Midge-Inferred Holocene Climate History of Two Subalpine Lakes in Southern British Columbia, Canada

The Holocene 14,2 (2004) pp. 258-271

Midge-inferred Holocene climate history of two subalpine lakes in southern British Columbia, Canada

Sandra M. Rosenberg,l 2* Ian R. Walker,1'2 Rolf W. Mathewes1 and Douglas J. Hallett"'3 ('Department of Biological Sciences, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada, V5A ]S6; 2Department of Biology, Okanagan University College, 3333 College Way, Kelowna, British Columbia, Canada, VI V 1 V7; 3Center for Environmental Sciences and Quaternary Sciences Program, PO Box 5694, Northern Arizona University, Flagstaff AZ 86011, USA)

Received 16 July 2002; revised manuscript accepted 13 February 2003

Abstract: To investigate postglacial environmental changes in both the coastal and interior wet belts of British Columbia, fossil midges were analysed from two subalpine lakes, one adjacent to the lower Fraser canyon (Frozen Lake), and the other in Mount Revelstoke National Park (Eagle Lake). The midge stratigraphy for Frozen Lake revealed an abundance of rheophilous chironomid taxa and Simuliidae larvae, reflecting the pres- ence of an inflowing stream. An abundance of Chaoborus mandibles and Microtendipes during the early Holo- cene (c. 10100-7700 'C years BP, c. 11500-8500 cal. years BP) suggests warmer temperatures. A subsequent decline in the warm indicators and relative increases in cold stenotherms (Heterotrissocladius and Diamesa) indicate cooling until present day. This climate reconstruction is consistent with other quantitative and qualitative evidence for past climatic change in southern British Columbia. At Eagle Lake the warm indicators, Dicrot- endipes and Polypedilum, are seen in the early Holocene (c. 8500-6730 1-C years BP, c. 9600-7600 cal. years BP), but are absent during the mid-Holocene when cooler temperatures probably prevailed. In the late Holocene HOLOCENE (c. 3800 'dC years BP to present, c. 4200 cal. years BP to present) there is a resurgence of warm indicators, RESEARCH which contrasts with the evidence of continued cooling typically seen in reconstructions of southern British PAPER Columbia summer temperatures. The Eagle Lake record therefore appears to be anomalous. Multiproxy and multisite investigations are needed to reconstruct Holocene climatic changes more reliably.

Key words: Chironomidae, midges, palaeoclimate, palaeolimnology, temperature reconstruction, climatic change, British Columbia, Holocene.

Introduction ditions (e.g., Walker, 1987; Hofmann, 1988; Walker and Mathewes, 1989; Walker et al., 1995; Lotter et al., 1997). Midge Palaeoecological techniques are powerful tools for reconstructing assemblages have been used to reconstruct hypolimnetic oxygen past environments, and can provide long and detailed records of content (Quinlan et al., 1998), acidity (Brodin and Gransberg, past climatic changes. They thus provide a window on natural 1993; Schnell and Willassen, 1996), salinity (Walker et al., 1995; climatic variations and on the role of climate in shaping terrestrial Heinrichs et al., 1997), trophic status (Lotter et al., 1998; Little ecosystems (Mathewes, 1985; Hebda, 1995; Walker and Pellatt, et al., 2000; Brooks et al., 2001) and, as is the focus of this paper, 2001; 2004). Chironomids (Order Diptera) have become important temperature (Walker and Mathewes, 1989; Walker, 1991; Lotter for palaeoclimatic studies because distinctive assemblages of et al., 1999; Walker et al., 1997; Olander et al., 1999). midges are good indicators of present and past environmental con- Several studies have used fossil midges to reconstruct past climatic oscillations (i.e., the Killarney and Younger Dryas events) *Author for correspondence. Present address: Biology Department, Langara College, in Atlantic Canada (i.e., Walker et al., 1991 a; 199 lb; Levesque 100 West 49th Avenue, Vancouver, BC, Canada V5Y 2Z6 (e-mail: et al., 1993a; 1993b; Wilson et al., 1993; Cwynar and Levesque, [email protected]) 1995). In southern British Columbia, however, palaeoclimato- Arnold 2004 10.1 191/0959683604h1703rp

Study area

For the purposes of Holocene climate reconstruction, two small subalpine lakes were chosen because treeline environments are known to be sites of high climate sensitivity (Luckman and Kear- ney, 1986; Clague and Mathewes, 1989; Walker and MacDonald, 1995; Pellatt and Mathewes, 1997; Pellatt et al., 1998; 2000; Bat- tarbee, 2000). Lower Frozen Lake (49°36'N, 121°28'W) is a small subalpine lake located in British Columbia's Coast Moun- tains, on the slopes of the lower Fraser Canyon (Figure 1). At 1180 m a.s.l., the lake occupies a basin in the forested subzone of the cool and moist Mountain Hemlock biogeoclimatic zone. The dominant trees in this zone are mountain hemlock (Tsuga mertensiana (Bong.) Carr.), Pacific silver fir (Abies amabilis - elevation contour lakes, rivers Dougl.) and yellow cedar (Chamaecyparis nootkatensis D. Don) N creeks (Brooke et al., 1970). Frozen Lake is approximately 3 ha in area tI _ ._Iroads with a maximum depth of 17 m and a bedrock sill controlling the a city/town Scale outlet. A small stream flows from Upper Frozen Lake through an m O 500 t OQO open meadow to the lower lake (Hallett et al., 2003). This inlet delivers a large volume of water to the lake during spring and summer melt. Figure 2 Map of Mount Revelstoke National Park region, indicating the Snow in the Mountain Hemlock zone begins to accumulate in location of Eagle Lake. October and deep snowpacks can persist until July. The late winter snowpack may be up to 3 m thick (Brooke et al., 1970; Hallett et al., 2003). Climate data at nearby Hell's Gate (49°47'N, environmental lapse rate of 0.6°C/100 m (Livingstone etal., 121°27'W, elevation 122 m) indicates a mean annual temperature 1999), the mean annual temperature and the mean July air tem- of 9.4°C and a mean annual precipitation of 1229 mm yr-' perature at Frozen Lake are expected to be about 2.8°C and (Environment Canada, 1993). It should be noted that daily mean 13.9°C, respectively. The actual precipitation at Frozen Lake temperatures and mean annual precipitation at Frozen Lake will could be >2X that recorded at the Hell's Gate station. differ considerably from the Hell's Gate station due to the Eagle Lake (51°3'N, 118°10'W) is located in Mount Revel- difference in elevation between the two sites. Assuming an stoke National Park, in the Selkirk Mountains (Figure 2). The

