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U.S. Geological Survey Open-File Report 00-306: Chapter 7
Holocene Palynology from Marion-Dufresne Cores MD99-2209 and 2207 from Chesapeake Bay: Impacts of Climate and Historic Land-Use Change
by Debra A. Willard and David A. Korejwo
U.S. Geological Survey, 926A National Center, Reston, Virginia 20192
Pollen records from cores MD99-2209 and 2207 provide a detailed record of vegetational responses in the Chesapeake Bay region to Holocene climatic shifts and altered land-use practices of the last few centuries. Pollen spectra from Chesapeake Bay indicate drier, probably warmer conditions than present throughout the early Holocene with increased moisture levels occurring between 6,600 yr BP and 6,100 yr BP. The middle Holocene record from about 5,800 to 2,300 yr BP is missing from core 2209; however increased percentages of Pinus (pine) pollen in the record from MD99-2207 suggests that wetter conditions were in place sometime during the middle Holocene. Relatively wet climate conditions continued into the late Holocene (2,300 yr BP to present). Holocene peaks in Pinus pollen abundance at about 8,200 yr BP, 6,800 yr BP, 6,200 yr BP, 2,800, yr BP, 1,400 yr BP, and ~500 yr BP, and perhaps ~4,000 yr BP, indicate periodic intervals of very wet and probably cool conditions. These millennial-scale climate events may correspond with hemispheric or global events recorded in deep-sea climate records of ice-rafting and sea-surface temperatures.
The timing of initial post-settlement land clearance in the 18th and 19th centuries is indicated by increases in Ambrosia (ragweed) pollen, with two successive Ambrosia maxima occurring in the mid-late 1800s and the middle 1900s due to agriculture and urbanization, respectively. Changing abundance of herbaceous and tree taxa also provide records of secondary succession in the watershed during 20th century reforestation and urbanization.
Paleoecological records of the impacts of past climate variability and land-use change on terrestrial and estuarine ecosystems can help understand their responses to future alterations in resource management. Sediment cores MD99-2209 and MD99-2207, collected on the IMAGES V cruise of the Marion-Dufresne to Chesapeake Bay in June, 1999, provide detailed records of vegetational response to climate changes in both the early and late Holocene as well as response to post-settlement land-use changes in the watershed. Although there are many late Quaternary pollen records from the eastern United States (see Delcourt and Delcourt, 1984; Watts and Hanson, 1994; Kneller and Peteet, 1999), these new cores are the first available that are dated with well-calibrated radiocarbon ages on marine shells. Moreover, no previous records approach the average sediment accumulation rates that characterized parts of Chesapeake Bay during the Holocene. For example, Kneller and Peteet (1999) described one of the highest resolution pollen records from eastern North America from Browns Pond in Virginia, which had an average accumulation rate of 70 cm/1000 years. At Marion-Dufresne core sites MD99-2209 and 2207, pre-colonial sedimentation rates averaged 250 and 100 cm/1000 yrs. The high sedimentation rates in certain intervals of core 2209 provide an opportunity for a detailed examination of vegetational responses to environmental and climatic changes on decadal to millennial scales for the periods 7600-5800 yr BP and 2300-0 yr BP. Site 2207 has a slower sedimentation rate than that at 2209; however, it contains a relatively continuous Holocene record, including the period between ~10,000 and 7,600 yr BP and the middle Holocene. Thus, these new cores provide an unprecedented opportunity to examine decadal and centennial-scale vegetational and climate variability in the mid-Atlantic region.
Similarly, general patterns of change in vegetation and sedimentation rate after colonization (Brush 1984; Defries 1986) have relied on cores from areas of Chesapeake Bay with relatively slow sedimentation rates. The new Marion-Dufresne cores, as well as those described by Cronin and others (1999, 2000) and Willard and others (1999), provide sub-decadal resolution of the last 200 years of the bay and its watershed because sedimentation rates increased at least fourfold during the 1800s and 1900s. In this report, we summarize Holocene vegetational patterns in the Chesapeake Bay watershed based on palynological analyses of these cores.
Pollen assemblages were isolated from 2-cm increments of sediment collected every 10 cm in core MD99-2209 and every 40 cm in core MD99-2207 using standard palynological techniques (see Traverse, 1988). Samples were dried and weighed; approximately 5 grams of material was processed for palynomorphs. One tablet of Lycopodium spores was added for calculation of absolute pollen concentration (Stockmarr, 1971). Samples then were treated with HCl and HF to remove carbonates and silicates. Samples in the upper 800 centimeters were acetolyzed (using 1 part sulfuric acid to 9 parts acetic anhydride) for 10 minutes in a boiling water bath, neutralized, and treated with 10% KOH for 10 minutes to remove humic material. Samples collected below 800 centimeters of core MD99-2209 and much of core MD99-2207 had abundant pyritic and organic material. Acetolysis does not remove such material, so these samples were oxidized in 15% HNO3 at room temperature for 15 minutes, neutralized, and soaked in 10% KOH for 5 minutes to remove humic material. After completion of these steps, samples were sieved using 150 µm and 10 µm mesh to remove coarse and clay-sized material, respectively. When necessary, the remaining residue was swirled to remove mineral matter. All residue was stained using Bismarck Brown and mounted on microscope slides using glycerine jelly.
