Late Pleistocene and early Holocene climate and ......ta: Diptera: Chironomidae)remains preserved in the sediments of a small, climatically sensitive Sierra Nevada lake to reconstruct

Late Pleistocene and early Holocene climate and limnologicalchanges in the Sierra Nevada, California, USA inferred from

midges (Insecta: Diptera: Chironomidae)

David F. Porinchu a;�, Glen M. MacDonald b;c, Amy M. Bloom d,Katrina A. Moser d

a Department of Geography, California State University at Long Beach, Long Beach, CA 90840-1101, USAb Department of Geography, University of California at Los Angeles, Los Angeles, CA 90095-1524, USA

c Department of Organismic Biology, Ecology and Evolution, University of California at Los Angeles, Los Angeles,CA 90095-1524, USA

d Department of Geography, University of Utah, Salt Lake City, UT 84112-9155, USA

Received 21 August 2002; accepted 26 May 2003

Abstract

Chironomid and stratigraphic analyses of a lake sediment core recovered from a high-elevation lake in the centralSierra Nevada, California, USA, was undertaken to assess chironomid community development during thePleistocene^Holocene transition and to quantitatively reconstruct the thermal regime that existed during this interval.Between 14 800 cal yr BP and 13 700 cal yr BP the chironomid community consisted almost exclusively ofHeterotrissocladius, suggesting this period was characterized by extremely cold climatic conditions. Evidence of post-glacial climatic amelioration, as manifested by increases in thermophilous chironomid taxa, head capsuleconcentrations, and taxon richness and diversity, commenced at approximately 13 300 cal yr BP. In order toquantify the magnitude of the water temperature changes associated with post-glacial climatic amelioration, a recentlydeveloped chironomid-based inference model for surface water temperature was applied to the subfossil chironomidassemblages. The one-component, weighted-averaging partial least squares (WA-PLS) model has an r2jack = 0.72,RMSEPjack = 1.1‡C and a maximum bias of 1.24‡C. The reconstructed surface water temperatures suggest a minimumwarming of approximately 4.7‡C occurred during the Pleistocene^Holocene transition. However, warming during thisinterval may not have been monotonic. Evidence of the Younger Dryas has been previously discovered in the studyarea and adjacent regions; Our research indicates that changes in chironomid community composition and an inferredshort-term cooling event occurred between 12 000 cal yr BP and 11 500 cal yr BP, which may represent a localmanifestation of the Younger Dryas. Chironomid community development during the Pleistocene^Holocenetransition appears to primarily reflect the influence of changing climate conditions.C 2003 Elsevier B.V. All rights reserved.

Keywords: chironomids; paleoclimate; paleolimnology; alpine lakes; Sierra Nevada, California; Younger Dryas

0031-0182 / 03 / $ ^ see front matter C 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0031-0182(03)00481-4

* Corresponding author. Tel: +1-310-825-6658; Fax: +1-310-206-5976.E-mail address: [email protected] (D.F. Porinchu).

PALAEO 3166 9-9-03

Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 403^422

www.elsevier.com/locate/palaeo

mailto:[email protected]://www.elsevier.com/locate/palaeo

1. Introduction

It is becoming apparent that high-latitude andhigh-elevation regions are extremely sensitive toclimate change (Douglas et al., 1994; IPCC,2001; Battarbee et al., 2002). One of the likelye¡ects that projected global warming will have isto raise epilimnetic water temperatures in lakesand reservoirs (Schindler et al., 1990). These high-er epilimnetic temperatures will likely a¡ect thestructure and function of freshwater ecosystems.Studies of the response of aquatic communities topast periods of climate change will improve ourunderstanding of climate dynamics and our abilityto assess the likely response of these communitiesto projected global warming.

The terrestrial and aquatic ecosystems of theSierra Nevada, California, USA, appear to havebeen strongly a¡ected by climate warming duringthe transition from the late Pleistocene to the earlyHolocene (14 000 and 9500 cal yr BP; all dates arereported as 14C years before AD 1950 calibratedto calendar years). Available evidence suggeststhat glacial retreat (Clark and Gillespie, 1997),changes in vegetation structure and composition(Cole, 1983; Davis et al., 1985; Anderson, 1990)and hydrological changes (Benson et al., 1996,1997) occurred in the Sierra Nevada and adjacentareas during this period. However, quantitativeestimates of temperature change and knowledgeof the nature and rate of aquatic ecosystem devel-opment in the Sierra Nevada during the Pleisto-cene^Holocene transition are still limited.

In addition to the general trend of unidirection-al post-glacial warming, recent work has providedevidence that late Pleistocene climatic oscillationstypical of the North Atlantic sector also impactedwestern North America (Reasoner et al., 1994;Menounos and Reasoner, 1997) including partsof the western USA (Benson et al., 1996, 1997).Work from Great Basin pluvial lakes suggeststhat Owens Lake, Pyramid Lake and MonoLake alternated between wet and dry phases dur-ing the late Pleistocene and early Holocene (Ben-son et al., 1996, 1998, 2002). Although dating isproblematic, some of these oscillations appear tocorrespond to previously documented events suchas the Younger Dryas stadial (13 000^11 600 cal

yr BP). A number of researchers have suggestedthat changes in ocean^atmosphere interaction inthe North Paci¢c are the proximate cause of themillennial-scale climate oscillations that a¡ectedCalifornia during this period (Stine, 1994; Bensonet al., 1997, 1998; Bradbury, 1997). The hypoth-esis that changes in ocean^atmosphere interac-tions in the North Paci¢c during the late Quater-nary impacted climate in California can only beaddressed if continuous, high-resolution paleocli-matic reconstructions can be developed for theregion.

Unfortunately, detailed, quantitative, high-res-olution reconstructions of the thermal regime thatexisted during the Pleistocene^Holocene transi-tion in the Sierra Nevada are not available. Paleo-limnology o¡ers one means of developing thesemuch-needed records. Paleolimnology is a multi-disciplinary and integrative science that reliesupon the physical, chemical and biological infor-mation preserved in lake sediment to reconstructpast environmental conditions in aquatic systems(Battarbee, 2000; Smol and Cumming, 2000). Pa-leolimnological reconstructions can illuminate as-pects of both the structure and development ofaquatic ecosystem communities and also provideevidence of the past environmental and climaticconditions that impacted these communities. Inthis study, we focus on using chironomid (Insec-ta: Diptera: Chironomidae) remains preserved inthe sediments of a small, climatically sensitiveSierra Nevada lake to reconstruct the thermal re-gime that existed in the region during the Pleisto-cene^Holocene transition.