Selkirk Mountains, together with the Monashee Range to the west Table 1 Frozen Lake samples containing fewer than 50 chironomid and Purcell Range to the east, compose the Columbia Mountains, head capsules a system of north-south trending ranges with bedrock consisting of a complex of metamorphic and igneous rocks. The Columbia Depth Taxa Number of Mountains are bordered on the west by the Interior Plateau and (cm) head capsules on the east by the Rocky Mountains. The Columbia Mountains form the interior wet belt of southern 245 CricotopusJOrthocladius 0.5 British Columbia. Moist air masses from the Pacific coast are Corynocera nr. ambigua 1 responsible for the mean annual precipitation of 950.2 mm yr- I Heterotrissocladius grimshawi type 1 at the Revelstoke climate station, 443 m a.s.l. Procladius 2 recorded 0.5 at Protanypus (Environment Canada, 1993). The mean annual temperature Tanytarsina (undifferentiated) 1 Revelstoke is 6.7°C. Eagle Lake, however, is located at 1845 m (Ephemeroptera mandibles) (2) a.s.l., well above the Revelstoke climate station; thus, the study (Unidentifiable) (3) site is correspondingly cooler and wetter. Assuming an environ- 250 Tanytarsina (unidifferentiated) 1 mental lapse rate of 0.6°C/100 m, the estimated mean annual tem- 270 Diamesa 1 perature at Eagle Lake is -1.7°C. Mean July air temperature is Psectrocladius 0.5 estimated at 9.8°C. A temperature record is also available for most 285 Diamesa 1 of the past decade from a BC Hydro climate station located on Pseudodiamesa 1 Mount Revelstoke (51°2'N', 1 18°8'W, 1830 m a.s.l.; BC Hydro, 290 Micropsectra atrofasciata type 2 personal communication). The record is much shorter than that used in calculating climate normals, but indicates a mean July air temperature of I 1.1°C. According to Coupe et al. (1991), up to diameter modified Livingstone piston corer (Wright, 1967). The 2200 mm of annual precipitation might be expected at Eagle Lake two core records were matched up based on the stratigraphic with 50-70% as snow, leading to a maximum snowpack of nearly lengths of each of the cores. The sediment stratigraphy consists 4 m. At this elevation, the snowpack may persist from mid- of partially laminated gyttja with a thick tephra layer at 96-81 October through to mid-July. In 1999, an exceptionally wet year, cm, a thinner tephra at 36-35 cm, and small amounts of woody extensive snow patches were still evident at Eagle Lake in mid- debris throughout the core. The core was subsampled every 1 cm August (Rosenberg et al., 2003). from 110 to 0 cm and subsequently stored in a cold room at Simon Eagle Lake has a maximum depth of 1.5 m, an area of 0.3 ha Fraser University at 4°C. Subsampling (58 levels) of half cubic and is located 1845 m a.s.l. (Donald and Alger, 1984). The lower centimetre sediment samples was needed to give greater than 50 elevations of Mount Revelstoke support forests of western hem- head capsules per interval. lock (Tsuga heterophylla (Raf.) Sarg.) and western red cedar Processing of each sediment sample involved head capsule iso- (Thuja plicata Donn), but vegetation at Eagle Lake represents a lation techniques outlined by Walker (1987; 2001) and Walker higher-elevation community and is classified as the ESSFvc (very etal. (199la). Treatment with 10% HCI was used to remove wet cold Engelmann Spruce-Subalpine Fir) biogeoclimatic carbonates from the moist sediment, then 5% KOH was added to subzone (Coupe et al., 1991). The climax trees of this region each sample and heated to approximately 40-45°C for 5-6 include Engelmann spruce (Picea engelmannii Parry) and subalp- minutes to complete deflocculation. The sediment was sub- ine fir (Abies lasiocarpa (Hook.) Nutt.), with a small component sequently sieved on a 95 ,um Nitex® mesh. The retained residue of mountain hemlock (Tsuga mertensiana (Bong.) Carr). Eagle was backwashed into a beaker with distilled water. Individual Lake lies near the transition from continuous forest to subalpine head capsules were picked from a Bogorov counting tray under parkland. Many small openings are present among the trees sur- a dissecting microscope at X 10-25 magnification. The midge rounding the site. remains were transferred to drops of water on a coverslip using Dumont #4 forceps. The coverslips were allowed to air-dry, and then mounted on glass slides using Entellan® mounting medium. Methods Dipteran remains were identified at X 100-400 magnification using an Olympus BH-2 compound microscope. Identifications Field and laboratory methods were based on descriptions and keys by Oliver and Roussel In 1997, a 300 cm core from the deepest part (17 m) of Frozen (1983), Wiederholm (1983), Walker (1988), Uutala (1990), an Lake was obtained using a lightweight percussion corer extensive photograph (i.e., reference) collection and the WWW (Reasoner, 1993). The core was then transported to Simon Fraser field guide to subfossil midges (Walker, 1996-2000). If the head University where it was subsequently stored in a cold room at capsules retained more than half of a complete mentum, they were 40C. counted as one head capsule. If they had exactly half of a mentum, The sediment stratigraphy consists of dark brown gyttja grading they were counted as one half. Head capsules retaining less than into greyish brown clay at 238 cm depth and to grey clay near half of the mentum were not counted. the core base at 240 cm. Tephra layers were found at 163-155 cm, 127 cm and 60 cm (Hallett et al., 2003). For midge analysis Statistical analysis the core was subsampled every 5 cm from 240 to 0 cm, to give TILIA version 2.0.b.4 (Grimm, 1993) was used to collate the raw 49 sediment subsamples of 2 cm3. The intervals from 320-240 data and generate a midge percentage diagram in TILIA-GRAPH cm had substantially lower than 50 chironomid head capsules and version 2.0.b.5 (Grimm, 1991). The program CONISS was used were sometimes barren (Table 1). These samples are considered to perform a stratigraphically constrained incremental sum-of- too small to be statistically useful (Palmer, 1998; Heiri and Lotter, squares cluster analysis (Grimm, 1987). This allowed intervals 2001; Larocque, 2001; Quinlan and Smol, 2001). with major changes in midge communities to be recognized A 110 cm core was removed from the deepest part of Eagle throughout the cores. The significance of the zones was assessed Lake on 9 August 1999 in 1.5 m of water. A modified Kajak- following the broken-stick procedure outlined by Bennett (1996; Brinkhurst surface corer, equipped with a Glew trigger mech- 2002) and implemented in the computer program PSIMPOLL anism (Glew, 1989), was used to capture the uppermost 16 cm of version 4.10. As suggested by Bennett, we designated significant the core. The sediment below 16 cm was collected using a 5 cm boundaries as separating zones.

