Introduction
Pollen analysis of Quaternary peat deposits and fine-grained sediments is a valuable method for reconstructing past vegetation changes on time scales of centuries to many thousands of years. Late Quaternary pollen records have been used to develop vegetation histories for many areas of Alaska (e.g., Ager 1983; Ager and Brubaker 1985; Heusser 1985; Anderson and Brubaker 1993; Anderson et al. 2004), but the Turnagain Arm area of upper Cook Inlet, southcentral Alaska (Fig. 1), has received little attention. The vegetation history of Turnagain Arm is of interest because it now has Pacific coastal forest at the east end, boreal forest at the far west end, and a mixture of the two vegetation types in between. In this paper, we reconstruct the history of these vegetation types in the region by providing the first radiocarbon-dated pollen records to be published from eastern Turnagain Arm. We then discuss these new records within a larger regional context of topographic and climatic influences.
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Turnagain Arm is an east-west-trending glacial fjord that has been largely filled with sediments since deglaciation (Fig. 1B). It is bordered along most of its length by the Chugach Mountains to the north and the Kenai Mountains to the south (Fig. 1B). Beyond the western flanks of the Chugach and Kenai mountains, near where Turnagain Arm joins with the main trough of Cook Inlet, the fjord is bordered to the north by the Anchorage Lowland and to the south by the northern Kenai Lowland (Fig. 1B). During the late Wisconsin glacial interval, ice expanding from the Alaska Range and the Kenai, Chugach, and Talkeetna mountains converged in Cook Inlet to cover much of the region several times (Fig. 1B; Karlstrom 1964; Schmoll and Yehle 1986; Reger and Pinney 1997; Schmoll et al. 1999; Reger et al. 2007).
Reconstructing the history of terrestrial ecosystem development following deglaciation of Turnagain Arm is also of interest because the region is one of climate transition, and a strong precipitation gradient exists today between northwest Prince William Sound and the Anchorage Lowland (Fig. 1B; Table 1). It is likely that this gradient also existed in the past and may explain the distribution of present and past vegetation types in upper and lower Turnagain Arm.
The only pollen records that have been previously published from the Turnagain Arm area include an analysis of an undated peat core collected near Girdwood (Heusser 1960; Fig. 1B, site 9; Fig. 2, Heusser site) and a dated vegetation history from a peat bog near Hope, west of Girdwood (Ager and Carrara 2006; Fig. 1B, site 7). Late Quaternary pollen records with at least some radiocarbon age control have been published for sites in the Anchorage Lowland (Point Woronzof area; Fig. 1B, site 2; Ager and Brubaker 1985; Ager and Carrara 2006); the northern Kenai Lowland (Swanson Fen; Fig. 1B, site 3; Jones et al. 2009; and Paradox Lake, Fig. 1B, site 4; Anderson et al. 2006); the western flank of the Kenai Mountains (Hidden Lake; Fig. 1B, site 5, Ager 1983); and in the central Kenai Mountains, near Tern Lake (Fig. 1B, site 6; Ager 2001). In addition, several dated and undated pollen records have been described from Prince William Sound, east of Turnagain Arm (Heusser 1955, 1960, 1983a, 1985). The longest published pollen record with radiocarbon age control from Prince William Sound is from Golden (Fig. 1B, site 10; Heusser 1983a). In the interior, northeast of Anchorage, pollen data from an exposure of peat and from lake sediments near Matanuska Glacier provide information about the approximate timing of initial postglacial colonization of the upper Matanuska Valley, first by tundra and then by boreal forest vegetation (Fig. 1B, site 11; Williams 1986; Yu et al. 2008).
Glacial history
The Cook Inlet region has undergone multiple glaciations during the late Tertiary and Quaternary (Karlstrom 1964; Pewe 1975; Hamilton and Thorson 1983; Schmoll and Yehle 1986; Reger et al. 1995; Reger and Pinney 1997; Schmoll et al. 1999; Reger et al. 2007). Present-day glaciers and icefields cover substantial areas of the Chugach and Kenai mountains of south-central Alaska (Molnia 2008; Fig. 1B); but during past glacial intervals, glaciers occupied vastly greater areas, covering most of the Cook Inlet region and the mountain ranges of south-central Alaska and extending southward onto the continental shelf (Schmoll and Yehle 1986; Molnia 1986; Kaufman and Manley 2004). For the purposes of the present study of postglacial ecosystem development, the most relevant major glacial events in the Cook Inlet occurred during the late Wisconsin glacial interval ~30 000 - 11000 cal years BP (Reger et al. 2007).
