Abstract A survey of the modern physical set-ting of Lake El’gygytgyn, northeastern Siberia, ispresented here to facilitate interpretation of a250,000-year climate record derived from sedi-ment cores from the lake bottom. The lake liesinside a meteorite impact crater that is approxi-mately 18 km in diameter, with a total watershedarea of 293 km2, 110 km2 of which is lake surface.The only surface water entering the lake comesfrom the approximately 50 streams draining fromwithin the crater rim; a numbering system forthese inlet streams is adopted to facilitate scien-tific discussion. We created a digital elevationmodel for the watershed and used it to createhypsometries, channel networks, and drainagearea statistics for each of the inlet streams. Many

of the streams enter shallow lagoons dammed bygravel berms at the lakeshore; these lagoons mayplay a significant role in the thermal and biolog-ical dynamics of the lake due to their higher watertemperatures (>6!C). The lake itself is approxi-mately 12 km wide and 175 m deep, with a vol-ume of 14.1 km3. Water temperature within acolumn of water near the center of this oligo-trophic, monomictic lake never exceeded 4!Cover a 2.5 year record, though the shallow shelves(< 10 m) surrounding the lake can reach 5!C insummer. Though thermally stratified in winter,the water appears completely mixed shortly afterlake ice breakup in July. Mean annual air tem-perature measured about 200 m from the lakewas –10.3!C in 2002, and an unshielded rain gagethere recorded 70 mm of rain in summer of 2002.End of winter snow water equivalent on the lakewas approximately 110 mm in May 2002. Analysisof NCEP reanalysis air temperatures (1948–2002)reveals that the 8 warmest years and 10 warmestwinters have occurred since 1989, with the num-ber of days below –30!C dropping from a pre-1989 mean of 35 to near 0 in recent years. Thecrater region is windy as well as cold, with hourlywind speeds exceeding 13.4 m s–1 (30 mph) typi-cally at least once each month and 17.8 m s–1

(40 mph) in winter months, with only a few calmdays per month; wind may also play an importantrole in controlling the modern shape of the lake.Numerous lines of evidence suggest that the

This is the second in a series of eleven papers published inthis special issue dedicated to initial studies of El’gygytgynCrater Lake and its catchment in NE Russia. JulieBrigham-Grette, Martin Melles, Pavel Minyukwere guest editors of this special issue.

M. Nolan (&)Institute of Northern Engineering, University ofAlaska Fairbanks, 525 Duckering Bldg, Fairbanks,AK 99775-5860, USAe-mail: Matt.Nolan@uaf.edu

J. Brigham-GretteDepartment of Geosciences, University ofMassachusetts, 611 N. Pleasant Street, Morrill ScienceBuilding, Amherst, MA 01003, USA

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DOI 10.1007/s10933-006-9020-y

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ORIGINAL PAPER

Basic hydrology, limnology, and meteorology of modernLake El’gygytgyn, Siberia

Matt Nolan Æ Julie Brigham-Grette

Received: 20 February 2004 /Accepted: 1 May 2006 / Published online: 12 December 2006" Springer Science+Business Media B.V. 2006

physical hydrology and limnology of the lake haschanged substantially over the past 3.6 millionyears, and some of the implications of thesechanges on paleoclimate reconstructions arediscussed.

Keywords Arctic Æ Hydrology Æ Limnology ÆLake ice Æ Climate

Introduction

Lake El’gygtgyn lies inside of a meteorite impactcrater dated at 3.6 million year BP (Layer 2000)and holds a promising paleoclimate record withinits sediments (Brigham-Grette et al. 2001, 2007;Nolan et al. 2000; Nowaczyk et al. 2002). Locatedin northeastern Siberia (67.5 N, 172 E), this lakehas already yielded the oldest continuous terres-trial climate record in the Arctic, based on a short13 m long core dated to over 250,000 years BP, asthoroughly described elsewhere in this special is-sue (Brigham-Grette et al. 2007; Melles et al.2007; Nowaczyk and Melles 2007). However, littleis known in the western world about the physicalsetting of this lake. The name itself—‘‘El’gy-gytgyn’’— is Siberian Chukchi that has beentranslated to ‘white lake’ or ‘lake that neverthaws’, hinting at the intriguing possibility thatperhaps within human oral history that it has re-mained ice covered through the summer; such anice cover would have a major influence on thephysical and chemical dynamics of the lake, asdescribed later. This paper therefore describessome of the basic hydrological, limnological, andmeteorologic characteristics of the lake and itscrater-watershed, with the goal of providing acontext against which other modern-processstudies and paleo-conditions can be compared;this paper is the second in a series towards thatgoal (Nolan et al. 2003). The results describedhere are based on observations from three US–German–Russian field expeditions (2 May–15 May 1998; 7 Aug–6 Sept 2000; and April–Sept2003), remote sensing covering roughly the sameperiod, and data from loggers measuring contin-uously from 2000–2003 located around and withinthe lake.

Crater hydrology

Lake El’gygytgyn lies within a meteorite impactcrater, north of the Arctic Circle, in a region ofcontinuous permafrost in the Russian Far East(Brigham-Grette et al. 2007). The crater isroughly 18 km in diameter and is likely the bestpreserved on Earth for its size (Dence 1972; Dietzand McHone 1976). The crater lies along the re-gional continental divide, with the single outlet(the Enmyvaam River) flowing south and eastinto the Bering Sea, whereas just over the northedge of the crater rim water flows into the ArcticOcean. The lake is surrounded by continuouspermafrost, with local depths likely in therange of 100–300 m (Glushkova 1993). Vegeta-tion is sparse here, dominated by moss tundra(Oacease–Cyperaceae–Artemisia with someprostrate Salix spp). At the time of the impact,however, there was no permafrost in this region,and forests extended all the way to theArctic coast(Brigham-Grette and Carter 1992).

Numerical delineation of stream channels

A digital elevation model (DEM) of the craterregion was created by digitizing the contoursof commercially available topographic mapswith 20 m contour intervals at 1:100,000 scale(Mezyerynnet Q-59-19,20 and Otvegyrgyn Q-59-9.10).The resulting DEM had the usual artifacts asso-ciated with such a digitizing process, such as step-like elevations in flat areas and map splice errors,but in general was suitable for basic hydrologicalstudy and for ortho-rectification of satelliteimagery. This DEM, used in the hydrologicalanalyses here, has a 30 m spatial resolution, 10 mvertical accuracy, and 1 cm vertical resolution.