A midge-temperature weighted averaging inference model, Chaoboridae, family Ceratopogonidae and family Simuliidae), as based on surface sample data from 51 British Columbia lakes, was well as Ephemeroptera. For taxa where estimates of temperature developed by Palmer et al. (2002). In addition to Chironomidae, optima have been determined, the midge taxa are arranged from Chaoborus were used in the model, and in other statistical analy- cold-tolerant to warm-adapted. Stratigraphies for the remaining ses. Ceratopogonidae and Simuliidae were too rare to be included. chironomids are appended to the end of the diagram. To infer mean July air temperatures at our study sites, we used a Fifty-one chironomid taxa were identified from the Frozen Lake slightly modified version of this model incorporating higher taxo- sediments. The computer programs CONISS (Grimm, 1987) and nomic resolution. Although midges probably respond more PSIMPOLL (Bennett, 2002) indicated two significant zones in the closely to changes in water temperature, we reconstructed air Frozen Lake core (Table 3). temperature because water-temperature data were not available. Air temperatures are strongly correlated with lake surface-water Midge assemblaqe zone 1 (FRal, 240-153 cm, temperature and are of greater interest to palaeoclimatologiststhan c. 10100-6500 4C years BP, c. 11500-7500 cal. years water temperatures (Livingstone and Lotter, 1998). BP) This modified model was developed using CALIBRATE ver- Members of the Subtribe Tanytarsina are the dominant midge taxa sion 0.70 (Juggins, 1997) and WA-PLS version 1.1 (Juggins and throughout the core including zone FRO-1 (Figure 3). Other taxa ter Braak, 1996). The model revealed a significant relationship common in zone FRO-1 include Microtendipes, Chironomus, between mean July air temperatures and midge assemblages, with Tribe Pentaneurini, Heterotrissocladius Sergentia, Limnophyes, a jack-knifed r2 of 0.73 and a jack-knifed root mean squared error Corynoneura & Thienemanniella, Rheocricotopus, Psectrocladius of prediction (RMSEP) of 1 .87°C. As a further assessment of the and Chaoborus. Chaoborus mandibles (mainly Chaoborus potential usefulness of this model, we performed a partial con- trivitattus) are abundant throughout the zone and are often more strained CCA of the square-root transformed midge data, with the numerous than the remains of all other midges combined. Chiron- first axis constrained to represent mean July air temperature. All omid taxa which are found sporadically and at low abundance other axes were unconstrained. The resulting eigenvalues for the (<2%) include Tanytarsus chinyensis, Parochlus, Cladopelma, first (XI) and second (X2) axes are 0.303 and 0.282, respectively, Cryptochironomus, Endochironomus, Pagastiella, Phaenopsectra, yielding X11X2 of 1.07. This high ratio for the first and second Stictochironomus, Hydrobaenus, Potthastia and Protanypus. eigenvalues suggests that temperature explains a high proportion Psectrocladius (Allo. or Meso.) is present only in zone FRO-1. of the variance in the species data and lends confidence to the Ephemeroptera mandibles were abundant in early zone FRO-1 temperature inference model. sediments but become less common throughout the zone (Figure Using the squared chord distance as a dissimilarity coefficient, 3). Microtendipes decreases towards the top of FRO-1. Slight we used the computer program ANALOG (H.J.B. Birks and J.M. increases are apparent in Corynoneura & Thienemanniella (to Line, unpublished software) to determine if adequate analogues 15%) and Eukiefferiella & Tvetenia abundance (to 8%) towards existed in our surface sample data for each interval in the core the top of this zone. (Overpeck etal., 1985; Laing et al., 1999). Those core intervals exceeding the 95% confidence interval were assumed to have no Midge assemblage zone 2 (FRa2, 153-0 cm, c. 6500 14C analogues, whereas those samples lying between the 75 and 95% BP to present, c. 7500 cal. years BP to present) confidence intervals were considered to have poor analogues. A decrease in the average abundance of Chironomini, and Samples within the 75% confidence interval were considered to especially Chaoborus mandibles, characterizes zone FRO-2. The have good analogues in the surface sample training set (Laing Orthocladiinae increase slightly and Microtendipes is now rare. etal., 1999). Sergentia and Heterotrissocladiuscontinue to be present through- To further assess the temperature reconstructions, correspon- out this zone, whereas another cold stenotherm, Diamesa, makes dence analyses (CA) were performed on the fossil data. The its first appearance late in zone FRO-2 but remains rare. Although strength of the correlation between the CA axis 1 sample scores Rheocricotopus is less abundant than deeper in the core, there and the inferred temperature values indicates to what extent the are slight increases in most rheophilous taxa including principal changes in midge community structure can be explained Corynoneura & Thienemanniella, Doithrix & Pseudorthocladius by the temperature inferences (Laird et al., 1998). Eukiefferiella & Tvetenia and Limnophyes. Members of the family Ceratopogonidae are uncommon although they occur more fre- quently towards the core top. Heterotrissocladius marcidus type Results also increases in abundance towards the top of this zone. Frozen Lake: chronology and stratigraphy Temperature inferences and analogue comparisons CALIB 4.3 (Stuiver et al., 1998) was used to convert radiocarbon Marked fluctuations are evident in the reconstructed temperatures ('4C) ages to calibrated years. Extensive AMS radiocarbon dating for Frozen Lake (Figure 4). In FRO-1, inferred mean July air of plant macrofossils provides the chronology for the Frozen Lake temperature appears to gradually decrease with a maximum core (Table 2). A basal date of 10020 ± 50 "'C years BP inferred temperature of 15.6 + 2.1°C at 235 cm and a minimum (c. 11500 cal. years BP) indicates that the midge samples span inferred temperature of 13.1 + 2.0 at 170 cm. Reconstructed tem- the Holocene. Three ash layers were also found in Frozen Lake peratures remain cool, ranging from 11.6 ± 2.0 to 14.2 + 2.0°C, (Table 2). The lowermost tephra (163-155 cm) was identified as in the second zone. Although good analogues are available for Mt Mazama ash (6730 ± 40 "'C years BP, c. 7600 cal. years BP; most of the FRO-1 midge assemblages, only poor analogues are Hallett etal., 1997). The second tephra (127 cm) was found by available for most of the FRO-2 midge communities (Figure 5). sieving (Hallett etal., 2001) and subsequently identified using A high correlation (r = 0.85, p < 0.01) is evident between the electron microprobe analysis as being from a Glacier Peak erup- CA axis 1 sample scores and the temperature inferences. This tion (5000-5080 "4C years BP, c. 5800 cal. years BP). The third indicates that the temperature inferences can explain much of the ash at 60 cm was identified as Bridge River tephra, giving an variability in the Holocene midge communities. additional date of 2435 ± 26 "'C years BP (c. 2500 cal. years BP; Clague et al., 1995). Eagle Lake: chronology and stratigraphy Figure 3 depicts the percentage of total identifiable Chironomi- Two Picea and one Abies needle from 106-105 cm provided an dae, non-chironomiddipteran remains (i.e., members of the family AMS date of 8330 ± 40 "'C years BP (c. 9400 cal. years BP)

