CLAY MINERALOGY OF ESTUARINE CORES COLLECTED FROM THE TIDAL REACHES OF THE RAPPAHANNOCK RIVER, TRIBUTARY TO CHESAPEAKE BAY

Clay flocculation and mud deposition is an important process that marks the transition from the proximal to central estuary. Recent studies of the physical and biological characteristics of cores collected from the tidal reaches of the Rappahannock River indicate a late 20^th century freshening and deterioration of the salinity gradient in the Northern Neck region of Virginia. Our primary objective of this study is to evaluate whether or not the clay flocculation and the clay mineralogy was impacted by this apparent freshening in sediment cores collected at the fresh-saltwater transition zone.

Coring sites were chosen at the transition from proximal to central estuary at the upper limit of fresh and salt water mixing. At the time of sampling, salinity values ranged from 0-5‰ in the upper waters and ~5-15‰ in bottom waters. The sediment cores comprise gray mud and clay, with < 15% TOC, which range from 65-87cm thick. Previous Cs¹³⁷ studies at the coring sites indicate basal ages of early 20^th century. Microfossils comprise both fresh and brackish water assemblages of agglutinated foraminifera and nonmarine ostracoda that record a marked freshening upward trend in all cores.

To evaluate the clay mineralogy, cores were split lengthwise and 1cm³ of sediment was extracted from the top, middle, and bottom of each core. Sodium hexametaphosphate (1.5mL) was added as a deflocculant and samples were ultrasonically disaggregated and centrifuged for 20 min. at 750 rpms to separate the < 2 μm equivalent spherical-diameter fraction. Oriented samples were prepared on glass slides after vacuum filtration using a smear method. All powder samples were X-rayed in the air-dried state and after treatment with ethylene glycol by vapor salvation in a heated chamber (60^oC). Results indicate discrete kaolinite and illite based on the following reflections: kaolinite 001(~7Ǻ), 002(~3.5Ǻ) and 003(~2.3Ǻ); illite 001(~10Ǻ) and 002(~5Ǻ). Chlorite [001(~14.2Ǻ), 003(~4.7Ǻ) and 005(~2.84Ǻ)] and quartz [3.34Ǻ, 4.26Ǻ and 1.82Ǻ] are also present in all samples.

The XRD data indicate there are no apparent temporal or spatial variations in clay mineralogy within or between the coring sites. Thus, the freshening upward trend was not significant enough to impact the clay flocculation process or clay mineralogy during the past century.