Late Quaternary diatom and chemical profiles from a meromictic lake in Quebec, Canada

Chemical Geology, 44 (1984) 267--286

267

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

LATE QUATERNARY DIATOM AND CHEMICAL PROFILES FROM A MEROMICTIC LAKE IN QUEBEC, CANADA

R O G E R J O N E S ' , MICHAEL D. DICKMAN 2, R O B E R T J. MOTT 3 and MARCEL OUELLET"

1Department of Biology, Trent University, Peterborough, Ont. K9J 7B8 (Canada) 2Department of Biological Sciences, Brock University, St. Catharines, Ont. L2S 3A1 (Canada) 3 Geological Survey of Canada, Ottawa, Ont. K I A OE8 (Canada) 4Institut National de la Recherche Scientifique, INRS--Eau, Universit$ du Qudbec, Quebec, Que. G1 V 4C7 (Canada) (Accepted for publication November 30, 1983)

ABSTRACT Jones, R., Dickman, M.D., Mort, R.J. and Ouellet, M., 1984. Late Quaternary diatom and chemical profiles from a meromictic lake in Quebec, Canada. In: J.A. Robbins (GuestEditor), Geochronology of Recent Deposits. Chem. Geol., 44: 267--286. An investigation of the chemistry and diatoms in the sediments of a small hardwater meromictic lake in Quebec indicated remains of both freshwater and brackish (halophilic) water diatoms in the organic sediments laid down after the lake was isolated by isostatic rebound from the Champlain Sea, ~ 11,000 yr. ago. Water in deeper strata of the lake evidently was not completely fresh, giving rise to ectogenic meromixis. The loss of marine salts from the lake is believed to have taken 3000 yr. or so and that as this happened, salts of biological origin gradually accumulated in the monimolimnion so that maintenance of meromixis became biogenic. The sediment chemistry reflects primary successional changes in the vegetation of the area rather than a decline of ectogenie meromixis. As forest cover developed and soils stabilised on the rebounding land, erosional processes gave way to leaching processes as the source of nutrients to the lake. The appearance of eutrophic indicator diatoms, and an increased flux of nutrients to the sediments, suggest that a cycle of eutrophication occurred in the lake following the decline of hemlock, ~ 4 8 0 0 yr. ago. The recent sediments show an increase in concentrations of Cu, Zn and Pb as a result of anthropogenic activities.

INTRODUCTION

Pink Lake is situated in the Gatineau Park, Quebec, near Ottawa (Fig. 1). It is a small lake (surface area 9 ha; maximum depth 21 m) occupying a bedrock basin in crystalline limestone along the southern boundary of the Canadian Shield (Mott and Farley-Gill, 1981). The topography of the catchm e n t basin {area ~ 9 5 ha) is rugged with local relief up to 30 m above the lake. A small stream flows into the Gatineau River. Upland sites around the

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© 1984 Elsevier Science Publishers B.V.

268

QUEBEC

//~

M ont real,az~ ONTARIO

I/

?

.

2?o

M - Pink

Loke-

Fig. 1. Location of and morphometric map of Pink Lake, Quebec. lake are covered by semi-mature to mature closed stands of hardwoods (e.g., Acer saccharum (sugar maple), Ostrya virginiana (hop hornbeam), Quercus rubra (red oak), Fraxinus americana (white ash), Populus tremuloides (aspen), and Betula papyrifera (white birch). Small numbers of coniferous species (e.g., Picea glauca (white spruce), Tsuga canadensis (hemlock)) occur in the understories at some sites (Lopoukhine, 1974; Mott and Farley-Gill, 1981). Pink Lake is a hardwater lake in which carbonates are precipitated annually in the epilimnion in late spring (Dickman, 1979) and on leaves of Potamogeton amplifolius in summer (Aiken and Gillett, 1975). A calciphile mollusc, Pisidium rotundatum, lives in the lake (Grimm, 1975). The lake is also meromictic (Dickman et al., 1975; Dickman, 1979) which means that at the time of winter cooling there is only partial circulation down to a depth determined by density stratification (Rutner, 1963). The deeper, more dense part of the lake not involved in the annual circulation is termed the monimolimnion. The increased density of the anaerobic monimolimnion in Pink Lake is apparently caused by the accumulation of salts of biogenic origin (Dickman and Hartman, 1979) because the shelter afforded by the steep-sided basin is sufficient to prevent wind-generated currents from inducing complete overturn when the lake is isothermal in spring and fall. Pink Lake is classed as an endogenic meromictic lake (a type-IV lake, having a relative depth of 5.35%; K.F. Walker and Likens, 1975) and in this respect is similar to the majority of meromictic lakes in North America. The present elevation of Pink Lake of 162 m a.s.1. (above sea level) is below the postglacial marine limit of 198 m a.s.1, of the area (Romanelli, 1975}, indicating that the lake basin was inundated by the Champlain Sea ~ 1 3 , 0 0 0 yr. B.P.

