ABSTRACT
HOLOCENE AND TERMINAL PLEISTOCENE CLIMATE IN THE PACIFIC NORTHWEST: A POLLEN RECORD FROM LAKE LOUISE, WASHINGTON
Pollen extracted from lake sediments provides the ability to reconstruct Holocene vegetation history, providing a proxy for changes in temperature, precipitation and fire- or erosion-related disturbance at Lake Louise, near the southern terminus of Puget Sound, Washington State, USA. The resulting assemblage is divided into four zones. Z4, the oldest of these, and dominated by Pinus, suggests a cold climate. Z3 exhibits an increase in Pseudotsuga menziesii and Alnus rubra, indicating warmer conditions and increased disturbance, likely due to increased fire frequency. In Z2, Thuja and Tsuga increase, as Alnus declines, and Pseudotsuga declines slightly. This suggests higher precipitation, slightly cooler conditions, and decreased disturbance. Finally, Z1 shows an increase in Alnus, a slight increase in Pseudotsuga, no change in Tsuga, and a decrease in Thuja, signaling slightly increased temperatures, with increased disturbance, and consistent high precipitation. The general trends detected are similar to others observed in similar studies across the region.
Jeremy Matthew Scherr August 2015
HOLOCENE AND TERMINAL PLEISTOCENE CLIMATE IN THE PACIFIC NORTHWEST: A POLLEN RECORD FROM LAKE LOUISE, WASHINGTON
by Jeremy Matthew Scherr
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology in the College of Science and Mathematics California State University, Fresno August 2015 APPROVED For the Department of Earth and Environmental Sciences:
We, the undersigned, certify that the thesis of the following student meets the required standards of scholarship, format, and style of the university and the student's graduate degree program for the awarding of the master's degree.
Jeremy Matthew Scherr Thesis Author
Peter Van de Water (Chair) Earth and Environmental Sciences
Mathieu Richaud Earth and Environmental Sciences
Robert Dundas Earth and Environmental Sciences
For the University Graduate Committee:
Dean, Division of Graduate Studies AUTHORIZATION FOR REPRODUCTION OF MASTER’S THESIS
X I grant permission for the reproduction of this thesis in part or in its entirety without further authorization from me, on the condition that the person or agency requesting reproduction absorbs the cost and provides proper acknowledgment of authorship.
Permission to reproduce this thesis in part or in its entirety must be obtained from me.
Signature of thesis author: ACKNOWLEDGMENTS I would like to thank my advisor, Professor Peter Van de Water, for approximately 99 instances of assistance through every step along the way. Without his patience, insight, and clarity, this work would not have been possible. I would like to thank my committee members, Professors Mathieu Richaud and Robert Dundas. Through their diligence, this paper has gone from unremarkable raw stone, through cutting and polishing, to achieve its full potential. Thanks to Kena Fox-Dobbs, Jeff Tepper and their students, for their work in acquisition of the core itself, and for other forms of analysis that add significantly to overall understanding of the Lake Louise record. Finally, thanks to Judy Stone, Mary Phillips, Emily Michel, and all of the friends I have made in the EES department of CSU Fresno, for all the support that they have given me in this undertaking. TABLE OF CONTENTS Page
LIST OF TABLES ...... vi
LIST OF FIGURES ...... 1
INTRODUCTION ...... 1
LITERATURE REVIEW ...... 5
Overview ...... 5
Puget Coastal Rain Shadow ...... 7
Southern Puget Lowland ...... 10
Olympic Peninsula West Coast ...... 11
Surrounding Areas ...... 13
METHODS ...... 15
RESULTS ...... 18
DISCUSSION ...... 22
Overview ...... 22
Terminal Pleistocene and Early Holocene ...... 23
Middle Holocene ...... 27
Late Holocene ...... 28
WORKS CITED ...... 30
APPENDIX: POLLEN SITES OF THE PACIFIC NORTHWEST ...... 36
LIST OF TABLES
Page
Table 1. Pollen Sites From Previous Research in Region Surrounding Lake Louise ...... 7
LIST OF FIGURES
Page
Figure 1. View of western Washington state, with previous pollen research sites from literature review shown...... 2
Figure 2. Approximate location of the Lake Louise sediment core sample, USGS 7.5" Steilacoom topographic quadrangle...... 4
Figure 3. Lake Louise pollen percentage diagram showing percentage abundance of conifer and other arboreal taxa, plotted against core depth...... 19
Figure 4. Lake Louise pollen percentage diagram showing non-arboreal, aquatic and other pollen, as well as abundance summation and lycopodium to pollen ratio, plotted against depth...... 20
Figure 5. Ternary diagram showing covariance, with end members Pinus (cold) Pseudotsuga (warm) and Thuja (wet)...... 23
INTRODUCTION
The Pleistocene-Holocene boundary marks the transition from glacial conditions, and the rise of our current interglacial environment (Walker et al., 2009). Within this interglacial, climate has fluctuated to varying degrees (Mayewski et al., 2004). These shifts affect our ecosystems and thus result in changes to the floral assemblage in a given region over time. By examining pollen extracted from sediment, the history of changes in a floral assemblage can determine, with sufficient precision to be useful, a proxy for changes in the regional climate. The nature of such changes can be further constrained by examining charcoal extracted from sediment. During the terminal Pleistocene, the glacial terminus of the western Cordilleran glacial ice mass occurred in western Washington state (Figure 1). The region thereby provides key insights in understanding Holocene climate (Tsukada et al., 1981). The southern terminus of the Continental ice sheet in the Puget lowlands has borne comparably little scrutiny of the available pollen sites. The Puget Trough is a low elevation region, whose northern interface sinks below current sea level and coincides with that of Puget Sound. The trough extends southward through Washington (Barnosky, 1985) and northern Oregon, at which point it becomes the Willamette Valley. It is bounded on the west by the coastal ranges, and on the east by the Cascades (Barnosky, 1985). The northern extent of the region was predominantly physiographically influenced by the Puget Lobe of the Cordilleran ice sheet during the Vashon stade of the Fraser Glaciation (Hibbert, 1979). The formation of the Puget Sound occurred as the Cordilleran ice sheet retreated. Based on a time-distance curve of glacial advance and retreat, supported by radiocarbon dating of specific sites (Porter and Swanson, 1998), the region currently occupied by Lake Louise would have been free of ice around 16.6 ka. 2 2
Figure 1. View of western Washington state, with previous pollen research sites from literature review shown. Note: Lake Louise is represented by LL. For the key to other sites, see Appendix.
