SURFICIAL MAGNETIC SUSCEPTIBILITY TRENDS IN VARIABLY COLORED RELICT CRASSOSTREA VIRGINICA SHELLS, NEW JERSEY COAST

Similar to many coastal barriers along the U.S. Atlantic coast, a large proportion (>70%) of eastern oyster (Crassostrea virginica) shells reworked onto the beaches of New Jersey exhibit partial or complete surface darkening. Common in Quaternary estuarine deposits, these species have been used in a variety of paleoecological studies. This research examines the nature of persistent coloration by applying bulk low-field magnetic susceptibility (MS). Thirteen altered (dark) and 7 unaltered (light) relict shells ranging from 5-12 cm in length were analyzed in triplicate along longitudinal and transverse segments of shell interior and exterior using a high-frequency directional sensor. Given slight variations in shell thickness, their curvature is the key factor in determining MS response due to varying proportions of valve surface in contact with the sensor. Altered C. virginica samples yielded mean MS values of -10±5 μSI (unaltered: -20±5 μSI), as expected for diamagnetic carbonates. Outlying values of slightly positive MS (~5 μSI) were observed in some altered shells and, upon variably oriented spot measurements of samples with abnormally high susceptibility, exceeded +40 μSI. Such anomalous values suggest a degree of anisotropy associated with a paramagnetic component, e.g. iron sulfides. Prior studies on shell discoloration suggest an organic component as the coloring agent, but this alone does not explain the paramagnetic signal. Geochemical analyses will aid in identifying the cause of these anomalous values, and their relation to shell discoloration, but preliminary research suggests the potential of using MS for characterizing relict shells. The effects of valve curvature can be minimized by reducing each sample to equally sized fragments and by pulverizing. There are also indications from sectioning experiments that black color is not confined to valve surface. In addition to paleoecological reconstruction (residence time in original reduced back-barrier conditions or during burial on the shoreface), our findings have implications for lithostratigraphic research, where comminuted shell fragments comprise substantial portions of sedimentary horizons.