269

(years before present). Isostatic rebound data calculated by Romanelli (1975) suggests t h a t the Pink Lake basin was isolated from the Champlain Sea ~ 1 1 , 0 0 0 yr. ago. Such isolation would lead to a gradual process of desalination (freshening) as fresh water entered the lake. During freshening, inflowing water would form a surface lens of fresh water on top of the denser salt water. The latter may have gradually eroded away with time. It is assumed that the lake was sheltered to roughly the same extent as t o d a y and that the underlying salt water beneath the overlying fresh water prevented complete mixing. Meromixis in this case would have been ectogenic as a result of salt water remaining beneath a layer of fresh water (Hutchinson, 1957). However, there is no evidence of marine salts in the present-day monimolimnion (Dickman, 1974) although a marine relict fish, the three-spined stickleback (Gasterosteus aculeatus) is found in the lake (Rubec, 1975). To investigate the paleoecology of Pink Lake and its drainage basin since the lake basin was isolated from the Champlain Sea, ~-11,000 yr. B.P., sediment cores were collected from the deepest part of the lake for pollen, diatom and chemical analysis. Preliminary pollen and diatom studies were reported by Dickman et al. {1975) and a detailed pollen profile for Pink Lake and an interpretation of late Quaternary vegetation and climatic change in the area have been made by Mott and Farley-Gill (1981). This paper discusses the results of diatom analyses and sediment chemistry in relation to the late Quaternary history of Pink Lake and its drainage basin. MATERIALS AND METHODS

Core collection and su bsampling A Livingstone styled coring device (Mott, 1966) was used to collect two sediment cores, each 320 cm in length, from near the deepest part (21 m) of the lake and a Brown (1956) corer to sample across the mud-water interface. The top meter of sediment was taken to the laboratory to be frozen because of its very high water content. This frozen section was cut into 1 0 ~ m segments which were kept frozen until subsampled. The rest of the c o r e was extruded, cut into 0.5-m lengths, wrapped in Saran ® and foil and stored in a cold r o o m until subsampled for analysis. Subsamples, each 2 cm in length, were taken from one core at intervals of 10 cm for pollen and diatom analysis, except the top 10 cm and basal 10 cm of organic sediment where closer intervals were sampled for pollen analysis. Subsamples, each 2 cm in thickness, were taken from the top 20 cm of sediment for heavy-metal analysis, while 5-cm sections at 10-cm intervals were used for the rest of the core for heavy-metal and other chemical analyses.

270

Sample analysis Sediments for diatom analysis were treated with a combination of h o t concentrated H2SO4 and HNO3 in combination with K2Cr~O7 (Patrick and Reimer, 1966). Diatoms were mounted on glass slides in Hyrax ® mounting medium and counts, each of which exceeded 500 diatom frustules, were made using a Leitz ® ortholux inverted (plankton) microscope. Identifications were based upon the descriptions of Bailey (1924), Cleve-Euler (1951--1955), Foged (1970), and Patrick and Reimer (1966, 1975). Samples for chemical analysis were dried for 24 hr. at l l 0 ° C and then ground (agate mortar and pestle) to pass through an 80 mesh sieve. Organic matter (OM) was determined on three replicates of each sample by the Walkley--Black wet oxidation method (Jackson, 1958). The Bremner (1965) procedure was used for total Kjeldahl nitrogen digestion, and then the ammonia content of the digest determined with a specific-ion electrode (Bremner and Tabatabai, 1972). Carbonates were determined by loss on ignition at 950°C after correcting for wet oxidation of organic matter. Samples for total~element analysis (three replicates) were fused with sodium carbonate (3 g Na~COs to 0.25-g sample) and the melt dissolved in 50 ml 6 N HC1, which was then brought to 250 ml with deionized water. K was determined by flame emission spectrophotometry and Ca, Mg, Fe and Mn by atomic absorption spectrophotometry (Unicam ® SP 90). Lanthanum chloride (to give a final dilution of 1%) was added to samples for Ca determinations to prevent interference by P. A modification of the chlorostannous-reduced molybdophosphoric blue method in hydrochloric acid was used to determine phosphorus (Jones, 1975). Cu, Pb and Zn were analyzed according to the method of Kjensmo (1968} and Pinchette and Guimont (1975). RESULTS

Stratigraphy, radiocarbon dates and sedimentation rates The total depth of sediment in Pink Lake is not known, but a clearly recognizable stratigraphic change (Table I) occurred between a grey clay and the upper 300 cm of soft algal gyttja that accumulated during the late Quaternary biologically active phase in the drainage basin. The clay contained silt and sand layers and some pebbles of crystalline limestone (Mott and Farley-Gill, 1981). Radiocarbon dates were obtained for three samples, two of which were at pollen zone boundaries (Table I) and one was just above the boundary between Pollen Zones 8 and 9. By comparing the pollen sequences of Pink Lake and nearby holomictic Ramsay Lake and by using the age--depth diagrams for both lakes, Mott and Farley-Gill (1981) have suggested dates

271 TABLEI Description of sediment core from Pink Lake, Quebec, including dates, sedimentation rates and pollen zones (after Mott and Farley-Gill, 1981) DEPTH

DATES (yr.B,P.)