3 3
Regional climate west of the Cascade mountains alternates between dry summers and wet winters. The Puget Trough, located in a partial rain shadow caused by the Olympic Mountains to its west, experiences reduced precipitation compared to the coastal region. Precipitation also increases significantly to the east as elevation rises in the Cascades. Lake Louise, a circular depression of 0.16 km2 of surface area, is interpreted as a kettle lake, with maximum depth of 10.7 m (Wolcott, 1961). It is located approximately 3.2 km east of the nearest extension of Puget Sound and the town of Steilacoom, Washington (Figures 1, 2). The deepest sediments represent the lake in its infancy, in a basin exposed, and then filled with melt-water from recently retreated glaciers, as the Vashon stade gave way to the present interglacial period around 13.5 ka. The core represents deposition from then until modern conditions, with the uppermost samples marked by the presence of non-native Taraxacum spp. pollen. Time is constrained at three points in the core. The first date uses 210Pb in the sediment at 0.163 m, and is reported at 1863 (Datt, 2012). The second date used radiocarbon dating, with an age of 2840 ± 40 cal YBP (Datt, 2012, OS- 97736) at 2.79 m. Tephra from Mt. Mazama, Oregon occurs at 4.2 m (Datt, 2012), and has been dated at 7627 ± 150 cal YBP (Bacon, 1983; Zdanowicz et al., 1999). Utilizing average sediment deposition rate from the latter two defined points to extrapolate the age of the oldest sediments, this study encompasses sediment estimated to cover approximately 15,600 years (Datt, 2012), with a temporal resolution of 300 years for each sample. This is slightly lower than estimation glacial retreat exposing the site at 16.6 ka (Porter and Swanson, 1998), suggesting an uneven sedimentation rate, a gap between exposure and preserved deposition, or a combination of the two. 4 4
Figure 2. Approximate location of the Lake Louise sediment core sample, USGS 7.5" Steilacoom topographic quadrangle. LITERATURE REVIEW
Overview Topography in the Lake Louise area resulted from glacial action. During the height of the last glacial maximum, the study site was buried by the Puget Lobe of the Cordilleran ice sheet (Heusser, 1977). Glacial advance initiated about 25,000 14C yr BP (Booth et al., 2003), or 29,500 cal yr BP (Reimer et al., 2009). During the Evans Creek stade, 25,000 cal yr BP, the ice sheet advanced into the Fraser Lowlands of British Columbia (Booth et al., 2003). Moist conditions associated with climatic amelioration following the Evans Creek stade are thought to have potentially increased snowpack, leading to renewed glacial advance in the Vashon stade (Whitlock, 1992). This advance covered the study area, with modern topographic features tied to its subsequent retreat. The northernmost dated site along a flow line intersecting the study area is 18,925 cal YBP, with its southernmost terminus at 16,850 ± 100 cal YBP (Porter and Swanson, 1998). Glacial lakes formed during the glacial retreat. While most coalesced into a single lake occupying the majority of the present day Puget Sound (Booth et al., 2003), Lake Louise was not among them (Porter and Swanson, 1998). Rather, Lake Louise and Waughop Lake, 0.6 km to its north, occupy one of numerous north-south trending valleys excised by glacial action (Booth et al., 2003). A general consensus regarding the succession of vegetation in the area was established by previous research, though several significant individual variations exist. The broad interpretation of this succession is that at the end of the previous glacial advance in the Pacific Northwest, vegetation succession is typically marked with the transition from parkland and tundra to forest consisting largely of Pinus. This Pinus is generally assumed, due to lack of other pine species in the 6 6 area, to be Pinus contorta, the lodgepole pine. This represents a warming, but still cool environment (Heusser, 1977; Tsukada et al., 1981). Between 11 and 8 ka, increased Pteridium, Pseudotsuga menziesii and Alnus, particularly Alnus rubra indicate either the opening of forest canopy, a climate shift to a warmer, drier state, with potential for increased fire frequency, or a combination of the two (Leopold et al., 1982; Tsukada and Sugita, 1982; Cwynar, 1987). Between 8 and 3 ka, increases in the abundance of hydrophytic plants, such as Thuja, a type of cypress (Cupressaceae), and Tsuga heterophylla mark a cooling, moistening trend. This resulted in the present day climate regime (McLachlan and Brubaker, 1995; Spooner et al., 2007). Lake Louise conforms to this general trend, but unusual variations in Thuja, Tsuga and Pseudotsuga are observed in the aftermath of the Holocene Climatic Optimum (a warm period during roughly the interval 9 ka to 5 ka). Further, increased Alnus, mildly increased Tsuga and Pseudotsuga, and decreased Thuja, is observed in the most recent Holocene samples. Regional initiation of each stage of climate change varies between sites. No trend was observed tying initiation time to elevation, but this may be due to the scarcity of high-elevation sites. A general correlation was found with latitude. More northerly sites tend to show changes associated with cooling earliest, and changes associated with warming latest, in comparison to southern areas. Broadly, research in this area can be grouped in one of three regions. First among these is the Puget Coastal Rain Shadow, encompassing sites along both the eastern and western coastal margins of Puget Sound. The second region, extending in a wedge narrowing southward from the southernmost Puget Sound, is the Southern Puget Lowlands. The last region is the Olympic Peninsula. Lake Louise occurs at the boundary between the Puget Coastal Rain Shadow and Southern Puget Lowlands. All three are subject to wet winters and dry summers 7 7
(Hansen and Easterbrook, 1974, Hibbert, 1979), but the total precipitation varies significantly. The Olympic Peninsula experiences the heaviest precipitation on the windward side, but creates a rain shadow to its interior. The Southern Puget Lowlands and Puget Coastal Rain Shadow are on the windward side of the Cascades, which results in significantly increased precipitation. The site locations, elevations and dates of relevant pollen initiation from previous research, where available, is summarized in Table 1. Asterisks represent elevation approximations from Google Earth.
