A CENTENNIAL RECORD OF PALEOSALINITY CHANGE IN THE TIDAL REACHES OF THE CHESAPEAKE BAY

Microfossil paleoecological trends and oxygen isotope values observed in cores collected from the Rappahannock and Potomac Estuaries record evidence of changing salt-wedge gradients in the tidal reaches of the Chesapeake Bay pre- and post-industrial revolution.

Gravity and push cores from the Potomac (134 cm) and Rapphannock (141 cm) Estuaries were collected from the central and proximal estuarine zones, respectively. The lowermost microfossil associations in both cores comprise relative abundance alternations with respect to the ostracodes Cyprideis salebrosa and Cytheromorpha spp. with three notable population excursions. The uppermost microfossil association gives way to an oligohaline association of thecamoebians (Difflugia and Centropyxis) and the freshwater ostracodes Heterocypris, Limnocythere, Darwinula stevensoni, and Candona. δ¹⁸O stable isotope values obtained from the ostracode Cyprideis salebrosa are variable, ranging between -6.6 to -3.2‰ VPDB. δ¹⁸O values for Cytheromorpha spp. from the Potomac range from -8.2 to -3.2‰. Positive excursions of δ¹⁸O correspond with population peaks in both Cyprideis and Cytheromorpha likely indicating increased marine influence and/or higher salinities. This hypothesis is corroborated by pore-shape morphological analysis of Cyprideis salebrosa that demonstrate a relative percentage in irregularly shaped sieve pores during positive δ¹⁸O excursions.

Microfossil and oxygen isotope trends indicate a recent trend towards gradual freshening and deterioration of the salinity structure in both the Rappahannock and Potomac Estuaries with a marked shift during the mid-to-late 19^th century. We attribute these trends to the combined influences of anthropogenically forced changes in the fluvial processes and short term climate changes that might include the last vestiges of the Little Ice Age and prolonged droughts during the late 19^th and early-to-middle 20 centuries.