SEDIMENTS

SEDIMENTATION RATES ( mm yr."1)

POLLEN

ZONES

(cm) 0

0.64

1 Ambrosia Pollen Zone

0.37

Mixed Hardwood 2 Pollen Zone

125e.__

w

--

---

1300~--

BROWN ALGAL GYTTJA

3 Fa~lus ~lrandifolia

0.38

Pollen Zone

100m

--

3270

+260 m

Betula- Ouercus

0.26

4 Pollen Zone

0.16

5 Pollen Zone

0.25

6

-- 5500e-200--

Tsuga canadensis

7750 +170

m

-- 9800 O .............................. -- 10200 ~-

300-

SILT

-- 10700 ~-

GREY CLAY

.

.

0.16 .

.

.

.

Pollen Zone

-,, 7 Picea Pollen Z o n e

0.53 _

Pinus

.

.

.

.

.

p.

8 P()pulus Pollen-Zone .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

9 Herb Pollen.Zone

tFrom

Mott and Farley-Gill (1981)

for the boundaries between pollen zones in the Pink Lake sediment profile which were n o t actually radiocarbon dated (Table I). These dates, along with the two radiocarbon dates were used to determine the sedimentation rates shown in Table I. It can be seen t h a t sedimentation rates have varied markedly during the period since vegetation became established around the lake. Diatom stratigraphy Diatom frustules were present to a depth of 300 cm where the transition from an overlying algal gyttja to the grey clay occurred. Only a few diatom frustules were f o u n d in this clay, suggesting that abrasive mechanical forces may have destroyed the diatoms in the earliest part of the late Quaternary. The thin-walled planktonic diatom, Asterionella formosa was

272 SEDIMENT

DEPTH

(cm)

PC)L LEN ZONES

0

5o

I00

150

200

250

I

I

1

t

I

I

I

4

3

2

5

300 ,

6

I ?

ALKALIPHILOUS Cymbello

cymblforme$

Frogilarla Frog:~or

-41 H ~

brevtstrJot~ ~

Navicu!o

~onstruens

;onceolo~o gibbo

Rhopolodio Synedro

A v

u[no

•

-

I

HALOPHILOU S Amphora

avails

Epithemia

var

afflnis

adonata

Gyrosigma

attenuotum

Mostooloia

grevlllei

Mosto~loia

smlthli

Opeophora

martyi

Stouroneis

ignoranto

m

II IN

¢

=

II

EUTROPHIC Asterionello

n

formosa

Frogilorio

crotonensi$

Nitzschia

E]

Stephanodlscus

II

-

El-

paled astrea

TobeHar~o

fenestrota

Tobeltorio

flocculosa

EURYTOPIC Amphipteura Cyctotei~a Eunotia FrogHari~

aurora vulpinlo

Neidium

iridis

IIL~--

[~--

leptostouron

Na v i c u l a

Synedro

j-

praerup~a

Naviculo

Pinflu~orla

n

peHucida kuetzlnglona

t

# ¢

microslouron

0

nano

"v"

