Pyrite and Vivianite Intervals in the Bottom Sediments of Eutrophic

s93

The Canadi an M iner alo g ist Vol. 37, pp 593-601(1999)

PYRITEAND VIVIANITE INTERVALS IN THEBOTTOM SEDIMENTS OF EUTROPHICBAPTISTE LAKE. ALBERTA. CANADA

PHILIP G. MANNING

National Water Research Institute, 867 Lakeshore Road, Burlin1ton, Ontario L7R 4A6, Canada

ELLIE E. PREPAS' AND MARK S. SEREDIAK

Department of Biological Sciencesand Meanook Biological Research Station, Unive rs ity of Albe rta, Edmonton, Alberta T6G 2E9, Canada

ABSTRACT

The highly eutrophic statusofBaptiste Lake, Alberta, may be related to alteredgroundwater flow resulting from land-clearing activities since colonial settlement approximately 80 yr B.P. Sections from the top 42 cm of sediment cores from Baptiste Lake, which represent 150 yr B.P. to the present, were analyzed for forms and concentrations of iron and phosphorus The 3042 cm interval (90-150 yr B.P.) is marked by intensive formation of vivianite; concentrationsofpotentially bioavailable orthophosphate ion in some sections exceed I wtTo.This probably reflects a period when inflowing groundwater contained high concentrations of iron and phosphorus, which precipitated at the sediment surface under oxic conditions that likely prevailed at the time. In contrast, the top 25 cm (80 yr B.P to the present) show sedimentary formation of pyrite consistent with anoxic conditions at the sediment-water interface, the deposition of organically enriched sediments, and severe eutrophication. The waters of Baptiste Lake have become more eutrophic since colonial settlement.

Keywords: pyrite, vivianite, lake sediments,phosphorus, groundwater, trophic level, Baptiste Lake, Alberta

Soulrarnn

Le statut fortement eutrophique du lac Baptiste, en Alberta, pourrait Otrela cons6quenced'un rdseaumodifi6 de flux d'eau souterraine 1i6 au d6frichement des terres suite i la colonisation, il y a environ quafte-vingt ans. Les 6chantillons prdlev6s des 42 cm de carottes de la partie sup6rieurede la section s6dimentaireau lac Baptiste, intervalle repr6sentantcent-cinquante ans de s6dimentation,ont 6t6 analys6safin de connaitre la sp6ciation et les concentrationsdu fer et du phosphore. L'interualle de 30 i 42 cm (90-150 ans avant aujoud'hui) se distingue par la formation massive de vivianite; les concenffations des ions orthophosphate potentiellement biodisponibles dans certains 6chantillons ddpassent7Vo par pords.Cet enrichissementt6moignerait d'une p6riode oi les nappes d'eau souterraines qui nourissaient le lac avaient des teneurs 6lev6es de fer et de phosphore, qui ont pr6cipit6 d la surface du s6diment sous conditions oxyques pr6conis6esd cette p6riode. En revanche, sur les 25 cm au haut de la colonne stratigraphique (80 ans jusqu'd pr6sent), il y a eu formation sddimentaire de pyrite, ce qui implique des conditions anoxyquesd I'interface s6diment--eau,la d6position de s6dimentsenrichis en matibre organique, et une eutrophication s6vbre Les eaux du lac Baptiste sont devenuesplus fortement eutrophiques depuis l,dre coloniale.

(Traduit par la R6daction)

Mots-clds: pyrite, vivianite, s6dimentslacustres, phosphore, eau souteffaine, niveau trophique, lac Baptiste, Alberta.

INrnooucrrox deposit. The watershed,310 kmz in area,is 587oforest, l6Vo agricultwal land, 25Vooflakes, bogs and marshes, Baptiste Lake, Alberta, is a naturally eutrophic lake and 7Vo cottage shoreline development (Mitchell & (Hickman et al. 1.990,Mitchell & Prepas 1990) 10 km2 Prepas 1990). The watershedwas basically undisturbed in surfacearea, situated 165 km northwest of Edmonton, prior to the first large influx of settlersin 1909 to 7914. Alberta. The lake lies in a glacial meltwater channel. Baptiste Lake has two basins, the south basin being The underlying bedrock is marine shale of the La Biche the deeper,joined by a relatively shallow neck (Fig. 1). Formation. Clayey glacial till is the dominant surficial Both basins are thermally stratified in summer and in

I E-mail address: [email protected]

