|U.S. Geological Survey Open-File Report 2004-1350|
Chapter 1. Introduction to Pocomoke Sound Sediments
|T. M. Cronin 1, D.A. Willard 1, W. Newell 1, C. Holmes 2, J. Halka 3, M. Robertson 1|
1U.S. Geological Survey, Reston, Virginia 20192; 2U.S. Geological Survey, St. Petersburg, Florida 33701; 3Maryland Geological Survey, Baltimore, Maryland 21218
Sediment carried by rivers and eroded from shorelines is a major environmental problem in Chesapeake Bay and its tributaries due to its direct effect on water clarity and indirect effect on living resources, notably submerged aquatic vegetation (SAV) (Orth and Moore, 1983; Batiuk and others, 2000). Although there have been numerous studies of sedimentary processes in Chesapeake Bay and surrounding regions during the past 50 years, there is still considerable uncertainty about the relative contributions of river-borne and shoreline sources of sediment, the contribution of oceanic-source sediment in the southern bay, and the impact of land-use changes on the sediment budgets (Langland and Cronin, 2003). An improved understanding of patterns of sediment transport in the bay is particularly important for future management decisions about remedial strategies in various tributaries and watersheds, and for improving watershed modeling efforts to simulate the effect of best management practices on sediment flux.
The present study was designed to determine patterns of sedimentation and sediment-related processes in Pocomoke Sound using a variety of geological methods. Pocomoke Sound is a large body of water located in the southeastern part of Chesapeake Bay (Figure 1.1) with several characteristics suitable for the study of temporal and spatial sedimentary patterns. Its main channel (~24 m water depth) is separated from the deep channel of the bay's mainstem and the deep channel in Tangiers Sound, located to the north, by a relatively shallow (<2 to 12 m) region in the vicinity of Watts Island. The shallow areas in the Pocomoke and Tangiers Sound region are critical habitats for SAV, however both Tangiers and Pocomoke Sounds experienced declines in SAV acreage between 1992 and 1999 (Orth and others, 2002). More generally, it is generally believed that significant declines in SAV during the 20th century has been caused by land-use changes in the bay's watershed. To more fully understand the relationship between SAV, water clarity and sediment processes, a long-term perspective regarding the sources of sediment in Pocomoke Sound is needed. This chapter provides a brief overview of previous studies, methods used, and modern hydrography for Pocomoke Sound.
The Pocomoke Sound channel is the site of a thick (10-25 m) sequence of finegrained Holocene sediments deposited over the past 8,000 years, which preserves a record of sedimentation both before and after colonial land clearance and under different climatic and hydrological extremes. Sediment consists of particulate material entering Pocomoke Sound from four potential sources: river-borne sediment from the Pocomoke River, sediment eroded from shorelines (mainly tidal marsh sediments), sediment entering the sound from the south where the Pocomoke Sound channel extends to near the mouth of the bay (see Langland and others, 2003), and biogenic particulate material formed by phytoplankton and other organisms living in the sound. The first three sources of sediment are the main focus of this report. Biogenic material produced in the water column by organisms is not directly considered in the current study, however some of our results regarding climatically-driven changes in Pocomoke Sound salinity are pertinent to the inflow of nutrients and primary productivity over decadal and centennial timescales.
The amount of river-borne sediment entering the sound will be a function mainly of the watershed topography, geology, land cover, climatological and hydrological characteristics. The Pocomoke River basin consists of an 813 mi2 watershed, which is relatively small compared to other major tributaries to Chesapeake Bay. The Pocomoke watershed is currently comprised of 48 % forested, 34 % agriculture, 14 % wetlands, and 4 % urban land-use. It is comprised of a relatively low-lying coastal plain and, in contrast to watersheds of bay tributaries on the western side of the bay, does not drain parts of the Piedmont and Appalachian physiographic provinces. The Pocomoke River, which enters Pocomoke Sound in its the northeastern corner, is also a relatively small river in terms of mean annual discharge. In these ways, the Pocomoke system represents a distinct contrast with the larger tidal tributaries located on the west side of the mainstem bay such as the Potomac, James, Rappahannock, and Patuxent Rivers, which are subestuaries characterized by strong vertical salinity gradients and turbidity maximum zones like those in the mainstem bay.