Chironomids, which are also known as midge£ies, have long been used as biotic indicators toclassify lakes in terms of trophic conditions andhypolimnetic oxygen concentration (Thienemann,1918; Brundin, 1949). They also have a long his-tory of being used to qualitatively interpret late-glacial climatic conditions (Andersen, 1938), andmore recently they have been used to developquantitative paleoclimatic reconstructions (re-viewed in Porinchu and MacDonald, 2003). Ear-lier work in the Sierra Nevada has demonstratedthat summer surface lake water temperatures ac-count for a large and statistically signi¢cant pro-portion of the variance present in the modern

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D.F. Porinchu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 403^422404

distribution of chironomids in this region (Porin-chu et al., 2002). A number of studies, using thecalibration dataset approach and associated mul-tivariate statistical techniques (reviewed in Smoland Cumming, 2000), have demonstrated thatsummer surface water temperature has a statisti-cally signi¢cant relationship to the distribution ofchironomids (Walker et al., 1991a, 1997; Olanderet al., 1999). Application of quantitative transferfunctions, developed from these calibration data-sets, to subfossil chironomid assemblages pre-served in late Quaternary lake sediment have en-abled researchers to quantify changes in past

temperature in Atlantic Canada and the north-eastern USA during the late Pleistocene (Walkeret al., 1991b; Levesque at al., 1993, 1997; Cwynarand Levesque, 1995) and western Norway duringthe late Pleistocene and early Holocene (Brooksand Birks, 2001).

In this paper we present the ¢rst detailed anal-ysis of late Quaternary chironomid communitydevelopment in the Sierra Nevada of California.We use time-trend analysis to compare late Pleis-tocene and early Holocene midge communitycomposition and change to the modern distribu-tion of chironomids in this region. Lastly, we ap-

Fig. 1. Map of the study area indicating the distribution of the 56 lakes sampled for inclusion in the modern calibration dataset,including the location of Greenstone Lake.

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D.F. Porinchu et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 198 (2003) 403^422 405

ply a recently developed transfer function to thesubfossil chironomid assemblages preserved inGreenstone Lake to reconstruct the thermal re-gime for the central Sierra Nevada during thePleistocene^Holocene transition. This is the ¢rststudy to use the remains of subfossil chironomidsto reconstruct past temperature regimes in thewestern USA.

2. Study area

The study site, Greenstone Lake, California(37‡58P47QN, 119‡17P24QW), is located in the Hoo-ver Wilderness near Tioga Pass on the east side ofthe Sierra Nevada crest at an elevation of 3067 ma.s.l. (Fig. 1). At present, Greenstone Lake is asmall (8.1 ha), relatively shallow (maximum depthapproximately 5.10 m), slightly alkaline (pH = 8.1)lake. Greenstone Lake is located on a contactbetween metamorphosed sedimentary rocks of Ju-rassic age and metarhyodacite of Triassic age(Sietz, 1983). Sur¢cial talus and slopewash depos-its of Quaternary age are present on the south andsouthwest sides of the lake. According to D.Clark (Western Washington University, personalcommunication) the lake is located outside theRecess Peak moraines. The vegetation surround-ing Greenstone Lake is typi¢ed by sub-alpine bo-real vegetation dominated by Pinus contorta, Tsu-ga mertensiana and Pinus £exilis.

The Sierra Nevada is characterized by a mon-tane, Mediterranean climate, with hot, aridsummers and cool, humid winters (Major, 1988).The dominant factor, controlling climate in Cal-ifornia, is the presence of a semi-permanent high-pressure system in the north Paci¢c Ocean (Ra-phael and Mills, 1996). Topographic relief greatlya¡ects the distribution of temperature and precip-itation in the Sierra Nevada. Lee Vining, CA(37‡57PN, 119‡07PW), situated 15 km east ofGreenstone Lake, is located at the base of theeastern £ank of the Sierra Nevada at an elevationof 2073 m a.s.l. Over the climate normal period of1971^2000, Lee Vining has had an average Janu-ary temperature of 31.28‡C, an average July tem-perature of 21.67‡C and an average total precip-itation of 33.05 cm (Western Regional Climate

Center, 2002). There are no high-elevation mete-orological stations in the Sierra Nevada that haverecords covering the climate normal period (1971^2000). The nearest high-elevation station withcomparable records is located approximately 100km southeast of Lee Vining in the White Moun-tains (37‡35PN, 118‡14PW) at an elevation of 3780m a.s.l. This station has had an average Januarytemperature of 39.03‡C, an average July temper-ature of 7.67‡C and an average total precipitationof 51.03 cm during the climate normal period(Western Regional Climate Center, 2002).

Study of the late Quaternary glacial history ofthe Sierra Nevada began late in the 19th centuryand has continued to the present (Russell, 1889;Blackwelder, 1931; Matthes, 1939; Birman, 1964;Curry, 1969; Burke and Birkeland, 1983; Phillipset al., 1996; Clark and Gillespie, 1997). Recentwork has revised the chronology of late Pleisto-cene and Holocene glaciations in the Sierra Neva-da (Clark and Gillespie, 1997; Konrad and Clark,1998). Based on detailed mapping of moraine de-posits and 14C dating of sediment cores recoveredfrom alpine lakes and meadows in the centralSierra Nevada, Clark and Gillespie (1997) suggestthat the last major late Pleistocene glacial advancewas the Recess Peak. Clark and Gillespie (1997)obtained dates of 11 190 P 7014C yr BP (V13 200cal yr BP) and 10 880 P 6014C yr BP (V13 000 calyr BP) for basal lake sediments from high-eleva-tion lakes in the eastern Sierra Nevada which aremorainally dammed by Recess Peak deposits.They suggest that these dates provide a minimumage estimate for the Recess Peak advance, indicat-ing that the Recess Peak advance likely occurredprior to 12 000 14C yr BP (V14 000 cal yr BP) andtherefore pre-dates the Younger Dryas event.

3. Methods

3.1. Field

Replicate sediment cores were recovered fromGreenstone Lake on 18 July 2001, using a modi-¢ed Livingstone piston sampler (Wright, 1991)deployed from a platform supported by in£atablerafts. Cores were recovered from the basin located

PALAEO 3166 9-9-03


in the northeast quadrant of the lake in 5.10 m ofwater (Fig. 2). A plastic tube ¢tted with a pistonwas used to recover the £occulent surface sedi-ment and ensure that the sediment^water interfacewas recovered intact. The sediment recovered withthe plastic tube was sub-sampled in the ¢eld at1-cm increments and stored in Whirl-paks0. Theremaining lake sediment was extruded in the ¢eld,described, wrapped in plastic ¢lm and aluminumfoil and transported to the laboratory where thecores were stored at 4‡C. Measurements of thelimnological characteristics of Greenstone Lakewere made on 9 August 1999 during an earliervisit to collect surface (0^1 cm) sediment for usein the modern calibration set.

3.2. Laboratory

The amount of organic carbon present in the

lake sediment was analyzed using loss-on-ignition(LOI) analysis (Dean, 1974). LOI analysis for thelate Pleistocene and early Holocene portion of theGreenstone lake core was undertaken at 0.5-cmresolution, with the analysis based on 1 cm3 ofsediment.