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winter under ice cover. However, mixing is incomplete We have collected sediment cores from the surface in spring and autumn in the south basin (Fig. 2), but is of the bottom sedimentsto a depth of 42 cm, which rep- incomplete in the north basin in the spring only. The resents approximately 150 years of deposition (Tumer hypolimnia are anoxic for much of the year, at which 1995). We determinethe main forms of phosphor-usand time phosphorusis massively releasedfrom the bottom iron in the top 42 centimeters of bottom sediment and sediments(Fig.2). This releasestimulates a blue-green relate them to the bioavailability of phosphorus and algal bloom on overturn. The main contributors to wa- trophic status of the lake. Our main aim is to use the ter inflow are: streams 67%, precrpitation l8%o, and sediment cores to track changesin the trophic level of groundwaterl27o (Crowe & Schwarz 1981, Shaw & the lake since before colonial settlement. Prepas 1989). The pattern of groundwater flow is un- usual in that flow into the lake bottom increaseswith ExpeR[4exrAr Dnrans distancefrom shore (Shaw & Prepas1989). This is consistent with an offshore hydrological connection be- Gravity cores of bottom sediment were collected tween the underlying aquifer and the bottom sediments. from the deepersouth basin in May 1991 and from both Prior to 1000years B.P., Baptiste Lake was probably basins in February 1992.The dark grey silty clay cores meromictic (Hickman et al. l99O). The vertical distribu- were sectioned into 1-cm slices down to a depth of 30 tions of pollen and pyrite spherules in the sediment sug- cm and into 2-cm slices thereafter. One core from the 210Pb gest that the lake has become more eutrophic with time south basin was collected and sectionedfor chro- and that the intensity of anaerobic conditions in the bot- nology (Turner 1995) using the method of Appleby & tom watershas been variable (Hickman et al. 1990).Py- Oldfield (1978). The method is based on the assump- rite is formed in reducing sediments intermittently tion of a variable rate of sedimentation.Discontinuities exposedto oxygen (Berner 1984).Human disturbancein in sediment porosity (as a function of sediment depth) the watershedhas increasedsince the earlv 1900s. indicated a variable rate of sedimentation.All sections

Frc. 1. Diagram depicting the deep hole and main sampling location in south basin of Baptiste Lake. Hatched areas representvarious developments.e 9.. cottages and campgrounds. Contour intervals are in meters.

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were frozen immediately on retrieval and freeze-dried programsof Stone (1967). Values of chi-squaredand within ten days. the visual examination of the spectrawere used as crite- Room-temperature Mrissbauer spectra were recorded ria of goodness of fit. The Mcissbauerspectra of two on a 5l2-channel spectrometer wrd analyzedusing the sections,24 cmand34-36 cm, alsowere recorded at 4 K to quantify poorly crystallized magnetically ordered oxides of iron (Murad 1988). Concentrationsof total iron, Al, Mn, and Ca in the dried sedimentswere measuredby, sequentially, diges- tion in aqua regia, evaporation to dryness, dissolution of the residuein dilute HCl, and atomic absorptionspec- ffometry using suitable standardsand controls. Concen- i trations of Fe2+,Fe3*, and FeS2 were then determined F L from arearatios in the Mcissbauerspectra and total iron u lq o- values, assuming equal recoil-free fractions for all forms. Total concentration of metals are reproducible to +5Voand are good to +10%. Concentrations of nonapatite inorganic phosphorus (NAIP), the principal pool of bioavailable phosphorus in sediments (Williams et al. 1980), insoluble apatite phosphorus, and organically bound phosphorus were measured by wet-chemical extraction (Williams et al. 1976). Values are reproducibleto+57o and are good to +70Va.Water sampleswere collected and analyzed for total phosphorus and dissolved phosphorus following (Menzel gt0 methods described earlier & Corwin 1965, I Prepas1983). c Concentrations of organic carbon and total sulfur u 15 o- (Sr) were determined with a Leco induction furnace (Kemp 1971); values are reproducible to +3Vo and are good to -t1%o.Concentrations are given as weight per- cent of dry sediment.

