A Record of Lateglacial and Early Holocene Environmental and Ecological Change from Southwestern Connecticut, USA

1 Analyses of a sediment core from Highstead Swamp in southwestern Connecticut, USA 2 reveal late-glacial and early-Holocene ecological and hydrological changes. Late-glacial pollen 3 assemblages are dominated by Picea and Pinus subg. Pinus , and the onset of the Younger Dryas 4 (YD) cold interval is evidenced by higher abundance of Abies and Alnus viridis subsp. crispa . 5 As climate warmed at the end of the YD, Picea and Abies declined and Pinus strobus became the 6 dominant upland tree species. A shift from lacustrine sediment to organic peat at the YD-7 Holocene boundary suggests that the lake that existed in the basin during the late-glacial interval 8 developed into a swamp in response to reduced effective moisture. A change in wetland 9 vegetation from Myrica gale to Alnus incana subsp. rugosa and Sphagnum is consistent with this 10 interpretation of environmental changes at the beginning of the Holocene.


Introduction
Environmental and ecological changes associated with the Younger Dryas (YD) climatic oscillation (12,,600 calibrated 14 C years before present; cal yr BP) have been studied at many sites in eastern North America using a variety of approaches (e.g., Peteet et al., 1990;Levesque et al., 1993;Mayle et al., 1993;Cwynar and Levesque, 1995;Shemesh and Peteet, 1998;Yu and Eicher, 1998;Lavoie and Richard, 2000;Newby et al., 2000;Cwynar and Spear, 2001;Huang et al., 2002;Shuman et al., 2001Shuman et al., , 2002;;Hou et al., 2007;Lindbladh et al., 2007;Yu, 2007).Pollen records typically feature an increase in cold-tolerant taxa at the beginning of the YD and a shift to taxa indicative of warmer conditions at the YD-Holocene boundary (e.g., Shuman et al., 2002).Quantitative reconstructions of temperature for this interval yield generally consistent results, indicating ~5 C shifts at the beginning and end of the YD (Shemesh and Peteet, 1998;Yu et al., 1998;Cwynar and Spear, 2001;Yu, 2007). Lae-glacial changes in moisture balance, on the other hand, have received less study.Lake-level reconstructions from southern Québec (Lavoie and Richard, 2000) and southeastern Massachusetts (Newby et al., 2000;Shuman et al., 2001) indicate relatively wet conditions during the YD and drier climate at the beginning of the Holocene, but other records of moisture-balance shifts associated with the YD have not been developed.In this paper we report on a late-glacial sedimentary record from a swamp in southwestern Connecticut, USA.Analyses of pollen and organic content provide additional insights into changes in moisture balance at the end of the YD.

Study Area
Highstead Swamp (41 19.5' N, 73 23.75' W) is located at Highstead, a 150-acre woodland preserve in Redding, Connecticut.This area of southwestern Connecticut falls within the Northeastern Coastal Zone, an ecoregion that extends across southern New England (Griffith et al., 1994).
The swamp is part of a 4-ha seasonally flooded basin; it has an intermittent outlet stream that drains into a 1-ha artificial pond before continuing to the southeast.Soils range from muck to poorly drained and stony (Wolf, 1981).The swamp is bounded to the west by a rugged northeast-to-southwest trending ridge of Ordovician-age schist and granitic gneiss; to the east is a smooth, northwest trending drumlin composed of Wisconsinan glacial till overlying Illinoian till (Rodgers, 1985;Stone et al., 2005).The vegetation of the swamp features Acer rubrum and Betula alleghaniensis in the overstory, Clethra alnifolia, Lindera benzoin, and Ilex verticillata in the understory, and a ground layer of Symplocarpus foetidus, Osmunda cinnamomea, and Carex species.Dry, upland forest to the west consists of 70-90 year old Quercus rubra, Q. coccinea, and Q. prinus, with dense Kalmia latifolia in the understory.To the east, moist Acer rubrum, Fraxinus americana, and Liriodendron tulipifera forest (45-85 years old) occurs on fine-grained soils.The Quercus forest was continuously forested during the European settlement period but was cut heavily for wood products, while the Acer-Fraxinus forest was open pasture during the settlement period and reverted back to woodland only in the twentieth century.

