|U.S. Geological Survey Open-File Report 2004-1350|
Chapter 3. Sediment transport in Pocomoke Sound, Maryland Interfered from Microfossils in Surface Sediments
|D.A. Willard 1, T.M. Cronin 1, C.E. Bernhardt 1, and J. Damon 1|
1U.S. Geological Survey, Reston, Virginia 20192
The distribution of pollen and benthic foraminifers in surface sediments of estuaries and adjacent tributaries can provide a tool to evaluate shoreline erosion and transport of sediment of various grain sizes from coastal marshes into the main channels of estuaries. Pollen grains produced by plants living in tidal marshes adjacent to estuaries can be transported from the location of their production, and, when compared to wind-blown pollen from terrestrial plants (i.e., oak, pine pollen), can provide evidence transport of sedimentary particles in the size range of 10-150 microns. Like marsh pollen, species of benthic foraminifers that, due to their ecology exclusively inhabit coastal marshes, can be transported by waves and currents into deeper water and provide evidence for transport of sedimentary particles in the size range of ~150 to 1000 microns. This chapter evaluates the distribution of pollen and benthic foraminifers in surface sediments in Pocomoke Sound (Figure 3.1) to assess the relative influx of sediments from shoreline regions.
The Pocomoke River is a blackwater, low-energy, low-gradient, Coastal Plain river on the Eastern Shore of Maryland, Delaware, and Virginia with a drainage area of 1,997 km2. The river is ~50 km long, and the upper 25 km were channelized in the 1920's to facilitate agricultural activities in that region. As early as 1939, much of the upper reaches of the watershed had been lumbered, drained and cultivated, but the areas near its tidal reaches are thought to be similar to the original forest (Beaven and Oosting, 1939). The Pocomoke watershed lies within the Oak-Pine forest region, which was dominated by oaks with abundant pines and hickories before land clearance (Braun, 1950). Adjacent to the river are bottomland swamps that ranged from <1 - 3 km wide in the 1920's. These swamps are dominated by Taxodium distichum (cypress) and Nyssa biflora (swamp black gum), with Acer rubrum (red maple) abundant in the understory. Fraxinus pennsylvanica (green ash) is common in the understory, and Liquidambar styraciflora is more abundant near the transition to upland forests (Beaven and Oosting, 1939; Alexander, 2003).
Pocomoke Sound is an oligohaline to lower polyhaline (<5 to >18 ppt), tidally-influenced embayment situated between the mouth of the Pocomoke River and the southern part of Chesapeake Bay. The main channel in the southern portion of the sound ranges from ~5-28 m in water depths. The main channel is surrounded on the north, west and east by very broad shallow areas < 4 m water depth. The Pocomoke Sound coastline consists mainly of tidal marshes, which provide habitats for marsh-dwelling foraminifera (Ellison and Nichols 1976).
Material and Methods
Two sets of surface samples were collected for analysis of microfossils, consisting of one set of estuarine samples for analysis of pollen and calcareous microfossils and the other a set of floodplain samples for analysis of pollen. Estuarine samples were collected during May 2001 in a series of transects in Pocomoke Sound, Maryland (Figure 3.1). Some of the samples were collected using the R/V Kerhin and a Van Veen sampler, and others were collected using a Ponar grab sampler (Table 3.1). From each sample, the upper 2 cm were collected for microfossil analysis. Approximately 10 grams (wet weight) were sampled for pollen analysis, and 30-40 grams were processed for benthic foraminifera.
Floodplain samples were also collected at eight sites along the length of the river between 1998 and 2000 as part of a broader research project on modern pollen deposition within forested wetlands (Figure 3.2). At each floodplain site, a series of transects was laid out perpendicular to the river channel; clay pads were placed at approximately 50 m intervals along each transect. At the ends and midpoints of each transect, we collected "mini-cores" of the upper 1-2 cm of sediment using 50 ml centrifuge tubes. After clay pads had been in place for one year, we also collected sediment that accumulated on them for comparison with the adjacent surface sediment. The surrounding vegetation was described to correlate pollen assemblages with source vegetation.
Pollen was isolated from sediments using standard palynological preparation techniques (Traverse, 1988; Willard and others, 2003). After drying the sediment, one tablet of Lycopodium spores was added to each sample. 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 water bath at 70° C. After neutralization, residues were sieved with 149 µm and 10 µm nylon mesh 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 were counted from each sample to determine percent abundance and concentration of palynomorphs.