Table 1. Pollen Sites from Previous Research in Region Surrounding Lake Louise Latitude (N) Longitude (W) Elevation (meters) Site Name Pseudotsuga Initiation (ka) Thuja, Tsuga Initiation (ka) Primary Author Year Puget Coastal Rain Shadow 49.31 122.55 305 Marion (AKA Jacobs) Lake 10.5 7.6 Matthewes 1973 49.32 122.56 540 Surprise Lake 10.5 7.6 Matthewes 1973 49 122.34 49 Pangborn Bog 9.9 7.1 Hansen 1974 48.77 122.12 200 Mosquito Lake 9.9 7.1 Hansen 1974 47.81 122.31 104 Hall Lake 10 7 Tsukada 1981 48.04 123.08 165 Manis Mastodon 9.6 n/a Petersen 1983 47.94 122.88 60 Crocker Lake, Cedar Swamp 11 7 McLachlan 1995 47.67 122.26 0 Lake Washington 10 7 Leopold 1982 Olympic West Coast 48.01 124.53 37 Wentworth Lake 8 3 Heusser 1973 48.16 124.57 20 Wessler Bog 8 3 Heusser 1973 47.97 124.49 73 Soleduck Bog 8 3 Heusser 1973 48.05 124.46 421 Thunder Lake 9 6 Spooner 2007 47.86 124.31 120 Bogachiel River Valley 10 3 Heusser 1978 47.68 124.02 710 Yahoo Lake 11.4 7 Gavin 2013 47.81 124.16 164 Hoh River Valley 8.5 5 Heusser 1974 47.23 123.96 45* Humptulips 10.5 n/a Heusser 1983 Southern Puget Lowland 47.03 122.63 75 Nisqually Lake 10 7.6 Hibbert 1979 46.77 122.95 60 Zenkner Valley 8 3 Heusser 1977 46.73 122.17 436 Mineral Lake 10 8.5 Tsukada 1981 46.54 122.25 282 Davis Lake 10 5.5 Barnosky 1981 45.8 122.49 155 Battle Ground Lake 11 6 Whitlock 1992 45.89 122.52 206 Fargher Lake n/a n/a Heusser 1983 Western Cascade 48.24 121.62 190 Kirk Lake 11.2 7.6 Cwynar 1987 46.77 121.76 1311 Jay Bath n/a 3 Dunwiddie 1986 46.77 121.76 1361 Log Wallow n/a 3 Dunwiddie 1986 46.77 121.76 1482 Reflection Pond 1 n/a 3 Dunwiddie 1986 Olympic Highland 47.71 123.54 1415 Martins Lake 10 6.7 Gavin 2001 47.88 123.35 1508 Moose Lake 10 6.7 Gavin 2001
Puget Coastal Rain Shadow Most of Puget Sound lies within the rain shadow of the Olympic Mountains. While insufficient to cause complete removal of atmospheric 8 8 moisture, this range significantly decreases precipitation in the lowland to the east, resulting in comparatively moderate total precipitation, with modern averages of 940 mm/yr at Seattle Tacoma International Airport, and 966 mm/yr in Everett, Washington (National Weather Service, 2014). In contrast, on the western side of the Olympic Mountains, average precipitation at the Quillayute State Airport is 2498 mm/yr (National Weather Service, 2014). Precipitation then increases eastward, toward the Cascade range, where increasing elevation wrings moisture out as air masses move over the Cascade mountains towards the east (Hibbert, 1979). An example of this can be seen at Snoqualmie Falls, where average precipitation is 1557 mm/yr (National Weather Service, 2014). Tsukada et al. (1981) sampled Hall and Mineral lakes. From 12.5-10 ka, the presence of Pinus and Tsuga indicated cold and wet conditions, described as taiga. From 10-8.5 ka, Pseudotsuga, Alnus rubra and Pteridium, coupled with elevated charcoal abundance, suggested an unstable, transitional open forest, subject to warming and frequent fires. From 8.5-7 ka, dominant pollen types included Pseudotsuga, Alnus rubra, Pteridium and Thuja, with the latter indicating increased moisture. From 7-5 ka, Pseudotsuga and Thuja comprised the key members of the assemblage. Decreasing Alnus suggested less disturbance, and hence, a more stable environment. From 5 ka to recent times, the record has shown abundance of Tsuga, Pseudotsuga and Thuja consistent with modern cool, moist conditions. Within the last century, anthropogenic effects have removed local forests. Bracken fern spores rose dramatically in the aftermath, as did Alnus, an effective pioneer species quick to take advantage of cleared land (Tsukada et al., 1981). Petersen et al. (1983) and McLachlan and Brubaker (1995) worked at sites in the northeastern Olympic Peninsula. Petersen et al. (1983) examined records 9 9 from the Manis Mastodon site. Instead of the usual postglacial Pinus, from 12-11 ka, recovered pollen types imply open herb and shrubland, including cactus. Temperatures were noted to resemble current conditions, so this early assemblage is thought more indicative of a local rain shadow effect than a regional xeric state. Fisher (2013) noted that at present, this is the driest part of the Olympic Peninsula. From 11 ka to 9.6 ka, the area was colonized by conifer forest, including Pinus and Picea, indicating increasing precipitation as glaciers retreated. Records from Crocker lake (McLachlan and Brubaker 1995) indicate Pinus dominated earliest Holocene vegetation, suggesting a cool climate. This dominance was not evident at Cedar Swamp, despite a separation of only 3 km (McLachlan and Brubaker 1995). This discrepancy illustrates the importance of utilizing multiple sites to obtain a complete picture of regional deglaciation patterns and climate. Around 11 ka, vegetation shifted to Alnus rubra and Pseudotsuga menziesii, adapted to warmth and environmental disturbance. Results resembling the modern assemblage of Tsuga and Thuja were established around 7 ka, indicating cooling and increased precipitation. The distinct change in Alnus abundance observed at Crocker Lake, the northern site, was much less evident at Cedar Swamp. Additional details were provided by Mathewes (1973), Hansen and Easterbrook (1974), and Leopold et al. (1982). Mathewes (1973) investigated Marion and Surprise lakes, in southwestern British Columbia, noting that vegetation in this locale recolonized quickly after deglaciation, with diverse flora evident by 12,690 ± 190 cal YBP. Hansen and Easterbrook (1974) examined Pangborn bog and Mosquito lake, in the vicinity of the northeastern Sound, finding a similar cohort of species, comprised of Pinus, followed by Pseudotsuga, and then Thuja/Tsuga. Leopold et al., (1982) studied Lake Washington, near the eastern Sound. Lignin analysis, examining the chemical fingerprint left by 10 10 residual phenolic polymers found in all vascular plants (Sarkanen and Ludwig, 1971), suggests that this Pinus pollen was derived from a different source region, likely arriving in the study site via long distance atmospheric transport. High abundance of alder, Douglas fir and bracken fern spores mark the most notable pre-Mazama trend, suggesting the region was either open forest, or warmer, drier, and prone to more frequent disturbance. A sharp increase in Thuja was noted around 7.2 ka, following the eruption of Mt. Mazama, persisting to present.