Chironomid analysis follows methods outlinedin Walker (1987). A minimum of 45 head capsuleswere identi¢ed and enumerated from 26 of the 29samples analyzed (range: 45^231 head capsules)(Quinlan and Smol, 2001; Heiri and Lotter,2001); chironomid analysis was based on 40head capsules for three samples (231, 234 and254 cm). The amount of sediment processed andtreated to obtain the minimum number of headcapsules varied greatly between samples (range:0.25^6.5 ml). Chironomid remains that consistedof less than half a head capsule were not enum-erated, those that consisted of greater than half a

Fig. 2. Map showing the position of Greenstone Lake in relation to the nearby Conness Glacier. Coring location depicted by X.

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head capsule were enumerated as a whole headcapsule, while those that were half a head capsulewere enumerated as half a head capsule.

Identi¢cations were based predominately onSimpson and Bode (1980), Cranston (1982), Wie-derholm (1983), Oliver and Roussel (1983),Walker (1988, 2000) and an extensive Universityof California at Los Angeles (UCLA) referencecollection. In general, identi¢cation of subfossilchironomid remains is made at the generic level.However, in certain cases species-type level iden-ti¢cations are possible, e.g., Corynocera oliveri-type, while in other cases broader taxonomicgrouping are required, e.g., grouping Hydrobaenusand Oliveridia together. Corynocera-type 1 has amentum intermediate between Corynocera oliveriand Corynocera nr. ambigua, short distallyrounded spurs on the antennal pedestal and alarge dorsal tooth present on the mandibles. Ta-nytarsus group 3 is characterized by a broad tri-dent-shaped median tooth and lateral teeth ar-rangement that is similar to C. nr. ambigua.Tanytarsus group 4 is characterized by a mentumvery similar to C. nr. ambigua and a distinctivesharp apical projection on the antennal pedestal.Micropsectra radialis-type and Micropsectra insig-nilobus-type were separated based on the presenceof a distinct post-occipital plate associated withthe M. insignilobus-type specimens.

3.3. Statistical analyses

3.3.1. Modern calibration datasetThe modern distribution of chironomids in the

eastern Sierra Nevada was established by analyz-ing the subfossil chironomid remains preserved inthe surface sediment of 56 lakes in this region

(Porinchu et al., 2002) (Fig. 1). These lakes weresampled during late July and early August 1999,July 2000 and July 2001. Surface sediment wasrecovered using a mini-Glew gravity corer(Glew, 1991). During surface sediment collectioneach lake was characterized by a number of phys-ical and limnological variables, e.g., pH, maxi-mum depth, surface water temperature, speci¢cconductivity. At this time water samples werealso collected for water chemistry analysis (seePorinchu et al., 2002 for further details). Thelakes incorporated in the calibration set spannedan elevation gradient of 1360 m, were generallysmall (mean surface area = 10 ha), circum-neutralto alkaline (pH range: 6.7^8.8) and relativelyshallow (mean maximum depth= 10.50 m). In to-tal each lake was characterized by 31 environmen-tal variables.

Redundancy analysis (RDA) with forward se-lection of the 56-lake dataset identi¢ed that ¢ve ofthe 31 measured environmental variables have alarge, independent and statistically signi¢cant re-lationship to the distribution of chironomids inthe study lakes (Porinchu et al., 2002). These var-iables were surface water temperature, depth, ele-vation, strontium and particulate organic carbon(POC). Surface water temperature accounted for11.2% of the variance in the chironomid data. Apartial RDA with surface water temperature asthe sole environmental variable and depth, eleva-tion, Sr and POC as co-variables revealed thatsurface water temperature independently accountsfor 6.8% of the variance present in the calibrationdataset.

A one-component weighted-averaging partialleast squares (WA-PLS) model based on 44 lakesand all taxa present in the calibration set was

Table 1Details of the radiocarbon dates obtained for Greenstone Lake

Sample Lab number Depth Material Age 2c age range Relative area underdistribution

Calibrated age:age^depth model

(cm) (14C yr BP) (cal yr) (cal yr BP)

GreenstoneLake

Beta-157868 245^246 bulk 10 700P 80 12 980^12 610* 0.855 12 80012 490^12 350 0.145

Beta-157867 229^231 bulk 9 270P 60 10 580^10 360* 0.742 10 47010 360^10 240 0.245

Dates based on AMS analysis carried out by Beta Analytic Incorporated. *These ranges had the highest probability distributionalthough they were 6 95%. The midpoint of these ranges was used in constructing the chronologies.

PALAEO 3166 9-9-03


developed (12 lakes were removed from the train-ing set, including Greenstone Lake, based onscreening criteria discussed in Porinchu et al.,2002). The r2jack between measured and predictedsurface water temperature was 0.72, theRMSEPjack was 1.1‡C and the maximum bias ofthe WA-PLS model was 1.24‡C.

3.3.2. Comparison and model application tosubfossil assemblages

Time-trend analysis, which relies on a space-for-time substitution, was used to assess the tra-jectory of chironomid community development inGreenstone Lake (Porinchu and Cwynar, 2002).Changes in the composition of subfossil chirono-mid assemblages were compared to the moderndistribution of chironomids in this region usingcorrespondence analysis (CA). The compositionof the subfossil chironomid communities presentin Greenstone Lake during the Pleistocene^Holo-cene transition was averaged at 250-year intervalsand passively ordinated with abundance data de-scribing the contemporaneous distribution of chi-ronomids in the Sierra Nevada. The ¢rst two or-dination axes were constrained using the 44 lakesthat are incorporated in the chironomid-based in-ference model (Porinchu et al., 2002). All taxapresent in both the subfossil assemblages andthe calibration set were included in this analysis.

A one-component WA-PLS model was appliedto the subfossil chironomid assemblages recoveredfrom Greenstone Lake to develop a quantitativeestimate of temperature change during the Pleis-tocene^Holocene transition. One means of assess-ing the reliability of a quantitative paleoenviron-mental reconstruction is to determine for eachsubfossil assemblage the total percentage of taxapresent that do not appear in the modern calibra-tion dataset. According to Birks (1998) recon-structions that are based on subfossil assemblagesthat have s 95% of the subfossil taxa present inthe calibration set are very reliable. Of the 35chironomid taxa present in the late Pleistocene^early Holocene portion of Greenstone Lake, 32(or 91%) are present in the calibration set. Thetaxa that are not present in the calibration set,Pseudodiamesa, Apedilum and Glyptotendipes, areextremely rare in the Greenstone Lake chirono-

mid stratigraphy and never exceed 5% of the totalidenti¢able chironomids at any level. Sample-spe-ci¢c standard errors were estimated for each fossilassemblage by Monte Carlo simulation usingleave-one-out cross-validation as implemented bythe program WA-PLS. A LOWESS smoother(span = 0.20) was applied to the reconstructionto highlight the main trends in surface lake watertemperature during the Pleistocene^Holocenetransition.