Using the commercial software RiverTools#,we calculated a number of basic hydrologicalstatistics from this DEM. The entire basin areadraining into the lake (including the lake) is293 km2, with 183 km2 of land area draining intothe lake itself (lake surface area 110 km2).Approximately 50 streams drain from the craterrim into the lake. To facilitate research, a com-mon numbering system for these streams wasdeveloped during the 2000 field season following

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upon the work of Glushkova (pers. comm., 2000).These numbers begin with 0 at the outlet stream(southeast corner) and increase clockwise aroundthe lake (Fig. 1). The drainage area and relieffor each of these streams was calculated andpresented in Table 1. Minor inlet streams exist inbetween these 50 numbered streams, particularlynear their entry into the lake, so the sum of the 50watershed areas in Table 1 is only about 85% ofthe total watershed area. Though not labeled onthe Fig. 1, it is suggested that future publicationsrefer to these minor streams using decimal nota-tion based on rough percentage distance betweentwo major streams (e.g., use ‘‘Stream 16.4’’ torefer to a stream inlet located about half-waybetween Streams 16 and 17 but closer to Stream16, noting the inlet’s spatial coordinate ifavailable).

Validation for the drainage areas was com-pleted primarily by comparing computer gener-ated channel networks with Ikonos satellite images(4 m spatial resolution) and with stream vectorstaken from the paper topographic maps (presum-ably derived originally from air photos). The visualcorrelation between these data sets was excellent,despite the Ikonos image not being ortho-rectifiedusing the DEM and projection differencesbetween the DEM and the topographic maps.

Stream geomorphology

Landcover patterns are typical of arctic terrain.Higher elevations with steeper terrain aredominated by frost-shattered rock. These descendto lower-slope hills dominated by cryoturbationfeatures such as stone stripes and frost boils. Asslopes drop below about 5!, they are covered bypatches of tussock tundra that become increas-ingly more contiguous towards the lake. Gravel-bedded streams exist at the bottoms of most val-leys, with all but the largest streams being only afew meters across. The rocks in the lake wa-tershed are largely Cretaceous metavolcanics ofthe Okhotsk-Chukchi volcanic belt and the lake islargely surrounded by the Pykvaam Formationconsisting of ignimbrite and tuff (Layer 2000).

The majority of the land surface draining intothe lake is located along the broad western shore(Fig. 1). Why the lake itself is offset from the

crater rim remains to be fully explained. Informalhypotheses include a low angle impact, pre-impacttopographic controls (higher mountains in thewest), post-impact tectonic tilting, and more ero-dable terrain with prograding deltas in the west.While the first two possibilities are outside thescope of this paper, we consider the hydrologicalaspects of the last two here.

We altered the DEM to synthetically dam themodern outlet stream in the DEM, filled the lakesynthetically with more water, and tilted it pseu-do-tectonically to look for low spots that may havebeen outlets previously. We found that the onlyfeasible alternate exit point is a low pass (~36 mabove modern lake level) near Stream 36. Thisarea was examined in the field and later with airphotos and revealed no signs of ever being a lakeoutlet, despite the appearance of linear features atroughly this elevation on the hills near Streams1–3 (Glushkova and Smirnov 2007). Due to mil-lennia of cryoturbation in this area, most evidenceof paleo-shorelines has been destroyed and suchlinear features could also be features of perma-frost or locally emphasize structural geology.

The idea of prograding deltas, however, isquite plausible. The Ikonos image in Fig. 2 clearlyshows that Streams 12–14 are part of the samedelta system, which can also be seen extendingunderwater as a shallow shelf. Part of the expla-nation for why the eastern shore is not similarlymigrating towards the center may be a geologiccontrol—a fault runs through the southeast cor-ner of the crater and the rock here is more com-petent, actually outcropping on the shoreline nearStream 48. Seismic measurements made in thelake in 2003 indicate that these deltas continuequite deep into the sediment package beneath thelake (Niessen et al. 2007); these measurementsalso show that at least 400 m of lake sedimentoverlie the impact surface. Thus sediment haslikely been gradually displacing the lake fromboth the western shore and from below, suggest-ing that the original lake volume may have beenmore than quadrupled in volume (three times asdeep and 1.5 times more surface area).

The presence of water tracks on the westernshore has several implications for the dominanthydrological processes in effect today. While be-low the resolution of our DEM, these small

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hydrological features are visible on the Ikonosimage (Fig. 2) by the slightly different vegetationcolor, in this case a darker green.Water tracks are afeature found in permafrost terrain across the

Arctic and are thought to be related to the imma-ture drainage systems imposed by the frozenground (McNamara et al. 1998). Little erosion ispossible here because snowmelt, typically the

Table 1 Watershedstatistics of the 50 majorstreams draining in LakeEl’gygytgyn

Bold rows indicate thefive largestsub-watersheds. Questionmarks indicate lack offield observations ofwhether lagoons werepresent. Coordinates areincluded here to preventambiguities or confusionin the field and indicatelocation of stream mouthat lake level, reported inthe UTM 59 projection

Streamnumber

Area(sq. km)

Relief(m)

Sizerank

Lagoonin 2000?