Southern Puget Lowland While the Southern Puget Lowland experiences some of the same rain shadow effects of the Puget Sound region, the coastal range south of the Olympic Peninsula is neither particularly contiguous nor of the same caliber of elevation as the Olympic or Vancouver Island ranges, and is therefore less effective at precipitation segregation (Galewsky, 2009). Areas to the west of Mt. Rainier have been studied by several researchers. Barnosky (1981) developed a record from Davis Lake. Initial Pleistocene- Holocene transition is shown by the invasion of the Davis Lake region by Pinus, which was followed by Abies lasiocarpa and Alnus, indicating a cool, dry climate. Pseudotsuga arrived approximately 10 ka as climate warmed, with Abies grandis, Pinus monticola and Thuja following. Tsuga arrived sometime between 11 and 9 ka, indicating an increase in precipitation. Around 5.5 ka, Thuja expanded at the expense of Pseudotsuga and Alnus, and is indicative of additional precipitation. Hibbert (1979) examined Nisqually Lake. In addition to the usual Pinus, the record shows Picea, around 12.7 ka. As climate warmed around 10 ka, the forest shifted to Abies and Pinus, with the former becoming increasingly dominant. Expansion of Quercus and Tsuga denote the period of post-Mazama cooling. 11 11
Tsukada and Sugita (1982) sampled Mineral Lake, where the forest in the area was likely open, as evidenced by Pteridium spore abundance from 10.7 ka to present. Near the Oregon border, Heusser (1983) sampled Fargher Lake. Fargher Lake's record was partially destroyed on top, due to tilling. While the data-set terminated prior to significant overlap with the temporal subject of the current paper, the end-Pleistocene conditions were recorded. Prior to 12.5 ka, Picea, Pinus, Tsuga, Graminae and Compositae suggest a cool, moist climate with open grasslands. From 12.5 ka to 10 ka, Pinus, Abies and Alnus occur, suggesting increasing disturbance or opening of the forest canopy. Whitlock (1992) (nee Barnosky, 1985) studied Battle Ground Lake to the south. The record showed end-Pleistocene dominance of Pinus and Alnus sinuata, with Graminae and Artemisia indicating cool, dry conditions. During the Holocene Climatic Optimum, local flora shifted to an assemblage typically seen in the more southerly Willamette Valley, consisting of Pseudotsuga, Alnus and Pteridium, with Quercus, Chrysolepis, and herbs. This suggests a warmer, drier climate. Around 6 ka, Thuja became more abundant, suggesting a decrease in drought frequency or intensity.
Olympic Peninsula West Coast The western side of the Olympic coastline experiences extremely heavy precipitation events. This is caused by the combination of the prevailing Westerlies gathering moisture over the Pacific, and the coastal mountain range acting as a lifting force that results in condensation and precipitation. Many of the same trends were observed here as in other locales in the inland region. Some studies, however, indicate onset of climatic shifts was delayed compared to regional trends, with the Holocene Climactic Optimum recorded beginning around 12 12
8 ka (Heusser, 1973; Heusser, 1974), and ending around 3 ka (Heusser, 1973, Heusser, 1978). Heusser (1973) studied three sites in the northwestern Olympic Peninsula: Wentworth Lake, Wessler Bog and Soleduck bog. At the terminal Pleistocene, the region was colonized by Pinus, Alnus and Picea. Transition of flora to Picea and Tsuga indicated a brief cool spell, ending around 10 ka. Increasing Pseudotsuga, and a lack of Tsuga provided evidence of a warmer, drier environment from 8-3 ka. Alnus dominated at this time, indicating disturbance and/or opening of the canopy. Heusser (1974) also studied the northwestern Olympic Peninsula's Hoh River Valley. This site was late to lose Pleistocene tundra, only giving way to Pinus at around 10 ka. Pinus declined in favor of Pseudotsuga and Alnus somewhat in excess of 8 ka. Of note, Thuja was never present in significant quantity. Heusser (1978) researched the Bogachiel River Valley. As the Pleistocene ended, parkland, represented by Graminae, Compositae and Cyperaceae pollen, comprised the majority of the taxa. These taxa dwindled in relation to arboreal pollen, initiated by Pinus, followed by Picea and then Alnus, as is consistent with climate warming. Through the Holocene, Heusser found that Picea, Alnus rubra and Pseudotsuga acted as pioneer species. The presence of Pteridium, which lacks shade tolerance, indicated open forest at this time (Crane, 1990). Total pollen abundance decreased at approximately 3 ka, possibly as a result of compositional shifts from open canopy to a closed canopy forest. More recently, anthropogenic activity resulted in increased Alnus. Further, Heusser (1983) also studied sites in the Humptulips area, located in the southwestern Olympic Peninsula. From the latest Pleistocene, at 15 ka, to 10.5 ka, Pinus and Alnus dominated, suggesting a cool climate, with open forest or frequent disturbance. 13 13
More recently, Spooner et al. (2007) examined the record at Thunder Lake, finding Picea, Alnus, Abies and Tsuga, interpreted as a cool, dry postglacial climate. This record, unlike some others, did not support additional cooling in the late Holocene. Gavin et al. (2013) analyzed Yahoo Lake, in the western central Olympic Peninsula. Before 7 ka, charcoal abundance and copious Alnus pollen indicate climate was highly variable, and prone to a large numbers of fires.
