ORIGIN OF ANOMALOUS MAGNETIC SUSCEPTIBILITY AND COLORATION IN RELICT ATLANTIC OYSTER SHELLS: GEOCHEMICAL INSIGHTS

Thin organic-rich coatings containing small amounts of paramagnetic minerals contribute to dark-gray to black color of shell surfaces and growth lines of reworked Atlantic oyster (Crassostrea virginica) and several other bivalve species. This also explains the low-field magnetic susceptibility (MS) values that are unexpectedly high for diamagnetic carbonates. The Mann-Whitney test indicates that darker (lower mean grayscale) valves have significantly higher MS (p<0.03) than lighter ones, with median values of 1.8 and -15 µSI, respectively (n=31). SEM Energy Dispersive (EDS) analyses of darkened spots reveal heterogeneous surfaces with C/Ca atomic ratios ranging from ~1 (pure CaCO₃) to 3 (carbon enriched). Analysis by X-ray photoelectron spectroscopy (XPS) indicated that darkened valves had higher proportions of carbon-carbon bonding and a normalized C/Ca peak ratio of 28:1 compared to 4:1 in lighter shells, confirming the presence of an organic-rich coating. In addition to calcite or aragonite peaks at ~280/206, 710, 1085, and 1435 cm^-1, 785 nm excitation Raman spectra of dark shells produced peaks at ~1114, 1226, 1378, 1500, and 1885 cm^-1, which may result from organic single and double carbon-carbon bonds, methyl, or carbonyl groups. These spectra also revealed pyrite (340 and 376 cm^-1) on some darkened surfaces. The average total iron concentration on darker surfaces is slightly greater than that on lighter sections of C. virginica. While XPS analysis did not reveal the presence of Fe on the surface, sulfur was detected. Pyrite and other paramagnetic sulfide minerals may precipitate under anoxic conditions required for preservation of organic matter, thereby enhancing the MS signal. Therefore, systematic susceptibility variations in marginal marine sequences might be produced, in part, by changes in the amount of reworking of diagenetically altered mollusk shells from anoxic offshore or lagoonal environments. Our findings emphasize the need for utilizing a suite of geochemical techniques to explain the genesis of dark color in relict coastal mollusks, with implications to their paleoecology, taphonomy, and contribution to an otherwise “pure” carbonate-dominated bioclastic fraction of Mid-Atlantic coastal deposits.