Methods
We collected a sediment core from Highstead Swamp in June of 2006.We accessed the center of the swamp using an established boardwalk and collected a 256-cm-long core using a modified Livingston piston sediment sampler.Core segments were extruded horizontally in the field, wrapped in plastic and aluminum foil, and subsequently refrigerated.
The analyses presented here were performed on the interval of the core from 256 to 94 cm; the upper interval of the core appeared to be disturbed and therefore was not analyzed.
Sediment samples of 1 cm 3 were prepared for pollen analysis following standard procedures (Faegri and Iversen, 1989), and tablets containing Lycopodium clavatum spores were added during processing to allow calculation of pollen and spore concentrations (Stockmarr, 1971).
Pollen residues were mounted in silicone oil and analyzed at 400x magnification.At least 500 pollen grains and spores of upland plant taxa were counted for each sample, and pollen percentages were calculated relative to that sum.Myrica-type pollen, which is very abundant in samples from 134 and 138 cm, was not included in the sum used to calculate percentage values.
Sediment organic content was estimated for 1-cm 3 samples at selected depths by percent weight loss-on-ignition (LOI) at 550 °C.
Chronological control is provided by accelerator mass spectrometry 14 C analysis of four woody plant macrofossils sieved from the sediment (Table 1).Dates were converted to calibrated 14 C years before present (cal yr BP) with CALIB 5.0 (Stuiver and Reimer, 1993).The date of the uppermost sample (~11,900 cal yr BP; 109 cm) is inconsistent with the age-depth relationship of the other dates and data from other sites, and is therefore rejected.

Discussion
The changes observed in the Highstead Swamp record at ~13,000 cal yr BP, including declining percentages of Ostrya-Carpinus pollen and higher abundances of Abies and Alnus (presumably A. viridis subsp.crispa), are consistent with pollen data from sites in southern New England (e.g., Davis, 1969;Suter, 1985;Lindbladh et al., 2007) and elsewhere in eastern North America (e.g., Mayle et al., 1993;Peteet et al., 1993).The ~5 C drop in temperatures at the beginning of the YD (Shemesh and Peteet, 1998;Yu et al., 1998;Cwynar and Spear, 2001;Yu, 2007) appears to have shifted vegetation assemblages across the region towards cold-tolerant taxa (Shuman et al. 2002), although some ecological changes may have been underway in advance of the onset of YD cooling (Lindbladh et al., 2007).
The end of the YD cold interval in the Highstead Swamp record is also marked by changes in vegetation seen at other sites.Picea pollen percentages decline abruptly at ~11,500 cal yr BP, and boreal taxa such as Abies are replaced by temperate taxa including Pinus strobus, Tsuga canadensis, and Quercus (e.g., Mayle et al., 2003;Shuman et al. 2002;Lindbladh et al., 2007).A comparison of pollen and paleoclimatic data by Williams et al. (2002) indicates that vegetation responded quickly to rising temperatures at the beginning of the Holocene.The peak in Alnus pollen percentages at the YD-Holocene boundary in the Highstead Swamp record, however, is not a feature that is normally observed in other pollen records from eastern North America (but see Newby et al., 2002).In fact, the decline of Alnus is typically seen as a distinguishing marker of the end of the YD (Mayle et al., 1993).We interpret the increase in Alnus abundance, as well as the rise in Myrica-type pollen occurring just prior to the Alnus peak, as evidence for changes in the composition of the local vegetation in response to a shift in moisture availability and hydrological conditions at the beginning of the Holocene.
Several lines of evidence from the Highstead Swamp record indicate a sequence of hydrological and ecological changes during the transition from the YD to the Holocene.The decline in the abundance of Nuphar, an aquatic plant, and shift from lacustrine sediment to peat suggest that the lake that occupied the basin during the late-glacial interval became a swamp after ~11,500 cal yr BP.A similar transition in the sediments of Makepeace Cedar Swamp, located in southeastern Massachusetts, was also interpreted as a shift from lake to swamp at the beginning of the Holocene (Newby et al., 2000).The changes in wetland vegetation at Highstead Swamp are consistent with this interpretation.The wet substrate of the swamp was initially dominated by Myrica gale, as evidenced by the high abundance of Myrica-type pollen, and subsequent development of the swamp likely allowed Alnus and Sphagnum to become prevalent.We suspect that the Alnus pollen represents the presence of Alnus incana subsp.rugosa, which can occur with Sphagnum in bogs and swamps in eastern North America (e.g., Cronan and DesMeules, 1985).
Taken together, the changes observed in the sedimentary record from Highstead Swamp are consistent with lake-level studies from eastern North America (Lavoie and Richard, 2000;Newby et al., 2000;Shuman et al., 2001), which suggest that water levels declined ~11,500 cal yr BP in response to declining effective moisture.Those dry conditions prevailed in New England until ~8000 cal yr BP, when the deterioration of the Laurentide Ice Sheet brought wetter climate to the region (e.g., Shuman et al., 2006).USA.Inset map shows location of Rogers Lake (Davis, 1969).(light gray is lacustrine sediment, dark gray is peat), and pollen and spore concentrations for selected taxa from the Highstead Swamp sediment record.Note changing scale for x-axes.
Horizontal lines are depths of 14 C samples; the 2 calibrated 14 C age ranges are shown.

Figure captions Figure 1 .
Figure captions

Figure 2 .
Figure 2. Pollen percentage diagram for selected taxa from the Highstead Swamp sediment record.Note changing scale for x-axes.Horizontal lines are depths of 14 C samples; the 2 calibrated 14 C age ranges are shown.