Table 2 AMS radiocarbon and tephra dates for the Frozen Lake sediment core

Depth (cm) Lab. number Uncalibrated age Calibrated age* Relative area under Material dated (CAMS) (lC years BP) (cal. years BP) probability distribution

26-25 45980 1570 ± 60 1590-1580 0.009 conifer needle (Abies amabilis) 1570-1330 0.986 1320-1310 0.004 36-35 45981 1950 + 40 1990-1960 0.093 conifer needle (Tsuga mertensiana) 1950-1820 0.907 61-60 2435 ± 26 2710-2630 0.271 Bridge River tephra 2620-2590 0.050 2540-2530 0.015 2500-2350 0.664 90-89 45983 3560 ± 40 3970-3950 0.061 conifer needle (Tsuga mertensiana) 3930-3800 0.647 3800-3720 0.292 120-119 45984 4530 ± 50 5320-5030 0.974 conifer needle (Tsuga mertensiana) 5010-4980 0.026 148-147 45985 6170 + 40 7220-7220 0.001 twig fragment 7210-7170 0.092 7160-6940 0.903 6920-6910 0.004 163-155 6730 ± 40 7670-7560 0.852 Mazama tephra 7540-7510 0.148 189-188 45986 8180 ± 50 9400-9390 0.006 conifer needle (Abies amabilis) 9370-9360 0.001 9280-9010 0.993 222-221 45987 9390 + 70 11060-11020 0.030 twig fragment 11010-10960 0.034 10840-10830 0.001 10770-10400 0.929 10340-10330 0.001 10320-10310 0.002 10300-10290 0.003 236-235 45988 10020 ± 50 11920-11860 0.035 conifer needle 11700-11260 0.965 (Abies lasiocarpa)

*Based on Stuiver et al. (1998).

from near the base of the Eagle Lake core. Therefore Eagle Lake Midge assemblage zone 1 (EAG-1, 110 cm - Mazama ash, does not contain a complete Holocene sequence. The sediment- c. 8500-6730 C years BP, c. 9600-7600 cal. years BP) accumulation rate is estimated by linear regression and the basal The basal sediments of the Eagle Lake core are dominated by age is estimated by extrapolation because no macrofossils were Dicrotendipes, Polypedilum and Tanytarsina (Figure 6). The Chi- found beneath the 106-105 cm interval. ronomini are more abundant in zone EAG-1 than elsewhere in the Two tephra layers are also identified in the Eagle Lake core core. Other common taxa, including Procladius, Psectrocladius (Table 4). Dr F.F. Foit Jr at the Microbeam Facility, Washington and Chaoborus trivittatus, are found throughout the zone. Some State University, analysed the glass chemistry of each tephra. The of the rarer taxa in zone EAG-1 include Sergentia, Corynoneura lowermost tephra, from 96-81 cm, is identified as Mt Mazama & Thienemanniella, Limnophyes, Acari and Ceratopogonidae ash (6730 + 40 14C years BP; c. 7600 cal. years BP; Hallett et al., (Bezzia type). 1997). The second tephra, at 36-35 cm, is the Mt St Helens Yn tephra, providing an approximate date of 3400 '4C yr BP (c. 3700 Midge assembqlae zone 2 (EAG-2, Mazama ash - 47 cm, cal. years BP; Luckman et al., 1986; Mullineaux, 1986). c. 6730-3800 C years BP, c. 7600-4200 cal. years BP) Twenty-eight chironomid taxa were identified from the Eagle There is a large increase in Tanytarsina and a pronounced Lake core (Figure 6). Five distinct midge assemblage zones are decrease in the Chironomini in EAG-2. Tanytarsina and identified in this core by CONISS (Table 5). All zone boundaries Dicrotendipes are the dominant taxa. Polypedilum is also common are significant as assessed in PSIMPOLL (Bennett, 2002). We in this zone but its abundance fluctuates greatly with two distinct ignored a sixth significant zone boundary separating the sample peaks at 75 and 65 cm. The maximum of Microtendipes and first above 1.5 cm from the sample immediately below, because we appearance of Endochironomus and Tribelos also occur. did not feel it useful to include two zones consisting of one Cold-stenothermous taxa (e.g., Sergentia, Stictochironomus and sample each. Heterotrissocladius are rare in the Eagle Lake core, but are more