I

(in

TIME B.P. radio carbon

~

]]-Found

II

I

3270 4.260

years) in

abundoncll

only

in

7750 +_.170 1971-73

algol

samples

10600 ± 150

273

absent from the sediment record over its entire 10,500 yr. This is probably due to its lack of preservation in the sediments (Stoermer, 1977) as it was abundant in the plankton samples taken from the lake during 1971-1973 (Dickman and Johnson, 1975). There were 167 diatom species identified, of which 7 (17%) were c o m m o n to abundant over some part of the core (Fig. 2). The 28 species have been grouped in Fig. 2 according to their tolerance to environmental conditions. Diatoms that can tolerate a wide range of environmental conditions (Patrick and Reimer, 1966, 1975) are referred to as eurytopic. Species that tend to be indicative of increased production in lakes are said to be eutrophic indicators (Foged, 1954; Stoermer, 1977). Species tolerant to low salinity were referred to as halophilous (Bradbury, 1975) while species indicating alkaline waters were referred to as alkaliphilous (Hustedt, 1930). However, there is much controversy in the literature regarding the indicator status of particular taxa (e.g., Tailing, 1955; Stoermer and Yang, 1968; Stockner, 1971; Haworth, 1972; Birks et al., 1976). In this regard Asterionella formosa, Tabellaria fenestrata and T. flocculosa might equally well be c o n s i d e r e d eurytopic in place of eutrophic. The first diatom to appear above trace levels was a halophilic thick-walled benthic species Opeophora martyi which apparently was more resistant to mechanical damage than the thinner-walled planktonic taxa that appeared later in the sediment core. Opeophora martyi first occurs in Pollen Zone 7 along with 3 other halophilic species. While halophilic species were particularly predominant in Pollen Zone 6 they are accompanied by eurytopic and alkaliphilic species. This suggests that a substantial lens of fresh water was floating on denser brackish water below. However, by the end of Pollen Zone 6 (7750 + 170 yr. B.P.) all of the halophilic taxa had fallen to trace levels with the exception of Gyrosigma attenuatum which is capable of living also in fresh water. Eutrophic indicator species appear in Pollen Zone-5 sediments but the dominant species was the alkaliphilic diatom, Rhopalodia gibba. In Pollen Zones 2--4, no particular group of diatoms was predominant. However, the eutrophic species Fragilaria crotonensis became dominant in the latter part of Pollen Zone 4 and continued its dominance into the first part of Pollen Zone 3 before declining to trace levels.

Fig. 2. Relative abundance of 28 dominant diatom species. Diatom species consisting of less than 1% of the total diatom flora in any particular sample are indicated with a straight line. Species constituting 1--10% of the total diatom population are represented by a narrow divergence; species constituting 11--30% were termed " c o m m o n " and are depicted with a moderate divergence. The widest separation of lines represents abundant or dominant species, i.e. those constituting more than 30% of the sample. The nine pollen zones of Mott and Farley-Gill (1981) and the Pink Lake radiocarbon dates are superimposed for purposes of comparison.

274

Rectangles in Fig. 2 represent the relative abundance of the dominant diatoms in the phytoplankton in Pink Lake, during the May--September period from 1971 to 1973, when bi-weekly plankton samples were taken. The dominance of eutrophic species (Fig. 2) during this period is coincident with high nutrient loading, high algal standing crop and low Secchi transparency (Dickman and Dorais, 1977).

Sediment chemistry The lowest water content (Fig. 3) occurred in the compact clay of Pollen Zone 9 and was highest in the surficial sediments. There are several trends in sediment organic matter concentrations through the core. The organic matter concentration was lowest (1.8%) in the clay of Pollen Zone 9 but then as forest vegetation established and the climate ameliorated through Pollen Zones 8--5, the organic content increased consistently, with some minor fluctuations. The highest concentrations occurred in Pollen Zone 5 (73.5%) and at the boundary of Pollen Zones 4 and 5 (73.8%). The decline to 43.6% in Pollen Zone 4 was interWater Content % fresh wt. Depth am.

0 O

Carbonote$ % dry wt.

20

40

60

80

I

I

I

I

Orgonic Matter % dry wt.

~1

Pollen Zones I

r

2

I00

0

2

4

6

8

I0

0

I0

20

30

40

50

60

70

)

5

I00 -

4

200-

5

6 7 300 -

8 9 I

I

0

0"5

J

I

1,0

0

I

I

4

I

I

8

mg cm "2 y r f l

Fig. 3, W a t e r c o n t e n t as % f r e s h w e i g h t c a r b o n a t e s a n d o r g a n i c m a t t e r as % d r y w e i g h t a n d f l u x (A A) o f c a r b o n a t e s a n d o r g a n i c m a t t e r as m g c m -2 y r . -~ in P i n k L a k e sediments.

275

rupted by an increase to 65% at a depth of 162.5 cm. The trend from the lowest concentration (43.6%) was for an increase in organic matter in the upper part of Pollen Zone 4, and through Pollen Zone 3 (with minor fluctuations) to 58.6% at the boundary between Pollen Zones 3 and 2. A decline to 45.7% organic matter in the middle of Pollen Zone 2 is followed by another increase into the surficial sediments. Using the age--depth diagram determined for Pink Lake by Mott and Farley-Gill (1981) and a solid density of 2.5 g cm -s (Lerman, 1979) (because the density of Pink Lake sediments was not determined), the flux to the sediments of chemical species can be estimated (Lerman, 1979, p. 341). The flux of organic matter apparently increased by the greatest amount in the Picea Pollen Zone (Fig. 3), as spruce trees rapidly replaced poplar/aspen on the landscape (Mott and Farley-Gill, 1981). The flux of organic matter then declined in Pollen Zone 6, was lowest in Pollen Zone 5 and then gradually increased, with minor fluctuations into Pollen Zone 1. Carbonate concentrations (Fig. 3) were low in Pollen Zones 7--9 and increased from 1.8% to 9.0% in Pollen Zone 6 when Pinus species were predominant. A decline in carbonate concentrations into the middle of Pollen Zone 5 was followed by an increase to the highest concentration (9.6%) in the sediment profile at the boundary of Pollen Zones 4 and 5. Nitrogen rng/g dry wt Depth cm

eho,~

N/OM xlO-5

P/O~.

dry wt

xlO - o ~km

0

0

I

2

3

0

I

2

3

4

5

0

2

4

6

0

4 i

)

6 i

8

I0

i

i

I

2

3

I00.

