LUngsee: A geochemical history of meromixis

Rodney V. HarmsworthEnvironmental Sciences & Planning, VTN Consolidated Inc., 2301 Campus Drive, Irvine, CA 92713, U.S.A.

Keywords: geochemistry, lake sediments, meromixis

Abstract

Langsee lies in the southern Austrian alps and exhibits permanent meromixis. The lake sediment profileconsists of a layer of glacial clay, then gyttja and an upper layer of sapropel. The 7 m core was analyzed for 24metallic and 4 biophilic elements. The geochemical principles of isomorphism and atomic substitution wereused to explain the association of chemically dissimilar element pairs such as K, Ba and Mg, Ni. Highconcentrations of S early in the gyttja zone indicate that meromixis occurred early in the history of the lake.

Introduction

Langsee is of interest to limnologists because ofits meromictic character. Like several other Austri-an lakes, Langsee exhibits a strongly reduced mo-nimolimnion and associated chemical characteris-tics of high sulphide, ammonia and ferrous ions(Findenegg, 1935, 1947; Lbffler, 1973). Meromixishas been classified by Findenegg (1935) as staticwhen it is due to the geologically determined pres-ence of saline water in the monimolimnion or dy-namic when the ordinary processes of decomposi-tion and solution of the lake sediments lead to thecondition. Hutchinson(1957) classified meromicticlakes as ectogenic when due to external events in-troducing saline water or surficial freshwater, cre-nogenic when due to saline springs, and biogenic,which corresponds to Findenegg's dynamic type.

The stratigraphy of the Langsee sediments (Fig.1) shows the appearance of sapropel in the uppertwo metres of sediment. Frey (1955) in his classicwork on the history of Langsee regards the beginn-ing of the sapropel zone as the proximate start ofmeromixis. Frey also considers a triptogenic factor,which is the clearing of land for agriculture andsubsequent runoff of morainal clay, as the trigger

mechanism for the onset of meromixis. Further-more, the lake was already predisposed to mero-mixis because of topographic features and low windvelocity during the fall, thereby enhancing mero-mictic stability. Frey, therefore, started from a bas-ic assumption that meromixis originated concur-rently with the sapropel sediments.

Based upon the disappearance of the ostracods,decline of the chironomids, and appearance ofChaoborus, Lffler (1975) believes that Langseebecame meromictic by the beginning of the Prebo-real, which predates the sapropel sediments.

The geochemical observations in this paper pro-vide additional evidence in the search for the origin,causes and consequences of meromixis in Lingsee.

Methods

Samples of sediment were shipped from corescollected and described by Lffler (1975). Four orfive of the original samples taken at 5 cm intervalswere composited to obtain sufficient sediment forthe analyses. Each data point therefore representsan average over a considerable period of time.

Cation analyses were undertaken using atomic

Hydrobiologia 108, 219-231 (1984).© Dr W. Junk Publishers, The Hague. Printed in The Netherlands.

Results and discussion of the elements

The elements analyzed have been grouped ap-proximately in Goldschmidt's (1954) classificationaccording to their geochemical affinities, but manyelements exhibit some tendency to other groups.

1

2

3

4-I

SAPROPEL

CLAY-SAPROPEL

GYTTJA

CLAY-GYTTJA

5

- CLAY

6

Fig. 1. The stratigraphy and the sample loci.

absorption spectroscopy following complete diges-tion of the sample with HNO 3 and addition ofH2SO4 and HF when necessary.

Phosphorus was also analyzed on the digestateusing standard methods (APHA, 1975). Sulphurwas analyzed using a Fischer Scientific sulphuranalyzer with a high temperature furnace and anamperometric titration technique. Silica and alum-ina were measured using the ASTM methods(ASTM, 1981). Carbon was analyzed with a Beck-man carbon analyzer, and nitrogen was analyzedusing the Kjeldahl technique (APHA, 1975).

The siderophile elements: Fe, Co, Ni, Mo

Iron. Fe (Fig. 2) shows a decreasing pattern belowthe lithospheric average throughout the clay zone.In the sapropel zone iron increases above the li-thospheric average and then decreases in the upperlevels. The early minima of Fe and its general lowabundance until the sapropel zone must be due toinsufficient reduction and/ or acidity of the soils topromote the mobilization of ferrous ions. However,during the middle sapropel period, conditions musthave been more conducive to the transport of Fe.