Microfossil samples were washed using a 63 µm sieve; 100-300 foraminifera per sample were identified and counted from the >150 µm fraction to calculate percent abundance. Sediment between 63 and 150 µm were also scanned for qualitative analysis of smaller benthic foraminifera. We used the taxonomy of Ellison and Nichols (1976) to identify the foraminiferal species; descriptions of Chesapeake Bay foraminifera can be found in Cronin and others (1999), also available online at http://pubs.usgs.gov/pdf/of/of99-45/. Additional ecological information and references are provided in Ellison (1972), Buzas (1969, 1974), Cronin and others (2000) and Karlsen and others (2000). Pollen and foraminiferal data are available online at http://geology.er.usgs.gov/eespteam/Atlantic/index.htm. Pollen data also are available from the North American Pollen Database (http://www.ngdc.noaa.gov/paleo/pollen.html).
Results and Discussion
Pollen: Pocomoke Sound
Pollen assemblages were dominated uniformly by Pinus pollen (41-71%) with Quercus pollen subdominant (9-29%). Other trees consistently present in low percentages include Carya (1.5-6%) and Liquidambar (sweet gum) (1-9%) (Table 3.2). Nyssa pollen is present in low percentages (usually <1%) in most samples. Pollen of TCT (Taxodiaceae/Cupressaceae/Taxaceae) is most abundant in samples near the river mouth. Pollen of marsh taxa, including members of the Poaceae (grasses), Cyperaceae (sedges), Asteraceae (excluding Ambrosia (ragweed)), and Typha (cattail), exceed 10% abundance in all sites with water depth = 2 m and typically comprise <5% at water depths >3 m (Figure 3.3). Ambrosia pollen, a common indicator of land clearance, is present in all samples (0.3-5.3%) but shows no apparent correlation with water depth or distance from shore.
Pocomoke Sound pollen assemblages correspond well with the regional pollen rain preserved at other sites in the southern half of the bay in containing >40% Pinus pollen and ~10-30% Quercus pollen. In contrast, sites from northern Chesapeake Bay have <40% Pinus pollen and >30% Quercus pollen (Willard and others, in review). When compared to assemblages from deep-water sites in Pocomoke Sound and elsewhere in the bay, assemblages from shallow-water sites (<2 mwd) are distinguished by the greater abundance of pollen of herbaceous taxa, particularly the non-Ambrosia Asteraceae and Poaceae. These represent marsh taxa growing on or near the shore, and the greater abundance of their pollen in shallow, nearshore surface sediments in Pocomoke Sound indicates that their pollen is not usually transported far from the shoreline source. A similar pattern was noted using Chenopodiaceae pollen in the Port Tobacco Estuary of the Potomac River (Defries, 1986). We also noted that Ambrosia pollen is less abundant in most Pocomoke Sound surface samples (0-5.3%) than in the mainstem of Chesapeake Bay (typically 3-14%). Because Pocomoke Sound is surrounded by marshes and forests, the lower Ambrosia abundance in the Sound probably reflects the relatively small amount of land clearance and agricultural activity in the area surrounding the Sound.
Pollen: Pocomoke River Floodplain Sites
Floodplain pollen assemblages are dominated by a combination of Liquidambar, Pinus, and Quercus pollen, depending on their location within the watershed. In general, upstream sites have greater percentages of Liquidambar, whereas downstream sites have greater abundance of Pinus and Quercus pollen (Table 3.2). North of Whitons Crossing, the river has been channelized extensively to improve drainage for agriculture. At sites in this part of the river (Delaware Crossing, Cypress, Willards, Whitons Crossing), Liquidambar pollen abundance is greatest (25-61%), and Acer pollen is relatively common (4-7%); Pinus and Quercus pollen comprise 10-24% and 6-21% of assemblages, respectively. At non-channelized sites farther downstream, Pinus and Quercus average 43% and 14% of the assemblages, respectively, whereas Liquidambar pollen comprises only 4-11% of assemblages. At all of these floodplain sites, Nyssa pollen is abundant (up to 33%) and Taxodium common (up to 17%). Herbaceous pollen (Chenopodiaceae, Asteraceae, Poaceae) is relatively rare (1-3%), and Ambrosia pollen comprises up to 12% of assemblages.
