Late Holocene Sedimentation and Paleoenvironmental History for the Tidal Marshes of the Potomac and Rappahannock Rivers, Tributaries to Chesapeake Bay

Abstract

Instrumental tide gauge records indicate that the modern rates of sea-level rise in the Chesapeake Bay more than double the global average of 1.2-1.5 mm yr-1. The primary objective for this study is to establish a relative depositional history for the tidal marshes of the Potomac and Rappahannock Rivers that will help us improve our understanding of processes that influence sedimentation in the proximal tributaries of Chesapeake Bay. Marsh cores were collected from Blandfield Point VA, Tappahannock VA, and Potomac Creek VA. The sedimentary facies include: 1) a lower unit of organic-poor, grey clay with fine sand and silt layers and estuarine foraminifera; and 2) an upper unit of organic-rich clay and peat with abundant brackish to freshwater marsh foraminifera and thecamoebians. AMS 14C dating of bulk marsh sediments yield sedimentation rates at Potomac Creek ranging from 3.04-4.20 mm yr-1 for the past 2500 years. Rates of sedimentation calculated for Blandfield Point indicate 1.37-2.19 mm yr-1 in the basal clays and peat for the past ~3000 years. Foraminiferal census counts indicate a freshening upward trend with a transition from an estuarine Ammobaculites crassus assemblage to a marsh Ammoastuta salsa assemblage with abundant freshwater Thecamoebians. The late Holocene history of sedimentation for the marshes indicates that differential compaction, recent land use practices, and climate change have contributed to the resultant freshening-upward environmental trend and variability in sediment accumulation rates between coring sites. Corresponding author: Neil E. Tibert ntibert@umw.edu 92 VIRGINIA JOURNAL OF SCIENCE INTRODUCTION The Chesapeake Bay watershed comprises numerous tributaries draining from the eastern Appalachian Mountains. The central axis to the Chesapeake has been evaluated in the context of decadal, centennial, and millennial climate changes (Cronin and others 2005, 2010). In the historic Northern Neck region of Virginia, the tidal reaches of the Rappahannock and Potomac Rivers (Fig. 1) have received little detailed study with respect to the nature of the sedimentary record spanning the past several thousand years. Recent estimates for eustatic sea level are estimated to be as high as 1.5-1.88 mm yr (Church and White 2006, Nerem and others 2006) whereas the instrumental tidal -1 FIGURE 1. Location map for the tidal reaches of the Potomac and Rappahannock Rivers. Table1 lists the coordinates and detailed coring information for Sites A-C. Table 2 list coordinates and details for the tide gauge stations (Sites 1-4). Inset shows our location along the eastern Atlantic coast of the USA. Late Holocene Sedimentation 93 records from the Chesapeake Bay indicate rates as high as ~3-4 mm yr (Boon 2012). -1 The disparity between global and regional base level change in the Chesapeake Bay is not well understood and likely reflects the combined effects of allogenic, autogenic, and anthropogenic processes in the region (Cronin 2012). The primary objective for this paper is to establish a late Holocene sedimentation and paleoenvironmental history for the tidal reaches of the Potomac and Rappahannock Rivers in the Northern Neck region of Virginia, USA. Our primary analytical tools include physical stratigraphy (loss on ignition, grain size, and magnetic susceptibility), foraminiferal paleoecology, and AMS C geochronology applied to cores collected from the central estuarine region of the 14 tidal Potomac and Rappahannock Rivers. BACKGROUND The Chesapeake Bay is the largest estuary in the United States, with shores bordering the states of Virginia, Maryland, and the District of Columbia. The watershed area of this coastal plain estuary is 167,000 km that includes the following major 2 tributaries: Susquehanna, Potomac, Rappahannock, York, and James Rivers (Boesh and others 2001). The Chesapeake Bay is the product of Holocene sea-level rise formed by fluvial incision coupled with the inundation of river valleys following the terminus of the last glacial maximum (Schubel and Pritchard 1986). The Chesapeake Bay is located in an apparently inactive tectonic region on the North American passive margin. However, many Cretaceous age faults have been identified in close proximity to our localities in the Fredericksburg, VA (Table 1) which marks the transition from the Piedmont region (west) to the coastal plain (east) in Virginia (Fig. 1) (Berquist and Bailey 1999). Lower Tertiary sedimentary deposits in the region include fine-to coarse glauconitic quartz sand and clay-silt of the Lower Tertiary Pamunkey Group (Brightseat, Aquia, Marlboro, Nanjemoy, and Piney Point formations) (Mixon and others 1989). TABLE 1. List of sampling localities from the Potomac and Rappahannock tidewater region of Virginia and Maryland. Site Location Longitude Latitude Geographic Info