A MICROSCOPIC CHARACTERIZATION OF VARVED SEDIMENTS, PETTAQUAMSCUTT RIVER ESTUARY, RI

The Pettaquamscutt River Estuary, RI (PRE) provides a unique setting for paleoclimate research because of its high-fidelity sediment record and boundary location between oceanic and continental climate influences. Density stratification in two ice block depressions results in anoxia at depth, allowing laminated sediments to accrete undisturbed. Comparison of laminae counts with independent age constraints (210Pb, 137Cs, 14C) has shown PRE couplets to be varves, valuable to paleoclimate research as high-resolution timekeepers and archives of seasonal environmental signals. Modern laminae were identified as light or dark based on appearance in thin section under cross-polarized light. The previously proposed model explains light laminae as dominated by mineral clasts transported to the basin by runoff. Light laminae thickness correlates to rainfall, making it a useful climate proxy. Dark laminae are thought to form through spring deposition of biogenic sediment promoted by increased productivity in the water column. Here microscopic analyses were conducted to better characterize varve components and further our understanding of their potential as climate proxies. Thin sections and loose sediment were examined using BSEI. Light and dark laminae show relatively equal distribution of diatom tests, unidentified bioclasts, framboidal pyrite, scant silt and clay-sized minerals, and abundant amorphous material. Because no distinction was observed between laminae at this scale, petrographic microscope analysis was undertaken. Couplets were examined at 1.5 mm intervals along laminae to determine the volume percent and size of mineral clasts larger than 0.02 mm. No difference in size was observed, but light laminae were found to contain a significantly higher percent of minerals than dark. Therefore, fewer mineral clasts are deposited during dark laminae formation. Although the percent of minerals is not large enough to fully explain color differences within couplets, we suggest that this effect is combined with the darkening effect of photosynthetic pigments in biogenic sediment. This study provides quantitative support for the proposed model of PRE varve formation and contributes data useful to the interpretation of varved estuarine sediment and the paleoclimate record it preserves.