Rgsulrs

Spectral resolution

Visual examination of the higher-energy absorptior in the room-temperatureMcissbauer spectra, as a function of depth, indicates that two types of spectrum are present.Thus, the envelope attributed to ferrous iron is gt0 more intense and has an additional sharply defined

F shoulderin spectraof sectionsof core deeperthan 30 cm c u 1( (Fig. 3). A second core from the south basin, collected o- in February 1992,displayed quantitatively similar spectral features. The main iron compounds entering the lake are clay minerals and hydrated ferric oxides in surface waters and ferrous ions in solution in groundwater. Hence, the two types of spectrareflect different diagenetic processesoccuring over different periods J SMA within 1982 1983 the sediment.Redox potentials within the sedimentsare strongly dependenton the rate of breakdown of organic Fto.2. Isopleths depicting (a) concentrations of dissolved matter and, hence,on algal productivity in the lake. The .C, oxygen (mg L-r), O) temperarure and (c) concentra- Mrjssbauer spectrawere resolved by invoking two dif- tions of total phosphorus (mg L-,) in the south basin of ferent schemes.For upper sectionsof the sedimentcol- Baptiste Lake as functions of depth and time Summer av- umn, which show two visually obvious doublets, three erage epilimnetic concentrations of phosphorus and chlo- doublets were invoked marking ferrous ions in struc- rophyll a exceed 40 and 20 pg L-r, respectively. tural sitesin clay minerals, ferric ions in oxides and clay

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F o- E m s

3210-1-2-3 VELOCITYmms-l

Frc. 3. Room-temperature Mdssbauer spectra of (a) 2 to 4 cm section of sediment core, and (b) 34 to 36 cm section. The spectra show, respectively, the development of pyrite and vivianite sisnatures.

minerals, and Fe2t ions in pyrite, FeS2(Manning el al. pyrite doublet were constrainedat values determinedfor 19'19, 1994). The absorptions of ferric ions and pyrite >957o-ptre crystals of pyrite, i.e., at v aluescorrespond- are superimposedand are not resolved by eye in room- ing to an isomer shift of 0.30 mm s-1,a quadrupolesplit- l, temperature spectra (Manning et al. 1979).ln all com- ting of 0.60 mm s and a half-width of 0.30 mm s-r putations, the peak positions and half-widths of the (Morice et al. 1969). Optical microscopy and semi-

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quantitative X-ray diffraction showed that opaque at an energy corresponding to the Fe2+1and Fe2+nab- framboids are particularly abundantin the 8-20 cm sec- sorptions in vivianite (Manning et al. 1991, Nembrini tions of core. Electron microscopy confirmed the FeS2 et al. 1983). Vivianite, Fe3(POa)2.8H2O,is a major sink composition of the framboids. The presenceof pyrite for inorganic phosphorus in moderately reducing sedi- was further confirmed by the narrowing, as a function ments in which iron and inorganic phosphorus are of depth of section, of the composite "ferric" doublet in highly mobile and in which the sulfide ion is not pro- a two-doublet fit (one ferrous and one ferric doublet) duced in high concentration (Manning et al. l99l). (Manning et al. 1979). Thus the half,widths of 0.58 + Vivianite was confirmed by X-ray diffraction, and also I 0.02 mm s measuredfor the 0-1 and l*2 cm sections by noting the considerably reduced ferrous absorption are more typical of ferric ions in oxides and clays, on washing the34-36 and 36-38 cm sectionsin dilute 1 whereas the value of 0.37 + 0.02 mm s measuredfor HCl, in which vivianite is soluble. The concentrationof the 14-15 cm section is more indicative of pyrite (Man- Fe2+(clay) in washed samplesis consistentwith that in ning et al. 1979). Most of the sulfur in the too 25 cm of the top 20 cm of core (Table 1). Hence, most of the fer- core is clearly in pyrite (Table | ). rous iron absorption in the deeper sectionsof south ba- Sections 34 cm and 9-10 cm showed little change sin core is due to vivianite. The Fe2+1and Fe2*11 in the Mrissbauerspectrum after an acid wash, henie higher-energyabsorptions were clearly resolvedin spec- poorly crystalline FeS compounds are relativelv minor tra of sediment retrieved from Narrow Lake (Manning phases.This is consistentwith their low cohcentrations et al. 7991), a mesotrophic lake located in a glacial in the strongly reducing sedimentsof other prairie lakes meltwater channel in close proximity to Baptiste Lake. (Manning et al. 1994). The spectra of the 34-36 and 36-38 cm sections of The additional presenceof vivianite in sections of Baptiste Lake sediment were successfully resolved us- south basin core deeper than 30 cm is indicated by the ing four doublets, one ferric iron, two vivianite (Fe2+1 sharply defined bifurcate absorption at2.9 mm s-t, i."., and Fe2+1)and one clay (Fe2*;. The remaining spectra were then resolved by constraining all ferrous iron doublets (peakpositions and widths). This was deemednec-