All ordinations were implemented by the pro-gram CANOCO version 4.0 (ter Braak and Smi-lauer, 1998). The chironomid percentage diagramwas created using TILIA (version b2.0) developedby E. Grimm (Illinois State Museum, Spring¢eld,IL, USA). Zonation of the chironomid percentagediagrams was performed by ZONE version 1.2and based on optimal sum of squares partitioning(Juggins, 1991). The WA-PLS calibration modeland the sample-speci¢c errors associated with thereconstruction were developed using the programWA-PLS version 1.1 (Juggins and ter Braak,1996). The LOWESS smoother applied to thetemperature reconstruction was implemented byMINITAB version 13. The Shannon^Wiener di-versity measure, de¢ned by the equation:HP=3g(pi)(ln pi), where HP is the Shannon^Wie-ner index of diversity and pi is the proportion ofall individuals in the sample belonging to taxon i,was used as a measure of the diversity of thechironomid assemblages (Levesque et al., 1996).The diversity index was applied to abundancedata grouped at the generic level. Assemblagesthat consist of and/or are dominated by fewtaxa tend to have a relatively low index of diver-sity.

4. Results

4.1. Chronology and lake sediment characteristics

A 283-cm core was recovered from GreenstoneLake. The stratigraphy of the early Holocene por-tion of the core is typi¢ed by dark brown, organ-ic-rich gyttja between 225 cm and 232 cm (Fig. 3).Between 232 cm and 247 cm a grading of thegyttja into lighter-colored, less organic sediment

PALAEO 3166 9-9-03


Fig. 3. Chironomid stratigraphy and percent LOI for Greenstone Lake. Abbreviations for chironomid taxa: Psectrocladius (Allo/Meso) =Allopsectrocladius/Meso-psectrocladius. Taxa are grouped into ‘Cool-water Taxa’ or ‘Temperate Taxa’ according to their surface water temperature optima (as determined in Porinchu etal., 2002) with the exception of Heterotrissocladius subpilosus, Pseudodiamesa, Corynocera-type 1 and Glyptotendipes which are placed into groups based upon theirsurface water temperature optima as de¢ned in the literature. Surface water temperature optima values listed in parenthesis following each taxon. Optima for thetaxa are based on a simple WA regression and calibration model using classical de-shrinking. Optima are provided for the taxa that are present in greater than20% of the modern calibration dataset lakes. The number of May£y (Insecta: Ephemeroptera) mandibles encountered in Sediments also indicated.

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occurs with a gray silty clay band occurring be-tween 238 and 239 cm. A distinct transition tosilty clay occurs at 247 cm and likely re£ects thetransport of glacial £our from the nearby ConnessGlacier into Greenstone Lake. Application ofHCl to the late Pleistocene and early Holoceneportion of the core suggests that the sedimentdoes not contain carbonates; carbonates havebeen shown to adversely a¡ect 14C dating (Mac-Donald et al., 1991).

The chronology for Greenstone Lake is basedon two accelerator mass spectrometer (AMS) ra-diocarbon dates (Table 1). The age^depth func-tion developed for the core assumed a constantrate of sediment deposition between the 14C ra-diocarbon dates. The program CALIB version 4.3(Stuiver et al., 1998) and the atmospheric decadaldataset were used to convert the radiocarbondates to calibrated ages. The midpoint of the calBP age range with the highest probability of oc-

curring at the 2c level was used to create the calyr BP age scale.

The LOI curve shows a roughly sigmoidal rela-tionship with the immediate late-glacial periodcharacterized by extremely low organic content(V2%) (Fig. 3). At 13 200 cal yr BP an approx-imately three-fold increase in percent LOI occurs.The monotonic increase in LOI is interruptedonly once with a marked drop in LOI occurringcentered at 11 700 cal yr BP. The drop in LOIfrom approximately 12% to 7% at this depth isassociated with the presence of a 1 cm thick bandof silty clay sediment (238^239 cm) identi¢ed inthe sediment stratigraphy. This short-term rever-sal in the amount of organic matter present in thelake sediment is followed by increasing LOI val-ues with a local peak of approximately 15% at-tained at 10 900 cal yr BP. The modern LOI valuefor Greenstone Lake is V17%.

4.2. Chironomid community composition andtrajectory

The chironomid percentage diagram is dividedinto ¢ve zones (Fig. 3).

4.2.1. GSL-1 (14 800^13 700 cal yr BP;259^252 cm)

This zone is dominated by a single cold-waterchironomid taxon, Heterotrissocladius marcidus-type (V75%). Also present in this zone are othercold-water taxa such as Heterotrissocladius grim-shawi-type (V5%), Heterotrissocladius subpilosus-type (V15%), Pseudodiamesa (V3%) and Micro-psectra radialis-type (V1%), is also present inGSL-1, albeit at extremely low levels. Head cap-sule concentrations are extremely low throughoutGSL-1 averaging less than 10 head capsules/ml.The richness and diversity of the chironomid com-munity during this period are also low; richness£uctuates between 2 and 3, and diversity averagesapproximately 0.20 (Fig. 4).

4.2.2. GSL-2 (13 700^13 300 cal yr BP;250^248 cm)

This zone is characterized by a decrease in therelative abundance of the cold-water chironomidtaxon, Heterotrissocladius (V60%). Other cold-

Fig. 4. Richness and diversity measures for the chironomidcommunity from Greenstone Lake.

PALAEO 3166 9-9-03


water taxa, such as Sergentia (V10%), Hydrobae-nus/Oliveridia (V5%) and Corynocera oliveri-type(V15%), do appear, as does the temperate taxonDicrotendipes (V5%). Appearing for the ¢rst timein the chironomid stratigraphy is a member of theTanypodinae, Procladius. Head capsule concen-trations approximately double in this zone aver-aging 16 head capsules/ml. Taxon richness anddiversity also increase averaging 7.0 and 1.4 re-spectively.

4.2.3. GSL-3 (13 300^12 300 cal yr BP;248^242 cm)

Increases in cold-water taxa such as Sergentia(V20%), Corynocera oliveri-type (V30%) andCricotopus/Orthocladius (V10%), along with aninitial decrease in the dominant warm-water taxonDicrotendipes (V10%), characterize this zone.The appearance of Chironomus, Tanytarsina andmay£y (Ephemeroptera) mandibles and an in-crease in Procladius are also observed. Head cap-sule concentration increases dramatically in thiszone, reaching a zonal peak of 174 head capsu-les/ml at 246 cm. Both the richness and diversityof the chironomid community initially increase,reaching values of 14 and 2.3 respectively at 245cm.

4.2.4. GSL-4 (12 300^10 700 cal yr BP;242^231 cm)

GSL-4 is marked by a sharp decrease in theabundance of the Orthocladiinae, which is chie£ycomprised of cold-water chironomid taxa. Sergen-tia, Heterotrissocladius (V2%) and Corynoceraoliveri-type (V5%) are dramatically reduced whilethere is a corresponding increase in the relativeabundance of temperate chironomid taxa suchas Dicrotendipes (V25%), Psectrocladius (Psectro-cladius) (V10%) and Corynocera nr. ambigua(V5%). Midway through this zone, however, abrief increase in C. oliveri-type and total Ortho-cladiinae head capsules is observed. Also of noteis the appearance of Parametriocnemus/Paraphae-nocladius and the increase and continuous pres-ence of Corynocera-type 1 and may£y mandiblesin the sediment. The richness and diversity mea-sures, which are initially high, decrease throughthe zone. Head capsule concentrations increase

dramatically, reaching up to 232 head capsules/ml near the top of the zone.