Easting(m)

Northing(m)

1 0.40 85 43 ? 548442 74810012 2.39 93 20 No 547336 74806083 3.97 143 10 No 546522 74803604 2.22 147 22 No 545232 74800325 4.55 212 8 Yes 543972 74800316 2.61 211 18 No 543282 74802867 1.32 185 28 Yes 542682 74804098 4.44 188 9 Yes 541773 74814429 3.41 267 12 ? 541657 748163010 3.56 244 11 Yes 541386 748234511 1.56 51 27 ? 541360 748316112 9.92 316 3 Yes 541207 748430813 0.60 11 39 ? 540761 748580114 19.00 299 2 No 540595 748640215 3.00 134 15 ? 540253 748730016 7.21 306 5 Yes 539978 748759717 1.27 87 31 ? 539772 748817218 1.70 284 26 No 539861 748844219 2.16 264 23 No 539982 748868020 5.72 362 7 No 540517 748931721 8.56 156 4 No 541385 749009822 0.13 10 48 ? 541717 749039823 2.61 139 19 Yes 541841 749051224 0.34 17 45 ? 542081 749069125 7.18 310 6 No 542381 749090126 2.67 176 17 Yes 543037 749132627 0.25 9 47 ? 543363 749151028 3.02 264 14 Yes 543611 749165129 0.46 21 41 ? 544025 749183830 3.33 160 13 ? 544149 749189331 2.37 160 21 ? 544602 749216232 2.87 208 16 Yes 545411 749231133 0.97 144 33 ? 545832 749243234 2.00 92 24 ? 547151 749270135 0.85 39 35 ? 547393 749271336 1.19 53 32 ? 548562 749258237 0.02 115 50 ? 549403 749222338 0.28 167 46 ? 549525 749216439 0.61 93 38 ? 550272 749141640 0.53 101 40 ? 550515 749063141 1.28 152 30 ? 550463 749027142 0.06 34 49 ? 550523 748997143 0.69 149 36 ? 550722 748949344 0.66 159 37 ? 551538 748853745 0.87 146 34 ? 551846 748821446 0.36 120 44 ? 552371 748763047 1.79 162 25 ? 552552 748700148 0.43 134 42 ? 552584 748619249 27.43 282 1 No 552133 748436050 1.32 109 29 ? 551779 7483184

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dominant hydrological event of the year in Arcticwatersheds (Kane et al. 1992), occurs when theground is frozen near the surface. Rather thanscour soil, these water tracks have the character-istic feature of having slightly thicker vegetation,presumably due to the difference in soil moistureavailability there. This vegetation may then serveto slow erosion further. A common characteristicof these water tracks is that they tend to be paralleland follow the steepest gradient, with none of thebranching characteristic of drainage systems innon-permafrost terrain. Their existence hereconfirms that permafrost plays a dominant role inthe hydrology of the crater system and that snow-melt is likely the dominant hydrological event ofthe year.

Stream discharge

During the 2000 field season, we were able tomeasure the discharge of the outlet stream threetimes and many of the smaller inlet streams once.In late August, flows in all the inlet streams wereless than 1 m3 s–1, and often just a trickle. As ex-pected, streams with the largest drainage areas(Table 1) had the largest flows, but could easily be

crossed on foot. The outlet stream dischargedropped from 19.8 m3 s–1 on August 16, to 14.2m3 s–1 August 23, and to 11.6 m3 s–1 on September1. The maximum sill depth at the lake outlet wasabout 0.6 m, and the outlet was about 30 mwide atthe sill. The initial channel deepened to about1.5 m as it narrowed, before beginning to braidabout 200 m downstream. During the 3 weeks ofobservation, water level dropped approximately0.12 m. A recently abandoned outlet channel,presumably active during spring runoff, was pres-ent several hundred meters east of the presentchannel, with apparent spring shorelines approxi-mately 1 m higher than the late summer shoreline.

Lagoons

Many of the inlet streams were impounded bygravel bars at the shoreline. Figure 3 shows across-section of one such bar. As can be seen,lagoonal deposits are onlapped by gravels throwninland from the beach presumably during storms,a process observed in the field at other locationsalong the beach. As the lagoon water depth in-creases, it either tops over and erodes the bars orseeps out through the porous sand and gravel, orboth, as revealed through inspection of about 20such lagoons. About half of these streams werestill impounded in August 2000 (Table 1), andpresumably remained impounded until at leastthe following spring when they likely reach theirmaximum height due to snowmelt. Total lagoonarea in 2000 was 11.5 ± 1.0 km2, measured from a1 m pan-chromatic Ikonos image acquired thatsummer; during snowmelt, this area may double.Lagoon dynamics were observed to be similar in2003. Longshore drift driven by northernly windsduring a 7-day storm in August, 2003, closed offthe lake’s main outlet, but the lake continued toslowly drain through September via seepagethrough the gravel berm.

Physical limnology

Bathymetry

Our search revealed two different bathymetricmaps of the lake. The first is found on existingpaper topographic maps of the region and is

Fig. 2 Example close-up of Ikonos imagery (30 July2000). Note the prograding delta extending underwaterbetween streams 12 and 14, and the linear drainagefeatures in the tundra called water tracks. It is possiblethat the toe of this delta formed during lower lake levelsand was later submerged

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highly inaccurate. The second was found inad-vertently in Russia with no attributable author-ship (Fig. 4a). We brought this map into the fieldwith us, spot checked it with sonar and found it tobe as accurate as we could crudely determine withour own sonar and handheld positioning systems;significant errors may yet exist however. We thendigitized the contours of this map, gridded them,and merged it with the DEM created earlier. Wecreated the lake-shore contour that splices thesetwo maps together using a lake outline createdfrom the Ikonos image with 4 m spatial resolution.

Using this digital bathymetry and the Ikonosimage, we were able to calculate several valuablelake statistics. Lake surface area is 110 km2, orroughly one third of the total watershed area.Lake volume calculated from the bathymetryusing linear interpolation and 10 m horizontalslices is 14.1 km3 (Fig. 4b). Maximum lake depthis 174 ± 2 m, as measured by several techniques,with inter-annual lake level variations on the or-der of 1 m. For comparison, a circular lake ofconstant depth having the same surface area andvolume would have a diameter of 11.8 km and adepth of 128 m. Lake surface elevation is approx-imately 492 m above sea level. The lake bed ischaracterized by several shallow (< 10 m deep)shelves, remarkably steep sides, and a broad flat

bottom (Fig. 4a). The shelves and sides are typi-cally armored by gravels, making gravity coringdifficult, but the flat bed is typically a sandy-silt tosilt with rare cobbles; these shelves may havebeen paleo-shore lines. The deepest part of thelake is nearly in its center, but strong verticalrelief on the eastern shore, likely due to bedrockoutcroppings, puts most of the water towards theeastern side. The steep sides found around mostof the remaining basin, likely consisting of sedi-ments and former shorelines, occasionally slumpcreating debris flows as revealed by seismicmeasurements (Niessen et al. 2007). Between theshallow shelves and the deep basin (where slopesare less than 1!), typical slopes range 5–15! in thesouth and west shores and 15–30! in the east andnortheast shores; the highest slopes of greaterthan 40! were found along the eastern shorewhere bedrock outcrops into the lake. In the ab-sence of lake ice and thermal stratification, thefact that the lake is about 70 times wider than it isdeep favors easy mixing of the water column bythe strong winds present here.