Surrounding Areas
West Cascades A few study sites do not fit handily into the previously defined regions, but nonetheless introduce important information on the characteristics of the greater region. Cwynar (1987) and Dunwiddie (1986) provide studies of the Western Cascades, which experience heightened precipitation analogous to, though less severe than, the Olympic West Coast. Gavin et al. (2001) studied the Olympic Highland, showing that the succession of vegetation shown in other papers is significantly altered with sufficient elevation. The Martins Lake record, for example, does not record a shift to arboreal taxa until after the eruption of Mt. Mazama. Cwynar (1987) explored the record at Kirk Lake, near the central eastern Sound. Tsuga and Pinus were noted in the basal samples, indicating cool, moist climate prior to 12 ka. At 12 ka, pioneer species, including Alnus rubra, Alnus sinuata and Picea increased, arguing for a period of greater than normal disturbance. From 11.2 ka to 10.5 ka, expansion of Picea and Alnus sinuata indicated an opening of the canopy, with minor influence from fire, as recorded in the charcoal record. From 10.5 ka until the eruption of Mt. Mazama, Alnus rubra, Pseudotsuga and Pteridium spores, coupled with increased charcoal accumulation 14 14 rates are taken by Cwynar as indicating a closed, successionally young forest with high fire frequency. Increase in Thuja, Tsuga, and Pseudotsuga post-Mazama indicates a climate trend towards cooler and moister conditions of today. Recently at Kirk Lake, the site experienced enhanced peatland growth, leading to greater diversity that may not be indicative of the surrounding region. Dunwiddie (1986) scrutinized three ponds south of Mt. Rainier: Jay Bath pond, Log Wallow pond, and Reflection pond. These records extended 6 ka, and illustrated initially high abundance of Alnus, with appearance of Tsuga and Thuja at 5 ka at Log Willow and 3.7 ka at Jay Bath. This change was not observed at Reflection pond.
Olympic Highland Gavin et al. (2001) scrutinized Martins and Moose lakes, two high altitude sites in the central and north-central Olympic Peninsula. At the Pleistocene- Holocene transition, these environments were cold, dry, and close to treeless. Pollen assemblage consists of Compositae, Artemisia and Pteridium spores. Martins Lake did not have forest established even in the warm early Holocene. An abrupt transition to modern vegetation, consisting of Tsuga and Abies, occurred after the eruption of Mt. Mazama (7627 ± 150 cal YBP). Moose Lake had Abies forests in the early Holocene, and then shifted to Chamaecyparis, Tsuga and Pinus, from shortly after the Mt. Mazama eruption, indicating a cooling and moistening trend. From the mid Holocene to modern times, the region has shifted to parkland. METHODS
Pollen preparation involves several steps. Pollen grains are coated in an exceptionally stable polymer called sporopollenin (Piffanelli et al., 1998). This substance allows chemical preparation by immersion in a series of different acids, dissolving everything except the polymer portion of the pollen grain. Preparation techniques are broadly outlined by Faegri and Iversen (1989). Initial sediment samples, taken from the core every 5-20 cm, were immersed in water, with a small quantity of HCl. The sample was then “spiked” with the introduction of ten tablets of exotic Lycopodium pollen with a known quantity, per the technique proposed by Jorgensen (1967) and refined by Stockmarr (1971). The samples were mechanically stirred until the material disaggregated. The samples were transferred to test tubes using a swirling technique in which heavy material was allowed to settle, with the suspended particles poured into test tubes for further processing. The addition of Lycopodium pollen in a known quantity allows the estimation of population by comparing the ratio of introduced and recovered pollen grains. This method allows for the determination of absolute pollen abundance in a sample, through the following equation: Gc/Gt = Lc/Lt, or Gt = (Gc*Lt)/Lc Gc = grains counted Gt = total number of grains in sample Lc = Lycopodium grains counted Lt = total number of Lycopodium grains in sample Following filtration, immersion in HCl assured removal of the sample's carbonate fraction. Next, the samples were transferred to 50ml test tubes and 16 16 treated with HF to dissolve the silicate fraction. This step required a minimum of thirty hours. Additional exposure to HCl following the HF treatment prevented silicate recrystalization. Two rinsing cycles with hot distilled water prepared samples to begin the process of acetolysis. The application of 10% nitric acid continued sample disaggregation. Two rinsing cycles with distilled water removed any nitric acid remnant. Samples were immersed in glacial acetic, then acetolyzed using a mixture of acetic anhydride and sulfuric acid. At this point organic material was removed, leaving the sporopollenin-comprised exine intact for identification (Faegri and Iversen, 1989). To prevent chemical burning of the pollen grains, glacial acetic was applied to terminate the reaction. Two rinsing cycles with distilled water removed the majority of chemical residue. The samples were treated with 5% KOH to wash particulate matter for the acetolysis step. Repeated rinsing occurred with distilled water until the fluid fraction cleared. Addition of alcohol prepared samples for saffronin staining, for easier identification of pollen grains. Samples then required addition of one part tert butyl alcohol, two parts silicone oil mounting fluid, prior to transferring to vials. After manual stirring, samples were loosely covered. Open-air evaporation of alcohol resulted in a finished product, ready to be mounted in silicone oil on a microscope slide. A grain count measured pollen type abundance and concentration of a number of dominant constituent families, charcoal abundance, and its change over time. To achieve a representative sample, and to maintain a solid base for comparison with prior studies in the area, counting included a minimum of 300 terrestrial grains (Hill, 1996). Abundance was calculated as a percentage of terrestrial pollen grains. Counts were at 400x magnification. To increase the 17 17 precision of climate interpretation, an effort was made to distinguish between Alnus rubra and Alnus sinuata, as well as to distinguish between haploxylon and diploxylon variants of Pinus. Unfortunately, grain preservation was insufficient to the latter task. RESULTS