TABLE 1 ELEMENTAL CONCENTRATIONS*IN SECTIONSOF A essary because the Fe2+1,Fe2+11, and Fe2+(clay) SEDIMENT CORE,BAPTISTE LAKE, ALBERTA absorptions are superimposed and because their low- energy peaks lie beneaththe dominant envelope which pvto Deprh Fe, FeF Fe+ Fe(Sr) NAIp Ca sr PT Mn BP is due to ferric iron. The degree of oxidation of the vivianite is not known, hencethe ferric iron envelopeis a composite of ions bound in G--t 28 04 22 02 0061 58 039 ot6 031 2 clay, hydrated oxide and l-2 28 0 48 17 064 0086 6l 058 013 4 vivianite. Vivianite peaks are poorly developedin spec- 34 28 03I 17 074 005 l0l 016 9 tra of sedimentsfrom the north basin. +5 28 034 l6 084 0051 5 5 099 013 034 l0 The 4 K spectrashow that for the 24 cm and34-36 7-8 28 048 12 1lE 0055 76 14'l 014 049 17 cm sections of Baptiste Lake sediments,magnetically 9-10 28 0 36 1 t4 0049 176 o t'7 23 ordered ions contain, respectively, 64Vo and 68Vaof to' l2-t3 28 039 08 148 006 016 30 14-15 2A 034 I I 134 0032 122 ol7 037 37 tal spectral absorption. Inthe24 cm section, the mag- 19-20 395 038 21 148 0046 23 179 ot2 55 netic behavior is derived from ferric iron in oxides; the 24-2s 362 033 21 123 0056 t71 014 76 sharpnessof the pyrite absorption is accentuated.Fer- rous ions in vivianite also order magnetically at 4 K Depth Fer Fe'* NAIP Ca Sr PT Mn BP (Gonser & Grant 1967); hence the broad unresolved bands in the 4 K 28-30 513 212 3 t7 063 074 | 042 0E5 077 90 spectra of the 34-36 cm section un- 30-32 431 I 16 32 08 03 o44 057 99 doubtedly arise from ferric ions in both oxides and 32-34 1A1 069 ll 03 011 022 26 0 38 029 0 r7 112 vivianite. Most of the ferrous iron in the 34-36 cm sec- 34-36 497 229 27 190*rO'l t2 t2 t4 t28127 tion is in vivianite (Fig. 3), amounting to approximately 36-38 558 251 3 1 220** Oal 124 I 065 t33 138t42 30Voof the total iron (Table 1). 38-40 485 218 21 18 067 076 I 048 085 097157 40_{'2 302 124 t8 0r 03 019 14 036 02'7 052r70 M ds sbaue r spectral ass i gnments

* W€ight% of dry s€dimmts Total iron @nc€trbatioN for O to 15 m d@th ue 0 lO The spectraof all sectionsin the 0-25 cm depth in- wtolo Con€ntrations of apatiteboud phosphorus alotrg the lqgth of core ue 0 04+0 01 wt% Conctrtrations oforgdically boud phosphorus de 0 06+0 03% terval are satisfactorilyresolved on the basis ofthe Fe2*- and show no obvious tmd with deph of buial Concentrations of orguic C ue Fer+-FeS2scheme in which the ferrous iron is in clay 20 El 5 wtyo fot 0-25 cm deprh, 6d l4 qt | 3 vtyo for the 3H;2 m intnd ii (Fig. 3). Measured values of isomer Mesred by HCI *traction; @ncqflatiN of Fd. in residue de 0 4 fld 0 3 wt% shift, quadrupole for the 34-36 md 36-38 cm sectio4 respectively. Pw is equal to F€w divid€d by splitting and half-width are, respectively, 1.13 mm s-1, 2.7. In the 34-36 ffi md 36-38 @ sections, 80% of NAIP is qtlacted bv HCl. 2.60 mm s-l, and 0.42mm s-1for ferrous iron, and 0.40 VMmite wa not d€tected in the uppa 20 cm ofcore Coocsrtrations ofAi show no signifimt tod with depth ry@ging 2 O10 5 wt% Deprh is quoted in m Bpj mm s-1,0.70 mm s-1,and 0.55 mm s-1for ferric iron; all date in yetrs before presqt. values are good to +0.03 mm s-1. The corresponding