Lake shape

We hypothesize that the odd, somewhat squareshape of the lake is a result of at least several

Fig. 3 Cross-section ofbeach sediments whereStream 14 enters laketowards right. Stormdeposited gravels can beseen above lagoon-deposit sediments. Notebackpack for scale

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factors. If modeled as a square, the north and southshores are exactly perpendicular to the bearings ofdominant winds (335!, or 25! west of north), towithin the 5! measurement accuracy of both mea-surements (Fig. 5). Numerous researchers havefound that Arctic lakes are often oriented like thisto the wind (Burn 2002; Carson and Hussey 1959;Cote and Burns 2002; Harry and French 1983;Rosenfeld and Hussey 1958; Sellman et al. 1975),and others have found that a water-body underlainby deformable sediments in nearly any windyenvironment will tend to align shorelines in thismanner (Cooke 1940; Kaczorowski 1977; Rex 1961).The typical pattern found in smaller lakes is thatwinds force water along the dominant wind vector,

with maximum erosion occurring at the upwindcorners and deposition occurring on the downwindedge. This creates elliptically shaped lakes in theeasily erodable sediments of the Alaskan Arcticcoastal plain, but geological and lacustrine controlslikely prevent this shape from developing atEl’gygytgyn. For example, a wind from the southpiles water up along the northern shore. Tomaintain mass continuity this water then must re-turn towards the south, balancing gravity and thewind-driven surface current, such that it mostlikely turns at the corners and follows the shorelineback. This places the highest erosional forces on

Fig. 4 Lake bathymetry (a) and hypsometry (b). Bathy-metric contour interval is 10 m; though we believe thisbathymetry to be fairly accurate, substantial errors may yetexist

Fig. 5 Surface water dynamics schematic (a) and 2002hourly wind speeds and directions (b). The orientation ofthe lake is identical to the dominant wind direction(bearing 335 degrees) to within the measurement accuracyof 5!. How winds help shape the lake is described in thetext

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these corners, rounding them out. On the westernshore, elliptical growth is prevented by significantdelta formation at Streams 12–14 (Fig. 1). On theeastern shore, it is prevented by bedrock erosion atthe outcrops near Stream 45. A similar dynamicresults when winds blow from the north. The resultis a roughly square lakewith rounded corners and apinched center, oriented with the prevailing winds,which tend to blow either from the north or fromthe south along a bearing of 335!; this is shownschematically in Fig. 5a. Observations of long-shore drift support these hypothesized lateral re-turn flows (the outlet stream was observed to beclosed-off by gravel during a mid-August storm in2003 and never re-opened, as mentioned above),and observations during storms in 2000 and 2003revealed a pattern of sediment flow at the watersurface that also supports this hypothesis in theopen water. Further measurements and modelingare required, however, to be conclusive about theinfluence of wind on lake shape.

Lake temperature

El’gygytgyn is a cold, oligotrophic lake. We in-stalled a thermistor string in the deepest part ofthe lake in summer of 2000 which was recoveredin-tact in summer of 2003. This string was com-posed of five Tidbit# dataloggers (Onset Com-puter Corp) attached to a weighted wire-ropesuspended by floats about 4 m below the watersurface; the floats were distributed such that if therope broke at any location, the free portion wouldfloat to the surface for later recovery on shore.Records from these thermistors are shown inFig. 6 for 2002, which was typical of all years.These thermistors show that the water tempera-tures here did not exceed 4!C during the 3 yearsof record and that the water is thermally stratifiedin winter; measurements from another thermistorstring on the shallow shelves indicate that waterthere can exceed 4!C, as warm stream water andsolar radiation gain on the sediments warm thelake locally. At least some of the apparent dif-ferences in inter-annual winter water tempera-tures in Fig. 6 are due to changes in waterthickness between years (that is, the water and icesurface was closer to the top thermistor in 2003).

We did not measure phosphorous or nitrogencontents, but the waters of Lake El’gygytgyn are aclear, deep blue with little algae or vegetationvisible in the lake. These features are character-istic of oligotrophy and indicating that the oxygendemand is not high in general (Cremer andWagner 2003; Hobbie 1984; Wetzel 2001); secchidisk depths were measured to over 20 m.

El’gygytgyn is also a monomictic lake that isfully mixed by late summer, at least during themeasurement period 2000–2003 when the therm-istors indicate the same seasonal pattern ofwarming and cooling each year. The warming ofthe pelagic water column in spring starts at the iceinterface shortly after snowmelt begins on thelake in late May (Table 2), probably as a combi-nation of lateral movement of warmer shore-wa-ter, vertical percolation of snow-melt throughcracks in the ice, and solar gain through the iceonce the snow is completely melted as foundelsewhere (Ellis et al. 1991, 1997a; Malm et al.1998; Stefanovic and Stefan 2002). As thiswarming continues, density differences betweenlayers that are no-longer thermally-stratifiedbecome negligible and isothermal mixing likelybegins to propagate downward at a steady ratebeneath the ice (Fig. 6), in the manner suggestedby theory (Matthews and Heaney 1987). Thoughwe have not yet analyzed remote sensing from2002, a sudden decrease in surface-water tem-peratures in mid-July (Fig. 6) suggests that thelast lake ice melted suddenly at this time (likelydue to ‘candle ice’ tipping over and exposingincreased surface area, as is observed on manyice-covered lakes, including Lake El’gygytgyn in2003) and that within 10 days the entire lake wasnearly isothermal. Given that strong winds hereare common, as described later, with white-cappedwaves as high as 1 m across the 12 km fetch, thereis little doubt that the lake water is fully mixedduring August and September. No significantthermocline or summer stratification was ob-served by this string or by boat measurements in2000 and 2003. By early October water tempera-tures are dropping rapidly and typically ice coverforms by October 20 (Table 2), reducing windmixing and leading quickly to thermal stratifica-tion of the water column.

