ENVIRONMENTAL AND PHYSIOLOGICAL INFLUENCES ON ISOTOPIC AND ELEMENTAL COMPOSITIONS OF BRACHIOPOD SHELL CALCITE - IMPLICATIONS FOR THE ISOTOPIC EVOLUTION OF PALEOZOIC OCEANS

Brachiopods from the Demissa Bed (Middle Devonian, Hamilton Group) biologically regulated the incorporation of Mg, Sr and Na into their shell calcite. Significant taxonomic differences in the elemental contents of three species (Athyris spiriferoides, Mediospirifer audacula and Mucrospirifer mucronatus) may be related to differences in calcification processes. In contrast, no significant difference or "vital effects" were observed between the isotopic values of Mediospirifer audacula and Athyris spiriferoides from Erie (DELTA-O-18 = 0.03 parts per thousand, p = 0.949; DELTA-C-13 = 0.76 parts per thousand, p = 0.083) and Genessee Counties (DELTA-O-18 = 0.39 parts per thousand, p = 0.471; DELTA-C-13 = 0.06 parts per thousand, p = 0.854). This suggests that these brachiopods did not exert a biological control over their isotopic compositions, and that their shell calcites reflect ambient physicochemical conditions. Isotopic compositions in unaltered shell calcites of brachiopods from Genessee County, which was close to the basin depocentre, are heavy for carbon (delta-C-13, xBAR = + 5.01 parts per thousand, PDB) and oxygen (delta-O-18, xBAR = - 2.85 parts per thousand, PDB) compared to the species sampled at the basin's edge (Erie County; delta-C-13, xBAR = + 2.79 parts per thousand, PDB; delta-O-18, xBAR = - 3.83 parts per thousand, PDB). There is a significant separation in isotopic values between the deeper- and shallower-water brachiopods of the basin (DELTA-O-18 = 0.98 parts per thousand, p = 0.0005; DELTA-C-13 = 2.22 parts per thousand, p = 0.0005). The delta-O-18 variation suggests a temperature/salinity change with water depth, whereas the change in delta-C-13 composition probably records an enrichment/depletion of organic matter with water depth. This observation has significant implications for Paleozoic ocean isotopic-evolution studies, because many global changes in marine delta-O-18 and delta-C-13 are based on isotopic shifts of similar magnitude.