Percent abundance of taxa was based on counts of at least 300 pollen grains. Absolute pollen concentrations were calculated using the marker-grain method described by Benninghoff (1962). Marker tablets of Lycopodium spores were the source of the exotic grains, and the quantity of Lycopodium spores in the marker tablets was determined by the manufacturer with a Coulter Counter following the procedures of Stockmarr (1973). The concentration of spores in these tablets was 12,542 +/- 416. Absolute pollen concentration was calculated using the formula (Maher, 1981):
where: Pconc = pollen per gram dry sediment;
R=pollen grains counted/marker grains counted;
M=marker grains added; V=dry weight of sediment
Radiocarbon dates were obtained from 18 samples in core 2209 and ten samples in 2207. Age models are described by Colman and others (Chapter 6), who converted radiocarbon years into calendar years using the calibration model of Stuiver and Reimer (1993). Minor modifications can be made to the radicarbon chronology for core 2207 based on the pollen results presented here. Ages used in this paper are calendar years before present.
Early Holocene (~10,000 to 6,000 yr BP)
Early Holocene Pollen Biostratigraphy
The pollen records from cores 2209 and 2207 are shown in Figures 7.1 and 7.2, respectively. The interval 1710-900 cm in core 2209 is well-dated at 7,600 to 5,900 yr BP by a series of radiocarbon dates and the pollen assemblages from this interval support this age model. The lower nine meters of Core 2207 are dated by a wood radiocarbon date of 12,340 yr BP at 2051 cm, a date on probably reworked wood of 38,500 yr BP at 1796 cm, and a 10,130 yr BP date on mollusks from 1161 cm. Although these ages suggest this interval represents the late Pleistocene, perhaps including the Younger Dryas chronozone (~12,500-11,500 yr BP), the pollen record between 2050 cm and 1150 cm contradicts these dates, because it contains ~50 to 60 % Quercus pollen. In pollen records from eastern North America, low percentages of Quercus pollen (<5%) signifying cool moist periods characterize the late glacial and, in some records, the Younger Dryas age intervals (12,500 yr BP). In most records, Quercus pollen comprises ~10% % of pollen assemblages during the pre-Younger Dryas (14,000-12,500 yr BP) (Watts, 1979; Whitehead, 1981), and Quercus percentages >50% do not occur in eastern North American until after ~10,000 yr BP. Some examples are records from northeastern North Carolina (Whitehead, 1981), eastern Pennsylvania (Watts, 1979), the central Appalachian Mountains of Virginia (Kneller and Peteet, 1999), Connecticut (Davis, 1969), and New Jersey (Peteet and others, 1990).
It should be emphasized that the vegetational history in eastern North America during the Younger Dryas is extremely complex and Kneller and Peteet (1999) suggest that warm moist conditions existed about 12,300 yr BP (~11,280 C-14 yr BP) in the central Appalachians and other regions south of glaciated parts of eastern North America. Regardless of the details of late glacial pollen records, the greatest abundance of Quercus pollen prior to 10,000 yr BP is <15 %. Thus, the pollen record from 2207 suggests that the entire 2050 cm of this core must be younger than 10,000 yr BP, and we adopt this age model until additional radiocarbon dating is performed.
Early Holocene Pollen Assemblages
Pollen assemblages in 2209, between 1700 cm and 900 cm depth (7,600-5,900 yr BP), and in 2207, between 2051 to 750 cm (~10,000 - 6,000 yr BP) were dominated strongly by Quercus pollen (40-70%), with abundant Pinus (15-40%) and variable amounts of Carya (hickory) (5-20%) (Figs. 7.1, 7.2). Other taxa commonly present throughout this interval were Liquidambar (sweetgum), Tsuga (hemlock), Nyssa (tupelo), Picea (spruce), and Ulmus (elm). In core 2209, between 1700 and 1200 cm (7,200 to 6,500 yr BP), Fagus (beech) and Poaceae (grass) pollen were common, comprising >5% and >3% of the assemblages, respectively. Cyperaceae pollen was also consistently present in this interval. Between 1250 and 900 cm (~6,400 - 5,900 yr BP), Carya, Fagus , and Poaceae pollen each were less abundant, while Pinus abundance increased. Cyperaceae pollen was rare to absent between 1400 and 900 cm. A similar pattern is preserved in core 2207; below 800 cm (~6,600 yr BP), Tsuga, Ulmus, Alnus (alder), and herbaceous taxa (Poaceae (grasses), Chenopodiaceae (pigweeds), Asteraceae (asters), Cyperaceae (sedges) are common, with decreases above 800 cm. Carya abundance doubled and Liquidambar increased slightly above 1300 cm.
