PALEOECOLOGICAL IMPLICATIONS OF NITROGEN AND CARBON ISOTOPIC DATA FROM ORGANIC MATERIAL IN DEVONIAN RUGOSE CORALS

The Rugosa were important reef builders from the Middle Ordovician to the Late Permian and in some cases were present in extraordinary abundances. Rugose corals grow in an accretionary fashion and have been the target of a range of research that aims to a wide range of questions about ancient environmental conditions. The aim of this research is to develop techniques for the analysis of stable isotope signatures from skeleton-associated organic material in Rugose corals. Stable nitrogen and carbon isotope analyses of organic matter preserved in their calcium carbonate skeletons can provide insight into their paloecology and time-resolved changes in basinal conditions. Through the use of a nano-EA, a system that allows for analysis of nanomolar quantities of carbon and nitrogen, we can achieve sample resolutions that permit sclerochronological analyses of skeletal organic matter. Rugose corals were sampled from the Joshua Coral Bed in the Hamilton Group of Central New York near Skaneateles. These coral beds range in thickness from ~1 m to as many as 20 m and may have covered several hundred square kilometers of the sea floor during intervals of the Middle Devonian. Corals were physically and chemically cleaned to ensure that exogenous organic matter from host shales was removed. Organic material is evident in thin sections as thin filaments concentrated in coral tabula, and remains as whispy organic filaments in acidified residuals. The nitrogen and carbon isotopic compositions of coral-associated organic matter are significantly different from host shales, and vary along the coral skeleton. Sampled corals (Siphonophrentis) have δ¹⁵N values that range from 3.5 to 7.0‰ and were are 1-4 ‰ higher than the host rock organic matter (-2.5‰). Organic matter δ¹³C values range from -28.5 to -26.5‰ compared to -28.5‰ for the host shales. On average, these values are consistent with diets that comprised phytoplankton or zooplankton. The stable isotopic variability is consistent with either changing feeding habits or variability in basinal conditions. In general, these results suggest that Rugosan skeletal organic matter may prove to be excellent repositories of both paleoecological and paleoceanographic data, and that preserved organic material could provide substantial new insight into their diets and physiology.