Cobalt. The Co profile (Fig. 2) shows two markedmaxima, one in the mid-gyttja zone and the other inthe sapropel zone. A minimum occurs at the uppertransition zone. The Co maxima probably repres-ent periods of reduction in the alkalinity of the soilsand mobilization of Co ions. The stability field ofCo (Garrels & Christ, 1965) probably is such thateven a small decrease in alkalinity below pH 9 islikely to mobilize Co.

Nickel. The Ni profile (Fig. 2) is always below thelithospheric average (75 mg kg- ) and declines, likeMg, from a maximum in the early clay zone to a lowlevel at the beginning of the transition zone. Amaximum occurs early in the gyttja zone andanother later in the same zone. The profile thenlevels off at about half of the lithospheric averagethrough the sapropel zone. The profile follows Mgthrough the clay zone, probably as a result of itsisomorphous substitution in some Mg minerals andits deposition as hydrolysate sediments. The maxi-mum in the early gyttja zone may be due to precipi-tation as the sulphide.

Molybdenum. The Mo profile (Fig. 2) remains be-low its lithospheric average(l .5 mg kg') during theclay zone, then exhibits a maximum in the mid-gyt-tja zone and two subsequent maxima in the saprop-el zone. The Mo profile is difficult to interpret withany degree of precision. The maxima are probably

220

E

-C.a,

2

Ec o

221

Ej-8

1

2

3

4

5

6

Fig. 2. The siderophile elements Fe, Co, Ni and

due to the precipitation of MoS 2 under reducingconditions, but factors causing its mobilization inthe drainage basin are not readily apparent. Lind-say (1979) has shown that the stability of Mo min-erals in soils is complex. However, soil Mo appearsto be more soluble at higher pH and is unaffected byredox, although minerals controlling the solubilityof Mo may be effected by redox.

Tin. Sn was analyzed throughout the core, but wasat or below the limits of detection.

The chalcophile elements: Cu, Zn, Cd, Pb, As, Sb,Se

Copper. The Cu profile (Fig. 3) shows increasingCu levels in the upper clay and lower transitionzones. At the mid-point of the gyttja zone, Cureaches a minimum and increases again in the uppertransition to sapropel. Cu appears to be associated

Ao and the lithophile elements V, Cr, and Mn in mg kg.

more with the hydrolysate sediments and periods ofoxidation in the drainage basin than with otherpotential factors governing its distribution.

Zinc. The Zn profile (Fig. 3) shows high levels of Znthroughout the clay and early gyttja zones. A min-imum is reached in the mid-gyttja zone followed bya maximum in the transition to sapropel and a lowlevel throughout the sapropel zone. During the clayzone, Zn was weathered then deposited along withother hydrolysate elements in the clay and earlygyttja zone. The maxima at the upper transition tosapropel is a result of an increase in the hydrolysatesediments.

Cadmium. The Cd profile (Fig. 3) shows a constantlevel in the clay zone well above the lithosphericaverage, followed by a maximum in the gyttja. Inthe upper transition zone, there is a minimum fol-lowed by higher levels in the sapropel zone. Because

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E

0.4)M

1

2

3

4

5

6

Fig. 3. The chalcophile elements C

of the similarity in size between Cd2+ (97 pm) andMn 2+ (80 pm), manganiferous minerals may con-trol the solubility of Cd. Cd is probably depositedas the highly insoluble sulphide greenockite.

Lead. The Pb profile (Fig. 3) shows minor maximaabove and below a minimum in the mid-gyttja zone.The most outstanding feature of the profile is apronounced maximum late in the sapropel zone.The two minor maxima may be the result of deposi-tion with the hydrolysates. The major maximum inthe upper sapropel zone has no explanation at thistime.

Arsenic. The As profile (Fig. 3) shows two pro-nounced maxima, one in the lower gyttja zone andone in the lower sapropel zone. The As profile iswell above the lithospheric average throughout, in-dicating the presence of As minerals in the drainagebasin. The lower maximum is probably the result of

u, Zn, Cd, Pb, As, Sb and Se in mg kg.

mobilization under oxidizing conditions and depo-sition in a reducing environment. The upper maxi-mum may be the result of mobilization with Fe andsubsequent deposition with Fe (OH)3 .