The floodplain sites differ from the estuarine sites in their lower abundance of Pinus pollen (typically <45%) and much greater abundance of Liquidambar and Nyssa pollen (usually >10% and >5%, respectively). Pollen of Taxodium and Acer also are common in floodplain deposits, averaging 2-6% and 1-7% of assemblages, respectively. The comparatively poor representation of pollen of these taxa in sites distal to the forested wetland indicates that pollen of these plants is preferentially deposited under the forest canopy with minimal fluvial transport from the forested wetland downstream to the sound.
Foraminifera: Pocomoke Sound
Benthic foraminiferal assemblages from surface sediments from Pocomoke Sound reveal the salinity-influenced distribution pattern shown graphically in Figure 3.4. In the distal-most regions of the sound, where highest salinities are found nearest the marine source, Elphidium, a genus preferring polyhaline salinity in Chesapeake Bay, is the dominant taxon. As one moves northward into generally lower salinity regions, many samples have greater relative proportions of Ammonia, which prefers mesohaline salinities. Still closer to the Pocomoke River mouth, one finds increased proportions of Ammobaculites, a genus that thrives on organic substrates in salinities usually < 5-10 ppt. Some nearshore samples also contain low proportions of Miliammina fusca, a species which inhabits shallow, nearshore habitats off tidal marshes. Finally, in a sample taken close to shore in the northern part of the sound (TR-3), one finds marsh-dwelling species of Trochammina, with lesser amount of Miliammina, and Ammoastuta.
Although there are exceptions to this general pattern, these results are consistent with findings in other regions of the Chesapeake Bay region that the primary control of benthic foraminiferal distributions in Chesapeake Bay is the strong salinity gradient in the estuary. Ellison and Nichols (1970) observed nearly identical qualitative patterns in tributary-to-bay transects of foraminiferal samples taken in the Rappahannock River and by Ellison and Nichols (1976) in the James River. With the exception of relatively large proportions of Ammobaculites at PC-6B (station 2, Figure 3.4) and low proportions of marsh species at PC-3B (station 6b; Figure 3.4), the overall pattern indicates surface sediments contain assemblages of benthic foraminifers that lived at or near the sampling stations. Minimal transport of foraminiferal specimens from marshes and adjacent shallow water regions is indicated by these results.
The results of our analyses of pollen and foraminifera from Pocomoke Sound surface sediments suggest that neither the upstream forested wetlands nor coastal marshes bordering the sound have contributed appreciably to particulate matter in the 10- to 1000-micron size range that is currently being deposited in the sound. These results can be interpreted in light of monitoring records of SAV and coastal habitats from the Pocomoke region. In a comprehensive study using aerial and field analysis of SAV beds, water quality monitoring data, and spatial data on coastline from the Maryland Geological Survey (MGS), Orth and others (2002) conducted detailed analyses of SAV trends in Tangier and the northwestern border of Pocomoke Sound for the period 1992-1998. While acknowledging that coastal land loss can have both a negative and positive impact on SAV beds, they concluded that during that period there was no apparent correlation between land loss measured by the MGS spatial data and SAV coverage in the greater Tangier-northern Pocomoke Sound region.
Orth and others (2002 also conducted an analysis of shoreline history in the vicinity of Cedar, Clump and Big Islands along Pocomoke Sound's northern border. They showed that this region had either a stable or an accreting coast between the years 1942 and 1988. This situation is in sharp contrast with most regions of Tangier Sound, which experienced low to severe coastal land loss. Thus, the available evidence from historical maps of coastal regions in the northern part of our study region suggests that it is unlikely that major erosion of marshes occurred during the past few decades. Although long -term shoreline data were not analyzed for the Virginia portion of Pocomoke Sound, if the trends in Maryland apply to other parts of Pocomoke Sound, it would be consistent with our findings of minimal transport of marsh pollen and foraminifers to central parts of the sound.
We thank Rick Younger, Jeff Halka, and the crew of the R/V Kerhin for assistance in obtaining surface samples in Pocomoke Sound. Cliff Hupp, Mike Shenning, Patrick Buchanan, and Lisa Weimer helped collect surface samples from the Pocomoke River floodplain. We thank Tom Sheehan and Chris Nytch for laboratory assistance.
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