Shoreline erosion can contribute as much as 50 % of the total sediment load in some areas of Chesapeake Bay, depending on coastal characteristics and other factors (Miller, 1987). Like much of the eastern shore of Chesapeake Bay, low-lying tidal marshes form the margins of much of Pocomoke Sound, in contrast to parts of the western side of Chesapeake Bay, where coastal seacliffs, dunes and other features occur. Erosion of marsh habitats by tides, waves, storm surges and rising sea level could, in theory, contribute a large proportion of sediment to the Pocomoke Sound.
A large flux of sediment enters Chesapeake Bay at its mouth originating from the continental shelf (Ludwick, 1974; Byrne and others, 1983; Colman and Hobbs, 1988). Although much of this sediment is coarse-grained (sand) and there is evidence for both bayward and seaward transport, it is unclear how much net fine-grained (clays and silts) sediment comes from oceanic sources or how far north it is transported. In their comprehensive survey of surficial sediments in the Virginia portion of Chesapeake Bay, Byrne and others (1983) concluded that significant amounts of coarse-grained and probably fine-grained sediments derived from the continental shelf were advected northward from the baymouth region. Moreover, oceanographic studies of drifters (Norcross and Stanley, 1967) and bottom currents (Bumpas, 1973; Beardsley and Boicourt, 1981) indicate strong nearshore bottom flow into Chesapeake Bay. Therefore, Pocomoke Sound must be considered a potential sink for ocean sediments carried northward from the baymouth, as well as a catchment area for river-borne and shoreline sediments from the Pocomoke Sound area itself.
In summary, the current study is designed to provide a basic understanding of sediment deposition in this unique region of the greater Chesapeake area through the analysis of various micropaleontological, geochemical, and geochronological indicators found in surficial sediments and sediment cores collected in Pocomoke Sound.
Previous Studies of Pocomoke Sediments
Previous studies of the Pocomoke Sound applied sedimentological, geological, and geophysical (seismic) methods to determine the basic characteristics and distribution of the sediments deposited in this region. As part of a comprehensive analysis of surface sediment characteristics in the Virginia portion of Chesapeake Bay, Byrne and others (1983) found that the surface sediments in Pocomoke Sound generally conformed to patterns found through the mainstem (Kerhin and others, 1988); that is, courser grained sediment (mostly sand sized) blankets the shallower regions (< ~6-10 m water depth) and fine-grained sediment (clays and silts) covers the deeper regions in the channels. Hobbs, (1983, see also Byrne and others, 1983 for review of previous studies) analyzed the carbon and sulfur content of Chesapeake Bay surficial sediments, including some samples from the Pocomoke region. Hobbs (1983) concluded that finer grained sediments in Pocomoke Sound contain 1.5-2% carbon, and generally have higher proportions of carbon than course-grained sediments. The sulfur content in Pocomoke sediments was about 0.2 - 0.4%.
Byrne and others (1983, see also Byrne and Anderson 1977) also used 19th and 20th century (1850 versus 1950) bathymetric surveys, short sediment cores, and coastal geomorphological and geological data to construct a sediment budget for the southern part of Chesapeake Bay. Although the focus of their study was the mainstem of Chesapeake Bay, their observation that shoreline-derived sediment is highest in the vicinity of headlands and lowest near river mouths and low-lying marsh coastlines, suggests that the marsh-dominated Pocomoke coastline is not a major sediment source for Pocomoke Sound.
Colman and Hobbs (1988) used seismic profiles to delineate the positions of the Quaternary paleochannels in southern Chesapeake Bay and parts of Pocomoke Sound formed during glacial periods of low global sea level. They showed that the thickness of Holocene overlying the Cape Charles erosion surface was between 10 and > 25 m thick, similar to that in the mainstem of the bay. These geophysical data were used in the present study to select sites for sediment coring and analysis.
The lower portion of Pocomoke Sound was also studied by Hobbs (1988) who examined the prospect of mining fossil oyster shells near the mouth of the sound. Hobbs ran shallow seismic-reflection profile surveys through portions of Pocomoke and Tangiers Sounds under the assumption that oysters had likely grown along the walls of the Pocomoke channel at times when sea level was lower. Based on eleven vibracores ranging from 5.5 to 12 m of sediment recovery, Hobbs (1988) concluded that the oyster deposit near Parkers Rock in Pocomoke Sound is a low density, widely distributed, nearsurface deposit with approximately 178,450m3 of shell within the larger area of 1,003,200m2.