Antimony. The Sb profile(Fig. 3) shows concentra-tions below the lithospheric average during the clayzone, then a maximum in the gyttja zone and twoothers in the sapropel zone. Although not much isknown about Sb in the weathering cycle, the profileappears to reflect deposition in the reducing envir-onments, as Sb2S3 is very insoluble.

Selenium. The profile (Fig. 3) shows concentrationsof Se above the lithospheric average throughout thecore and a singular maximum to 34 mg kg' at theupper level of the clay zone. Because the stabilityfield for Se shows that very high oxidation poten-tials are required to mobilize selenates and selenites(Coleman & Deleveaux, 1957), strongly oxidizing

223

conditions may have prevailed during the Se max-imum.

The lithophile elements: Na, K, Mg, Ca, Sr, Ba, Ti,V, Cr, Mn, Al, Si

Sodium. The Na profile (Fig. 4) starts well belowthe lithospheric average of 28 300 mg kg-l in theclay zone, fluctuates, then decreases to a very lowamount at the start of the gyttja zone. Throughoutthe remainder of the core, Na remains low. Na isvery soluble and goes into ionic solution duringweathering. The presence of Na, therefore, in theclay zone, represents mechanically abraded rock-flour, which had not been chemically weathered.The period of low Na indicates an ameliorated cli-matic condition with little mechanical erosion.

Potassium. The K profile (Fig. 4) shows three max-ima. The first and greatest maximum lies in the clay

zone, followed by smaller maxima in the lowergyttja zone and in the upper transition zone. Themaxima are interpreted as periods of weatheringand deposition of hydrolysate sediments. The pro-file is at all times well below the lithospheric aver-age, which may be due to the greater availability ofthe more soluble biotite than the more insolublepotash feldspars.

Magnesium. The Mg profile (Fig. 4) stays above thelithospheric average of 20 900 mg kg - until thegyttja zone, where it falls to below half the averagethroughout the remainder of the core. The high Mgcontent of the clay and transition zone to gyttjaprobably represents the input of the hydrolysatefraction of Mg. The remainder of the core wassubject to less severe climatic conditions, and con-sequently the Mg would be in soluble form.

Calcium. The Ca profile (Fig. 4) approximates the

Fig. 4. The lithophile elements Na, K, Ba, Sr, Mg and Ca in mg kg.

224

lithospheric average of 36 300 mg kg- I through theearly part of the core until the upper transition tosapropel. In the sapropel zone, Ca increases to amaxima of 350 000 mg kg-'. The large amount ofCa in the profile, no doubt, represents photosyn-thetically induced carbonate deposition.

Strontium. The Sr profile (Fig. 4) remains below itslithospheric average of 375 mg kg' throughout thecore. The levels of Sr in the clay zone are the lowest,then increase in the lower transition and early gyttjazones. The Sr level decreases in the upper transitionzone before increasing again as the sapropel zone.The general increase in Sr in the sapropel zone isprobably related to coprecipitation with calcium,but the causes of the deposition in the gyttja zoneare less clear.

Barium. The Ba profile (Fig. 4) shows two majormaxima, one in the clay zone and the other in the

E o- o 0

1

2

3

4

5

6

upper transition to sapropel. These maxima and thetwo minor maxima in the gyttja zone and uppersapropel zone are coincident with similar maximain the potassium profile and quite unlike the pro-files for the other alkaline earths. The Ba maxima,therefore, represent periods of increased depositionof K and aluminosilicate minerals.

Titanium. Ti, with a lithospheric average of4 400 mg kg-', is present at low levels throughoutthe core, with its maximum of 670 mg kg' low inthe gyttja zone (Fig. 5).

Vanadium. The V profile (Fig. 2) shows two distinctmaxima at the extremes of the gyttja zone and lowconcentrations (<7 mgkg-') throughout the re-mainder of the core. The initial maximum is inter-preted as a period when the soil was in the oxidizedstate, with an Eh > 0.2 V and pH of approximately8 so that the metavanadate ions (V401 2 4) would be

o0

Fig. 5. The lithophile elements Si, Al and Ti in mg kg and the AI:Ti ratio.

225

mobilized (Garrels & Christ, 1965). With a decreasein soil oxidation potential during the gyttja zone, Vwould be present as insoluble oxides and immobile,which would account for the minima. The soil con-ditions must have then become more oxidizingthereby releasing V. In the lake, precipitation intothe sediments would have occurred as the insolubledioxide or as sulphides.