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Table 3 Zonation of the Frozen Lake chironomid stratigraphy (ages (c. 11500-7800 cal. years BP), often called the xerothermic per- rounded to the nearest century) iod, is considered to have been warmer with less precipitation than present (Mathewes and Heusser, 1981). The second phase Chironomid Depth Uncalibrated age Calibrated age* from 7000 to 3500 "'C years BP (c. 7800-3800 cal. years BP) is assemblage zone (cm) (radiocarbon (calibrated thought to have been a period of climatic transition, and in south- years BP) years BP) ern British Columbia has been called the mesothermic period (Hebda, 1995). This transitional period begins with warm, moist FRO-2 0-153 Present - 6500 Present - 7300 conditions and ends with cool, moist conditions (Mathewes, FRO-1 153-240 6500-10100 7300-11500 1985). This cooling marks the beginning of the Neoglacial interval, a period of renewed glacial activity, which is believed to con- *Based on Stuiver et al. (1998). tinue through the third phase, from 3500 "'C years BP to present (c. 3800 cal. years BP to present). According to Pellatt and common in zone EAG-2 than elsewhere. Among the rheophilous Mathewes (1997), this Neoglacial cooling allowed for the estab- taxa Limnophyes is the most abundant. Tanytarsus lugens + Cory- lishment of modern subalpine conditions. This last phase is nocera oliveri and Monopsectrocladius also peak in zone EAG- characterized by cool temperatures, maximum precipitation and 2, coinciding with the disappearance of Psectrocladius (Allo. or includes the Tiedemann and 'Little Ice Age' (LIA) glacial Meso.). advances (Clague and Mathewes, 1996; Ryder and Thomson, 1986). Midge assemblage zone 3 (EAG-3, 47-29 cm, c. 3800- 2400 14C years BP, c. 4200-2400 cal. years BP) Frozen Lake A resurgence of the dominant Chironomini, most notably Dicrot- The Frozen Lake core is characterized by very diverse midge endipes and Polypedilum, occurs in conjunction with a decrease assemblages, including many rheophilous taxa (e.g., Brillia & of Tanytarsina. Microtendipes and Glyptotendipes both decline. Euryhapsis, Corynoneura & Thienemanniella, Eukiefferiella & Tvetenia, Limnophyes and Rheocricotopus). These taxa are typical Midge assemblage zone 4 (EAG4, 29-3 cm, c. 2400-400 inhabitants of streams (Oliver and Roussel, 1983; Wiederholm, 14C years BP, c. 2400-500 cal. years BP) 1983; Walker, 1988; Ruck et al., 1998). Rheophilous taxa are also Although remains of Psectrocladius (Allo. or Meso.) type are rare abundant in the sediments of Marion and Tugulnuit Lakes in Bri- in EAG-4, other Psectrocladius remains increase markedly at the tish Columbia (Walker and Mathewes, 1987; Ruck et al., 1998). beginning of this zone. Among the less common taxa, the declines This abundance of rheophilous midges reflects substantial stream of Microtendipes and Limnophyes and an increase in Corynoneura & inflow. This inference is further supported by the fragmentary Thienemanniella are most apparent. record of larval Simuliidae (black flies). Simuliid remains were consistently present in low numbers throughout the length of the Midge assemblage zone 5 (EAG-5, 3-0 cm, c. 400 14C Frozen Lake core and are considered good indicators of flowing years BP to present, c. 500 cal. years BP to present) water (Ruck etal., 1998; Walker, 2001). Increases of Tanytarsina, Polypedilum and the Tribe Pentaneurini Although Tanytarsina dominated in FRO-1, the great abun- are noted at the top of the core. Dicrotendipes and other Chirono- dance of Chaoborus mandibles, accompanied by Microtendipes mini decrease in EAG-5. Although the rheophilous taxon Cory- and other temperate taxa, suggests warm early-Holocene tempera- noneura & Thienemanniella occurs in EAG-5, there are no cold- tures (Figure 3; Walker et al., 1997), as reflected in the midge- stenothermous taxa present. inferred mean July air temperatures ranging from 13.1 ± 2.0 to 15.6 ± 2.1°C (Figure 4). A comparison with other midge-inferred Temperature inferences and analogue comparisons mean July air-temperature records from southern British Colum- The temperature reconstruction for Eagle Lake reveals substantial bia indicates that the Frozen Lake temperatures are typical for fluctuations in Holocene air temperature ranging from 11.6 ± 2.0 early-Holocene treeline sites (Figure 9a; Palmer et al., 2002; Pel- to 16.6 + 2.2°C (Figure 7). In EAG-1, inferred mean July air latt et al., 2000). The inferred mean July air temperature decreases temperatures range from 13.7 ± 2.0 to 16.6 ± 2.2°C. In the somewhat through zone FRO-1. This trend continues through second zone, EAG-2, the inferred temperatures are mostly cooler, zone FRO-2. ranging from 11.6 ± 2.0 to 14.6 ± 2.1°C (the high temperature Mathewes and Heusser (1981) suggested that a period of higher of 15.4 ± 2.1°C is excluded from this zone as it corresponds with precipitation and cooler temperatures began before the eruption a poor analogue, Figure 8). Higher temperatures are inferred for of Mt Mazama, also recorded by Palmer et al. (2002) and Smith EAG-3, 4 and 5 ranging between 14.4 ± 2.2 and 16.2 ± 2.20C. et al. (1998). The interval following the Mazama eruption is often Overall, the reconstructed values suggest little temperature differ- called the mesothermic interval, a period of decreasing tempera- ence between the early and late Holocene, but indicate a pro- ture and increased moisture (Hebda, 1995). Although this mid- longed cool interval in the mid-Holocene. Further analysis (Figure Holocene cooling appears to persist longer at other subalpine sites 8) reveals that the surface sample training set contained good (e.g., Cabin Lake and 3M Pond; Pellatt et al., 2000), the overall analogues for most mid-Holocene assemblages. The available ana- trend at Frozen Lake is similar. logues for the early- and late-Holocene assemblages are typically Similarly, the late Holocene is considered a time of cooling, poor. A very strong correlation (r = 0.99, p < 0.01) is evident with Neoglacial advances in the Coast, Cascade and Rocky Moun- between the CA axis 1 sample scores and the inferred Eagle tains. It is recorded as a cooling in the midge records of Cabin Lake temperatures. Lake, 3M Pond (Smith et al., 1998; Pellatt et al., 2000), Lake-of- the-Woods and North Crater Lake (Figure 9a; Palmer et al., 2002). Higher-resolution sampling would be needed to resolve Discussion LIA events such as those described by Luckman and Kearney (1986) and Clague and Mathewes (1996). Earlier palaeoecological studies led to the recognition of three Although a decreasing inferred mean July temperature trend is main Holocene (10000 "4C years BP to present, c. 11500 cal. evident in the midge reconstruction, and fits well with diverse years BP to present) climate phases in southern British Columbia. other sources of palaeoclimate data, the sample specific errors for The early-Holocene phase from 10000 to 7000 "'C years BP the samples all overlap, indicating that higher-resolution details

10 20 30 40 50

70 80' 90' 100 110* E 120 130- 140 a 150 160 .$ 170 -.Nm 180 2.h 190 4 200- ----I 210- -- f 220 I I 230 I 240

9 10 11 12 13 14 15 16 17 18 Inferred Mean July Air Temperature (°C) Figure 4 Chironomid-inferred mean July air temperatures for Frozen Lake. Bars indicate the sample specific errors in the inferred temperatures.