The pollen record from Lake Louise can be broadly divided into four zones. Unfortunately, exact chronologic data is somewhat scarce. The first zone represents the late Pleistocene and early Holocene, beginning approximately 15.6 ka. The next represents the Holocene Climatic Optimum, which lasted until slightly after the eruption of Mt. Mazama (7627 ± 150 cal YBP). The third zone is interpreted as post-optimum precipitation increase, and lasted until after 2840 ± 40 cal YBP. The last zone indicates recently increased warming and disturbance. Arboreal pollen dominates throughout the sequence. Figures 3 and 4 represent the arboreal and non-arboreal constituents of the pollen record, respectively. The oldest of the zones, Z4, is denoted by the high abundance of Pinus pollen, which declines over time, from a peak of 81% to a low of 33% (Figure 3). Significant quantities of Alnus rubra (8%-28%) and Pseudotsuga (0-20%), are also observed, along with Thuja (3%-9%). Fern spores, represented by monolete and trilete varieties (Figure 4), increases midway through the zone, from a low of 1% to a maximum of 17%. Overall pollen concentration is high, but declines throughout much of the zone, rebounding somewhat shortly before the transition to Z3. Charcoal abundance is at its highest, but is roughly synchronous with pollen abundance throughout the sequence. The zone ends as Thuja, Alnus rubra, and Pseudotsuga increase. The Holocene Climatic Optimum zone, Z3, shows increased abundance in Thuja, ranging from 9 to 12%, Tsuga (10%-12%), Pseudotsuga (8%-28%), and Alnus rubra, (25%-44%), as Pinus declines to a minimum of 7%, stabilizing around that value for the rest of the Holocene (Figure 3). Fern spores remain high, peaking at 18%, before declining mid-way through the zone to the low level, 19 19
Z4
Z3
Z2
Z1
1863 Zone
+/- 2840 40 Mazama Tephra - 7627 Mazama Tephra
Sambucus
Ericaceae
sinuata Alnus 40
Arboreal Shrubs and
20 Alnus rubra Alnus
Quercus
Corylus
Acer 20
Betula
Fraxinus Salix
Populus Picea
20
Tsuga 40
20 Conifers
Pseudotsuga diplox Pinus
haplox Pinus 80
60
40
20 Pinus undifferentiated Pinus
40
20 Thuja
Abies
0
50
700
650
600
550
500
450
400
350
300
250
200
150 100
Figure 3. Lake Louise pollen percentage diagram showing percentage abundance of conifer and other arboreal taxa, plotted against(cm) Depth core depth.
20 20
Z4
Z3
Z2
Z1
1863
Zone
2840 +/-40 2840
Mazama Tephra -7627 Tephra Mazama
200
150
100
50
Charcoal abundance Charcoal
50
40
30
20
10
Pollen per cubic cm (1000x) cm cubic per Pollen
Pediastrum % Pediastrum
Other sum Other
20
Aquatic sum Aquatic
20
Non-arboreal sum Non-arboreal
100
80
Other
60
40
20 Aquatic
Arboreal sum Arboreal
Unknown
Unidentifiable
Typha
20
Trilete
Monolete
Drosera
Polygonaceae
Oxyria
Moraceae
Sagittaria
Rosaceae
Non-arboreal Caryophyllaceae
Polemoniaceae
Amaranthaceae
Umbeliferae
Equisetum
Ambrosia
Ligulaflorae
High Spine High
Low Spine Low
Lotus
Ranunculus
Rumex
Artemisia
Graminae
0
50
700
650
600
550
500
450
400
350
300
250
200 150 100 Figure 4. Lake Louise pollen percentage diagram showing non-arboreal, aquatic and other pollen, as well as abundance summation and(cm) Depth lycopodium to pollen ratio, plotted against depth. 21 21 ranging from 2% to 7% observed for the rest of the Holocene. Overall pollen concentration is somewhat more stable, higher than subsequent zones, but significantly lower than the previous zone on average. Concentration decreases toward the upper terminus of Z3 (Figure 4). Average charcoal abundance declines slightly, but remains significantly higher than in subsequent zones. Post-Holocene Climatic Optimum, Z2, records a significant fluctuation in Thuja. This fluctuation shows itself to be at the expense of Pseudotsuga and Tsuga, and, to a much lesser degree, Betula. However, it is not observed in Alnus rubra. With only two dates available at or near Z2, there is insufficient temporal constraint to determine whether these fluctuations are truly periodic. Because of the degree of variation, data from this section is better described in trends than in maxima and minima. The overall trend of Z2 includes increased abundance in Thuja and Tsuga compared to their previous abundances. Compared to Z3, average abundance of Thuja shifts from 5% to 23%. Tsuga increases from a Z3 average of 4%, to 10% in Z2. Pseudotsuga remains equivalent, on average, to its Z3 abundance. Alnus rubra decreases, from an average of 34% to 18%. Pollen and charcoal concentration are lower than in previous zones. Compared to Z2, the most recent zone, Z1, is characterized by an increase in Alnus rubra, ranging from 15% to 42%, a slight increase in Tsuga (1%-21%) and Pseudotsuga (8%-32%), and a decrease in Thuja (5%-29%). Overall pollen and charcoal abundance are at their lowest, but vary significantly between samples. Anthropogenic influence can be observed in the most recent samples through the presence of trace quantities of non-native Liguliflorae, and is also likely responsible for the spike in charcoal abundance in the uppermost sediment samples. DISCUSSION
Overview Figure 5 presents a ternary diagram showing the major drivers of climate in each pollen zone. In Z4, high Pinus abundance suggests that cold conditions were limiting, consistent with cold conditions elsewhere. Z4 is the most ordered zone, with other influences only minimally present. In the next zone, Z3, increased Pseudotsuga is indicative of warming (Leopold et al., 1982). Z2 is consistent with increasing prevalence of precipitation, though wide dispersion across the graph suggests an unstable climate in which warmth and precipitation vary as to which influences vegetation more. Finally, Z1 shows warming once again as the driving force, with trends similar to others in the region (Leopold et al., 1982; McLachlan and Brubaker, 1995; Tsukada et al., 1981). This study can be divided into three time periods. In the early Holocene, encompassing Z4 and Z3, the record largely corroborates previous studies (Hansen and Easterbrook, 1974; Whitlock, 1992). Further, because even broadly similar studies show some differences, each additional study improves the potential resolution of regional climate history reconstruction. The middle Holocene, represented by Z2, shows intense fluctuation in Thuja, with inverse fluctuation in Pseudotsuga, Tsuga, and, to a lesser degree, Betula. Finally, the late Holocene, Z1, shows a significant increase in sedimentation rate (Figures 3, 4). Similar trends in pollen have been observed during this time (Leopold et al., 1982; Tsukada et al., 1981), but these comprised a comparatively minute portion of the cores in which they occurred, and were not discussed.