Materials and Methods
The current study involves several approaches to the study of Pocomoke Sound Study. The objectives and methods of each approach are briefly summarized here and discussed in more detail with the results in later chapters. Lithological, chronological, micropaleontological and X-raadiograph data can be viewed and obtained from the website http://geology.er.usgs.gov/eespteams/Atlantic/index.htm
Sediment coring. Sediment cores were taken aboard the vessel R/V Kerhin, May 15-16, 2001, and September, 17-19, 2001. Piston and gravity corers were collected from shallow-deep-shallow transects in the southern, central and northern regions of the sound. Sediment recovery ranged from 73 to 200 cm in May 2001 and longer cores (500 cm) were taken at sites PC-2B and PC-6B in September 2001 (Table 1.1). All sites were located using Differential Global Positioning System (DGPS), and cores were stored in 6.7 cm diameter cellulose acetate butyrate (CAB) core liners.
Twenty-eight surficial sediment samples were also collected along transects in Pocomoke Sound (Table1.2), and replicates of these were collected during the May and September, 2001 cruises for the purpose of evaluating the sources of modern Pocomoke Sound sediments. The surficial sediments were collected with a Van Veen sampler, and subsampled with 4.25-inch diameter core liners, before sectioning the upper 4 cm into 1- cm intervals. During May 14-16, 2001, a smaller support boat collected additional surface samples in the shallow waters of Pocomoke Sound that could not be reached with the R/V Kerhin. This sampling involved the use of a Ponar grab sampler that homogenized the upper few centimeters of sediment.
X-radiograph analysis. X-radiographs of bay sediments provide valuable information on sedimentary structures (burrowing, laminations etc), grain size, the occurrence of gas, and other properties that are important in the interpretation of the chronology for the core and microfossil and geochemical patterns. Sediment cores were x-radiographed in their liners at the Maryland Geological Survey (MGS) using a Xerox 125 xeroradiograph processor. X-radiographs assist in delineating small-scale internal structures in the cores such as clam burrows, shells or gas voids. On the negative film, denser material, such as a shell, appears lighter whereas the less dense material, such as a gas void or burrow, appears darker. After X-radiography, sediment was extruded from the core liner, split along the axis, digitally photographed, examined and described. The physical characteristics and sedimentary structures were logged, and the core descriptions emphasized qualitative grain size. Visible shells were immediately sampled from selected horizons for radiocarbon dating.
Radiocarbon and lead-210 dating. Establishing an accurate age-model for the deposition of Pocomoke sediments is essential for interpreting temporal patterns of sedimentation. Three methods were used to date Holocene sediments from the cores: radiocarbon (14C), lead-210 (210Pb) radiometric dating, and pollen stratigraphy (see below). Radiocarbon ages on shells of estuarine mollusks (Cronin and others, 2000; Colman and others, 2002) can provide ages on Holocene sediments older than about 400- 500 years. Lead-210 dating was used in one core to establish the rate of sedimentation over the past century and with pollen stratigraphy, especially the relative proportion of Ambrosia (ragweed) pollen, it was used for chronology over the past 200 years. It was also possible to correlate cores from within Pocomoke Sound and to those from other regions of the bay using benthic foraminifers and ostracode faunal assemblages and stable isotopic ratios (Cronin and others, 2000; Karlsen and others, 2000; Cronin and Vann, 2003). The results of the various dating and correlation methods are discussed in the following chapters.
Microfossil Processing. Following X-radiography, cores were sampled for calcareous microfossil and pollen analysis to reconstruct the region's ecological history. To analyze fossilized shells of calcareous organisms (benthic foraminifera and ostracodes) as indicators of paleoecological history, sediment samples taken every 2 to 10-cms were washed through a 63-µm sieve. The remaining sand-sized portion was dried at ~52°C in a convection oven for twenty-four hours. After 24 hours, the sediment was stored in 8 ml clear glass vials, and the portion of sediment > 150 µm s was picked for foraminifera and ostracodes under a microscope using a fine brush (see Karlsen and others, 2000 for details).