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parameters for pyrite are constrained. The measured Forms ofphosphorus parametersare consistent with the presenceof ferrous ions in structural sites in clay minerals and with ferric Concentrationsof NAIP over the top 25 cm of north ions in similar sites and also in hydrated oxides (Coey basin sediment are relatively low (Table 1) and compa- et al. l9l4,Manning et al. 1991).These consistent reso- rable to concentrationsin sedimentsof other eutrophic lutions confirm (Table 1) the conversion of ferric iron prairie lakes (Manning et al. 1994). The sediments in into pyrite with time, i.e., on burial, a trend observedin the 3042 cm depth range are massive sinks for NAIP the reducing sediments of other lakes (Manning er a/. (Table 1), and their Mcissbauerspectra correspondingly 1979, 1994). Sediments laid down in approximately show clear evidence for vivianite (Fig. 3); in support, 1920 (Turner 1995), now buried to depth of 25 cm most of this NAIP is releasedon washing with 0.2 M (Table l), became sufficiently enriched in organic mat- HCI (Table 1). Moreover, these deeper sections show ter so as to promote the later production of sulfide ion no enrichment in the inert elements Al and apatite-P and pyrite; lower redox potentials reflect increasing al- (Table 1), elements derived largely from erosional in- gal productivity within the lake consequentupon settle- puts. The elevated concentrationsof organically bound ment. The high concentrations of pyrite reflect the phosphorus in the 32-34 cm and 34-36 cm sections currently highly eutrophic condition of Baptiste Lake. (Table 1) may not be real. Concentrations of organic The resolution of the Mtissbauer spectra of the phosphorus are measured as the difference between con- deeper sedimentsyielded hyperfine parametersfor fer- centrations of total P and NAIP + apatite-P (Williams et ric iron that are similar to those measuredfor the shal- al. 1976). For these two sections,the concentrationsof lower sections. Measured concentrations of pyrite are organic P are subject to the great error inherent in the minor at less than 5Voof total iron. Few framboids were measuring of small differences between two large values observedunder the optical microscope, hence the com- (Table 1). puter-derived content of pyrite may be spurious. The Soluble reactive phosphorusis releasedmassively to sedimentslaid down prior to approximately 1920, i.e., the hypolimnion during summer and winter anoxia in the depth interval 3G-40 cm (Tumer 1995), are en- (Fig.2). The sediments show no visual indication of a riched in total iron, ferrous iron (strongly), ferric iron, brown oxidized surface layer of hydrated ferric oxide, manganeseand NAIP, when measuredagainst concen- nor are concentrations of ferric iron significantly ele- trations of apatite,relative to their concentrationsin the vated over the top centimeter (Table 1): phosphorusre- 0-25 cm section of core (Table 1). In contrast, concen- lease is probably due to the breakdown of freshly trations of organic carbon are lower than in the later deposited algal remains rather than to the reduction of sediments (Table l). Elevated concentrations of iron, oxide-phosphate comPounds. manganese and inorganic phosphorus are consistent with sedimenthorizons formed under conditions of high Core chronology redox potential (Manning et al. 1991, Nembrini er a/. 210Pb 1983); in contrast,thin bandsof sedimentcontaining >1 The profile confirms that sedimentation has wt% NAIP underlie oligotrophic Lake Ontario and me- been continuous and undisturbed over the top 42 cm of sotrophic Narrow Lake. The latter lake is in the same core, or over approximately 150 years of deposition meltwater channel as Baptiste Lake, but has a smaller Oable l). Rates of sedimentation are 0.63 cm yr-r and drainage ratio and has experienced less disturbance in 200 gma yrr (Turner 1995). the watershed. Whereas most of the iron in the Fe-P horizon in Lake Ontario sedimentsis present as poorly DlscussroN crystallized hydrated ferric oxide (Manning et al. 1983), in Narrow Lake sediments,of higher organic content, The contrasting forms and behavior of iron, phos- the Fe3+-phosphatecompound precipitated initially is phorus, sulfur, and carbon compounds in the 0-20 and rapidly converted into vivianite (Manning et al. l99l). 3042 cm sections of sediment reflect different redox Vivianite is formed under mildly reducing conditions regimes following deposition over the past 150 years and under high concentrations of available iron and (Table 1). This further reflects periods of different dy- phosphateions (Nriagu &Dell 1974). The watersheds namics of phosphorus and, possibly, different trophic of Baptiste and Narrow lakes drain similar terrain. In levels in the lake. The sulfate-reducing horizon (0-25 addition, groundwater flow into the offshore sediments cm) is consistentwith highly anoxic sediment,a conse- represents an important contribution to the total inflow quence of oxygen depletion in the bottom waters and (l2%o and 37Vo,rcspectively, for Baptiste and Narrow richly organic sediments. Baptiste Lake is cuffently lakes; Shaw & Prepas 1989, Shaw et al. 1990). The highly eutrophic, and limited agriculture in the water- deeper sediments,at3042 cm, with a nutrient compo- shed has likely contributed to that condition (Cooke & sition of low-grade fertilizer, probably mark a period in Prepas1998). the history of Baptiste Lake when trophic conditions The high concentrations of iron, Mn, NAIP, and were similar to those currentlv in mesotrophic Narrow vivianite inthe 3042 cm sections are probably the re- Lake. sult of orocessessimilar to those now occurring in the