Four significant glacial advances during the late Wisconsin have been recognized in the Cook Inlet region (Karlstrom 1964; Reger and Pinney 1997; Reger et al. 2007). The initial interpretations of the Naptowne (late Wisconsin) glacial advances in the Cook Inlet region as defined by Karlstrom (1964) have been refined and reinterpreted by other researchers in light of additional mapping, radiocarbon dating, and stratigraphic studies in the region (e.g., Reger et al. 1995; Reger and Pinney 1997; Schmoll et al. 1999; Reger et al. 2007). For the purposes of this paper, we follow the terminology of late Wisconsin glacial events in upper Cook Inlet as summarized by Reger et al. (2007). The four recognized late Wisconsin glacial advances covered successively smaller areas of the Cook Inlet region, and the later three glacial advances appear to have been of much shorter duration than the earliest, most extensive ice advance. Reger et al. (2007) retain Karlstrom's original nomenclature for three of the original four names for the earliest three Naptowne glacial advances: the Moosehorn (the oldest, most areally extensive, and longest enduring), the Killey, and the Skilak. The fourth Naptowne glacial advance has been renamed the Elmendorf advance, after the prominent Elmendorf moraines that cover part of the northern Anchorage Lowland, the lower Susitna Valley, and the lower Matanuska Valley (Fig. 1B). Karlstrom's chronology for these glacial events has been updated in light of extensive field investigations in the region during the past several decades, as summarized in Reger et al. (2007).
Our investigations focus on the development of terrestrial ecosystems following glacial retreat from the maximum ice positions achieved during the Elmendorf glacial advance. Major glacial recession during the waning stages of the preceding Skilak stade was accompanied by flooding of much of Cook Inlet, including Turnagain Arm, by marine waters. During the following Elmendorf stade, glaciers readvanced into Turnagain Arm from tributary valleys in the Chugach and northern Kenai mountains (Fig. 1B). The trunk glacier in Turnagain Arm flowed westward to reach maximum positions near Hope ~15 100 cal years BP and occupied another somewhat later terminal position near Bird Creek (Fig. 1B; Kachadoorian et al. 1977; Reger and Pinney 1997; Schmoll et al. 1999; Ager and Carrara 2006; Reger et al. 2007). Retreat of glacial ice of the Elmendorf stade from its maximum terminal positions probably began sometime before 14 100 cal years BP, based on radiocarbon ages of organic sediments at the base of sediment cores from Lake Lorraine, a kettle lake on the Elmendorf moraine west of Knik Arm in upper Cook Inlet (Fig. 1B, site 1; Kathan et al. 2004). Other radiocarbon-dated deposits providing minimum ages for the retreat of glacial ice from the Elmendorf moraine near Anchorage have yielded similar ages (Schmoll et al. 1999). The final retreat of glacial ice from Turnagain Arm and its tributary valleys set the stage for the beginnings of postglacial terrestrial ecosystem development in the deglaciated parts of the Chugach and Kenai mountains. Reconstructing by means of pollen analysis the sequence of vegetation changes following deglaciation is the subject of this paper.
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Paleoclimates
A broad outline of postglacial climates of south-central Alaska can be reconstructed from a variety of published evidence. The transition from arid, full-glacial climates to a somewhat warmer, wetter climate may have begun as early as 16 400 cal years BP in the Kenai Peninsula region, as inferred from pollen evidence (Ager and Brubaker 1985). Increases in marine biological productivity in paleoceanographic records from the Gulf of Alaska indicate warmer ocean waters during the late glacial (Bolling-Allerod) climatic warming (~14 700 - 12 900 cal years BP; Barron et al. 2009). This was followed by a climatic cooling event (Younger Dryas: ~12 900 - 11700 cal years BP), which caused some vegetation responses in coastal sites, most notably a drop in fern spore percentages (Peteet and Mann 1994). Other evidence for Younger Dryas cooling includes a drop in marine biological productivity, an increase in sea ice in the Gulf of Alaska (Barron et al. 2009), and a negative ~2 [per thousands] [[delta].sup.18]O in a lacustrine record near Matanuska Glacier (Fig. 1B, site 11; Yu et al. 2008). During the early Holocene, warmer climates beginning as early as ~11 000 cal years BP are inferred from various climate proxies (Kaufman et …
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