STABLE ISOTOPE VARIATION IN MIDDLE DEVONIAN BRACHIOPODS FROM THE HAMILTON GROUP OF CENTRAL NEW YORK; PHYLETIC AND ENVIRONMENTAL EFFECTS

Stable isotope ratio of oxygen in brachiopod shell carbonate has been used widely as a proxy for seawater d18O through the Phanerozoic (e.g. Veizer, et al, 1997). Based on the assumption that low metabolic rate and slow growth would lead to equilibrium precipitation of shell carbonate in brachiopods, it has been suggested that the generally negative (-2 to -6 per mil) d18O in Devonian brachiopods reliably records the seawater isotope composition, barring evidence of recrystallization of shell calcite. In the middle Devonian Hamilton Group of New York State, previous studies (Bates and Brand, 1991; Popp, et al, 1986) have suggested that brachiopods were generally in isotopic equilibrium with ambient seawater, and that phyletic and environmental differences were minimal in well-preserved samples.

In this study, stable isotope ratios of carbon and oxygen were obtained on well-preserved brachiopod shells across an environmental and stratigraphic spectrum to assess the possible effects of water column stratification and freshwater input from the Acadian Mountains source area. Our sample set includes multiple specimens of Spinocyrtia and Mucrospirifer. Spinocyrtia is a relatively thick-shelled genus that is found in slightly shallower sandy or firm-bottom environments, whereas thinner-shelled Mucrospirifer had a somewhat broader environmental tolerance including more offshore, muddy substrates. Consistent differences in stable isotope ratios between the two genera (Spino. mean 13C=1.9; 18O=-5.8; Mucro. mean 13C=1.9; 18O=-7.6; all PDB) suggest possible disequilibrium growth driven by kinetics during shell precipitation (e.g Auclair, et al, 2003), vital effects and/or as yet unrecognized diagenetic alteration. These interspecific differences occur between samples from the same depositional bed, and thus are probably not related to differences in seawater isotope chemistry or diagenesis.