Table 4 AMS radiocarbon and tephra dates for the Eagle Lake sediment core

Depth (cm) Lab. number Uncalibrated age Calibrated age* Relative area under Material dated (CAMS) (radiocarbon years BP) (calibrated years BP) probability distribution

36-35 3390 ± 130 3980-3940 0.016 Mt St Helens Yn tephra 3930-3360 0.984 96-81 6730 ± 40 7670-7560 0.852 Mazama tephra 7540-7510 0.148 106-105 74623 8330 ± 40 9470-9440 0.091 conifer needle fragments 9440-9260 0.864 9170-9150 0.045

*Based on Stuiver et al. (1998). of this reconstruction are not statistically significant. This is not In British Columbia, a late-Holocene warming is only weakly surprising given the errors associated with individual temperature evident in the temperature reconstruction of Mathewes and inferences (Walker et al., 1997; Lotter et al., 1999), and prior evi- Heusser (1981), based on pollen results from Marion Lake in the dence that Holocene temperature changes were of a much smaller southern Coast Mountains. The inferred warming at Marion Lake magnitude than those recorded in longer Quaternary sections. is small. Evidence for such warming is contrary to the bulk of Consequently, reliable midge-based temperature reconstructions palaeoenvironmental data from southern British Columbia, for the Holocene will need to be calculated by averaging the including numerous pollen records, and Mathewes' (1973) own results from several sites. The average or 'consensus' temperature subjective interpretation of Marion Lake's pollen. Bennett et al. record will likely provide a more reliable indication of the true (2001) note that palaeoclimate records from British Columbia do pattern of Holocene climatic changes. not provide an entirely coherent picture of late-Holocene climatic change. They suggest that a variety of factors, including the com- Eagle Lake plex topography of British Columbia and local variations in cli- Eagle Lake sediments included approximately half the number of matic change, may account for discrepancies among records. taxa found at Frozen Lake. Midge-inferred mean July air tempera- These discrepancies, however, are much more evident in precipi- tures for Eagle Lake have a larger range than those calculated for tation and salinity inferences than in palaeotemperature records Frozen Lake (Figure 7). (Heinrichs et al., 2001). In the early-Holocene (EAG-1) or 'xerothermic' period, warm Figure 9a shows that the Eagle Lake reconstruction does not summer air temperatures are inferred, ranging from 13.7 + 2.0- fit the typical midge-inferred temperature profile as obtained from 16.6 + 2.2°C. A cooler climate is inferred for the mid-Holocene, other subalpine lakes in southern British Columbia (Palmer et al., or 'mesothermic' period, but the inferred return to warm con- 2002). The late-Holocene reconstructedtemperature anomalies are ditions at the core top directly contradicts the cooler temperatures much warmer than those inferred at other sites, and much warmer ordinarily inferred for the late Holocene. than current temperatures at Eagle Lake. We also note that the

75% C. 1. 95% C.I. radocwbon do7t- 12

n~ ~~

25- --- -I 1570±0 ''C BP 1960A0G'C BP 50 - Irm oBrdge RWr ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~I-2435:26 4C BP 75 II FR0 2

I~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~. 3560*40 'C BP . I-I E 1251 Ia 4630550 "C BP -C _ 0) 150 6170*40 "C BP - Mazama I K- 6730*40 'C BP 175 h

I 81 0*0 "C BP . . m .,Mfl,ft, - FRO-1

9390+70 'C BP 225 10020*0 'C BP

i - . I 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Squared Chord Distance Figure 5 Comparison of available analogues with Frozen Lake chironomid fossil samples (squared chord distance from the fossil sample to the best available modem surface sample analogues. Confidence intervals are shown.

'--COLD WARM--> ei cl,~~~~~~0 O0'"O sS<1 NIPt iA-X k C,

'± ~~vro~~-~~e'\'- V tj'V~~~~" '~~~' ~~'~~'~' Zone CONISS

20EA4 30. MItSI.HelensYn 3400-

70- EAG

Mozomo 6730*40. 90 100EA-I 8330±40. 2 6 -:-:--"': 2 240 60 40 20 50100015000 2 4 Totol sum of squres

gytijo tephro cloy

Figure 6 Midge stratigraphy for Eagle Lake. Chironomid and non-chironomid (Chaoborus, Ceratopogonidae and Acari) taxon abundances are expressed as percentages of the total identifiable Chironomidae (regions in black); expanded regions illustrate a X 10 exaggeration. Species are ranked in accordance with their temperature optima derived from the weighted averaging model. Taxa that occurred in two or fewer samples, or that never exceeded 2% abundance, are excluded. available analogues are unsatisfactory for most of the Eagle Lake to account for the poor analogue situation. A multitude of factors core, with the exception of EAG-2. Although Eagle Lake is shal- other than temperature can affect midge communities, and it low, many of the lakes included in the surface sample training set seems likely that these other effects may be driving Holocene are .2 m deep; thus, Eagle Lake's current depth is insufficient changes at Eagle Lake. Other factors influencing midge distri-

Table 5 Zonation of the Eagle Lake chironomid stratigraphy (ages rounded to the nearest century)

Chironomid Depth Uncalibrated age Calibrated age* assemblage (cm) (radiocarbon (calibrated zone years BP) years BP)

EAG-5 0-3 Present-400 Present-500 EAG-4 3-29 400-2400 500-2400 EAG-3 29-47 2400-3800 2400-4200 EAG-2 47-88.5 3800-6700 4200-7600 EAG-1 88.5-110 6700-8500 7600-9600

*Based on Stuiver et al. (1998). 55 a 60 O) 65 -_ EAG-2

0 EAG-5 F 70 5 75 10 80 15 - EAG4 85 Mazuma O 20 90 8O7 "C BP 25 95 30 100 _ ~IEAG-1 35 -r- 105 - 8=0 Ic BP 40 -EAG-3 45 .- 0 0.2 0.4 a.6 0.8

50 _F Squared Chord Distance Figure 8 Comparison of available analogues with Eagle Lake chironomid .c60 fossil samples (squared chord distance from the fossil sample to the best 65 EAG.2 available modern surface sample analogues. Confidence intervals are shown.

mally co-vary with mean July air temperature (e.g., winter 02), may be exerting an independent influence on the midge fauna.