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Dissolved oxygen

Lake El’gygytgyn’s biogeochemical and deposi-tional setting is dominated by the influence of icecover on the dissolved oxygen level (Melles et al.2007). Multi-year ice cover can affect dissolvedoxygen levels in some lakes by sealing the waterfrom the atmosphere, allowing biota within thelake (particularly in the lake sediments) to grad-ually consume it; indeed, even seasonal ice coversare known to lead to massive fish-kills in smallerlakes (Ellis et al. 1991, 1997b; Ellis and Stefan

1989). Preliminary core analyses (Brigham-Gretteet al. 2001; Cosby et al. 2000; Nowaczyk et al.2002) clearly shows anoxic conditions duringglacial cycles at the core-site in the deepest partof the lake, indicating that the presence or ab-sence of lake ice cover throughout summer is thesingle largest driver of the biogeochemical envi-ronment there (Brigham-Grette et al. 2007;Melles et al. 2007).

In the modern environment, the lake appearsfully saturated with dissolved oxygen. During the1998 winter coring expedition, small larvae were

Fig. 6 Lake El’gygytgyn water temperature at five depthsin 2002. A 24-point (1 day) running mean has been appliedto these data. The lake is thermally stratified during thewinter, begins mixing shortly after snow melt begins in

mid-May (Table 2), and is completely mixed shortly afterthe lake ice disintegrates completely in mid-July. Watertemperatures at all depths were colder in winter 2002–2003than the previous one

Table 2 Important dates of lake ice dynamics derived from SAR (Nolan et al. 2003)

Winter Onset of lakeice freezing

Onset of lakeice snow melt

Completion of lakeice snow melt

Onset of lakeice moat formation

Completion oflake ice melt

1997–1998 No Data < 8 July < 8 July < 8 July 8 July–9 Aug1998–1999 >6 Oct 17–18 May 31 May–4 June 24 June–4 July 28 July–13 Aug1999–2000 16–19 Oct 8–11 May 23 June–2 July 23 June–2 July 16–19 July2000–2001 18–20 Oct >14 May No data No data No data

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found in the lake sediments at 170 m depth,indicating that there is currently some mid-wintersupply of oxygen there. Crude chemical test-stripsindicated 7 mg/l at the top of sediment core inMay 1998 and in summer 2000 digital measure-ments indicated the water was fully oxygen-saturatedthroughout the pelagic column (Cremer andWagner 2003). Remote sensing (Nolan et al.2003) also indicates a high level of biologicalproductivity near the deepest part of the lake, asdescribed next.

Lake ice and biological productivity

Satellite Synthetic Aperture Radar (SAR)observations have yielded several insights intoboth lake ice dynamics and biological productiv-ity (Nolan et al. 2003), as well as possible mid-winter mixing dynamics. Nolan et al. (2003)modeled the interaction between weather, snowmelt, and lake ice breakup, using NCEP globally-gridded weather data (Kalnay et al. 1996) andSAR observations as validation with good suc-cess; remote sensing from 1998–2001 indicates aconsistent annual pattern where lake ice forms inlate-October, snow melt begins in mid-May, amoat forms around the lake in mid-June, and thelake is clear of ice by mid-July (Table 2). Perhapsmore interestingly, these SAR measurementsrevealed that the spatial distribution of bubbleswithin the lake ice is non-uniform, and that theyconsistently form most heavily over the shallowshelves and the deepest part of the lake (Fig. 7).Nolan et al. (2003) argued that these brightnesscontrasts can only be explained as differences inbubble content observed by microwave penetra-tion through the dry winter snow, as the patterndisappears as soon as snow-melt begins, prevent-ing microwave penetration. Over the shallowshelves, it is clear that the warmer sedimentsthere (as measured in the field and caused byincreased solar gain) support higher rates of bio-logical productivity and higher associated rates ofrespiration and decomposition, which leads to theincrease in bubble generation there.

Three plausible explanations were postulatedfor the central concentration of bubbles (Nolanet al. 2003), but data from other researchers ac-quired since then likely rule out two of them. The

unresolved issue at that time was the structure ofthe impact crater underlying the lake. Large impactcraters often have either central uplift features(conically shaped) or uplifted ring structures withor without a central uplift. Recent seismic mea-surements (Gebhardt et al. 2006) suggest that aring structure is likely, and that any uplift features(ring or cone) are located several kilometers fur-ther west than the deepest part of the lake. Thisfinding decreases the likelihood of the twohypotheses fromNolan et al. (2003) that dependedon the uplift structure being located directly be-neath this deepest part of the lake to describe thegaseous-source of the lake-ice bubbles found there.

Thus the most likely explanation currently forthese central-lake bubbles is a toroidally shapedconvection cell within the lake that supplies warm(4!C) water to the deepest part of the lake, wherehigher rates of respiration and decomposition canthen be supported compared to the surroundingarea, similar to the dynamics on the warmershelves that are the source of the water. In earlywinter, shelf sediments release heat causing thewater to increase in density and to sink, as hasbeen found in many ice-covered lakes (Ellis et al.

Fig. 7 Radarsat SAR scene with several overlaid bathy-metric contours (as marked). This image was acquiredwithin about a week of lake ice formation. Differences inbrightness of the lake are related to the distribution oflake-ice bubbles, as described in the text. Note the closecorrespondence of the central bright spot with the deepestpart of the lake (~175 m). In general, the bubbleconcentration reflects bathymetric features and some ofthe brightest regions of the shore may correspond to landthat was recently submerged by rising lake levels

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1991, 1997a; Hondzo and Stefan 1993; Likens andRagotzkie 1965; Matthews and Heaney 1987;Stefanovic and Stefan 2002). This prior researchsuggests that these currents would be limited to< 1 mm/s with a thickness of < 0.4 m along thesediment interface. To our knowledge, thoughthis toroidal convection cell has been postulatedmany times on other lakes, no one has evermeasured the upwelling that must occur at thecenter to supply the return flow, thus our SARobservations may be the strongest support forsuch an upwelling yet described. There is somepossibility that if this convection cell does existhere that it could be substantially shallower than175 m and exist just below the ice surface; how-ever we then have no explanation for why thebubble pattern mimics the 170 m bathymetriccontour so well (Fig. 7), and we thus believe it tobe an unlikely possibility. Water chemistry mea-surements in 2000 also indicate that the waterbelow 170 m in depth is characteristically differ-ent than the bulk water column (Cremer andWagner 2003), further supporting a deep-waterconvection cell. Note that because the flows are soshallow and slow that this does not conflict withthe observations of a thermally stratified watercolumn (Fig. 6) for the bulk of the lake water,however further research is required to validatethe existence and dynamics of this mixing.