Pollen was isolated from sediments at 10-cm intervals using standard palynological preparation techniques (Traverse, 1988; Willard and Korejwo, 2000). One tablet of Lycopodium spores was added to each sample (5-7 g dry weight) to calculate its pollen concentration. Samples were processed with HCL and HF to remove carbonates and silicates respectively, acetolyzed (1 part sulfuric acid: 9 parts acetic anhydride) in a boiling water bath for 10 minutes, neutralized, and treated with 10% KOH for 10 minutes in a 70°C water bath. After neutralization, residues were sieved with 149 µm and 10 µm nylon meshes to remove the coarse and clay fractions, respectively. When necessary, samples were swirled in a watch glass to remove mineral matter. After staining with Bismarck Brown, palynomorph residues were mounted on microscope slides in glycerin jelly. At least 300 pollen grains and 300 dinocysts were counted from each sample to determine percent abundance and concentration of palynomorphs.
Beryllium-7 sampling. The temporal and spatial distribution of berylium-7 (7Be) can be used to identify sites of recent deposition and short-term variability in sediment delivery and deposition rates in Pocomoke Sound. 7Be is formed in the atmosphere by the interaction of cosmic rays and nitrogen and has a very short half-life of 53 days. This reactive element adheres to sediment immediately; thus, its presence in the sediment is a useful indication that the sediment was in contact with the atmosphere during the previous few months and indicates a recent influx of new material or resuspension of existing sediments. The spatial distribution of 7Be in the surface sediments of Pocomoke Sound was analyzed for samples taken during cruises in May and September, 2001 (see Chapter 2).
Hydrography of Pocomoke Sound
Hydrographic conditions of Pocomoke Sound have been monitored at the Chesapeake Bay Program's (CBP) monitoring stations EE3.3 and EE3.4 since 1984 (Figure 1.1) and are available at the CBP Chesapeake Information Monitoring System (CIMS) website (http://www.chesapeakebay.net/cims/). CBP data were used to examine the characteristics of the hydrography of the sound, including its levels of total suspended sediments. These stations monitor only a small region in the sound and only provide data to water depths of 7 and 8 m, and therefore a complete analysis of the spatial and temporal variability in Pocomoke Sound physical parameters and water quality is not available. Nonetheless, they provide a long-term (20 year) record of conditions that can be compared to other regions of the bay.
Figure 1.2 illustrates the mean annual values for water temperature, salinity, and surface dissolved oxygen at CBP station EE3.3 for the period of record. The figure shows a typical annual temperature cycle from ~ 5 to > 25°C, and a salinity cyclereflecting lowest salinities during the spring runoff (~15 ppt) and highest salinities during the fall (~19 ppt). Although monitoring data are not available from most of the sound, we expect lower salinity closer to the mouth of the Pocomoke River and higher salinity nearer the southern end of the sound, reflecting the mixing of fresh and marine waters.
Figure 1.3 shows a vertical profile of temperature, salinity and dissolved oxygen from CBP stations EE3.3 and EE3.4 from spring and fall, 2000. They illustrate relatively small differences in temperature, salinity, and dissolved oxygen within the upper 2.5 to 4.0 m of water at each site. Monitoring data below 4 m water depth is not available.
Figure 1.4 compares the seasonal, interannual, and depth related (2-4 m versus 0.5 m) variability in total suspended solids (TSS) at Pocomoke Sound station EE3.3 to the same data from a mainstem bay station (CB4.4). One sees that TSS in Pocomoke exhibits a large amount of variability with generally (but not exclusively) the highest values occurring during spring months. The patterns for shallow water depths (2-4 m) and surface zones (0.5 m) are generally similar ranging from 10-40 mg L-1, reflecting their proximity to one another. In contrast, there is a strong contrast between surface (0.5 m) and deep bottom (24.5-31 m) TSS patterns at mainstem station CB4.4. It is notable that the bottom TSS levels at station CB4.4 are typical of other deep channel sites where strong wintertime pulses of TSS exceeding 80 mg L-1 rise above a baseline level of < 10 mg L-1.
Seasonal depletion of dissolved oxygen (DO) is a major environmental problem in Chesapeake Bay and DO is monitored monthly or bimonthly at many stations around the bay. Graphical depiction of spatial and temporal patterns of hypoxia available from the Chesapeake Bay Program show dissolved oxygen in the southern part of Pocomoke Sound can be mildly hypoxic (1-3 mg L-1) during relatively wet years with high river runoff, although most years Pocomoke Sound is well oxygenated. However, monitoring sites in Pocomoke Sound are sparse compared to the rest of the mainstem bay, which experiences complete hypoxia and anoxia during many years.
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