Regional assessment of temperature inferences As discussed in relation to the Frozen Lake results, good Holo- , , 6 v cene reconstructions of temperature should not be based on a sin- .Bl~ 833D44 X4C Bp gle core, but rather on the results obtained from as many lakes as I _ r _ 1~~I ... 1t fl F 1 Ir r _r_ I IV -i fi possible. Figure 9b offers the consensus reconstruction for six 9 10 11 12 13 14 15 16 17 18 19 lakes in southern British Columbia: 3M Pond, Cabin Lake, Crater Inferred Mean July Air Temperature (0C) Lake, Lake-of-the-Woods, Eagle Lake and Frozen Lake. During Figure 7 Chironomid-inferred mean July air temperatures for Eagle Lake. the lateglacial period, from approximately 11000 through 9800 Bars indicate the sample specific errors in the inferred temperatures. cal. years BC (C. 10000 14C years BP) temperatures 1 to 5°C cooler than today are inferred. This cold period is not recorded in 3M Pond, Eagle or Frozen Lakes as these lakes were not formed until butions and diversity may include dispersal ability and changes in the early Holocene. water chemistry, benthic substrates and predators (Hoffman et al., During the early Holocene (9600-7000 cal. years BC, c. 9800- 1996). Hoffman et al. (1996) noted that the number of invertebrate 8000 "4C years BP), mean July air temperatures, as inferred from taxa decreases across the transition from forest to alpine lake eco- midges, were 1 to 5°C warmer than present. This reconstruction systems. Changes in substrate, which can affect organism abun- provides independent confirmation of the early-Holocene warmth dance and diversity, also tend to change with elevation from more qualitatively inferred from palaeobotanical evidence (Mathewes, organic substrates at lower elevations (forest/subalpine lakes) to 1985; Hebda, 1995). Temperatures gradually decreased through- more inorganic substrates at higher elevations (alpine lakes). out the mid-Holocene. The coolest Holocene summer tempera- Declining lake depth can also influence water temperature and tures begin at around 1000 cal. years BC (c. 2800 "4C years BP) nutrient cycling. Eagle Lake water depth has been known to fluc- and persist until the present day. 3M Pond shows the greatest tuate up to 1.0 m (Donald and Alger, 1984). We could be seeing temperature change, with high-inferred temperatures in the early a response of midge assemblages to variables other than simply Holocene and the lowest temperatures during the late Holocene. temperature changes (Lotter et al., 1997; Walker et al., 1997). Eagle Lake does not follow the pattern typical of other lakes. It is especially worth noting that the midge fauna of Eagle Lake During the mid-Holocene interval, Eagle Lake temperatures are is very unusual relative to other high-elevation British Columbia inferred to be a few degrees lower than during late-Holocene lakes, including a higher proportion of presumed 'thermophilous' times. Although the pattern of temperature change may have species than anticipated. The Eagle Lake fauna is, therefore, a varied somewhat regionally (e.g., on a northeast to southwest clear 'outlier' relative to other lakes we have sampled. From this axis), we prefer to maintain a conservative interpretation of the palaeoenvironmental assessment, it is impossible to discern the data, at least, until additional data from the Columbia Mountains reason for the unusual fauna. One or more key variables that nor- can be obtained.

(a) Age (radiocarbon years) 10000 8000 6000 4000 2000 0 8

6 E 0 _- 4 ez 2

._oCL 0

E -2 0 -4

-6 12000 10000 8000 6000 4000 2000 0 2000 4 Age (calendar years)

(b) Age (radiocarbon years) 10000 8000 6000 4000 2000 0 8-.

iE _ 0 4 2: .°0W da_0 :

120

-4

12000 *10000 8000 6000 4000 2000 0 2000 Age (calendar years) Figure 9 (a) Chironomid-inferred palaeotemperature records for six lakes in southern British Columbia. (b) Five-point running averages of palaeotemperature records for southern British Columbia with and without the Eagle Lake data.

The average palaeotemperature reconstruction, with or without Cordilleran ice sheet was rapidly disintegrating, causing an the Eagle Lake results, generally parallels the qualitative infer- abrupt decrease in surface albedo. This change in albedo would ences derived from palaeobotanical data (Figure 9b; Mathewes, have markedly reinforced the shift from glacial temperatures 1985; Barnosky et al., 1987; Vance, 1987; Hebda, 1995; Walker to a warm Holocene climate. We also note that the maximum and Pellatt, 2001; 2004). Our reconstruction clearly reveals the Holocene temperatures coincide with the summer solar in- rapidity of the warming at the Pleistocene/Holocene boundary, solation maximum c. 8000 BC (c. 8800 l4C years BP; Berger c. 9600 cal. years BC (C. 10000 14C years BP). In contrast, the and Loutre, 1991). The cooling trend through the mid- mid-Holocene cooling is indicated as a very gradual transition Holocene parallels a decline in summer solar insolation from xerothermic warmth to current summer temperatures. Conse- (Barnosky etal., 1987; Vance, 1987; Thompson etal., 1993). quently, the upper and lower bounds of the 'Mesothermic' are not We concur with Kutzbach et al. (1993) that changes in easily defined. Nevertheless, the term 'Mesothermic' is useful in Holocene summer temperatures were driven largely by changes reference to this slow transition between the warm 'Xerothermic' in summer solar insolation. Unfortunately, good proxy records (prior to c. 6000 to 7000 cal. years BC; c. 7000 to 8000 "4C years for winter palaeotemperatures do not exist. Since winter solar BP) and cool 'Neoglacial' (after c. 2500 cal. years BC; c. 4000 14C insolation was at a minimum in the early Holocene, winter tem- years BP) intervals in British Columbia. peratures changes may have followed the opposite trajectory - The rapid warming at the end of the Pleistocene is well i.e., from cold early-Holocene to mild late-Holocene winter documented throughout the Northern Hemisphere. Locally, the temperatures.