Local weather

In summer of 2000, we installed several weatherstations around the lake. The main station is lo-cated near the outlet stream on old fluvial gravelbeds a few meters above lake level, and measuresair temperature and relative humidity at twoheights in shielded housings, wind speed anddirection, net solar radiation, barometric pres-sure, rainfall via an unshielded tipping bucket,snow depth via sonic ranger, and soil tempera-tures and moisture down to approximately 0.6 m.The unit is powered by deep cycle batteries andsolar panels and was designed to run unattendedfor at least 3 years. Unfortunately, the day afterour departure from the lake in 2000 the datalog-ger was permanently ‘‘disabled’’ by a recreational‘‘hunter’’ from the nearest town (200 km away);

this event was not discovered by us until the fol-lowing summer. At that time we replaced thedatalogger and the station logged continuouslyuntil it was downloaded in September of 2003,recovering more than 2 years of data, with 2002being the only complete calendar year to date; thestation was left running in 2003 with several yearsof data capacity. Two other stations were estab-lished, one on the north side of the lake at shorelevel, and another on a peak approximately 300 mabove lake level on the south side; these mea-sured rainfall and air temperature. Unfortunatelythese units stopped working within severalmonths of deployment, apparently caused bycombinations of lightening, bear attacks, andfaulty components.

Air temperature

The crater region is cold, but apparently gettingwarmer over the last half-century. Our localmeasurements show mean annual air temperature(MAAT) at the lake in 2002 was –10.3!C(Fig. 8a). Extremes in 2002 ranged from –40!C inwinter to as high as +26!C in summer, withoccasional mid-winter warming approaching 0!C.We compared these local measurements with thenearest grid-cell of the 2002 NCEP reanalysisdata (Kalnay et al. 1996) for this region, whichindicates a MAAT of –8.3!C. Such discrepanciesare typical for reanalysis data, which use a widevariety of sources to arrive at a globally griddeddata set with 2.5! (geographic) resolution cells torepresent all of the points within that large cell.However, the reanalysis data does capture thetrends in temperature reasonably well; thus whilecomparisons between NCEP and local data interms of absolute values may not be worthwhile,trend analysis of the multi-year NCEP data wouldlikely have value at the local level. Figure 8b plotsthe entire 1948–2002 NCEP record, with dailyaverages, minimums, and maximums for each dayof the year; average MAAT during this periodwas –10.4 ± 1.1!C. However, closer inspection ofthis long-term record (Fig. 9) shows that 11 of theof 15 warmest years on record have occurredsince 1989 and that the past 3 years (2000, 2001,and 2002) have seen MAATs greater than twostandard deviations (about 2!C) warmer than the

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long-term mean. It is unknown whether some ofthis trend may be accounted for by changes in theamount or quality of assimilation data throughtime used to create the gridded data here.

These warmer MAATs are explained by war-mer winters. Shown in Fig. 9 are negative, positive,and net degree-days. Positive degree days (dd)were calculated by summing daily average tem-peratures on all days in 2002 when the averagedaily air temperature was above 0!C, negative

degree days by summing negative daily tempera-tures, and net degree days by adding positive andnegative degree days together. The degree-daymethod is a useful way to quantitatively comparetemporal and spatial trends in temperature be-cause it does not depend on arbitrary determina-tions of which months or days are consideredwinter or summer (e.g., June–July–August is typ-ically considered ‘summer’ in many climatologies)and quantitatively describes the magnitude of

Fig. 8 (a) Comparison ofmean daily airtemperatures measuredlocally with airtemperatures from theNCEP reanalysis. Trendscompare well, thoughNCEP indicates slightlywarmer temperatures.(b) Mean daily average,daily minimum, and dailymaximum airtemperatures derivedfrom NCEP reanalysisdata 1948–2002. Actualtemperatures may havebeen slightly colder, asindicated in (a)

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winter coldness or summer warmness in a singlenumber. The degree-daymethod is also used in ourlake-ice models and is a simple proxy for estimat-ing lake ice growth and decay and comparisonsbetween regions. As can be seen, there has been nolong-term trend in positive degree-days, which hasbeen +739 ± 127 dd for the entire record. Prior to1989, there was no trend in negative degree-days,which had been –4648 ± 251 dd. Since 1989, how-ever, the 10 warmest winters on record have oc-curred, with values reaching –3549 dd, which ismore than four standard deviations higher than thepre-1989 mean. This change in negative degree-days is greater in magnitude than the total positivedegrees. The reason for the warmer negative de-gree-days seems to be related to a steadily declin-ing number of days below –30!C (there was notrend in numbers of days between –20 and –30!C)as shown in Fig. 9; that is, the winter’s are not asextreme and are becoming more moderate. Be-cause rootstocks are sensitive to the extremes ofcold weather, such a decrease in extremes mayallow less hardy rootstocks to survive in winters

here, perhaps facilitating a new vegetative suc-cession that has not been seen for at least 50 years.Thus, assuming the reanalysis data are reliable, thetrend of warmer temperatures and increasingvariability found throughout the Arctic since 1989(Chapman and Walsh 1993) seems to have a re-lated manifestation at Lake El’gygytgyn.