Chromium. The chromium profile (Fig. 2) shows acontinuous decline from a maximum in the earlyclay zone through the gyttja zone. A maximumoccurs in the upper transition zone, and a low leveloccurs throughout the sapropel zone. The deposi-tion of Cr in the clay and upper transition zones isexpected because of its strong presence in the hy-drolysate sediments. The Cr decline in the gyttjazone reflects a more stable drainage basin.

Manganese. The Mn profile (Fig. 2) remains belowthe lithospheric average of 950 mg kg-' throughoutmost of the clay zone and into the gyttja zone.During the mid-gyttja zone, a maximum is reachedwhich is followed by a minimum. Another maxi-mum occurs in the sapropel zone, after which theprofile declines in the upper part of the sapropel.Evidently, the conditions in the soils become reduc-ing and perhaps sufficiently acidic to mobilize theMn but not the Fe. The pink mineral rhodochrosite(MnCO 3) also occurs in oxidation zones and may,in fact, occur in Langsee. Frey (pers. commun.) hasnoted pink layers in the inshore sediments, whichcould possibly be due to the precipitation of Mnunder the oxidizing conditions found in the inshorearea of the lake.

Silicon. The Si profile (Fig. 5) shows maximumconcentrations during the clay and lower gyttjazones, then a sharp decrease during the mid-gyttjazone. In the upper gyttja and transition zone tosapropel, the silicon profile increases again beforedecreasing in the sapropel zone. Si provides anindication of the rate of erosion of the parent rocksbecause of its predominance in the lithosphere. Theminimum in the gyttja zone must represent in-creased stability in the drainage basin and reducederosion, while the reduction the sapropel zone maybe a result of dilution by calcium carbonate.

Aluminum. The Al profile (Fig. 5) closely parellels

that of silicon, with maxima in the clay zone andupper transition to sapropel. A distinct minimumoccurs in the mid-gyttja zone, and very low levelsoccur throughout the sapropel zone. Like Si, Alalso provides an indication of the rates of erosionand deposition of hydrolysate sediments.

The biophile elements: C, N, P, S

Carbon. The carbon profile (Fig. 6) shows very lowlevels until the gyttja zone, where it increases to amaximum before declining to a steady levelthroughout the sapropel zone. The carbon repres-ents both autochthonous and allochthonous mate-rial, reflecting the productivity of both the lake andits drainage system. The maximum in the carbonprofile is coincident with the transition to sapropel,and is probably the result of forest clearance. Thelower concentrations of C in the sapropel are prob-ably a result of dilution, of approximately 4:1 bythe precipitated calcium carbonate.

Nitrogen. The nitrogen profile (Fig. 6) shows asteep increase in the gyttja zone and a stronglyfluctuating profile throughout the sapropel zone.The maximum concentration of 3 600 mgkg I

reached early in the gyttja, is below the sedimentarylevels of 8 000 mg kg- in the unproductive Butter-mere, and of 12000 mgkg-' in the productiveEsthwaite (Mackereth, 1966).

Phosphorus. The P profile(Fig. 6) shows a series oferratic fluctuations. After the mid-gyttja zone, theincreases and decreases of P tend to be synchronouswith those of Fe, which may be controlling its solu-bility in the drainage basin.

Sulphur. The S profile(Fig. 6) remains at about thelithospheric average throughout the clay zone. Atthe transition to the gyttja zone, S begins to in-crease, reaching a maximum of 36 000 mg kg-' inthe gyttja zone, indicating strong mobilization of Sin the drainage basin and deposition in the lake. Aminimum occurs in the upper part of the gyttjazone, followed by fluctuations in the sapropel zone.The increase in S in the gyttja zone represents in-creased geochemical mobilization of S as well as anincreased input from biological sources in thedrainage basin. The lake environment must havebeen anaerobic in the bottom layers to deposit S sogreatly in excess of its ratio to organic matter.

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E

JZ

1

2

3

4

5

6

Fig. 6. The biophile elei

Discussion

Major changes in geochemical processes resultedin the deposition of glacial clays largely throughexogenous forces, while the sapropel resulted large-ly from endogenous forces, with the gyttja beingsomewhat intermediate.