4

200. 5

6

7

~o-~

m

e_ 9

0'

.& .o~ .~ mg crn-2 yr71

. . .02. .

0

,04 m~ cm-2yr, -1

.06

Fig. 4. Nitrogen and phosphorus as mg/g dry weight (nitrogen)/(organic matter) and (phosphorus)/(organic matter) ratios, and flux ( A A) o f nitrogen and phosphorus as m g cm -2 yr. -~ in Pink Lake sediments.

276

From that time on (5500 yr. B.P.) the carbonate concentration gradually declined, with minor fluctuations to 4.3% in the surficial sediments of Pollen Zone 1. The flux of carbonates (Fig. 3) increased from Pollen Zone 4 and then remained relatively constant through to Pollen Zone 1. The total phosphorus concentration (Fig. 4} of the clay in Pollen Zone 9 ( ~ 2 mg g-l) is higher than the average for igneous rocks (1.18 mg g-t) reported by Rankama and Sahama (1949) probably because of the presence of apatite-rich rocks in the drainage basin (Hogarth, 1970). The highest concentrations of phosphorus in the sediments occurred through Pollen Zone 5 and most of Pollen Zone 4. The concentration declined significantly in the upper part of Pollen Zone 4 from ~ 3 . 5 to 1.0 mg g-t at the boundary of Pollen Zones 3 and 4. Then, it followed a gradual increase to 2.6 mg g-1 and another decline into the surficial sediments. After an initial increase of the flux of phosphorus to the sediments in Pollen Zone 7 (Fig. 4) and decline into Pollen Zone 6, the pattern of flux is similar to that exhibited by the concentration of phosphorus. The high P/OM ratio in the clay of Pollen Zone 9 can be attributed to the low concentration of sediment organic matter. The rapid decline of this ratio through Pollen Zones 8--6 reflects substantially increasing sediment organic matter concentrations and so dilution of the phosphorus. The nitrogen concentrations (Fig. 4) in the clay are very low (< 0.1 mg g-l) but then increase rapidly along with the organic matter content through Pollen Zones 6--8. There is, in fact, a significant correlation (P ~< 0.01) between sediment nitrogen and organic matter concentrations (Table II). The rapid rise in the N/OM ratio (Fig. 4) through Pollen Zones 9--7 and at the end of Pollen Zone 1 and in Pollen Zone 2 is due to a more rapid increase in the concentration of sediment nitrogen than sediment organic matter. The increase in the N/OM ratio at the end of Pollen Zone 1 and into Pollen Zone 2 reflects a greater proportionate increase of sediT A B L E II C o r r e l a t i o n m a t r i x f o r m a j o r e l e m e n t s in s e d i m e n t s o f P i n k Lake, Q u e b e c (33 s a m p l e s analyzed) OM OM

N P Ca Mg Fe K Mn

N

P

Ca

Mg

Fe

K

-0.4036*' --0.0524 - 0 . 6 9 2 6 *2 - - 0 . 4 0 5 2 .1 --0.6583*: --0.2956

---0.1547 --0.3296 --0.1597 --0.2647 - - 0 . 3 5 2 7 .1

0.2502 0.2757 0.3430 0 . 3 7 1 7 .1

0.7234*: 0.9348*: 0.6594*:

-0.7475*: 0.7108":

-0.6520*:

---

0.8351": 0 . 4 3 2 3 .1 --0.3156 --0.8488*: - - 0 . 5 8 9 4 *2 --0.8400*: - - 0 . 5 6 3 6 *2

OM = o r g a n i c m a t t e r . ,1 S i g n i f i c a n t at P < 0 . 0 5 ; ,2 s i g n i f i c a n t at P ~< 0.01.

