Stable isotope signature of middle devonian seawater from hamilton group brachiopods, central new york state

Abstract

Stable isotope ratios of oxygen in articulate brachiopod shell low-magnesium calcite have been widely used as a proxy for seawater δ 18O through the Phanerozoic. It has been suggested that the generally negative δ 18O in Devonian brachiopods reliably records seawater isotope composition. In this study, stable isotope ratios of carbon and oxygen were obtained on well-preserved brachiopod shells from the Middle Devonian Hamilton Group of Central New York State. The sample set includes multiple specimens of two common genera, Spìnocyrtia and Mucrospirifer. Spinocyrtia is a relatively thick-shelled genus that is found in sandy or firm- mud bottom facies, whereas thinner-shelled Mucrospirifer had a somewhat broader environmental tolerance including organic-rich muddy substrates. Optical and scanning electron microscope images of shell material show remarkably well-preserved orginal biogenic microstructures, suggesting minimal diagenetic recrystallization of calcite.