Chapter 7. Pollen Stratigraphy and Land-Use Change, Pocomoke Sound, Maryland

Introduction

The pollen record of land-use change in Chesapeake Bay is generally well-understood for the northern and central parts of Chesapeake Bay, showing increases in abundance of weedy species such as Ambrosia (ragweed) and Poaceae (grasses) occurring during times of maximum land clearance and preferential loss of hardwoods due to logging (Brush, 1984; Willard and others, 2003). Because Ambrosia aggressively colonizes cleared land within the first year of clearance (Bazzaz, 1974) and produces large quantities of pollen, it is an excellent biostratigraphic marker in eastern North American sediments and has recently been put to use in establishing post-Colonial sedimentation rates in estuarine and terrestrial sediments (Cronin and others, 2003; Townsend and others, 2002). The composition of pollen assemblages also provides information on sediment source. Earlier work in the Potomac River estuary (Defries, 1981) correlated abundance of Chenopodiaceae pollen with proximity to shore, and our current work in Pocomoke surface sediments showed similar patterns (Willard and others, Chapter 3, this volume). Here, we present results from a series of nine cores collected in Pocomoke Sound (Figure 7.1) summarizing both sedimentation patterns in the Sound and land-use patterns interpreted from pollen records.

Material and Methods

Pollen was analyzed from nine cores collected in Pocomoke Sound on the R/V Kerhin in September, 2001 (Table 7.1: PC-2B, PC-2B-3: 37° 53.429'N, 75° 448.408'W, 7.9 mwd; PC-3B: 37° 50.741'N, 75° 48.745'W, 11.4 mwd; PC-4B: 37° 8.300'N, 75° 50.301'W, 27.3 mwd; PC-4C: 37° 48.496'N, 75° 49.820'W, 7.3 mwd; PC-6B: 37° 44.910'N, 75° 52.334'W; 114.6 mwd; TR-1D: 37° 56.300'N, 75° 39.110'W; 1.5 mwd; TR-2D: 37° 56.473'N, 75° 40.257'W, 1.6 mwd; TR-5: 37° 57.086'N, 76° 38.880'W, 1.9 mwd). Cores PC-6B and PC-2B-3 were collected using a piston corer; the other cores were collected with a gravity corer. Cores were x-radiographed at the Maryland Geological Survey and described at the U.S. Geological Survey. Samples were collected as two-centimeter increments every ten centimeters within the core for palynological analysis.

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. Pollen data are available online at http://geology.er.usgs.gov/eespteam/Atlantic/index.htm or from the North American Pollen Database (http://www.ngdc.noaa.gov/paleo/pollen.html).

Results

Cores TR-1D and TR-2D

Cores TR-1D and TR-2D are 92 cm and 72 cm long, respectively, and each are strongly dominated (60-77%) by Pinus pollen, with Quercus and Carya pollen subdominant (Figure 7.2). In TR-1D, Liquidambar abundance increased in the upper 20 cm. Ambrosia is rare (<1%) throughout both cores, as are pollen of other herbaceous taxa.

Core TR-5

Core TR-5 is 95 cm long and is dominated by Pinus (28-59%) and Quercus (11-31%) pollen throughout (Figure 7.2). Carya and Liquidambar are present throughout the sequence, comprising 5-10% and 1-8% of the assemblages, respectively. Poaceae pollen abundance was greatest (4-20%) in the uppermost 30 cm. Ambrosia abundance increased in the upper 10 cm, but even at that interval, its abundance was low (<2.5%).

Core PC-2B-3

Core PC-2B-3 is 482 cm long and dominated by Pinus (36-75%) and Quercus (23%) pollen throughout (Figure 7.3). Ambrosia and Asteraceae pollen are common (~5% and 5-10%, respectively) between 200 cm and 480 cm. Above that point, both groups decrease in abundance, and Pinus pollen abundance increases, as does that of Liquidambar. Carya pollen also is present consistently in relatively low proportions (<5%).

Core PC-3B

Core PC-3B is 162 cm long and dominated by Pinus (45-60%) and Quercus (10-25%) pollen (Figure 7.4). Carya and Liquidambar pollen also are common, comprising 5-10% of the assemblage. There is a sharp decrease in Quercus pollen (from 25% to 15%) and increase in Pinus pollen (45% to 60%) between 120 cm and 130 cm depth. Ambrosia is rare throughout the core, comprising <5% of the assemblage.