23 23
Figure 5. Ternary diagram showing covariance, with end members Pinus (cold) Pseudotsuga (warm) and Thuja (wet).
Terminal Pleistocene and Early Holocene The extent of the pollen record at Lake Louise is likely not constrained to the Holocene, though earliest samples show a high quantity of Pinus pollen, with no evidence of the parkland biome that often immediately succeeds glacial retreat (Heusser, 1978; Petersen et al., 1983). While pollen evidence does not directly support site development immediately following glacial retreat, this stands somewhat in contrast to Datt's (2012) sedimentation based hypothesis of core 24 24 origin at 15.6 ka. Coupled with estimates of glacial evacuation of the site at approximately 16.6 ka by Porter and Swanson (1998), this suggests a brief, unrecorded period of parkland in the time between glacial retreat and sedimentation, an uneven sedimentation rate reducing the accuracy of Datt's estimate, or that the environment surrounding Lake Louise quickly transitioned from glacial burial to Pinus forest. While this study was of insufficient resolution to resolve Pinus to species level, other studies (Hibbert, 1979; Barnosky, 1981; McLachlan and Brubaker, 1995) suggest the most likely species type for the majority of early Pinus pollen is Pinus contorta. At the initiation of sediment record, the base of Z4, Pinus dominated the record. Similar dominance can be seen at an analogous time in other studies of the region (Heusser, 1973; Whitlock, 1992). Pinus contorta serves as an effective pioneer species, as it is quick to seed, and is able to tolerate a wide variance in precipitation, substrate stability, and length of growing season (Hibbert, 1979). Temperature tolerance for cool and cold conditions is fairly broad (Thompson et al., 1999). Coupled with initial lack of other pollen types, this dominance suggests an initially cold climate. Climatic amelioration is shown throughout Z4 by expansion of other species, particularly Pseudotsuga menziesii¸ a relative thermophile (Thompson et al., 1999), at the expense of Pinus, with comparable trends observed in other studies (Tsukada et al., 1981; Leopold et al., 1982). While other studies have used abundant Artemisia pollen (Whitlock, 1992; Gavin et al., 2001) and lack of glacial advance (Heine, 1998) in this stage to argue for dry conditions, pollen-based evidence of aridity in this study is limited to the absence of significant quantities of hydrophytic plants. Pinus, along with rapidly increasing Alnus rubra, argue strongly for ecosystem disturbance. High charcoal abundance links this disturbance to frequent fires from the immediate postglacial 25 25 to the Pleistocene-Holocene transition, as seen in other studies (Brown and Hebda, 2003; Gavin et al., 2013). Alnus rubra increase may also signal opening of the canopy, which is consistent with increasing fern spores occurring around mid Z4. Observations during this period are similar to most other sites in the region (Hansen and Easterbrook, 1974; Heusser, 1973; Whitlock, 1992), though a few high altitude sites present Pinus abundance increases which are comparatively weaker (Mineral Lake, Tsukada et al., 1981; Martins Lake, Gavin et al., 2001) or absent (Spooner et al., 2007). Figure 5 shows, through dominance of Pinus in the Pinus-Pseutotsuga-Thuja ternary model, that cold is the dominant driver of climate in the study site. Given the time frame immediately following glacial retreat, this would be expected as a regional trend. After the decline of Pinus, no single successor species achieved such a degree of dominance over the floral assemblage. Other sites do not share this characteristic (Leopold et al., 1982; Gavin et al., 2013). Z3 begins with an increase in Pseudotsuga menziesii, which, coupled with the decline in Pinus, suggests a warming climate (Thompson et al., 1999). In Z3, Thuja and Tsuga, two strong indicators of increased moisture (Thompson et al., 1999) were largely absent, suggesting xeric conditions, akin to those observed by McLachlan and Brubaker (1995). The increasing abundance of Alnus rubra, however, which has similar needs for temperature and precipitation as Thuja and Tsuga, (Thompson et al., 1999) challenges this suggestion. Alnus rubra is exceptionally tolerant of summer drought, but it requires high winter precipitation (Thompson et al., 1999). Some of this abundance may be explained if the canopy was more open, and is supported by increased presence of Pteridium and other fern spores, widely seen in other studies (Tsukada et al., 1981; Leopold et al., 1982; Whitlock, 1992; McLachlan and Brubaker, 1995; Spooner et al., 2007). In some of these studies, 26 26 however (Tsukada et al., 1981; Whitlock, 1992; Spooner et al., 2007) Alnus expansion and depletion is not particularly well aligned with that of Pteridium. In this study, Alnus rubra maintains relatively high abundance, even after Pteridium declines in mid Z3. Cwynar (1987) suggests the likeliest solution, which is supported by high charcoal abundance: Alnus rubra and Pseudotsuga are both tolerant to fire. Tsuga heterophylla, in contrast, is exceptionally sensitive (Tesky, 1992). Floral abundance, therefore, is representative of a young community, disturbed before successional maturity altered it. This line of evidence would suggest that, while the Holocene Climatic Optimum did represent a warmer time in the local record, it was not necessarily as xeric as many previous studies conclude. Cwynar (1987) suggests that a relatively small decrease in precipitation may have lead to a large increase in fire frequency. Others (Tsukada et al., 1981, Barnosky, 1985) stress that xeric conditions at this time were likely primarily attributable to more extreme summer drought, allowing for the expansion of Alnus, but resulting in increased fire in the region (Walsh et al., 2008). The general trend of warming and drying is seen in other sites in the region (McLachlan and Brubaker, 1995; Heusser, 1978; Leopold et al., 1982), with some charcoal records supporting increased fire frequency (Tsukada et al., 1981; Long et al., 1998; Brown and Hebda, 2003; Walsh et al., 2008; Prichard et al., 2009), though changes in this study are less marked than some other sites (Tsukada et al., 1981; Leopold et al., 1982). The initiation and terminal times for this period vary widely, however (see Table 1, p. 7). 