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bottom sedimentsof mesotrophic Narrow Lake (Man- the sediments,bearing in mind that the bottom waters ning et al. 1991). Both lakes lie within the same seo- are anoxic for much of the year and that sulfide forma- logical formation, and both receive measurableiniow tion is well entrenched beneath the top centimeter of of groundwater. Groundwater is an important sourceof sediment. inflow to Narrow Lake. Groundwater inflow has been How rapidly is this massive pool of vivianite-NAlP suggestedboth as a mechanism for enhancing p recy- releasedto the porewaters and hence to the lake? The cling from bottom sedimentsto the euphotic zone, and mesotrophic status of Narrow Lake suggeststhat the as a factor controlling major-ion chemistry in prairie vivianite is only slowly eroded. Hypolimnetic total lakes (Shaw et al. 1990). Whereas groundwater at Nar- phosphorus (TP) releaserates are much lower in Nar- row Lake averagesa higher portion of total annual in- row Lake (summer mean 1.4 + 0.3 mg --z 6-t1 than in flow compared to Baptiste Lake (37 versus l2To, eutrophic Baptiste and Amisk lakes (11.0 + 2.0 andT .7 respectively), groundwater input into Baptiste Lake + 0.3 mg m-2 d-1, respectively; Prepas& Burke 1997). fluctuates and has been estimatedto be as hish as22Vo Vivianite lies in deeper sediment in Baptiste Lake than of total water inflow (Trew er a1. lg87 ).In Naiow Lake, in Narrow Lake. Phosphorus is released in massive percolation of iron-rich groundwaters through the bot- quantities to the hypolimnion of Amisk Lake, in which tom sediments leads to the massive coprecipitation of the sediments are devoid of vivianite (Manning et al. hydrated ferric oxide and NAIP before the phosphate 1994). ion can become bioavailable (Manning et at. l99l). Whereas inorganic phosphorusis releasedfrom the Burial in the reducing sedimentsleads to the reduction sedimentsin summer (Fig. 2), iron is almost completely offerric oxides and the formation ofvivianite. It is prob- removed from the water column and deposited in the able that a similar process took place in Baptiste Lake sediments(Table 2). Thesetrends highlight the disasso- prior to the turn of the century, Le.,high concentrations ciation of iron and NAIP under strongly reducing con- of ferrous and phosphate ions in groundwaters perco- ditions, due in part to the precipitation of iron as poorly lating upward through the offshore sedimentswere pre- crystalline sulfides. Much of this iron is retained within cipitated at the sediment-water interface. Moreover, the sedimentsas pyrite. Highly eutrophic lakes that are sinceredox potentialswithin the sedimentswere not suf- strongly stratified over much of the year are therefore ficiently 1ow so as to generate sulfide ion, it is likely liable to becomedepleted in iron. that less organic matter was depositedin the sediments Concentrationsof extractableiron in the epilimnion (Table 1) and that the bottom water was better oxvsen- of Baptiste Lake in May (30 pg f1, Table 2), prior to ated. Current quality of water may be inferior toihat stratification and the release of sediment phosphorus, which existed for a period of at least 70 years prior to are a relatively small proportion of total iron. Over the approximately 1920. The change in water quality may summer, most of the total iron is removed from the water be related to a changein groundwater, or the pattern of column (Table 2), such that the concentration of total groundwater flow, which in turn may arise from land- iron is lower than that of inorganic phosphate ion clearing activities on settlement. (Fig.2). The removal of iron arisesfrom the settling out The high concentrationsof NAIP in the deeper sec- of particulate matter and the consequentprecipitation tions of core, e.g., the 1.2 wt%omeasured for th; 34-36 of poorly crystalline iron sulfide under anoxic condi- and 36-38 cm sections(Table 1), are largely accounted tions. In the epilimnion in both summer and winter in for by the precipitation of vivianite. However, the use Baptiste Lake, there is insufficient extractable (avail- of the concentrationof ferrous iron leads to an underes- able) iron to bind but a small fraction of the NAIP. Con- timation of the vivianite concentration becauseof the partial oxidation (Nembrini et al. 1983) of structural ferrous iron to ferric on exposure of samples to air. Hence, more NAIP is contained within the vivianire TABLE 2 CoNCENTRAfiONS (mg r) OF TOTAL IRON structurethan is indicated in Table 1. Unfortunatelv. the AS A FUNCTION OF 1VATERDEPTH IN BAPTISTE LAKE broad bandsin the 4 K Mdssbauer spectraare too broad to enable resolution of vivianite and ferric oxide com- Depth(n) ponents. Nevertheless, vivianite accounts for the bulk of the NAIP in the deeper sections of sediment. I 7 lt t5 17 2l To raise the concentration of NAIP over a l0-m- thick hypolimnion by 200 pg L-l is equivalent to the 7 May 032 037 038 034 0 36 035 035 035 2 4 Jme 029 034 031 038 031 031 release of 2 g P m of sediment, which is the total 035 30 Jue 032 ot7 042 071 0 t7 020 019 amount of NAIP containedin the 0t3 top 3 cm of sediment. 29 Jvly 006 010 OM 006 00ll 006 00E 001 This much NAIP would require the reduction of ap- 26 Aug 007 009 008 010 010 012 010 0t2 proximately 20 g Fel+ in hydratedoxide (Manning er 2l Oct 060 al. 1994), based on the classical model for the anoxic release of adsorbed NAIP (Mortimer 1941). It is un- Con@trati@s of qhactable irm (0 lM HCI) w@ 0 03 mg L-l or lowq in the watq likely that this much hydrared ferric oxide can exisr in dum in tlrc umer of 1992