Precipitation

The El’gygytgyn crater has a dry environment,typical of the Arctic. An unshielded tippingbucket near the main weather station recorded70 mm of rainfall in 2002, all between mid-Mayand late-September when air temperatures werenear or above freezing. In summer of 2000, weobserved at least four separate snowfall events inAugust and September, and in 2002 the sonicranger showed transient snowfalls greater than5 cm beginning in mid-July and lasting the rest ofthe summer; it is therefore likely that some of thetipping bucket catch was snow that melted intothe bucket. This combined with the strong and

Fig. 9 Time-series of airtemperature data derivedfrom NCEP reanalysis.Note that warmer winters(negative degree-days)rather than warmersummers (positivedegree-days) seem to beresponsible for theincrease in mean annualair temperatures over thepast 15 years

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regular winds likely led to undercatch at the tip-ping bucket. End of winter snow accumulation in2002 was 0.40 m, as measured by the sonic rangeron the weather station; this value is no doubtsomewhat influenced by drifting caused by thestation, but our observations at end of winter in1998 indicate that 0.3–0.5 m snow depths aretypical here, as in much of the Arctic (Hinzmanet al. 2000). We did not measure snow density,but assuming that 0.27 g/ml is a reasonable andconservative value, as it is on the North Slope ofAlaska (Liston and Sturm 2002), end of wintersnow water equivalent is about 108 mm at theweather station. Thus the total precipitationestimate of 178 mm from end of summer 2001 toend of summer 2002 is likely conservative; thesevalues are slightly lower than those made on theNorth Slope of Alaska (Bowling et al. 2003; Listonand Sturm 2002), which may be reasonable con-sidering that weather patterns cause the tworegions to have quite different moisture sourceareas. In 2002, snow melt at the weather stationoccurred quickly once air temperatures ap-proached freezing, lasting less than 1 week (24–29May) tomelt the snow and expose the bare ground.

Our automated lake stage measurements alsoindicate that our 2002 precipitation measurementsare likely conservative. If we assume that theaverage end of winter snowpack across the 183 km2

crater watershed was 108 mm water equivalent,then we should expect a maximum of 179 mm risein lake level if no evaporation or sublimation oc-curred !183 km2 " 110 km2 # 108mm$. Duringsnowmelt in 2002, lake level rose by about200 mm according to a pressure transducer sub-merged with the thermistor string and correctedfor barometric pressure changes using theweather station’s barometer. This pressure recordgradually increased over the winter (beforebreakup and the 200 mm increase), by an amountthat corresponds to about 150 mm of water, pre-sumably due to snow loading of the ice. Thus thisstage record suggests that our snow measure-ments at the weather station (108 mm w.e.) arelower than the average for the crater watershedand further that there is little storage of snowmelton the land surface, as seems reasonable for fro-zen ground with no lakes. Because there is noevidence of glacial activity here (Glushkova

1993), it is likely that winter precipitation duringglacial times was substantially less than todayand/or that it melted completely during summer,as even a 50 mm annual accumulation would leadto sizable glaciers over 10,000 years. Evidence forextreme aridity across western Beringia duringfull glacial conditions is widespread (Brigham-Grette et al. 2004).

Wind

El’gygytgyn is also windy. Winds are dominantlyeither from the north or south (Fig. 5), with windspeeds exceeding 13.4 m s–1 (30 mph) everymonth in 2002 and exceeding 17.8 m s–1 (40 mph)in six of the months. In 2002, the mean hourlywind speed was 5.6 m s–1 (12.5 mph), the maxi-mum mean hourly wind speed was 15.6 m s–1

(34.9 mph), and the maximum wind speed re-corded was 21.0 m s–1 (47 mph). The strongestwinds are typically in winter, and there were onlya handful of calm days in 2002. While these windsare strong, they are not extreme. What is perhapsmost relevant to understanding the lake is thatthey are strong and persistent, and this likely playsan important role in controlling lake shape, asdiscussed previously.

Relevance to paleoclimate reconstructions

We now have a reasonable understanding of themodern environment of El’gygytgyn crater, andthis presumably should aid in our understandingof paleoclimate reconstructions, even if it is bypointing towards research gaps that still need tobe filled.

Currently the hydrology of the crater is char-acteristic of permafrost terrains found elsewherein the Arctic, but this was likely not always thecase. Peak discharges in the modern environmenttypically occur during snow melt over an imma-ture drainage system consisting largely of watertracks leading to gravel-bedded streams in thevalley bottoms. Cryoturbation and sedimentationhas obscured many clues as to the paleoenviron-ment, but the wide valleys along the crater-rimmay have been formed in warmer climates thatpre-date the initiation of glacial–interglacial

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cycles 2.6 million years ago and the wide-spreadformation of permafrost. At this time, the crater-lake was likely more than four times larger involume, and the center of the lake at that time isnow buried beneath more than 500 m ofsediments on what is now the western shore. Thatmore sediments exist here than in the deepestpart of the current lake-center suggests a highersedimentation rate and therefore an increasedtemporal resolution of cores taken from what isnow land. Further, a core taken from this locationwould presumably regularly switch between lake-deposited to river-deposited sediments, whichmay yield new or complementary information toa core taken from the current lake environment,perhaps even recovering vegetation in situ.

The lagoons found in the modern environmentcould have played an important role in the lake’sbiogeochemical cycling throughout time. Watertemperature in the shallow lagoons (< 1 m waterdepth typically) was often >8!C, substantiallywarmer than the lake itself (~3!C). Fish fry wereobserved in several lagoons, and algae and dia-toms are also present. Lagoon beds were typicallycomposed of fine sediments. Thus these lagoonsappear to act as settling and warming ponds, aswell as comparatively warm and safe havens forsmall creatures. In spring, when lagoons are full-est, the influx of warm water from the lagoons tothe lake likely plays a role in both moat formationaround the lake ice and the destruction of anythermal bars (Wetzel 2001) within the lake waterthat would otherwise prevent wide-scale mixingand warming of the lake. Then, throughout thesummer, the constant influx of warm water likelycontinues to help drive thermally-induced lake-water circulation by warming the lake marginwater up to 3.9!C (the density maximum) andforcing it to sink towards the deepest part of thelake. Because the processes that create these la-goons (initiated by wind-driven currents) havelikely been present for much of the lake’s history,care should be taken when interpreting sedimentcore proxies, as some of these proxies may havecharacteristics inherited from within the lagoonsinhabited by biota preferring higher temperaturesthen those inhabiting the lake itself.

Our air temperature, water temperature, andremote sensing data provide some clues as to lake

dynamics during glacial times. They suggest thatmixing of the lake-water would be greatly reducedduring glacial times when a permanent ice coverwas present (Melles et al. 2007), as even in themodern environment a strong stratification formsbeneath winter ice covers. However, moat for-mations and solar absorption through the snow-free ice may have led to low-volume convectioncell caused as sediments release heat to the waterinto winter, as appears to be occurring today be-neath the ice. Determining whether such densityflows could have reached the deepest part of thelake requires further modeling and field work, as itdepends sensitively on the balance between ther-mal conductivity, heat storage, and thermal dif-fusivity within the lake (i.e., the dynamics of‘‘thermal bars’’). However, that this warming be-gins well before the ice cover disappears (as seenin our data) suggests that a similar warming mayhave occurred to some depth during glacial sum-mers, and therefore caused an increase in biolog-ical productivity in the lake during that time.