4.2.5. GSL-5 (10 700^9700 cal yr BP;225^231 cm)

This zone is characterized by the appearanceand/or increase of thermophilous chironomidtaxa such as Psectrocladius (Psectrocladius)(V10%), Microtendipes (V3%), Corynocera nr.ambigua (V15%) and Polypedilum (V1%) and adecrease in Dicrotendipes (V10%). Some cold-water taxa such as Heterotrissocladius and Cory-nocera oliveri-type are essentially extirpated in thiszone; Sergentia (V1%) remains present, albeit atvery low levels. Tanytarsus group 4 (V5%) andTanytarsus group 3 (V1%) are also present in thiszone at moderate levels. Taxon richness and di-versity do not change markedly from the previouszone. Following a peak in head capsule concen-tration that occurs at the base of GSL-5 (V320head capsules/ml), a decrease in head capsule con-centration to levels similar to GSL-3 occurs (V94head capsules/ml).

4.3. Time-trend analysis

The ordination diagram based on correspon-dence analysis (ca) illustrates the trajectory of chi-ronomid community development in GreenstoneLake during the Pleistocene^Holocene transition(Fig. 5). The composition of the initial chirono-mid community (V14 500^13 700 cal yr BP) ispositioned well outside the ordination space thatis described by the chironomid communities thatare incorporated in the chironomid-based infer-ence model. These initial chironomid assemblagesare dominated by the cold-water taxon Heterotris-socladius and likely represent colder water condi-tions than we have been able to capture in ourmodern calibration set (Porinchu et al., 2002). By13 500 cal yr BP the chironomid community isincreasingly typi¢ed by taxa that are commonlyassociated with the coldest calibration set lakestoday. At approximately 12 500 cal yr BP the tra-jectory of chironomid community developmentchanges dramatically with the presence and rela-tive abundance of thermophilous taxa increasing,suggesting warming climatic conditions. A shift in

PALAEO 3166 9-9-03


chironomid community composition occurs at ap-proximately 12 000 cal yr BP when the trajectoryof community development moves away from as-semblages associated with the warmest calibrationlakes. Of note is the brief reversal in the trajectorybetween 11 750 cal yr BP and 11 500 cal yr BPwhen the relative abundance of cold-water chiro-nomid taxa brie£y increases. Between 11 500 and10 000 cal yr BP the composition of the subfossilchironomid community appears to stabilize withlittle change apparent. However, at 10 000 cal yrBP the chironomid community trajectory movestowards a composition increasingly similar to thewarmest lakes in the Sierra Nevada calibrationdataset.

4.4. Temperature reconstruction

Application of a one-component WA-PLSmodel to the subfossil chironomid assemblagespreserved in the Greenstone Lake sediment sug-gests that surface water temperatures during the

immediate post-glacial interval (14 830 cal yr BP^12 570 cal yr BP) ranged between 12.0‡C and13.5‡C (Fig. 6). However, it is important to rec-ognize that the chironomid-based inferences forthe period between 14 800 and 13 500 cal yr BPare poorly constrained by the existing Sierra Ne-vada calibration dataset and that the chironomid-inferred temperatures are likely signi¢cantly over-estimated. This initial period of low water temper-ature was followed by £uctuating, upward-trend-ing surface water temperatures. Post-glacialclimatic amelioration was possibly interrupted be-tween approximately 12 000 cal yr BP and 11 500cal yr BP when surface water temperaturedropped to 14.9‡C from 15.5‡C. The LOWESSsmoother indicates that that post-glacial warmingwas essentially monotonic with a brief interrup-tion occurring between 11 000 cal yr BP and12 000 cal yr BP. However, it is important tonote that the decrease in surface water tempera-ture that is inferred to have occurred between12 000 and 11 500 cal yr BP cannot be considered

Fig. 5. Time-trend CA bi-plot comparing fossil chironomid assemblages from Greenstone Lake with the 44-lake calibration data-set from the Sierra Nevada, California. Calibration dataset lakes are grouped according to measured surface water temperature.Numeric values represent the fossil chironomid assemblages at 250-year intervals (U103).

PALAEO 3166 9-9-03


statistically signi¢cant as the sample-speci¢c er-rors, which varied between 1.2‡C and 1.4‡C, andthe root mean square error of prediction(RMSEP) of the WA-PLS model are larger thanthe drop in surface water temperature that oc-curred during this period (Fig. 6).

5. Discussion

5.1. Chironomid community development

The chironomid percentage diagram (Fig. 3)indicates that the initial post-glacial chironomidcommunity in Greenstone Lake is dominated byone taxon, Heterotrissocladius. Heterotrissocladiusis commonly found at high abundances in thelate-glacial sediment of temperate North Ameri-can and northern European lakes (Levesque et al.,

1993; Cwynar and Levesque, 1995; Smith et al.,1998; Brooks and Birks, 2000). Today, in the Pa-learctic and Nearctic, Heterotrissocladius is gener-ally associated with arctic and alpine lakes withhigh oxygen concentrations and deep, thermallystrati¢ed, oligotrophic temperate lakes (Walkerand Mathewes, 1989a; Walker et al., 1991a; Lot-ter et al., 1997; Olander et al., 1999; Larocque etal., 2001). In the eastern Sierra Nevada Hetero-trissocladius grimshawi-type is restricted almostentirely to cold, high-elevation lakes and has asurface water temperature optimum of 14.3‡C.Heterotrissocladius marcidus-type, although notrestricted to cold, high-elevation lakes, has a sur-face water temperature optimum of 14.0‡C, and ismost common in these lakes (Porinchu et al.,2002). The dominance of Heterotrissocladius inthe post-glacial chironomid assemblages indicatesthat it was present not only in the profundal zone

Fig. 6. Chironomid-based surface water temperature reconstruction for Greenstone Lake during the late-glacial and early Holo-cene. Points represent actual temperature inferences with error bars and dark continuous line represents a LOWESS smoother(span= 0.20) has been superimposed on the chironomid-inferred values.

PALAEO 3166 9-9-03


but also in the littoral zone of Greenstone Lake.This suggests that Heterotrissocladius was likelyresponding to cold lake water conditions ratherthan deep-water oxygen concentration. Alsopresent in zone GSL-1 is Pseudodiamesa, a taxonthat is associated with cold £owing water and theprofundal of still water and can be considered acold stenotherm (Wiederholm, 1983; Walker etal., 1997; Brooks and Birks, 2001; Larocque etal., 2001). In an eastern Canada calibration setPseudodiamesa has the second lowest surfacewater temperature optimum (6.7‡C) of all thetaxa identi¢ed (Walker et al., 1997). Pseudodiame-sa was likely deposited in Greenstone Lake byin£owing streams carrying meltwater from thenearby Conness glaciers ; and its presence furthersubstantiates the inference of cold conditions dur-ing this interval. The LOI values indicate thatlake productivity was low during this period andthat the surrounding catchment was poorly vege-tated, resulting in high amounts of minerogenicmaterial being deposited within the lake. Thelow head capsule concentrations and diversity val-ues are typical of late-glacial conditions and sus-tained periods of climatic deterioration such asthe Younger Dryas (Walker and Mathewes,1989b; Levesque et al., 1996; Brooks and Birks,2001; Palmer et al., 2002). Low water tempera-tures, depressed lake productivity and limitedamounts of potentially available habitat, i.e.,rooted macrophytes and other aquatic vegetation,are likely responsible for the depauperate chiro-nomid community present in GSL-1.

