A Centennial Record of Paleosalinity Change in the Tidal Reaches of the Potomac and Rappahannock Rivers, Tributaries to Chesapeake Bay

Abstract

Gravity and push cores from the Potomac and Rappahannock Rivers (Virginia Tidewater) were collected from central and proximal estuarine zones with known seasonal salinity stratification. The lowermost microfossil associations in the cores comprise alternating ostracode populations of Cyprideis salebrosa and Cytheromorpha. This microfossil association gives way to an oligohaline association dominated by the freshwater ostracode Darwinula stevensoni. Stable oxygen isotope values (ä O) of Rapphannock Cyprideis salebrosa are 18 highly variable ranging between -6.6 to -3.2‰ VPDB. ä O values for 18 Potomac Cytheromorpha fuscata range from -8.2 to -3.2‰ VPDB. Positive excursions in ä O values are synchronous with population peaks for both 18 Cyprideis and Cytheromorpha indicative of increased marine influence and/or higher salinities. Microfossil paleoecology coupled with oxygen isotope values record a marked shift towards gradual freshening and deterioration of the salinity structure in the tidal tributaries during the mid-to late 19 century. th We attribute these trends to both decadal climate trends and aggressive land use practices in the Chesapeake Bay watershed during the late 19 to middle th 20th centuries. INTRODUCTION Estuaries are physically, chemically, and biologically complex environments at the convergence of continental and marine processes. In the Chesapeake Bay, the recent combination of anthropogenic watershed modification and sea-level rise are forcing mechanisms that have potentially influenced mixing of fresh and marine waters in the tidal reaches of the major tributaries (Colman and Bratton 2003; Boon 2012). To test this hypothesis, sediment cores were collected from the proximal and central reaches Corresponding author: Neil E. Tibert, ntibert@umw.edu Virginia Journal of Science, Vol. 63, No. 3, 2012 http://digitalcommons.odu.edu/vjs/vol63/iss3 112 VIRGINIA JOURNAL OF SCIENCE of the Potomac and Rappahannock estuaries for microfossil and stable isotopic analyses. Paleosalinity indicators were established on the basis of ostracode paleoecology (population abundances and pore morphometrics) and oxygen isotope values (ä O). The paleosalinity trends were considered in the context of sedimentation 18 history based on Cs dating, organic matter concentrations, and magnetic 137 susceptibility of collected cores. Cumulative results of these analyses indicate that salinity gradients in both estuaries have changed markedly since the beginning of the 19 century which is suggestive of anthropogenic influence on estuarine processes in th the Chesapeake Bay. BACKGROUND Geographic Location The Chesapeake Bay is the largest estuarine system in the United States that is located between Virginia and Maryland on the Atlantic Coastal Plain (Colman and Mixon 1988). This study focuses on the upper tidal reaches of the Potomac and Rappahannock Rivers where they transition from estuarine-to fluvial conditions (Fig. 1). Sediment cores were collected near the boundary between proximal (oligohaline) and central estuary (mesohaline) near Aquia Creek in the Potomac estuary and Blandfield Point in the Rappahannock estuary (Ellison and Nichols 1970, 1976; Ellison 1972; USEPA 1998). Since the 1980s, these estuaries have produced the highest annual FIGURE 1. Sediment cores were collected from the Potomac and Rappahannock estuaries, both of which are tributaries to Chesapeake Bay, the largest estuary in the eastern United States (Colman and Mixon, 1988). Virginia Journal of Science, Vol. 63, No. 3, 2012 http://digitalcommons.odu.edu/vjs/vol63/iss3 Centennial Record of Paleosalinity Change 113 sediment yields for all major Chesapeake Bay tributaries (Langland and Cronin 2003). In general, the relatively high sedimentation rates in Chesapeake Bay are related to ongoing sea-level rise and anthropogenic watershed modification (Fig. 2) (Colman and Mixon 1988; Brush 1989; Colman and Bratton 2003; Boon and others 2010; Boon 2012). Estuarine Circulation The convergence of fluvial and marine waters creates a dynamic circulation pattern that impacts sedimentary processes in estuarine environments. Where freshwater outflow meets incoming saltwater in a partially mixed estuary like the Chesapeake Bay, the seasonal halocline (salinity gradient) forms a relatively impermeable surface to sediment transport that frequently coincides with the estuarine turbidity maximum (ETM)(Langland and Cronin 2003). In the Potomac estuary, the maximum extent of saltwater intrusion occurs near Aquia Creek (Elliott 1976). In the Rappahannock estuary, the maximum extent of saltwater intrusion occurs near Blandfield Point (Ellison and Nichols 1970). Our preliminary assessment of modern salinity structure revealed oligohaline conditions and/or weak haloclines near Aquia Creek and Blandfield Point during June 2009 (Fig. 3). Microfossils Ostracodes are aquatic crustaceans that are sensitive