The clay zone

Analyses of the clay zone illustrate several impor-tant geochemical separations that occur as a resultof weathering of the parent rock. Si and Al occur atabout their lithospheric averages, as would be ex-pected from the breakdown and reformation ofaluminosilicates, while the hydrolysate elementsMg, Ni and Zn all show profiles above the lithos-pheric average. The oxidate elements Fe and Mnshow declining profiles upward through the clayzone, while the resistate element, Ti, remains at atenth of its lithospheric average.

nents C, S, N and P in mg kg.

Correns(1937) has noted that Ti is located main-ly in the non-colloidal fraction of oceanic sedi-ments. Al, on the other hand, is associated with theformation of colloidal particles. Hence, the ratio ofAl:Ti should provide an indicator of conditionsfavoring the formation of colloids during periods ofweathering. Figure 5 shows the Al:Ti ratio, whichexhibits a maximum in the mid-clay zone and adecline in the upper clay zone. The implications forLangsee are that the early clay sediments were de-rived primarily from more erosive and mechanical-ly induced sedimentation, whereas the mid clayzone was formed by the reformation of aluminosili-cate minerals from ionic species. The K data, and inparticular, the maxima concurrent with maxima ofthe Al:Ti ratio, provide supporting data, because Kis strongly adsorbed by aluminosilicates, formingthe secondary mineral illite (KA14Si 7AI020[OH]4 ).

The presence of Na in the clay zone and its virtualabsence throughout the remainder of the core isalso indicative of strongly erosive conditions. Be-

227

cause Na is soluble and is not adsorbed so stronglyas K, it tends to remain in the solute phase andconcentrates in the ocean. On the other hand, K,which is strongly adsorbed by aluminosilicates,tends to remain with the hydrolysate sediments.This geochemical separation is a useful indicatorseparating periods of mechanical weathering.

Mackereth (1966) deduced from the K and Nacontent of sediments from lakes in the English LakeDistrict, that intense erosion prevents soil forma-tion and removes minerals locked in the clay miner-al lattice from the biosphere, thus reducing produc-tivity. In addition, the strong adsorbtive capacity ofclay minerals would remove other ions from solu-tion, which, combined with other factors such asturbidity and climate, would militate against a pro-ductive situation. Mackereth then supposes thatperiods of less intense erosion and more intenseleaching from developed soils are more conductiveto higher productivity. Comparing the Na and Kvalues of Langsee with those of Esthwaite (produc-tive) and Ennerdale (unproductive), the values inLangsee are far lower throughout the entire sedi-mentary column, indicating, perhaps, a predisposi-tion to a more highly productive situation in Lang-see. Moreover, the Na and K values from Cowgill &Hutchinson (1970) for Lago di Monterosi, classi-fied as meso- to eutrophic by Stella & Margaritora(1970), are far higher than for Ldingsee. The lowerrates of mechanical erosion in Langsee than in theother lakes imply a predisposition to eutrophy fromthe very inception of the lake.

Analysis of the biophilic elements show low lev-els of C and S, while P is increasing and N(600-1 100 mg kg-') is well above the lithosphericaverage of 20 mg kg-'. Similarly, high levels of Nwere found in the glacial clays of the English LakeDistrict by Mackereth. Clearly, the N is of secon-dary origin because of the high values compared tothe lithospheric average and the low C values. If thesource of N is not from the lithosphere or the bio-sphere, then by elimination, one must suppose anatmospheric origin. During electrical storms, smallquantities of atmospheric N are converted to NH 3,NO 3 and other species. However, all the N fromatmospheric sources is in a soluble form and there-fore will contain NH4+ and NO 3 ions, which maythen become adsorbed by clay minerals. In the claymineral structures the replacement of Si4 + by Al3+

in the tetrahedra or A 3P+ by Mg 2+ or Fe2 + in the

octahedra results in a permanent negative chargeon the clay mineral. These negative charges on clayparticles may be neutralized by positive ions such asNH4+ in the adsorption process. However, the ad-sorptive capacity depends on the type of clay min-eral. The cation exchange capacity of the tetrahed-ral kaolinite and illite is poor because of strongionic bonding, which makes the isomorphous sub-stitution of cations difficult. The exchange capacityof the octahedral smectites and vermiculites is fargreater, although the bonding is weaker. Inspectionof the N and K profiles in the clay zone shows anincreasing N concentration at the upper level whileK decreases. Although far from conclusive, thismay suggest more significant levels of N adsorptionwhen the K dominated illite decreased.