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centrationsoftotal iron reach 300 to 500 pg L-l during KBvp, A.L.W. (1971): Organic carbon and nitrogen in the sur- Erie and Huron. J. Sed. spring and fall overturns, and drop as low as 40 pg L-' face sediments of Lakes Ontario, 41,53'7-548 throughout the summer. As ferric ions in poorly crys- Petrol talline hydrated oxides bind approximately llvo by MANNTNG.P.G., Luu, K R. & BIRCHALL,T. (1983): Forms of phosphateion (Manning et al. 1983),Baptiste weight of iron, phosphorusand trace metal ions in a layered sediment Lake may be consideredto be deficient in iron. Fe con- core from Lake Ontario. Can. Mineral 2l' 121-128' centrationsin groundwater-fedwells ofthe region range from <40 to >15 pg L-r, hence disturbance in the wa- MuRpny, T P. & PREPAS,E.E. (1991): Intensive for- tershed may have influenced the flow patteffI of iron- mation of vivianite in the bottom sediments of mesohophic rich groundwater. These calculations, which estimate Narrow Lake, Alberta. Can Mineral.29,'77-85 the amount of Fe necessary for the reduction of the (1994): Forms of iron and NAIP in Baptiste Lake, may explain the highly & - of phosphorusin eutrophic Amisk Lake, eutrophic condition of this lake. the bioavailability Alberta. Can. Mineral. 32, 459-468. This work serves to highlight the fragility of the Narrow Lake (Manning unique mesotrophy of nearby WrllI,trr.rs. J.D.H., CrunI-roN, M N., AsH, L.A. & et aI. l99l). Interferencewith the pattem offlow ofiron- Brncnell, T. (1979): Miissbauer spectral studies of the rich groundwater through watersheddisturbance would diagenesisof iron in a sulfide-rich sediment corc' Nature likely render Narrow Lake eutrophic on account of nu- 280,134-136. trient runoff from prairie soil. MENZEL,D.W. & ConwrN, N. (1965): The measurementof to- AcxNowr-pocnveNrs tal phosphorus in seawaterbased on the liberation of organically bound fractions by persulfate oxidaion. Limnol- 10,280-282. We thank Janice M. Burke, Tom P. Murphy and Oceanogr. Xiaowa Wang for much assistanceand advice. We also Mrrcnsrr-, P. & PREPAs,E. (1990): Atlas of Alberta Lakes' reviewers for their helpful thank the two anonymous University of Alberta Press,Edmonton, Alberta. comments. Support for this project was provided in part from an NSERC Research grant to E.E. Prepas. Field Monrci, J.A, RBrs, L.V.C. & Rrcrrno, D.T. (1969): researchactivities were based at the Meanook Biologi- Mossbauer studies of iron sulfides. J. Inorg. Nucl. Chem. cal ResearchStation. 31,379'.7-3802.