27 27 Middle Holocene The transition to the middle Holocene, Zone 2 is marked by an abrupt increase in Thuja, at the expense of Alnus rubra. Tsuga increases less rapidly, but is also shown to increase abundance. Similar trends are seen in other papers (Leopold et al., 1982; Tsukada et al., 1981), though these often include a decline in Pseudotsuga, indicative of a cooling trend not observed locally, and lack the level of fluctuation detected in this study. Tsukada (1981) records a single instance of Thuja stands destroyed in a fire around this time and replaced by Tsuga. Taken together, these changes indicate an increase in precipitation, which in turn decreased fire frequency (Cwynar, 1987). An attempt was made to explain the fluctuations as due to increased fire severity. Increased precipitation, particularly in a highly seasonal precipitation regime such as Washington (Marlon et al., 2006), can result in decreased fire frequency, but increased fire intensity (Millspaugh et al., 2000). Lack of desiccation makes a fire harder to start in a wetter regime, but higher precipitation results in more biomass available for ignition (Millspaugh et al., 2000). This hypothesis is, unfortunately, not supported by the charcoal record from Lake Louise. Peaks in Thuja concentration are neither completely synchronous, nor completely asynchronous, with peaks in charcoal concentration. Additionally, average charcoal abundance is low, compared to previous zones. Regional records of fire history during this time vary significantly. Gavin et al. (2013) observed lower fire frequency during this time than at any other during the past 14 ka. In contrast, other studies (Long et al., 1998; Long and Whitlock, 2002; Brown and Hebda, 2003) found elevated fire occurrence during some or all of the zone. Hallett et al. (2003), recording multiple sites, recorded mixed results. A meta-study by Marlon et al. (2006) examined multiple previous studies, and 28 28 found that the overarching fire trend during Z2 was initially high, but peaked at 7,000 cal yr BP, and, with only slight exceptions, decreased until near the end of Z2. Other studies generally observe a cooling and moistening trend (Mathewes, 1973; Spooner et al., 2007; Hibbert, 1979). This cooling, and the preceding warming, correspond to Milankovitch forcing, which peaked around 9 ka (Kutzbach and Street-Perrott, 1985).
Late Holocene The most recent transition, to Z1, shows a small increase in Pseudotsuga. The mild warming this suggests contrasts with some records showing increased cooling into the Neoglacial (Dunwiddie, 1986; Osborn et al., 2007), but these observations are not universally observed (Tsukada, 1981; Barnosky, 1981). Further, such cooling is more likely to be observed in higher elevation alpine settings. The precipitation record is more complex. Alnus rubra expands, but Thuja declines, and Tsuga remains largely unchanged. The interpretation of this sequence is that precipitation remains largely unchanged from the previous zone, but disturbance increases. Regionally, the fire record is mixed, with some sites recording a marked increase (Hallett et al., 2003; Long et al., 1998), and others showing no such occurrence (McLachlan and Brubaker, 1995; Long and Whitlock, 2002; Prichard et al., 2002). In this case, disturbance is viewed as unlikely to be from fire, due to low charcoal abundance throughout the zone. Further, Tsuga would presumably be affected by fire to a greater degree than Thuja. Rather, Datt (2012) noted a sharp increase in sedimentation rate during this time, suggesting increased erosion and sedimentation as the source of disturbance. Total pollen 29 29 abundance declines concurrently, as expected. Similar increases in sedimentation during the late Holocene are present in some other studies (Barnosky, 1981; Marlon et al., 2006; Walsh et al., 2008), but absent in others (Long and Whitlock, 2002; Dunwiddie, 1986). Similar pollen trends are seen in Leopold et al. (1982), McLachlan and Brubaker (1995) and Tsukada et al. (1981), but with elevated Tsuga. Their magnitude is insufficient to warrant classification as a distinct zone at any of these sites. Of all zones in the assemblage, this one is least clear, and would benefit most from additional study. Z1 owes its existence as a separate classification to increased sedimentation at this site in the late Holocene. This increased sedimentation is unlikely to be anthropogenic in nature. Z1 begins significantly prior to European colonization, and low charcoal abundance does not support an increase in fire frequency, anthropogenic or otherwise, as the source of this erosion. In addition to higher resolution study of Z1, macrofossil and Pinus stomata analysis would allow better constraint on species observed, and lignin analysis may better constrain pollen source locales. Further, the charcoal record could be reinforced with a study of magnetic susceptibility, which has been shown to correlate to erosion, often resulting from higher fire frequency (Hallett et al., 2003; Long and Whitlock, 2002). WORKS CITED
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APPENDIX: POLLEN SITES OF THE PACIFIC NORTHWEST
37 37
1. Marion (Jacobs) Lake (Mathewes, 1973) 2. Pangborn Bog (Hansen and Easterbrook, 1974) 3. Mosquito Lake (Hansen and Easterbrook, 1974) 4. Hall Lake (Tsukada et al., 1981) 5. Manis Mastodon (Petersen et al., 1983) 6. Crocker Lake (McLachlan and Brubaker, 1995) 7. Lake Washington (Leopold et al., 1982) 8. Wentworth Lake (Heusser, 1973) 9. Wessler Bog (Heusser, 1973) 10. Thunder Lake (Spooner et al., 2007) 11. Bogachiel River Valley (Heusser, 1978) 12. Yahoo Lake (Gavin et al., 2013) 13. Hoh River Valley (Heusser, 1974) 14. Humptulips (Heusser, 1983) 15. Nisqually Lake (Hibbert, 1979) 16. Zenkner Valley (Heusser, 1977) 17. Mineral Lake (Tsukada et al., 1981) 18. Davis Lake (Barnosky, 1981) 19. Battle Ground Lake (Whitlock, 1992) 20. Fargher Lake (Heusser, 1983) 21. Kirk Lake (Cwynar, 1987) 22. Jay Bath (Dunwiddie, 1986) 23. Martins Lake (Gavin et al., 2001) 24. Moose Lake (Gavin et al., 2001) Fresno State
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