277

ment nitrogen compared to the increase in sediment organic matter. The nitrogen flux increased sharply through Pollen Zone 7 (Fig. 4) b u t then declined into Pollen Zone 6 and remained relatively constant through the following pollen zones. K and Mg were negatively correlated with the organic matter in the core (Table II) so they were expressed as mg/g inorganic matter (Fig. 5). The highest concentration of each element occurred in the grey clay of Pollen Zone 9 and remained relatively high, with fluctuations, into the middle of the Pinus Pollen Zone (Zone 6) when concentrations declined into Pollen Zone 5. The sharp increases shown by Mg in the Fagus grandifolia Pollen Zone (Zone 3) was not shown by K. A significant correlation did not exist between Ca and organic matter or between Ca and K or Mg (Table II). The substantial increase of Ca in Pollen Zone 6 is followed, as with K and Mg, by a decline into the Hemlock Pollen Zone (Zone 5). Further increases in Ca occurred in the early part of Pollen Zone 4 and the latter part of Pollen Zone 3 and then a gradual decline occurred into Pollen Zone 1. The flux of Mg, K and Ca increased in Pollen Zone 7 (Fig. 5), declined sharply through the following pollen zone and then follows a pattern similar to that for the respective element concentrations. M

agneaium

Potol s l u m rng/g inorganicwt,

Depth cm

4

8

12

I0

30

Calcium 50

80

40

120

O"

Polkm Zones I 2

3

IOO

4

200 5

6

7 300

8

% t 0

I 0.1

I 0'2

I 0.3

I 0.4

9 I 0

I 0.1 mg

I i I i I , 0.2 0.6 1.0 c m "2 yr I

I 0

I

I 1

I

I 2

Fig. 5. Magnesium, potassium and calcium as m g / g inorganic weight and flux (A of magnesium, potassium and calcium as m g cm -2 yr.-1 in Pink Lake sediments.

-)

278

Iron Depth cm

Fe/Mn

Mongonese

mg/g dry wt

0 20 Ol~lhi

40 60 , ,

0 0.4 0.6 0.8 ~ # ' ~ '

Pot len

2O 4O

6O 8O

Zones

5

,oo

J

200

300---

0

I

I

1

2

J

0 mg

J

x12_2_ 4

c m - 2 y r "1

Fig. 6. Iron and m a n g a n e s e as m g / g dry weight and F e / M n ratio, and flux (A iron and m a n g a n e s e as mg c m -2 yr.-I in Pink Lake s e d i m e n t s .

A) o f

The trend of sediment Fe and Mn through Pollen Zones 9--5 (Fig. 6) is of declining Fe and Mn. The Fe concentration then remains relatively constant to the surface while sediment Mn tends to increase through Pollen Zone 3. The Fe/Mn ratio reflects these broad trends. The flux of Fe (Fig. 6) declined towards the end of Pollen Zone 7, whereas that of Mn increased. After a decline into Pollen Zone 6, the flux of both elements remains relatively constant through the remainder of the core, except for slight increases in Pollen Zone 1. Cu, Zn and Pb were not included in the correlation matrix (Table II) because sediment samples used for their analysis were not taken from the same depths in the profile as the elements in Figs. 4--6. Cu and Zn concentrations {Fig. 7) increased in the post-glacial in association with sediment organic matter to peak concentrations in Pollen Zones 5 and 6. This association is particularly noticeable for Zn. Cu and Zn declined through Pollen Zone 4 along with sediment organic matter and

279

remained at relatively constant concentrations, with minor fluctuations until Pollen Zone 1. In this pollen zone, Cu, Zn and Pb exhibit sharp increases which may be attributed to anthropogenic sources. With the exception of Pollen Zone 7, the pattern of flux of Cu and Zn to the sediments follows the pattern for respective metal concentrations. The concentration of Pb through most of the core is very low and so its flux is n o t presented in Fig. 7. Depth Cu crn ppm 100200300

Zn pprn x

I00

4

12

8

16

Fro ppm

Pollen Zones

100200 300

~I'TI

m

4

5

6

I/J

7

300~ /

9 ,

i

o O.s lO-2mg

i

,

~

li

i

I

50

s

le

c m - 2 y r -1

Fig. 7. C o p p e r , zinc a n d lead as p p m dry w e i g h t a n d flux o f c o p p e r a n d zinc as m g c m -2 yr. -1 in P i n k Lake s e d i m e n t s . DISCUSSION

The basal clay (300--320 cm) in the cores did not contain any marine microfossils (Mott and Farley-Gill, 1981) so the complete sequence of sediments laid down since Pink Lake was isolated from the Champlain Sea had not been sampled. This clay, in fact, contained a pollen assemblage, indicating the presence of herb and shrub vegetation so that the clay was deposited by freshwater runoff. However, it is reasonable to suppose that the Pink Lake basin went through an estuarine phase and limited tidal phase before being isolated from the sea b y post-glacial uplift and that evidence of this phase exists in sediments below 320 cm. The presence of halophilic diatom species in the increasingly organic sediment of Pollen