RepEneNcns Monrnursn,C.H. (1941): The exchangeofdissolved substances between mud and water in lakes . J EcoI 29 ' 280-329 ' AppI-esv,P.G & Onrlem, F. (1978):The calculation of 2r0Pb datesassuming a constantrate of supplyof unsupported MuRAD, E. (1988): Properties and behavior of iron oxides as 210Pbto the sediment.Catena 5,7-8. determined by Mdssbauer spectroscopy. 1z Iron in Soils and Clay Minerals (J.W. Stucki, B.A Goodman & U. BBnNnn,R.A ( I 984): Sedimentarypyrite formation: an update Schwertmann, eds.). Reidel Publishing, Dordrecht, The Geochim.Cosmochim. Acta 48,605-615 Netherlands.

CoEV,JMD., ScHINor-sn,D.W & WBesn,F (1974):Iron NeraentN'r.G.P . CeposlANco,J.A., VIEL, M & WnI-rlus, A.F. compoundsin lakesediments. Can. J. Earth Sci.lI,1489' (1983): A Mdssbauer and chemical study of the formation t493. of vivianite in sediments from Lago Maggiore (Italy). Geochim Cosmochim.Acta 47, 1459-1464- Cooxn,S.E & PnBp.ts,EE. (1998):Stream phosphorus and nitrogenexport from agriculturaland forested watersheds Nnrecu, J.O & Dell, C.I. (1974): Diagenetic formation of iron on the BorealPlain. Can J. Fish. Aquat. Sci. 55,2292- phosphatesin recentlake sedimetts. Am. Mineral. 59'934- 2299 946.

CRowE,A S. & ScHw.r.nrz,F.W (1981):Simulation of lake Pnspes,E.E. (1983): Orthophosphateturnover time in shallow watershedsystem. II. Applicationto BaptisteLake, A1- productive lakes. Can. J. Fish. Aquat. Sci.40,1412-1418' berta.Can. J. Hydrol. 52,107-125. & Bunrn, J M. (1997): The effect of hypolimnetic GoNsBr,U. & Gnexr, R.W (1967):Determination of spindi- oxygenation on water quality in Amisk Lake' Alberta, a rectionsand electric field gradientaxes in vivianiteby po- deep eutrophic lake with high internal phosphorusloading larizedrecoil-free ^y-rays Phys. Stat. Sol.2l,337-342. rates. Can. J. Fish. Aquat. Sci. 54,2111-212O'

HTcKMAN.M., Scnwrcrn, C.E. & Krenrn, D.M. (1990): SHAw,R.D. & Pnspls, E.E (1989): Groundwater-lake interac- BaptisteLake, Alberta - a lateHolocene history ofchanges tions. II. Nearshore seepagepattems and the contribution in a lakeand its catchmentin the southernBoreal forest. "/. of groundwater to lakes in central Alberta. J. Hydrol. ll9' Pale olimno l. 4, 253-267 . r2t-t36.

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Snew, J.F.H., FRTcKER,H. & PREpAs,E.E. (1990): Wrr-lnrras,J.D.H., Jequnr, J.-M. & THoMAs,R.L. (1976): An integrated approach to quantify groundwater transport Formsof phosphorusin the surficial sedimentsof Lake ofphosphorus to Nanow Lake, Alberla. Limnol. Oceanogr. Erie J. Fish. Res.Board Can.33,413-429. 35, 870-886. SHBan,H. & Tuorra,q.s,R.L (1980): Availabilitv to Srom, A.J. (1967): Least-squaresfitting of Mdssbauer spec- Scenedesmus quadricauda of different forms of phospho- tra. Appendix to "Mdssbauer spectraof iron (III) diketone rus in sedimentary materials from the GreatLakes. Limnol. complexes" by G.M. Bancroft, A.G. Maddock, W.K. Ong, Oceanogr.25, l-11. R.H. Prince & A.J. Stone. "/. Chem. Soc. A1967 , 1966-19'l l .

TnBw, D.O., BeLrvsau, D.J. & YouNcr, E.I. (1987): The Baptiste Inke Study Technical Reporl. Alberta Envir., poll. Control Div., Water Quality Control Board, Edmonton, Alberta.

210Pb TuRNER,L.J. (1995): dating oflacustrine sedimentsfrom Baptiste Lake (Core 075), Alberta. Nat. Water Res.Inst ReceivedMarch 8, 1997, revised manuscript acceptedMarch Contrib.95-118. 6. 1999.

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