The P profile in the clay zone starts at approxi-mately half the lithospheric average and increasesto the average. The implications for the incipientproductivity levels of the lake are not clear, becausea part of the phosphate of most rocks is soluble,where as the predominant phosphate mineral, flu-orapatite, has a low solubility. One could assumethat a low sedimentary level of P would imply highdissolved P and, therefore, high productivity poten-tial. One could also imply high sedimentary P andhigh productivity. In comparison with the entirecores from several lakes of the English Lake Dis-trict Lake and Lago di Monterosi, Langsee's levelsof P are somewhat lower.

Mg, Cr, Ni and Zn are all deposited in the hy-drolysate sediments at levels above those found inthe remainder of the core. Because these metalsoccur in different groups of the periodic table andmoreover because Zn is chalcophile, Ni is sidero-phile, while Mg and Cr are lithophile, it is notimmediately apparent why these metals separateand deposit together in the hydrolysates. Inspectionof their atomic radii (Mg2 + 66 pm, Cr 3+ 63 pm,Ni2+ 67 pm, Zn2+ 74 pm) shows a close enough sim-ilarity to allow for isomorphous substitution in theparent rock during its formation and, therefore,their concurrent deposition in the hydrolysates.

Similarly, Ba (Ba 2 + 134 pm) follows K (K+

133 pm), showing a maximum in the mid-section ofthe clay zone. One other observation of importanceis the maximum in the Se profile. Se, althoughgenerally found with S, separates from it geochemi-cally because of their relative mobilities under dif-ferent oxidizing strengths. S is readily oxidized to

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SO -- at relatively low oxidation potentials (Garrels& Christ, 1965), and may be expected to be re-moved first, while Se is mobilized only under highoxidation potentials(Coleman& Deleveaux, 1957).The Se maximum, in the upper portions of the clayzone, indicates therefore that a strongly oxidizingcondition must have existed in the drainage basin.During the late stages of clay deposition, a lesseroxidation potential must have existed in bottomwaters of LAngsee, which lead to the deposition ofSe.

The gyttja zone

The gyttja zone is delineated by a lower transi-tional period from the clay zone and an upper tran-sitional period to the sapropel zone. A number ofcomplex geochemical events occurred in the gyttjazone, which provide clues to both the evolution ofthe drainage basin and the lake chemistry. Twoevents must occur for an element to be deposited inthe sediments, first mobilization and transporta-tion in the drainage basin followed by deposition inthe lake.

The major events that occurred during the gyttjazone are reflected in the biophilic elements. Carbonincreases steadily throughout the zone until it isover 40% of the sediment. In comparison with otherlakes, the organic matter is toward the higher endwhen compared with the unproductive Ennerdale(Mackereth, 1966), which has <10% carbon as alsodoes Petenxil (Cowgill & Hutchinson, 1966). Esth-waite, which is productive, has approximately15%C in its post-glacial sediments (Mackereth,1966), as does Lago di Monterosi (Cowgill & Hut-chinson, 1967). More comparable to Langsee is theeutrophic Blelham Tarn in the English Lake Dis-trict, which has a similarly high organic mattercontent in the 40% range for its post-glacial sedi-ments (Harmsworth, 1968). The N and S contentsof the sediments also increase in the gyttja zone,which in combination with the carbon suggest anincrease in the productivity of the lake. P on theother hand fluctuates erratically.

The S content of the sediments greatly exceedsthe amount that can be ascribed to the deposition ororganically bound forms. Hutchinson (1975) hasnoted that the range of S from various species ofmacrophytes is 1300 to 7 100 mg kg'. Mackereth(1966) gives a figure of 12 000 mg kg I of S for

Anabaena found in Lake Windermere, while otheralgae contained about 10000 mg kg', and thehighly siliceous Asterionella contained 6 000 mgkg -'. With the exception of 12 000 mg kg I of S inthe lowest section of the gyttja, concentrations el-sewhere in the gyttja greatly exceeded 12 000 mgkg . These data demonstrate the deposition of in-organic forms of S, most likely as metallic sul-phides, in a reducing environment. The possibilityof deposition as sulphate is unlikely because of thehigh solubility of the most commonly occurringsulphates. Even naturally occurring gypsum(CaSO 4, 2H 20) has a solubility of 2 410 mg kg- incold water, which is far greater than that of thecarbonate. The hypolimnetic waters, must, there-fore, have been strongly anaerobic, throughout thegyttja zone, at least on a seasonal, if not, permanentbasis.

