Biogenic calcites contain up to several thousand ppm of sulfate, structurally bound at the lattice position of the carbonate ion (Takano, 1985; Staudt et al., 1994). Theoretically, this sulfate should reflect the sulfur isotopic composition of seawater at the time of calcite precipitation and would, thus, represent a valuable source of information for studying the isotopic composition of seawater and its evolution through time.
The Jurassic and Cretaceous time interval has been selected for a detailed determination of the sulfur isotopic composition of seawater, present as trace sulfate in belemnites.
Whole belemnite specimens, ranging in weight from < 1g to 90g, were first etched with HCl in order to remove any outside contamination and then ground. Initial treatment of the rock powder with NaCl-solution was applied to remove any remaining true sulfate minerals, resulting for example from oxidation of pyrite. Pretreated samples were subjected to acid-digestion (HCl), followed by filtration of any insoluble residue and final precipitation of the dissolved sulfate as barium sulfate. Combustion of barium sulfate with a vanadium pentaoxide-silica glass mixture (Yanagisawa and Sakai, 1983) yielded SO2 for mass-spectrometric analyses of the sulfur isotopic composition.
Preparation of three different aliquots from a large belemnite displayed an isotopic variability of ± 0.3 , which is comparable to the error bar observed in sulfur isotope analysis.
At present, the sulfur isotope curve records a fairly constant range of values between +15 and +17 for the late Jurassic, followed by a decrease in 34S to an isotope minimum of +14 near the Aptian/Albian boundary and an evolution towards a late Cretaceous value around +20 (Claypool et al., 1980). This curve is constrained by results from evaporite deposits which are unequally distributed throughout this time interval and characterized by age uncertainties ranging from < ± 5 Ma to > ± 40 Ma.
In contrast to this rather poorly constrained isotope record (with respect to spacing and time resolution), we have studied belemnites with at least one specimen per stage (Harland et al., 1990), yielding a time resolution of < ± 5 Ma throughout the entire time interval. Independent studies of petrography and cathodoluminescence, of their trace element contents as well as their carbon, oxygen and strontium isotopic compositions have constrained sample quality (Podlaha, 1995).
Sulfate contents for these belemnites range from 600 to 7000 ppm, as deduced from the barium sulfate yields. As expected, the obtained sulfur isotope curve for the Jurassic and Cretaceous displays substantially more internal structure than the 'Claypool-curve'. This can be observed for the entire studied time interval as well as for shorter time periods. These variations are interpreted as primary changes in the seawater isotopic composition, previously unrecognized due to poor time resolution.
A new sulfur isotope curve for the Jurassic and Cretaceous has been obtained utilizing the structurally-bound sulfate in belemnite calcite. Stratigraphic control as well as time resolution is far superior to the existing curve based on evaporitic sulfate occurrences. Previously unrecognized variations reflect changes in the sulfur isotopic composition of seawater sulfate.
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