Core PC-4B

Core PC-4B is 110 cm long and dominated by Pinus (20-50%) and Quercus (20-30%) pollen. Several distinctive stratigraphic intervals are noteworthy in this core. Between 70 cm and 110 cm, Tsuga, TCT, and fern spores are exceptionally abundant (Figure 7.5). Poaceae pollen is abundant in that interval and in the overlying 20 cm. From 30-40 cm, pollen of Ambrosia and other Asteraceae reach their peak abundance, and Carya and Ulmus abundance increases, and in the upper from 0-20 cm, Pinus dominance of up to 60% abundance is reached, and Liquidambar abundance exceeded 10%.

Core PC-4C

Core PC-4C is 92 cm long and dominated primarily by Quercus pollen (25-45%), with Pinus pollen subdominant in all except the uppermost samples (15-43%) (Figure 7.5). Between 20 cm and 90 cm depth, Nyssa pollen is an abundant (6-22%) component of the assemblage, along with Liquidambar (4-14%) and TCT (3-4%). At 10 cm depth, Nyssa pollen virtually disappears from the assemblage, Liquidambar decreases by more than one-half, and Ambrosia, Asteraceae, and Pinus pollen each increase in abundance.

Core PC-6B

Core PC-6B is 480 cm long and is dominated by Pinus (46-76%) and Quercus (7-25%) throughout (Figure 7.6). Carya and Liquidambar also are present throughout the core, although Liquidambar increases in abundance beginning ~250 cm. Ambrosia was nearly absent at the base of the core but became a consistent component beginning at ~340 cm.

Discussion

Using the increase in the relative abundance of Ambrosia and Pinus pollen to approximate post-Colonial times, we calculated minimum post-Colonial sedimentation rates for each of these Pocomoke Sound core sites. Two are particularly low (0.1 cm yr^-1): (RV) TR-5 and PC-4C. (RV) TR-5 is located at the river mouth, and the agricultural horizon at ~10 cm coincides with an unconformable oxidized surface apparently representing an erosion surface. In core PC-4C, there is an equally abrupt change from organic-rich silt-clay in the lower 70 cm of the core, representing an period when the area was vegetated by cypress-gum swamps, to coarse quartz pebbles in the upper 24 cm, when the forests were logged and vegetation and sedimentation patterns changed. Site PC-4B also has fairly slow post-Colonial sedimentation rates of 0.4 cm yr^-1 and shows distinctive biostratigraphic zonation. The relatively high abundance of Poaceae and Tsuga pollen characteristic of the lower 40 cm is suggestive of an early Holocene or older age. The peak abundance of Ambrosia pollen around 40 cm corresponds to a sedimentological change from silty sand to unsorted clay-gravelly-silty sand, representing impacts of Colonial land clearance on the site. Further changes occurred by the mid-20^th century (~20 cm), with deposition of dark-gray silty clay and greater abundance of Liquidambar, suggesting further changes in the nearby vegetation.

The remaining cores all have post-Colonial sedimentation rates of 0.7 - >4.0 cm yr^-1. All pollen assemblages are dominated strongly (60-70%) by Pinus pollen, and one biostratigraphic horizon is present in addition to the Ambrosia peak: the 1940-1950 Liquidambar increase. Liquidambar pollen doubles in abundance in the mid-20^th century; because this windborne tree pollen is presented in Chesapeake Bay sediments even more consistently than Ambrosia pollen, this biostratigraphic marker is a particularly useful tool for dating geologically recent events.

Conclusions

Pollen assemblages from sediment cores in Pocomoke Sound document high sedimentation rates (0.7 - >4.0 cm yr^-1) at most sites throughout the Sound in post-Colonial time. At most sites, the entire sedimentary record recovered was post-Colonial, so little comparison may be made between pre- and post-Colonial vegetational composition in the region. At the river mouth (Core (RV) TR-5, oaks may have been logged from the general region, resulting in its reduced representation in the pollen record; little else may be gleaned from the pollen diagram. Prior to late 19^th to early 20^th century logging, the area surrounding the site PC-4C was a cypress-gum swamp; it subsequently was recolonized by pine and other early successional plants. The great abundance of Nyssa pollen in most of this core suggests a shoreline or very nearby source of sediment throughout much of the history of the site, because such great abundances of Nyssa pollen have been noted only in surface samples collected near the source plants (Willard and others, Chapter 3, this volume).

Acknowledgements

We gratefully thank Capt. Rick Younger and the crew of the R/V Kerhin for assistance in obtaining cores for use in this study. Tom Sheehan and Meredith Robertson assisted in sampling and laboratory preparation of samples.

References

U.S. Department of the Interior | U.S. Geological Survey
URL: http://pubsdata.usgs.gov/pubs/of/2004/1350/chapters/chapter7.htm
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