Other changes that occur at the inception of thegyttja zone includes peaks in V and As. Both theseelements are mobilized under oxidizing conditionsforming vanadates and arsenates, and both arereadily precipitated as sulphides under reducingconditions. It would appear, therefore, that thehighly oxidizing conditions in the drainage basinthat mobilized Se persisted until the lower levels ofthe gyttja. The hypolimnetic waters must have be-come reduced, leading first to the precipitation ofSe, then V, As, and S.

By the mid-gyttja zone, a maximum above thelithospheric average, occurred in the Mn profile.Manganese occurs primarily as insoluble pyrolusite(MnO 2), manganite (MnOOH), and hausmannite(Mn3 04 ) under oxidizing conditions. However,during reduction, soluble bivalent ions are formed.The soluble species are transported in the groundwater to the lake, where oxidizing conditions occurin the epilimnion. The Mn2 + ions then form therelatively insoluble rhodochrosite if there is suffi-cient CO2 and under more highly oxidizing condi-tions, manganite and pyrolusite. Frey (pers. com-mun.) has observed pinkish varved calcareoussediments in the inshore zone, which could be dueto the precipitation of rhodochrosite.

In addition, hydrated manganese dioxide mayform in the oxidized surface waters and may subse-quently be precipitated. Hydrated manganese diox-ide may exist in colloidal form, which carries anegative charge, and consequently will adsorbheavy metal cations and coprecipitate with them.

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Concurrent with the Mn maximum are pronouncedminima in Al, Si, Zn, and K, which taken togetherindicate an environment more conductive to leach-ing rather than erosion. More strongly reducedconditions in the drainage basin may well haveprevailed during the mid-gyttja zone.

After the Mn maximum, a series of events oc-curred indicating a changed depositional environ-ment in which Mn, Mo, Cd, and Co decreased whileSi, Al, Zn, K, Cr, Cu, Pb, Ni, and V all increased.The increase in the resistates Al and Si and thepresence of K are indicative of a more stronglyerosional period. The Al:Ti ratio reaches a maxi-mum as does Cr, which indicate the greater abun-dance of hydrolysate sediments. The increase in Vat this time must mean a higher oxidation potentialin the drainage basin. The decrease in Mn, which isimmobilized at higher oxidation potentials, is con-sistent with the mobilization of V.

The sapropel zone

At the end of the gyttja zone, there is a transitionof clay and sapropel layers to sapropel. The abruptchange in the depositional environment coincideswith several changes in the pollen spectra. Frey(1955) deduced the development of agriculturefrom increases in cereal and agriculturally relatedspecies concurrent with forest clearance. The pres-ence of clay in the transition zone, is consistent,with the sharp increase in Si and Al, and, in theAl:Ti ratio. The onset of the sapropel zone ismarked not only by the changes related to the in-creased sedimentation but also by a major increasein Ca. The Ca, when calculated as CaCO3, com-prises 80% of the dry weight of the sediment. Theprecipitation of CaCO3 is physically as well as pho-tosynthetically induced according to Von Berger(1973). Strontium is also precipitated with Ca be-cause of its correspondingly low solubility (stron-tanite 1.0 mg 0. 1 l, calcite 1.4 mg 0. I ', and arag-onite 1.5 mg 0.1 1 l), while the more soluble Ba andMg carbonates (magnesite 106 mg 0.1 1', hydro-magnesite4.0 mg0.I 1 l,andwitherite2.2 mg0.I 1')remain in solution.

Other changes include lower levels of Si, Al, andC. The decline in the major elements is probably theresult of dilution by CaCO3 rather than a real de-cline in input, thus implying more rapid sedimenta-tion rates during the sapropel zone by as much as

4:1 over the gyttja. The other major changes in thesapropel zone are the increased deposition of Feand Mn, which must be the result of a decrease inthe oxidation potential of the watershed and in-creased leaching of the reduced ions. Deposition ofthe Fe and Mn probably occurred as FeS and MnSin the highly reduced sapropel.