280 Zones 7 and 6, when spruce (Picea) and pine (Pinus) pollen were c o m m o n in the respective zones suggests that water in the deeper strata of Pink Lake was not completely fresh. The simultaneous presence of both fresh and brackish water (halophilic) diatom species in the deepest sediments of the lake (230--300 cm) suggests that a lens of less dense water was present on top of denser brackish water in the lake. By the end of the Pinus Pollen Zone (Zone 6), ~ 7 7 5 0 yr. B.P., most of the halophilic taxa were present in the sediment at only trace levels. It is believed that the reduction to trace levels of the halophilic species was due either to the depth of the freshwater lens exceeding their light compensation point and/or a decline in salinity below their tolerance limits. Initially convective circulation would have caused desalination of the uppermost waters of Pink Lake as suggested by Rust and Coakley (1970) in studies of the post-glacial desalination of Stanwell--Fletcher Lake on Somerset Island in the Arctic. However, it seems likely that the sheltered nature of the steep-sided Pink Lake would have greatly retarded wind induced turbulence to depths below the present-day chemocline. Calculations based on the formulae of Toth and Lerman (1975) indicate that simple diffusion processes would result in complete desalination of a lake the size and shape of Pink Lake in ~ 4 0 0 0 yr. This figure compares favourably with the 3000 yr. or so required for most of the halophilic diatom species to decline to trace levels, particularly if the slow process of simple diffusion was aided on occasion by the ventilation of the upper monimolimnion (Dickman and Artuz, 1978). It is postulated that in Pink Lake there was a gradual change from ectogenic to biogenic meromixis over a period of 3000--4000 yr. with a gradual decline in salts of marine origin and a gradual increase of salts of biogenic origin so that by the time hemlock pollen reached a maximum (Pollen Zone 5) the salts in the monimolimnion were predominantly of biological origin. Because desalination appears to have occurred over a very long period of time, some of the salts in the saline monimolimnion may have been gradually deposited in the accumulating sediments as well as being lost into the mixolimnion and subsequently from the lake via the outflow. It is thus improbable that a sudden change in the concentration of a particular element or elements in the sediment sequence would occur during desalination, but rather a gradual decline in concentration would be evident. While there is a trend, with interruptions, of declining concentrations of Mg and K in the mineral fraction of Pink Lake sediments during the postulated period of desalination, it is believed that concentrations of these elements in the sediments are also related to the intensity of erosion and leaching of soils (Mackereth, 1966), rather than simply to desalination. Ericsson (1973) found in the post-glacial sediments of some Swedish lakes that exchangeable Mg, and to some extent exchangeable Ca, showed very good agreement with former salinity conditions as inferred by diatom evidence. Ericsson also mentioned attempts by other authors to use chemical characteristics of sediments to provide evidence of paleosalinity, the best indicator apparently being sediment boron concentrations.

281 Sediment organic matter concentration increased nearly exponentially during the period of forest establishment and soil stabilisation with the flux increasing during the period when spruce replaced poplar and/or aspen (10,200--9800 yr. ago). A similar exponential increase in the organic matter c o n t e n t of the sediment was noted in several holomictic lakes during the post-glacial period (Hutchinson and Wollack, 1940; Mackereth, 1966; Wetzel, 1970; Likens and Davis, 1975). Adams and Duthie (1976) postulated a largely autochthonous origin, as a result of increased lake paleoproductivity, for the organic matter in the late Quaternary sediments of Sunfish Lake in southern Ontario. While paleoproduction may n o t have been as high as in a holomictic lake, because the monimolimnion acts as a trap for nutrients in a meromictic lake, it is reasonable to assume increasing paleoproductivity in Pink Lake with climatic amelioration during the late Quaternary when trees such as Pinus, Betula, Quercus, Ulmus and Fraxinus established in the Gatineau Park area. At the beginning of Pollen Zone 7, spruce rapidly replaced the declining poplar/aspen, although the spruce did n o t form a closed forest (Mott and Farley-Gill, 1981). The spruce gradually declined towards the t o p of the pollen zone b u t was not replaced immediately by other genera. While hardwood pollen is present in the sediments of this zone, Mott and Farley-Gill are n o t sure that the pollen originated from trees growing in the area. The flux of phosphorus, nitrogen and bases to the sediments increased through this pollen zone, suggesting that erosion from the unclosed forest and incompletely developed soils was an important process whereby nutrients entered the lake. Organic matter, too, may have been carried into the lake and, along with a u t o c h t h o n o u s organic matter, caused the increase in flux (Fig. 3) to the sediments. Towards the end of the following Pinus Pollen Zone, the zone in which halophilic diatom species were abundant, sediment carbonate flux increased and concentration quadrupled. There was also an increase in the flux of organic matter in this zone after the sharp decline of flux at the end of the previous Picea Pollen Zone. Adams and Duthie (1976) observed in Sunfish Lake sediments a decline in the sedimentation intensity (flux) of organic matter as spruce declined and then an increase in intensity as pine and several deciduous species increased. Brunskill ( 1 9 6 9 ) a n d B o y k o (1973) observed spring-time precipitation of carbonates in meromictic lakes which occurs also in Pink Lake (Dickman, 1979). Increased paleoproduction causing biological precipitation of carbonates may thus account for the increase in sediment carbonates in the Pinus Pollen Zone. Whitehead et al. (1973) believe that a cycle of eutrophication occurred in a New England lake which coincided with the decline of hemlock. While there was an increase in sediment phosphorus concentrations in the Hemlock Pollen Zone in Pink Lake (Fig. 5) there was not apparently an increase in flux to the sediment. The decline of hemlock, ~ 4 8 0 0 yr. ago (Mott and Farley-Gill, 1981) at Pink Lake was followed by an increase in flux of