The next zone, GSL-2 (13 700^13 300 cal yrBP), continues to be dominated by Heterotrisso-cladius, however the community diversity in-creases as other cold-water taxa such as Coryno-cera oliveri-type, Sergentia and Micropsectraradialis-type appear, as does a temperate taxon,Dicrotendipes. Corynocera oliveri-type has beenassociated with tundra lakes in northeast Siberiawhere its modern distribution is delineated bylatitudinal treeline (Porinchu and Cwynar, 2000).In the Sierra Nevada, C. oliveri-type, whilebroadly distributed, is most abundant in colderlakes and has a surface water temperature opti-mum of 13.9‡C (Porinchu et al., 2002). It has alsobeen identi¢ed as the dominant taxon in many

coastal subalpine lakes in British Columbia, Can-ada (Walker, 1988) and appears to show prefer-ence for tundra sites in northern Fennoscandia(Olander et al., 1999; Seppa« et al., 2001). Sergen-tia is a taxon that is commonly found in arcticand alpine lakes (Lotter et al., 1997; Walker etal., 1997; Olander et al., 1999; Larocque et al.,2001), but it is also found in the profundal ofdeep lakes (Korhola et al., 2000). Its modern dis-tribution in the Sierra Nevada indicates that it ismost abundant and most consistently found incold, high-elevation lakes (Porinchu et al., 2002).The appearance of Dicrotendipes, albeit at lowlevels, suggests that the littoral zone of Green-stone Lake had warmed su⁄ciently to supportits presence. Dicrotendipes is considered a littoral,warm-water taxon, and is generally associatedwith low-elevation lakes and lakes found southof latitudinal treeline (Wiederholm, 1983; Walkerand MacDonald, 1995; Porinchu and Cwynar,2000; Brooks, 2000). Head capsule concentrationsand LOI remained low in this zone suggestingthat lake water temperatures had not yet su⁄-ciently warmed to greatly impact lake productiv-ity.

Dramatic changes in the aquatic ecosystem oc-curred in zone GSL-3 (13 300^12 300 cal yr BP).In conjunction with the abrupt decrease in thecold-water stenotherm, Heterotrissocladius, is theappearance of warm-water taxa such as Psectro-cladius (Psectrocladius), Psectrocladius (Allopsec-trocladius/Mesopsectrocladius) and Corynocera nr.ambigua, and an increase in Dicrotendipes. How-ever, there is also a corresponding increase incool-water taxa such as Corynocera oliveri-type,Sergentia, Cricotopus/Orthocladius and Paracla-dius and the appearance of a cold stenothermictaxon, Synorthocladius. Paracladius, which hasbeen identi¢ed as a common constituent of coldtundra lakes and £owing water (Porinchu andCwynar, 2000; Oliver and Roussel, 1983), andthe presence of C. oliveri-type and Synorthocla-dius, both of which have the coldest temperatureoptima in the Sierra Nevada calibration dataset,indicate that water temperatures continue to becool throughout this zone. The relative propor-tion of the Tanypodinae also increases, with Pro-cladius and members of the tribe Pentaneurini

PALAEO 3166 9-9-03


present, both of which have temperature optimaof 15.3‡C. The presence of Procladius in this zoneis di⁄cult to interpret; it has been identi¢ed as acommon constituent of late-glacial chironomidcommunities in western North America (Smithet al., 1998), but it is broadly distributed over alarge range of trophic conditions and altitude andtemperature gradients (Lotter et al., 1998; Porin-chu et al., 2002). The appearance of Chironomusand Sergentia together with some warm-watertaxa suggests that lake strati¢cation and an in-crease in lake productivity may have occurredresulting in a reduction in hypolimnetic oxygenconcentration. Sergentia, which is generally asso-ciated with cold arctic/alpine lakes, possesses he-moglobin and can tolerate moderate oxygen de-pletion (Walker, 1990). Chironomus, commonlyreferred to as bloodworm due to the presence ofhemoglobin, is often found at high levels in thelittoral habitats of shallow, warm, low-elevationlakes and it is often used as a bioindicator ofeutrophication. The increase in head capsule con-centrations midway through the zone and the in-crease in taxon diversity towards the top of thiszone suggest that an expansion of available hab-itat likely occurred, enabling the colonization ofthe littoral zone by an increasingly diverse, ther-mophilous chironomid community.

In the time period represented by GSL-4(12 300^10 700 cal yr BP) the chironomid commu-nity becomes increasingly dominated by warm-water taxa such as Dicrotendipes, Corynocera nr.ambigua, Psectrocladius (Psectrocladius) and Co-rynocera-type 1. Cold-water taxa, with the excep-tion of Corynocera oliveri-type and Cricotopus/Or-thocladius, are present at levels less than 5%. Thepresence of C. nr. ambigua and Corynocera-type 1is signi¢cant as they are most abundant in thewarmest lakes of the modern calibration set. Co-rynocera-type 1 is an unidenti¢ed Corynocera thatis intermediate in appearance between Corynoceraoliveri and C. nr. ambigua. Remains similar toCorynocera-type 1 have been identi¢ed as an un-known Corynocera in the remains of late-glacialsediment from southern British Columbia, Cana-da (Palmer et al., 2002). Corynocera nr. ambigua,typi¢ed by a dark and variable mentum, has beenidenti¢ed in the cold, oligotrophic coastal lakes

and ponds of British Columbia; however, in theSierra Nevada it is most commonly associatedwith warm, low-elevation lakes (Porinchu et al.,2002). It is possible that a number of previouslyunidenti¢ed Corynocera species or types may bepresent in both the late Quaternary sediment fromGreenstone Lake and the modern calibration da-taset. Although Psectrocladius (Psectrocladius)and Psectrocladius (Allopsectrocladius/Mesopsec-trocladius) are eurythermic, the genus Psectrocla-dius has been identi¢ed as a common constituentof low-elevation lakes in British Columbia(Walker and Mathewes, 1989a) and has alsobeen used to distinguish forested lakes from tun-dra lakes in Siberia (Porinchu and Cwynar, 2000).Midway through this zone there is also a slightincrease in the proportion of cool-water taxapresent.