Site A Blandfield Point VA 76°54'40.436"W 38°0'6.911"N Blandfield Marsh on Rappahannock River (proximal estuarine zone 0-5 ppt) Site B Tappahannock Harbor VA 76°51'15.368"W 37°55'16.723"N Coleman's Island, Hoskin's Creek tributary to Rappahannock River (distal tributary to central estuarine zone) Site C Potomac Creek VA 77°20'7.619"W 38°21'6.972"N Potomac Creek tributary to Potomac River (central estuarine zone 5-15 ppt) 94 VIRGINIA JOURNAL OF SCIENCE During the past several decades, the National Oceanic and Atmospheric Administration (NOAA, 2009) has maintained tidal gauging stations at Colonial Beach and Washington DC (Table 1). The sea level rates calculated from the instrumental records on the Potomac River range from 3.16-4.78 mm yr from Washington DC and -1 Colonial Beach respectively (Table 2), which are significantly higher than eustatic values of 1.0-1.5 mm yr (Table 2) (NOAA 2009; Boon 2012). The instrumental -1 records from the lower Rappahannock at Sewell’s point record a relative sea-level rise of 4.44 mm yr spanning the past 84 years. -1 Cronin and others (2000, 2005, and 2010) and Cronin and Vann (2003) reported microfossils from cores (~2-6 m in thickness) located at the mouths of the major tributaries in the central regions of the bay (e.g., Patuxent, Choptank, and the Potomac Rivers). Willard and others (2003) and Cronin and others (2003) reported a highresolution historical microfossil record that apparently discriminates important anthropogenic events such as the Medieval Warm Period and deforestation of the bay region with the arrival of European settlers. METHODS Marsh cores were collected from the Rappahannock and Potomac Rivers that includes Blandfield Point (Site A), Tappahannock Harbor (Site B), and Potomac Creek (Site C) (Table 1) (Fig. 1). A square-rod piston coring device was used to collect continuous 1-meter long core drives down a single coring hole (Wright 1967). Individual core sections were split along a longitudinal axis to produce two equal halves. Potomac Creek cores were evaluated for microfossils at 10 cm intervals. Approximately eighty 1cm sediment samples were soaked in a beaker of warm water 3 and mild detergent to disperse the clays (Scott and Leckie 1990). Samples were rinsed over a 63 μm sieved and picked wet using conventional microfossil methods (Scott and Medioli 1980). Each sample was then examined for foraminifera and relative abundances were calculated for species and select genera to simplify the trends. Exceptionally preserved specimens were examined on the scanning electron microscope (SEM) for identification and illustration purposes. TABLE 2. Tidal gauge data for the Chesapeake Bay (NOAA, 2009). Locality Instrumental Records SL Rate mm yr-1 YBP Tidal Station & Data Set Info NOAA Monthly Mean 1 Washington DC 3.16+0.35 87 8594900 (1924-2006) 2 Colonial Beach VA 4.78+1.21 39 8635150 (1972-2003) 3 Sewells Point VA 4.44+0.27 84 8638610 (1927-2006) 4 Solomons Island MD 3..41+0.29 74 8577330 (1937-2006) x Global average 1.5+0.5 0 Late Holocene Sedimentation 95 The total organic matter (TOM) was determined by using loss on ignition (LOI) (Dean 1974). Grain size analyses were conducted using methods modified from McManus (1988). Volume magnetic susceptibility was conducted on sediments using a Bartington MS2E surface scanner following the method of split-core logging of Last and Smol (2001). Select bulk sediment 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 Accelerator 14 Mass Spectrometry Laboratory and the dates calibrated using Calib 6.1.0 (Reimer and others 2009). RESULTS Sedimentary Facies Grey Clay Facies: The basal sediments at all coring sites comprise clay and sparse interbeds of silt and sand (Fig. 2). The grey clay facies ranges in thickness from ~7.54.25 m at Potomac Creek to ~5.5-2.5 at Tappahannock Harbor (Fig. 2). TOM values in the organic-rich clay range from ~8-28%. Magnetic susceptibility values are relatively low with positive excursion peaks in the silt-rich layers. Grain size analyses at Tappahannock Harbor indicate a coarsening-up trend from mud-to-silt and fine sand (Fig. 2). Foraminifera in the organic-rich grey clay are dominated by Trochammina inflata, and Ammobaculites spp. in association with sparse Ammoastuta salsa and Miliammina fusca. (Fig. 3). Peat & Clay Facies: All cores contain an upper unit of alternating peat and grey clay with TOM values that range from ~20% to 85% (Fig. 2). Magnetic susceptibility values are relatively low with little variability. Microfossil populations in this facies are dominated by Ammoastuta salsa and Miliammina fusca. Trochammina inflata and Jadaminna macrescens are also common while Haplophragmoides is the least abundant (Fig. 3). Sedimentary cores from Blandfield Marsh and Potomac Creek (Fig. 2) are capped with an uppermost rooted zone of the grass Phragmites and the freshwater thecamoebian Arcellacea sp. (Figs. 2, 3). Core Chronology & Sedimentation Rates Accelerator Mass Spectrometry (AMS) C dat