Whether the entire water column, or just thewater–sediment interface, became anoxic in thepast remains an open question, as several pathwaysexist for oxygenation of ice-covered lakes, andevidence suggests that at least part of the El’gy-gytgyn water column remained oxygenated duringthe last 250,000 years. The lake is currentlyinhabited by several non-migrating salmonoidspecies: Salvelinus boganidae (length ~800 mm),Salvelinus elgyticus (~200 mm), and Salvethymussvetovidovi (~300 mm); these species are unique tothe lake (Bely and Chershnev 1993; Skopets 1992)and indicate that life and a reliable food chain havepersisted here through perhaps several glacialperiods despite the continuous ice cover andanoxicconditions near the bottom.There are at least threepossibilities to explain this. (1) Benthic demandwas insufficient to consume all of the dissolvedoxygen, perhaps due to the lack of mixing. Most ofthe oxygen in lake water is consumed within thesediments, not the water column (Wetzel 2001). Instill, thermally stratified water beneath an icecover, however, oxygen consumption within thesediments can effectively drop to zero, as a thinanoxic boundary layer develops between thesediments and the pelagic water column thatseverely reduces biological productivity within the

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sediments due to lack of oxygen (Ellis and Stefan1989); that is, the consumption of dissolved oxygenis partly a function of mechanical mixing andre-supply of dissolved oxygen to the benthos.(2) Photosynthesis in summer continued beneaththe ice in the uppermost water layers. This wouldrequire much of the lake to be free of snow inglacial summer (Ellis and Stefan 1989; Ellis et al.1991; Stefanovic and Stefan 2002). In May1998 wedirectly observed that large areas of icewere blownfree of snow on the north end of the lake (this wasconfirmed for several years using remote sensing(Nolan et al. 2003)), so the possibility of a com-pletely snow-free surface during much drier glacialconditions seems reasonable (Brigham-Gretteet al. 2007; Melles et al. 2007). (3) Oxygenresupply occurred during glacial episodes whensummer-time moats were open around the lake.The snow-free, dark-colored rocks and sedimentswould have heated in the Arctic summer sun evenin glacial times and tended to warm or melt ice incontact with it, leading tomoat formation.Moderndata from the permanently ice-covered lakes in theDry Valleys of Antarctica indicate a super-satu-ration of oxygen there, as streams carrying waterand oxygen into the lake via moats, but the watereventually sublimates via the lake ice leaving theoxygen behind (Wharton et al. 1986). These pos-sibilities make it difficult to explain the causes ofanoxic paleo-conditions based on modern-processdata alone.

Had lake levels varied substantially in the past,however, anoxic conditions might be easier toexplain due to lower water volumes and increasedbenthic productivity due to exposure to sunlighton the broad flat bottom, and any altered-chem-istry conditions easier to explain due to increasedconcentrations of solutes. Sublimation rates oflake-ice in the Dry Valleys have been measuredat 35 cm a–1 (Clow et al. 1988); in the absence ofany annual water inputs to the lake, such a sub-limation rate would completely dry up El’gy-gytgyn in only 500 years. Several core-proxieslead to interpretations of peak cold and dry con-ditions at Lake El’gygytgyn lasting much longerthan this (Melles et al. 2007), and even withan order of magnitude less net water loss(i.e., 3.5 cm a–1), it would only take 5000 years tocompletely dry up the lake. Decreased lake levels

would clearly explain the existence of the cobble-covered shallow shelves surrounding the lake asold shorelines and river deltas that have beenrecently submerged; lakes in the Dry Valleys arecurrently showing similar trends, as warmer con-ditions there are leading to increased inputs to thelake systems from glacier melt (Bomblies et al.2001). Many of these same Antarctic lakes showsigns of their former size in terms of waterchemistry—the water in the deeper pockets issubstantially different chemically and therefore indensity, preventing mixing with more recent freshwater that floats on top of it. At El’gygytgyn, wenoticed substantial differences in water chemistryin the bottom few meters of water. Slumping ofsediments into or within the lakes, as observedseismically (Niessen et al. 2007), might also beeasier to explain with a variable water-level, aswave-undercutting of a shallow lake and water-ing–dewatering processes on exposed slopesmight lead to the fairly steep side bathymetryleading from the current shelves to the flat bot-tom. Close inspection of the ice-bubble pattern in(Fig. 7) hints that the regions of brightest bubbleconcentrations at the margins may indicate areaswhere land has been recently claimed by risinglake levels. Thus the possibility of substantialvariations in lake levels, even to the point ofnearly complete loss of water, should not beoverlooked when interpreting core proxies overthe past 3.6 million years, though sediment coresfrom the lake dating back to 250 ka show noevidence of complete desiccation (Melles et al.2007).

In summary, in the 3.6 million years since LakeEl’gygytgyn was formed, substantial changes havelikely occurred to the crater’s physical hydrologyand limnology, and these changes will complicateinterpretations of changes in the local and re-gional climate, which are the ultimate goal of theproject. However, we now have a reasonableunderstanding of this physical setting and how itmay have changed over time. Thus while furtherresearch is necessary to understand the dynamicsof these physical changes, such research is rea-sonably straightforward and should continue toimprove the interpretations of sediments coresretrieved from this unique and interesting loca-tion in the Arctic.

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Acknowledgements We would like to thank the NationalScience Foundation for their initial support of this research(OPP Award 0075122 to MAN and OPP Award #96-15768,Atmospheric Sciences Award 99-05813, and OPP Award0002643 to JBG), as well as Kristin Scott Nolan and RogerGrette for several years of additional support. We wouldalso like to thank G.H. Apfelbaum for his help in the fieldand in reconnaissance missions prior to the field work. SARdata are copyright of RSI. All opinions and findingspresented in the paper are those of the authors and notnecessarily those of the National Science Foundation.

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