Half way into the sapropel, the Fe and Mn pro-files abruptly decline. Initially the Mn increase isconcurrent with the Fe increase, and they then fluc-tuate synchronously. The Mn profile remains abovethe lithospheric average for a longer interval thanFe, which may be explained by the fact that Ferequires a lower oxidation potential for mobiliza-tion. The drainage basin, therefore, became pro-gressively more reduced during the sapropel zone,then reoxidized toward the present day. Shortlyafter the reoxidation period marked by the succes-sive declines in Fe and Mn, there was a period ofslightly increased erosion marked by an increase inthe Al:Ti ratio and small increases in Al and Ba.This period correlates with the increase in non-ar-boreal pollen shown in Frey's (1955) work, and isrelated to agricultural practices.

The origin of meromixis in Ldngsee

Hutchinson (1957) has described three origins ofmeromixis: ectogenic and crenogenic, which do notapply in the case of Lingsee, and biogenic. Biogenicmeromixis appears to originate as a result of climat-ic, topographic, and morphometric factors. Thesefactors may contribute to reduced autumnal circu-lation, which may subsequently be reinforced bylimnochemical processes, leading to higher dis-solved solids and thus greater stability in the mo-nimolimnion. Findenegg(1935) noted the protect-ed character of Lingsee and the low wind velocityduring the autumn as contributory factors to themeromictic condition.

Because Langsee was 6 m deeper at the inceptionof the clay zone, it must have had a greater predis-position to meromixis in the past based solely onthe morphometric factor. However, the climate wasprobably subpolar during the clay zone, resultingperhaps in the cold monomictic lake type. The bio-logical evidence of an abundant Eutanytarsus andostracod fauna (L6ffler, 1973) are consistent withthe cold monomictic lake type or at least high dis-solved oxygen concentrations in the deep waters.

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Moreover, the geochemical evidence shows littlecarbon and an oxidized drainage basin.

The increase in sulphur, early in the gyttja zone,provides direct evidence for the early onset of anae-robiosis. The conclusion concerning the onset ofanaerobiosis is consistent with the increases in nit-rogen and organic carbon. In addition to the geo-chemical evidence, Lffler (1975) has shown thedisappearance of Candona, Cytherissa lacustrisand Limnocythere sanctipatricii, at about the sametime as the increase in the sulphur.

On the basis of the lighter coloured gyttja, itcould be argued, that anaerobiosis was seasonal,and that permanent meromixis did not occur untilthe black sapropelic sediments. However, black sa-propelic sediments occur in many productive dim-ictic lakes with blue-green algae blooms, and arenot restricted to meromictic lakes. At the start ofthe transition to sapropel, the sulphur and nitrogenlevels declined, although carbon continued to in-crease. The decrease in sulphur to 2 800 mg kg- cannot be ascribed to dilution by clay, sapropel orcalcium, although they all increased in the transi-tion to sapropel. The decrease in sulphur probablyrepresents a true decrease in sulphide precipitation.The higher levels of erosion as evidenced by theincrease in aluminum and silicon suggest lowerproductivity. The increase in organic carbon, con-current with the increased erosion, was probablydue to the forest clearance at that time. The thinclay layers in the transition to sapropel may repres-ent temporary periods with an aerobic hypolim-nion.

The sulphur concentration increased again in thesapropel showing a return to the permanently re-duced monimolimnion.

The occurrence of sapropel may not be related tomeromixis. The onset of agriculture suggests a sub-sequent increase in nutrients, followed perhaps bychanges in the algae community, and calcium car-bonate deposition.

The history of meromixis in Lingsee is morecomplex than originally envisioned. The lake mayhave been meromictic even before the gyttja zone,although there is no unequivocal evidence for thispossibility. The rapid onset of meromixis in theearly gyttja zone suggests climate, topography andmorphometry as the causal factors rather than thebiogenic concentration of solutes.

Acknowledgements

The work was undertaken in the analytical la-boratories of Camp, Dresser and McKee, Inc.,while the author was president of the Sciences Div-ision, where R. L. Wen and W. Gilgren undertookmuch of the analysis and Karen Little the typing.

The author is also grateful to D. G. Frey for hisencouragement and library materials and to H.Loffler for suggesting the project and supplying thesediments.

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Received I October 1982; in revised form I March 1983;accepted 10 May 1983.