282 phosphorus, organic matter and carbonates to the sediments. These observations, and the appearance of eutrophic indicator species such as Fragilaria crotonensis and Tabellaria flocculosa, suggest a similar cycle of eutrophication in Pink Lake. Sediment Mg and K are mainly associated with the mineral fraction rather than with the organic fraction in English lakes (Mackereth, 1966). This is also the case in Pink Lake where a significant (P ~< 0.01) negative correlation for these elements with sediment organic matter was noted. Mackereth (1966) suggested that high sediment Mg and K indicated that soil erosional processes were more intense than leaching and vice versa. The high concentrations of sediment Mg and K {Fig. 4) when the vegetation was composed of herbs and shrubs (Pollen Zone 8) suggests substantial erosional input into this steep-sided lake which continued in Pollen Zone 7 when the forest was not yet completely closed. The subsequent declines in concentration and flux of Mg and K in Pollen Zones 6 and 5 indicate that erosional processes were giving way to leaching as the forest became closed and more complex and as soils developed and stabilised. As well as a decline in erosional transport, it is likely that podzol formation beneath coniferous vegetation reduced the influx of dissolved bases into the lake (Rodin and Bazilevich, 1968). Soluble organic compounds with solvating and chelating abilities, produced in the accumulating coniferous litter, would leach bases and sesquioxides of Fe and A1 from the upper soil layers, depositing them in the B horizon (Messenger et al., 1972; Messenger, 1975; De Kimpe and Martel, 1976). High values of Ca in sediments have been associated with leaching of softs rather than with erosional processes (Mackereth, 1966) but it is likely that biological precipitation has been more important in the post-glacial sediments of Pink Lake. In the early part of the late Quaternary, when relatively few coniferous species were dominant (Pollen Zones 5--7), destructive fires may have occurred as flamable litter accumulated (D. Walker, 1981). Following such fires, increased erosional input of nutrients into lakes occurred until enough litter and humus accumulated under regenerating forests to stabilise the soil. The effects of fires is reduced by increased diversity of the vegetation (D. Walker, 1981) so that fires around Pink Lake are likely to have had a diminishing role in the input on nutrients into the lake during the past 5000 yr. A study of charcoal influx into Pink Lake sediments could indicate whether increases in the flux of Mg and K to the sediments is associated with periodic fire episodes. The interpretation of the stratigraphic distribution of Fe and Mn in lake sediments is complicated by the fact that these elements undergo transformations to different valency states in soils, lake waters and sediments, depending on whether the particular environment is oxidizing or reducing (Mulder and Gerretsen, 1952; Hutchinson, 1957), acid or alkaline (Goldschmidt, 1954). Furthermore, both elements can form soluble complexes

283

with organic matter (Shapiro, 1957). Thus Fe and Mn may enter lakes in erosional material or in solution. The concentration (0.1 mg 1-1) of Fe in the monimolimnion of Pink Lake is n o t important in maintaining mermoxis (Dickman and Hartman, 1979). The sediment cores did n o t exhibit any black deposits of ferrous sulphide b u t rather they were brown in colour (Dickman et al., 1975) and the estimates of the flux of Fe to the sediments (Fig. 6) indicate that the flux was almost constant above the Pinus Pollen Zone ( ~ 8 0 0 0 yr. B.P.). Because the lake is a hardwater lake, and the sedim e n t carbonate content increased rapidly during the early stages of forest establishment, it is probable that Fe in the monimolimnion has n o t played an important role in the meromictic history of the lake. The flux of Mn to the sediment follows an essentially similar pattern to that of Fe. The increase of sediment Cu and Zn (Fig. 7) as forests established may reflect increased mobilization of these elements from softs while the increase in concentration and flux of these metals and of Pb in the past 150 years is a result of anthropogenic activities. ACKNOWLEDGMENTS

The authors are grateful for the technical assistance provided by Dr. E. Krelina in the preparation of sediment core samples. Financial assistance for R. Jones by the Trent University Research Committee is gratefully acknowledged, as is the help of Mrs. B.J. McKeown and Mrs. M.E. Read in manuscript preparation. The research of M.D. Dickman was jointly funded by the National Capital Commission and the National Sciences and Engineering Research Council o f Canada.

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