety of biogenic and non-biogenic carbonates during this study. The prime objective was the development of a routine preparation procedure and the assessment of accuracy
and reproducibility. The technique involves leaching (NaCl solution), acid-digestion (HCl) of the carbonate and, thus, liberation of the structurally-bound sulfate. This was followed by precipitation of the dissolved sulfate as barium sulfate. While the "NaCl-fraction" frequently displays
34S-depleted values, likely reflecting oxidized sedimentary sulfides, the "HCl-fraction" yielded sulfur isotope values which are in good agreement with data previously published for coeval evaporite deposits (Claypool et al., 1980). This is not only true for recent marine bivalves, brachiopods and gastropods but also for biogenic (belemnites, brachiopods) and non-biogenic, whole-rock carbonates of Mesozoic and Paleozoic age.


The analysis of structurally-bound sulfate in carbonates offers an attractive alternative to the evaporite based sulfur isotope age curve. Its prime advantage is the utilization of (bio-)stratigraphically well constrained samples yielding a time resolution far superior to the existing evaporite curve and comparable to the marine isotope records of carbon, oxygen and strontium. Ultimately, these records will be truly comparable because they will be generated from the same specimen.


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