to changes in salinity and temperature (Frenzel and Boomer 2005) and their ecological associations have been used to make inferences about past environmental conditions in the Chesapeake Bay (Elliott and others 1966; Cronin and Grinbaum 1999; Cronin and others 2005, 2010). When used in conjunction with ä O values of their calcite carapaces, ostracodes can 18 be used to develop paleosalinity proxies in the context of both climate induced evaporation and mixing of marine and freshwaters (Anderson and Arthur 1983; Anadón FIGURE 2. Historic sea level data for Washington, D.C. illustrates a constant rise in sea level over the last century for the Potomac estuary (NOAA 2008). Virginia Journal of Science, Vol. 63, No. 3, 2012 http://digitalcommons.odu.edu/vjs/vol63/iss3 114 VIRGINIA JOURNAL OF SCIENCE and others 2002; Holmes and Chivas 2002; Ito and others 2003). Studies by Medley and others (2008) demonstrated that the shape of the sieve pores on the external surface of the carapace (Type C of Puri 1974) varies significantly with salinity which further improves the potential for past salinity determinations using ostracoda. METHODS Sediment cores were collected from the Rappahannock and Potomac estuaries with Ogeechee and Gravity (Wildco Inc.) corers (Fig. 1). The longer Ogeechee cores (>100 FIGURE 3. Late spring salinity (mg/L) and temperature ( C) profiles measured from o the (A) Potomac and (B) Rappahannock estuaries. This analysis was part of an unpublished assessment of water quality during June 2009. Virginia Journal of Science, Vol. 63, No. 3, 2012 http://digitalcommons.odu.edu/vjs/vol63/iss3 Centennial Record of Paleosalinity Change 115 cm) were analyzed with a Bartington MS2C Core Logging Sensor, and subsequently partitioned into 1 cm segments for loss on ignition and microfossil processing. The shorter gravity cores (40 cm) were divided into 2 cm segments and samples sent to Core Scientific International (Winnipeg, MB, Canada) for Cs analysis via gamma 137 spectrometry. Microfossil census counts were completed at 2 cm intervals using the methods outlined in Medley and others (2008). Sediment samples were rinsed on a 125 ìm mesh sieve and wet samples were examined with a stereoscopic zoom microscope (Nikon SMZ1500). Select ostracodes were photographed using a variable pressure scanning electron microscope (Hitachi S-3400N). Morphometric shape analysis was completed according to the methods described by Medley and others (2008). Sieve pores on external valves of Cyprideis were traced to determine the areas and shapes using ImageJ 1.3.1v (National Institutes of Health by Wayne Rasband). Values for circularity were calculated using the following formula: Circularity = 4ð (area/perimeter ). The 2 best-fit trendline of circularity versus area crossplots was used to determine the pore slope values. Ostracode carapaces were bathed in deionized water and sent to the Saskatchewan Isotope Laboratory (Saskatoon, SK, Canada) for oxygen and carbon isotope analysis via stable isotope ratio mass spectrometry (SIRMS). SIRMS preparation entails the heating of samples in vacuo to dissipate contaminants (e.g. organic matter and water that may influence isotope values) prior to analysis with a Finnigan Kiel-IV carbonate preparation device directly coupled to a Finnigan MAT 253 isotope ratio mass spectrometer. Data is expressed relative to the VPDB scale and calibrated to the NBS19 standard (ä C=1.95‰ VPDB; ä O=-2.2‰ VPDB). 13 18 Sediment accumulation rates were determined following the Cs analysis method 137 of Robbins and Edgington (1975). Cesium-137 is a radioactive isotope (half life=~30 years) that was released into the atmosphere during nuclear testing and simple gamma spectrometry can measure its concentration in sediment (USEPA 2010). Given that atmospheric levels of Cs peaked in 1963, a sampling site’s average sedimentation 137 rate can be calculated using the simple stratigraphic thickness above the peak divided by the time in years since 1963. To ensure accurate gamma spectroscopy results, at least 2 g of sediment were removed from the center of each core segment. Forty samples from two Potomac estuary cores were analyzed by Core Scientific International (Winnipeg, Manitoba). Select samples were pretreated for radiocarbon dating at the University of Pittsburgh following the methods outlined by Abbott and Stafford (1996). AMS C analyses were performed at the University of Arizona’s 14 Accelerator Mass Spectrometry Laboratory. RESULTS Physical Stratigraphy Core PT-09-C3 from Aquia Creek (Potomac River) comprises 134 cm of dark grey clay with a layer of fine sand in the basal 10 cm (Figs. 1, 4). Magnetic susceptibility values average 38.7 SI Units with a minimum value of 18.7 (124 cm) and a maximum value of 166.7 (134 cm). Total organic matter (TOM) averages 6.59 % with a minimum value of 1.42% (132 cm) and a maximum value of 9.68% (11 cm). The maximum concentration of Cs occurs between the 20-22 cm cored interval (Fig. 5). 137 Virginia Journal of Science, Vol. 63, No. 3, 2012 http://digit