Middle Holocene pollen assemblages (~6000-3000 yr BP)
In core 2207, the middle Holocene interval (~720-430 cm) contrasts with the early Holocene by increased abundance of Pinus pollen, and corresponding decreases in Tsuga, Picea (spruce), and Alnus and herbaceous pollen. In 2209, the interval between 900 cm and 800 cm represents a poorly dated middle Holocene (with one date of 4,340 yr BP at 820 cm) when sedimentation at this site was interrupted and/or slowed considerably. There probably is an unconformity near 800-850 cm. The abundance of Pinus and Carya pollen peaked (~40% and 12%, respectively), and Quercus abundance decreased relative to the early Holocene. In sum, middle Holocene assemblages suggest wetter, possibly cooler conditions than in the early Holocene, though additional work is necessary on pollen trends in core 2207.
Late Holocene pollen assemblages (~3000 yr BP to present)
Pre-colonization: 3,000 yr BP - 250 yr BP
The interval between 800 cm and 290 cm in core 2209 provides the most detailed record of vegetation in the Chesapeake area yet available for the period between 2,300 and 250 yr BP. This interval is dated by both radiocarbon dates and an interpretation of colonial land clearance from the pollen record. Pollen assemblages from this interval are characterized by greater abundance of Pinus (30-60%), lower abundance of Quercus (25-55%), and relatively constant abundances of Carya (8-12%) pollen (Fig. 7.1). Liquidambar and Tsuga were less abundant in this interval than in the early to middle Holocene assemblages, each averaging about 1% of the assemblage. Great variability in abundance of Pinus pollen occurs throughout this interval and in the middle Holocene interval. Peaks 10-20% greater than adjacent samples occur between 890 cm and 850 cm (~5,900 yr BP), between 630 cm and 600 cm (~1,400 yr BP), and between 430 cm and 378 cm (640-490 yr BP). Between 500 cm and 300 cm depth (~1100-1750 AD), Picea, Ulmus, TCT (Taxodicaceae/Cupressaceae/ Taxaceae: cypress/cedar/yew families), Nyssa, and Poaceae pollen are absent. Ambrosia pollen comprises ~2% of these assemblages, with concentrations typically <1000 grains/gram dry sediment (Fig. 7.3).
Early Colonization: 250-150 yr BP
Important land-use changes after colonization in the Chesapeake Bay watershed are reflected in the pollen record of core 2209. We divide the record informally into early and late colonization. The early interval from 290 cm to 240 cm is characterized by increased abundance of Ambrosia, Pinus, and Poaceae pollen and lower abundances of Quercus and Carya. The onset of this interval is marked by the increased abundance of Ambrosia pollen from <2% to between 5% and 9%; its concentration also increased to between 2,000 and 4,000 grains/gram dry sediment (Fig. 7.3). Quercus and Carya pollen decreased to <40% and <7%, respectively, whereas Pinus pollen increased to >40%. Poaceae pollen comprised up to 3% of the assemblages, and Plantago pollen is a minor but consistent element of this interval.
Late Colonization: 150 yr BP to Present
The onset of this interval in core 2209 (240 cm to 0 cm) is marked by another increase in Ambrosia pollen to >10% abundance and concentrations of 4,000 to 18,000 grains/gram dry sediment (Fig. 7.3). Pinus pollen dominated most assemblages, ranging from 30-63%, and Quercus comprised 16-40% of the assemblages (Fig. 7.1). Juglans pollen was present consistently and in greater abundances than in lower depths of the core. Above 200 centimeters, Liquidambar, Nyssa, and Ulmus abundance increased. Ambrosia abundance fluctuated during this time, with a second peak abundance during the 20th century occurring between 90 and 60 cm (Fig. 7.2). Above 60 cm, slight increases in Betula, Poaceae, and Plantago abundance occurred.