The proportion of temperate chironomid taxaincluding Microtendipes, Polypedilum, Tanytarsusspp. A/C and Parakie¡eriella cf. bathophila con-tinue to increase in GSL-5 (10 700^9700 cal yrBP). Thermophilous taxa such as Microtendipes,Glyptotendipes and Polypedilum, generally associ-ated with the littoral zone of warm, mid- and low-elevation sites, have a limited distribution in arcticand alpine lakes (Walker and Mathewes, 1989a;Walker and MacDonald, 1995; Porinchu andCwynar, 2000; Olander et al., 1999; Brooks andBirks, 2001; Larocque et al., 2001; Porinchu etal., 2002). Tanytarsina spp. A/C, Microtendipesand Polypedilum have the highest temperature op-tima of all taxa that appear in greater than 20% ofthe Sierra Nevada calibration lakes. Corynocera-type 1, which is only present in two of the cali-bration dataset lakes, has an extremely high tem-perature optimum (19.5‡C) and its presence inGSL-5 in relatively high amounts suggests contin-ued climatic amelioration. However, it is interest-ing to note that the uppermost samples in thiszone see the disappearance of may£y remains, adecrease in head capsule concentration and thereappearance of cold-water taxa such as Sergen-tia, Heterotrissocladius spp. and Corynoneura/Thienemanniella. The reappearance of cold-watertaxa at approximately 9500 cal yr BP has beendocumented elsewhere in the Sierra Nevada (Po-rinchu and MacDonald, unpublished).

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5.2. Paleoclimatic reconstruction and implications

The date 10 700 P 80 14C yr BP (12 800 cal yrBP) that we obtained on basal organic sedimentsfrom Greenstone Lake provides a minimum datefor the deglaciation of this area. Clark and Gilles-pie (1997) have obtained similar dates from otherhigh-elevation lakes in the Sierra Nevada andhave suggested that the Sierra Nevada was essen-tially deglaciated between 14 000 and 15 000 cal yrBP. The basal date we obtained for GreenstoneLake is slightly younger than this. However, thisdate is consistent with the chronology of latePleistocene deglaciation proposed by Clark andGillespie (1997), given that it is based on a samplelocated V40 cm above the bottom-most sedimentin the core and that Greenstone Lake is a high-elevation lake, located near a modern glacier(Conness Glacier) which was much larger duringthe late Pleistocene, as evidenced by the nearbyRecess Peak deposits.

The modern chironomid assemblage found inGreenstone Lake was not incorporated in the in-ference model that was applied in this study dueto its high absolute residual (predicted water tem-perature minus measured water temperature) (seePorinchu et al., 2002 for further details). Themeasured surface water temperature for Green-stone Lake at the time of surface sediment collec-tion was 11.9‡C while measured ambient airtemperature was 15.0‡C. When the 44-lake chiro-nomid-based inference model is applied to themodern chironomid assemblage found in Green-stone Lake a surface water temperature of 13.3‡Cis inferred with a sample-speci¢c error estimate of1.2‡C. The di¡erence between the model-inferredsurface water temperature and the measuredwater temperature is slightly greater than the sam-ple speci¢c error estimate. A number of authorshave pointed out that a single measurement ofsurface water temperature does not adequatelycharacterize the mean surface water temperatureat a given site (Hann et al., 1992; Brooks andBirks, 2001). Recent work has demonstratedthat chironomid-based inference models formean summer or mean July air temperature oftenoutperform chironomid-based inference modelsfor surface lake water temperature. However,

the highly complex and varied topography ofthe Sierra Nevada and the paucity of high-eleva-tion meteorological stations with adequately longrecords in this region have limited our ability tomake meaningful estimates of mean July air tem-perature and hence develop a chironomid-basedinference model for air temperature (Porinchu etal., 2002). The closest modern analogues currentlyfound in the Sierra Nevada for the surface samplerecovered from Greenstone Lake are from BoxLake, Long Lake 2 (uno⁄cial name, Porinchu etal., 2002), Lower Sardine Lake and Big PotholeLake; these lakes have measured surface watertemperatures of 14.3‡C, 14.3‡C, 14.0‡C and13.3‡C, respectively (Porinchu et al., 2002). Wesuggest that based on its modern chironomid as-semblage, the surface water temperature measuredat Greenstone Lake on 9 August 1999 is likely anunderestimate of the mean July surface water tem-perature.

The time-trend analysis reveals that the compo-sition of the chironomid community at Green-stone Lake between 14 500 cal yr BP andV13 700 cal yr BP is not encompassed by themodern calibration dataset. Currently, there areno lakes in the Sierra Nevada calibration set inwhich the proportional representation of Hetero-trissocladius taxa exceeds 30%. Late-glacial sedi-ments from northern European lakes are com-monly characterized by high levels of Hetero-trissocladius, as well as other cold stenothermoustaxa such as Sergentia coracina and Tanytarsuslugens (Brundin, 1949). However, contiguous sub-fossil chironomid assemblages exclusively consist-ing of Heterotrissocladius have yet to be docu-mented. As far as we are aware the onlymodern analogue for the chironomid assemblagein GSL-1 comes from Chlorine Lake, a high-ele-vation lake located in southwestern British Co-lumbia, Canada (Walker and Mathewes, 1989a).A surface sample collected from this lake consistsalmost entirely of Heterotrissocladius. The mea-sured surface water temperature of ChlorineLake was 2‡C (Walker and Mathewes, 1989a).Pseudodiamesa, which is also present in GSL-1,has a surface water temperature optimum of ap-proximately 7‡C in both eastern North Americaand northwestern Europe (Walker et al., 1997;

PALAEO 3166 9-9-03


Brooks and Birks, 2001). Given the poor calibra-tion of the late-glacial samples in GSL-1, it isquite likely, based on the limited ecological dataavailable from Chlorine Lake and the presence ofPseudodiamesa, that any chironomid-inferred tem-peratures using the current Sierra Nevada calibra-tion dataset are signi¢cantly overestimated priorto 13 700 cal yr BP. It is for this reason that noquantitative chironomid-based temperature recon-struction is available for the interval between14 800 cal yr BP and 13 700 cal yr BP. Expansionof the modern calibration dataset will enable us toincorporate colder lakes into the calibration set,which should improve the accuracy of the chiro-nomid-based temperature inferences for the im-mediate post-glacial period.

Much of what is known about late Pleistoceneand early Holocene environments and climate inthe central Sierra Nevada is based upon macro-fossil and pollen analyses of packrat middens andlake and meadow sediment. Based on packrat evi-dence Cole (1983) suggests that the late Pleisto-cene and early Holocene in the western SierraNevada was characterized by an increasingly con-tinental climate that was colder and drier than atpresent. In general, the palynological research car-ried out in the central Sierra Nevada illustrates asimilar trend of late Pleistocene and early Holo-cene aridity (Adam, 1967; Davis et al., 1985; Da-vis and Moratto, 1988; Anderson, 1990; Ander-son and Smith, 1994). Based on palynologicalstudies of sediment recovered from lakes locatedin the sub-alpine and upper montane forest of theeastern Sierra Nevada, Anderson (1990) deter-mined that prior to 10 000 14C yr BP trees wereabsent or sparsely established around these sites.Open ground vegetation, such as Chrysolepis sem-pervirens, Cercocarpus, Arctostaphylos and otherCompositae, was more abundant during the latePleistocene and early Holocene further substanti-ating the existence of xerothermic conditions dur-ing this interval. However, little is still knownabout the speci¢cs in terms of the magnitudeand rate of change of the thermal regime duringthe Pleistocene^Holocene transition period.

