EFFECT OF PRESERVATION ON THE GEOCHEMISTRY OF LATE CRETACEOUS MOLLUSKS FROM THE WESTERN INTERIOR SEAWAY OF NORTH AMERICA. PART I: SR, O AND C ISOTOPES

Evaluating the effects of preservation on isotopic signatures of Sr, O, and C is critical to the use of these tracers in reconstructing salinity and temperature gradients in ancient oceans. We have analyzed a series of samples from a single site in the Baculites compressus zone of the Pierre Shale (Campanian) from South Dakota. Preservation was evaluated using light microscopy and scanning electron microscopy (SEM). An SEM preservation index (PI) was established based on comparison of the microstructure of fossil material with that of modern analogs. For example, the nacreous microstructure of cephalopod shells was rated excellent (PI=5) if, by comparison with modern Nautilus, the nacreous plates were well defined with discrete, angular boundaries. In contrast, samples showing fused nacreous plates with indistinct boundaries were rated poor (PI=1). Samples from a variety of species within a single concretion show that 87Sr/86Sr ratios vary with preservation and average .707323 ± .000214 (n= 4) for PI = 1 to 2, .707573 ± .000054 (n=9) for PI = 3 to 4, and .707656 ± .000041 (n=4) for PI = 4.5 to 5. Oxygen and carbon isotopic analyses of the same specimens show similar trends with preservation. The average value of 87Sr/86Sr in shell material with PI = 4.5 to 5 in the concretion is comparable to that in other similarly well-preserved material from the same site (average 87Sr/86Sr = .707678 ±.000016; n=2). The 87Sr/86Sr values in shell material with PI = 4.5 to 5 are equivalent to those obtained in prior studies of specimens from the B. compressus zone, and indicate that SEM preservational criteria are effective in screening shell material that records original isotopic signatures.