When viewed together, the Holocene pollen record from cores MD99-2207 and 2209 present a picture of significant pre- and post-colonization variability in vegetation in the region surrounding Chesapeake Bay. Some of the most notable features of the record are the alternating periods of sparse (10-15 %) and moderately abundant (25-40 %) Pinus pollen that occur over millennial timescales. Well-dated cool and wet periods indicated by Pinus maxima occur at approximately 8,200, 6,800, 6,200, 2,800, and 1,400 yr BP, with an additional poorly dated pine maximum near 4,000 yr BP. During certain events, Carya abundances increased slightly, while Quercus pollen decreased during these events. Although the dating of some events needs improvement, some of these may correspond to ice-rafting events described by Bond and others (1997) for the North Atlantic Ocean. The 8,200 yr BP event is particularly noteworthy because has become recognized as a global climatic cooling (Alley and others, 1997) and was also recognized at Brown Pond, Virginia by Kneller and Peteet (1999).
Another important discovery is the significant change in pollen assemblages between about 6,600 and 6,100 yr BP (1319 cm to 1000 cm in 2209), which may indicate a regional or hemispheric climatic event, with the greatest change occurring ~6,400 - 6,250 yr B.P. The increase in Pinus abundance, decrease in Carya, and disappearance of Liquidambar, and Ulmus pollen may signify wetter conditions due to changes in atmospheric circulation patterns.
The Late Holocene is characterized by high percentages of Pinus pollen throughout the entire interval, indicating wetter conditions than during the early Holocene. An increase in Pinus abundance has been documented in other terrestrial records from the mid-Atlantic region (Craig, 1969; Kneller and Peteet, 1993, 1999) and attributed to a generally wetter climate (Watts, 1979).
The fourfold increase in sedimentation rates after colonization allows us to examine vegetational response to human activities over the last few centuries at a subdecadal timescale. Land clearance in the 18th and 19th century is marked in the pollen record by sharp increases in Ambrosia pollen abundance and smaller increases in Poaceae and Plantago pollen abundance. The Ambrosia pollen increase is a two-step event in core MD99-2209. The initial increase between ~1750 and 1850 A.D. (290-240 cm) reflects early land clearance for tobacco farming in the watershed, and the peak abundance between ~1850 and 1900 A.D. (240-200 cm) represents changes associated with maximum land clearance and changing agricultural practices (see Brush, 1984; DeFries, 1986; Wolman, 1967). Throughout the 20th century, however, Ambrosia pollen fluctuated greatly in abundance (Fig. 7.3), reflecting changes in land use and forest density in the region.
Increased abundance of Poaceae and Plantago pollen also is consistent with increased agricultural activity and other disturbance in the watershed. Both Poaceae and Ambrosia are known to be early pioneer species in old-field succession (Bazzaz, 1974; Bazzaz, 1979; Keever, 1983; Solomon and Kroener, 1971), and Plantago has been particularly useful as an indicator of the existence of pastures and other plowed or mown lands (Makohonienko and others, 1998; Odgaard and Rasmussen, 1998). The greatest increases in Plantago and Poaceae abundance in the upper 50 cm (past 30 or 40 years), as well as a second peak in Ambrosia abundance, likely reflect some combination of field abandonment and urbanization of the region. In addition to the increase in weedy species abundance, sub-canopy trees such as Liquidambar, Alnus, and Juglans became more abundant after initial land clearance (Fig. 7.1). Subsequent increases in tree pollen abundance occurred in the early to mid-20th century as farms were abandoned and reforestation occurred. Taxa increasing in abundance at this time include Betula, Ulmus, and, to a lesser extent, Nyssa.
Pollen evidence from core MD99-2209 provides unprecedented detail on early to late Holocene vegetational trends and variability in response to both climatic and anthropogenic changes. These records indicate:
- Early Holocene climate was drier than today, indicated by strong dominance of Quercus pollen.
- Around 6,600-6,100 yr BP, increased Pinus abundance may represent an interval of slightly wetter and cooler conditions tied to regional or hemispheric climatic changes.
- The late Holocene (2,300 yr BP - present) was considerably wetter, and probably cooler than the early Holocene.
- Periodic intervals of wetter, cooler conditions, represented by Pinus peaks at approximately 8,200, 6,800, 6,200, 2,800, 1,400, 500 yr BP, and perhaps 4,000 yr BP, reveal millennial-scale Holocene variability that may correspond hemispheric or global climate events.
- Land clearance in the 18th-19th centuries is documented by a sharp increase in Ambrosia pollen
- Early 20th century field abandonment, forest recovery, and urbanization in the 1950s and 1960s are documented in the pollen record.
We thank the crew of the Marion-Dufresne and the IMAGES V program for providing the cores and other assistance with the cruise. We thank L.Weimer, J. Murray, T. Sheehan, and A. Fagenholz for technical support in processing samples and L. Weimer for assistance in pollen counts. This research was supported by the Place-Based Studies, Earth Surface Dynamics, and Coastal and Marine Geology Programs of the U.S. Geological Survey.
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