Conclusions climatic change at Big Lake, south-central British Columbia, Canada. Quaternary Research 55, 332-43. Bennett, K.D. 1996: Determination of the number of zones in a bio- Both Frozen and Eagle Lakes record similar timing of climatic stratigraphical sequence. New Phytologist 132, 155-70. change throughout the Holocene even though the direction of tem- 2002: Documentation for psimpoll 4.10 and pscombl.03. C pro- perature fluctuations do not concur. The midge-temperature infer- grams for plotting pollen diagrams and analysing pollen data. Uppsala: ences indicate high mean July air temperatures during the early Quaternary Geology, Uppsala Universitet, 25 pp. Holocene for both subalpine lakes. Lower mean July air tempera- Berger, A. and Loutre, M.F. 1991: Insolation values for the climate of tures were inferred for the mid-Holocene. Continued low-tem- the last 10 million years. Quaternary Science Reviews 10, 297-317. perature inferences were obtained from Frozen Lake through to Brodin, Y.W. and Gransberg, M. 1993: Responses of insects, especially the present day, but during this same late-Holocene period Eagle Chironomidae (Diptera), and mites to 130 years of acidification in a Scott- ish lake. Lake temperature inferences increase contrary to most other Hydrobiologia 250, 201-12. Brooke, R.C., Peterson, E.B. and Krajina, VJ. The reconstructions from southern British Columbia. The Eagle Lake 1970: subalpine mountain hemlock zone: subalpine vegetation in southwestern British temperature record may not be providing an accurate reconstruc- Columbia, its climatic characteristics, soils, ecosystems and environmental tion of Holocene temperature trends. It seems likely that other relationships. In Krajina, V.J. and Brooke, R.C., editors, Ecology of factors apart from temperature are affecting the fauna. Alterna- western North America, Vancouver: Department of Botany, University of tively, the climate situation surrounding Eagle Lake may be British Columbia, 147-349. regionally complex. A comparison with multiproxy data from the Brooks, SJ., Bennion, H. and Birks, HJ.B. 2001: Tracing lake trophic same, and other sites, is needed to substantiate the temperature history with chironomid - total phosphorus inference model. Freshwater inferences. Biology 46, 513-33. Due to the relatively small magnitude of temperature changes Cawker, K.B. 1983: Fire history and grassland vegetation change: three throughout the Holocene, midge-based climate reconstructions pollen diagrams from southern British Columbia. Canadian Journal of Botany 1126-39. based on single cores must be interpreted with caution. By averag- 61, Clague, JJ. and Mathewes, R.W. 1989: Early Holocene thermal ing the results from several sites in southern British Columbia, a maximum in western North America new evidence from Castle Peak, more reliable estimate of the Holocene temperature changes was British Columbia. Geology 17, 277-80. obtained. Nevertheless, changes in midge communities are not 1996: Neoglaciation, glacier-dammed lakes, and vegetation change driven only by events related to climate. Changes in water depth, in northwestern British Columbia, Canada. Arctic and Alpine Research chemistry or lake productivity may interfere with midge-based 28, 10-24. temperature reconstructions. Independent evidence from other Clague, JJ., Evans, S.G., Rampton, V.N. and Woodsworth, GJ. 1995: proxy climate indicators (e.g., in multiproxy studies) must be care- Improved age estimates for the White River and Bridge River tephras, fully considered when evaluating the results. western Canada. Canadian Journal of Earth Sciences 32, 1172-79. Coupe, R., Stewart, A.C. and Wikeem, B.M. 1991: Engelmann spruce- subalpine fir zone. In Meidinger, D. and Pojar, J., editors, Ecosystems of British Columbia, Victoria: Research Branch, Ministry of Forests, Acknowledgements Province of British Columbia, 223-36. Cwynar, L.C. and Levesque, AJ. 1995: Chironomid evidence for late- Funding for this project was provided by a grant from the Natural glacial climatic reversals in Maine. Quaternary Research 43, 405-13. Donald, D.B. and Alger, DJ. 1984: Limnological studies in Glacier and Sciences and Engineering Research Council of Canada to Ian R. Mount Revelstoke National Parks. Edmonton: Parks Canada, Canadian Walker. Thanks also to Murray Peterson and Parks Canada Wildlife Service. Revelstoke for permission to sample within Mount Revelstoke Environment Canada 1993: Canadian climate normals, 1961-1990 National Park. 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Quinlan, R., Smol, J.P. and Hall, R.I. 1998: Quantitative inferences of Livingstone, D.M., Lotter, A.F. and Walker, I.R. 1999: The decrease past hypolimnetic anoxia in south-central Ontario lakes using fossil in summer surface water temperature with altitude in Swiss Alpine lakes: midges (Diptera: Chironomidae). Canadian Journal of Fisheries and a comparison with air temperature lapse rates. Arctic, Antarctic andAlpine Aquatic Sciences 55, 587-96. Research 31, 341-52. Reasoner, M.A. 1993: Equipment and procedure improvements for a Lotter, A.F., Birks, HJ.B., Hofmann, W. and Marchetto, A. 1997: lightweight, inexpensive, percussion core sampling system. Journal of Modern diatom, cladocera, chironomid, and chrysophyte cyst assemblages Paleolimnology 8, 273-81. as quantitative indicators for ihe reconstruction of past environmental con- Reasoner, M.A. and Hickman, M. 1989: Late Quaternary environmental ditions in the Alps. I. Climate. Journal of Paleolimnology 18, 395-420. change in the Lake O'Hara region, Yoho National Park, British Columbia. 1998. Modern diatom, cladocera, chironomid, and chrysophyte cyst Palaeogeography, Palaeoclimatology, Palaeoecology 72, 291-316. assemblages as quantitative indicators for the reconstruction of past Rosenberg, S.M., Walker, I.R. and Mathewes, R.W. 2003: Postglacial environmental conditions in the Alps. II. Nutrients. Journal of Paleo- spread of hemlock (Tsuga) and vegetation history in Mount Revelstoke limnology 19, 443-63. National Park, British Columbia, Canada. Canadian Journal ofBotany 81, Lotter, A.F., Walker, I.R., Brooks, SJ. and Hofmann, W. 1999: An 139-51. intercontinental comparison of chironomid palaeotemperature inference Ruck, A., Walker, I.R. and Hebda, RJ. 1998: A palaeolimnological

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