The chironomid-based quantitative surfacewater temperature reconstruction (Fig. 6) suggeststhat post-glacial climatic amelioration in the vi-

cinity of Greenstone Lake was essentially sigmoi-dal rising from approximately 12‡C to approxi-mately 17‡C between 13 400 cal yr BP and10 600 cal yr BP. However, the immediate post-glacial period (14 800^13 700 cal yr BP) was char-acterized by extremely low lake water tempera-tures, which were likely 6 5‡C. Glacial meltwaterfrom the nearby Conness Glacier was likely £ow-ing into Greenstone Lake during this period de-pressing lake water temperatures. Between 13 300cal yr BP and 12 700 cal yr BP an approximately1‡C increase in lake water temperature occurredwith surface water temperatures reaching 13.4‡Cby 12 700 cal yr BP. This warming trend contin-ued to approximately 12 000 cal yr BP at whichtime surface water temperatures had reached apost-glacial high of 15.5‡C. However, the intervalbetween 12 000 cal yr BP and 11 500 cal yr BPmay have been characterized by decreasing sur-face water temperatures, with chironomid-inferredwater temperatures dropping to 14.9‡C by 11 500cal yr BP. A brief increase in lake water temper-ature was followed by another short-term coolingevent, which ended at 11 000 cal yr BP. By 10 500cal yr BP surface water temperatures at Green-stone Lake had reached 16.7‡C.

The slight decline in surface water temperaturebetween 12 000 cal yr BP and 11 500 cal yr BP,during which time surface water temperaturedropped from 15.5‡C to 14.9‡C, is potentially im-portant. It is possible that this chironomid-in-ferred cooling is a manifestation of the YoungerDryas. Recent work has provided evidence thatthe Younger Dryas climatic oscillation impactedwestern North America (Reasoner et al., 1994;Menounos and Reasoner, 1997), eastern Califor-nia (Benson et al., 1996, 1997) and coastal Cali-fornia (Kennett and Ingram, 1995). However,Benson’s work has not resolved whether the neg-ative hydrologic balance that Owens Lake experi-enced during this period was a result of reducedruno¡ from the Sierra Nevada or increased evap-oration attributable to higher summer tempera-tures, or some combination of both of these fac-tors.

It is di⁄cult to make any de¢nitive conclusionsabout the cause and signi¢cance of the coolingthat is inferred to have occurred between 11 200

PALAEO 3166 9-9-03


and 10 700 cal yr BP; the timing of this event doesnot correspond well to any known climatic eventsin the Sierra Nevada. The pause in post-glacialclimatic amelioration that may have occurred be-tween 12 000 cal yr BP and 11 500 cal yr BP maybe correlative to the Younger Dryas. However, itis equally likely that this event may simply be aresult of localized changes in climate or an in-crease in glacial meltwater inputs from the nearbyConness Glacier to Greenstone Lake. Althoughthe evidence from Greenstone Lake in terms ofsediment lithology and faunal changes does notunambiguously support the presence of an ex-tended period of cooling during the YoungerDryas, it is also interesting to note that a similarlow-amplitude change in water temperature dur-ing the Younger Dryas chronozone has been re-ported from high-elevation lakes in eastern NorthAmerica. Cwynar and Spear (2001) have deter-mined that high-elevation sites in the WhiteMountains of New Hampshire, USA, did notregister a strong Younger Dryas signal, whilemid-elevation sites, which had experienced signi¢-cant warming prior to the Younger Dryas, didregister a cooling of approximately 5‡C. Furtherwork is required on a variety of sites at di¡erentelevations in the Sierra Nevada to determine iflate Pleistocene climatic oscillations similar tothose detected in the circum-North Atlantic andelsewhere are unequivocally present in California.

6. Conclusion

This is the ¢rst quantitative chironomid-basedreconstruction of late Pleistocene and early Holo-cene surface water temperatures for the westernUSA. The chironomid-based temperature recon-struction suggests that the immediate post-glacialperiod (14 800^13 700 cal yr BP) was characterizedby extremely cold surface water temperatures,which were likely 6 5‡C. Between 13 700 and12 000 cal yr BP surface water temperatures roseapproximately 2‡C reaching a post-glacial high of15.5‡C at 12 000 cal yr BP. This was followed by a500-year period of depressed water temperatures,which may be correlative with the YoungerDryas. The early Holocene was characterized by

warmer conditions with surface water tempera-tures £uctuating between 15.5‡C and 16.5‡C. Ex-pansion of the modern calibration dataset, to in-clude additional lakes with lower surface watertemperatures than currently exist in dataset,should enable us to make more accurate, quanti-tative inferences of water temperature during theimmediate post-glacial period. However, thisstudy does substantiate the use of chironomidsin deriving quantitative estimates of past thermalregimes, which should prove quite valuable in im-proving our understanding of late Quaternary en-vironments and climate in the Sierra Nevada andthe surrounding region.

Acknowledgements

We would like to thank Jose Arias, Anna Kre-menetski, Desmond Moser, Joanne MacDonald,Alex MacDonald and Hilary MacDonald fortheir help in the ¢eld, Paddy Hardy at the U.S.Forest Service for facilitating our access to Green-stone Lake and Doug Clark for providing unpub-lished information about the bathymetry and ageof Greenstoke Lake. We would like to thank thesta¡ at the University of Utah, Department ofGeography DIGIT Lab for providing a basemap of the study area (Figure 1) and Chase Lang-ford (Department of Geography, UCLA) for cre-ating Figure 2. Funds to support this researchwere provided by a National Science Foundation(NSF) Regional Sciences grant to G.M.M. andK.A.M. (BCS-9905350), a NSF doctoral disserta-tion improvement grant to D.F.P. and G.M.M.(BCS-0101268) and a Geological Society of Amer-ica (GSA) graduate student research grant toD.F.P.

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Late Pleistocene and early Holocene climate and limnological changes in the Sierra Nevada, California, USA inferred from m...IntroductionStudy areaMethodsFieldLaboratoryStatistical analysesModern calibration datasetComparison and model application to subfossil assemblages

ResultsChronology and lake sediment characteristicsChironomid community composition and trajectoryGSL-1 (14 800-13 700 cal yr BP; 259-252 cm)GSL-2 (13 700-13 300 cal yr BP; 250-248 cm)GSL-3 (13 300-12 300 cal yr BP; 248-242 cm)GSL-4 (12 300-10 700 cal yr BP; 242-231 cm)GSL-5 (10 700-9700 cal yr BP; 225-231 cm)

Time-trend analysisTemperature reconstruction

DiscussionChironomid community developmentPaleoclimatic reconstruction and implications

ConclusionAcknowledgementsReferences

Documents

Late Pleistocene and early Holocene climate and ......ta: Diptera: Chironomidae)remains preserved in the sediments of a small, climatically sensitive Sierra Nevada lake to reconstruct