Isotopic and geochemical composition of marl lake

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Isotopic and geochemical composition of marl lake waters and implications for radiocarbon dating of marl lake sediments J. V. TURNER,' P. FRITZ,P. F. KARROW,AND B. G. W A R N E R ~ Department of Earth Sciences, University of Waterloo, Waterloo, Ont., Canada N2L 3G1 Received April 29, 1982 Revision accepted November 24, 1982

Radiocarbon dates on organic and calcareous fractions of sediment cores from marl lakes may yield anomalous ages due to the assumption of a constant hardwater correction factor along the sediment sequence. A study of eight marl lakes in southern Ontario that are actively precipitating calcium carbonate was conducted in order to assess those isotopic and aqueous geochemical parameters in modem lakes that may be utilized to estimate the history and extent of variations in the hardwater effect along such sediment sequences. Results show an increase in the 813C composition of lake DIC (dissolved inorganic carbon) as approach to isotopic equilibrium with atmospheric C 0 2 occurs. Differences in the extent to which this equilibrium is established also appear responsible for observed differences in the 14C activity of DIC between lakes of as much as 20 pmc (percent modern carbon). These variations have been related to the relative residence times of water in each lake by examination of their corresponding seasonal variations in 1 8 0 and 2~ content. Consequently 613c and 81s0 of marl and molluscs have been used to identify variations in the hardwater effect along the sediment profile. A profile of radiocarbon dates on marl from Little Lake in southern Ontario shows satisfactory agreement with an independently determined pollen chronology. Where certain criteria are met, marl deposits appear to be suitable material for establishing Quaternary chronology. Les datations au radiocarbone des fractions organique et calcaire de carottes de stdiments prClevCes dans les lacs de marne peuvent fournir des lges anormaux a cause de l'application hypothttique du meme facteur de correction pour la duretC de l'eau dans toute la sCquence ~Cdimentaire.Une etude de huit lacs de marne dans le sud de I'Ontario, oh il y aprkcipitation de carbonate de calcium, fut entreprise pour Cvaluer les paramktres isotopiques et de la gkochimie de I'eau de ces lacs actuels et dont les rksultats pourront servir a retracer I'histoire et Cgalement dans quelle mesure varie l'effet de la duretC de I'eau dans de telles sCquences ~Cdimentaires.Les rksultats indiquent une augmentation du 813c de la composition du carbone inorganique soluble (CIS) des lacs lorsqu'on s'approche des conditions d'tquilibre isotopique avec le C 0 2 atmosphCrique. Les diffkrences dans les valeurs correspondant aux conditions d'kquilibre semblent kgalement responsables pour les diffkrences observkes dans I'activitC du 14Cdu CIS entre les lacs jusqu'a autant que 20 pour-cent du carbone actuel (pca). Ces variations sont relikes au temps de skjour . consCquent, de l'eau dans chaque lac tel que rkvC1C par l'observation des variations saisonnikres des teneurs en ''0 et 2 ~ Par 613Cet 8180 de la marne et des mollusques ont semi il reconnaitre les variations dans I'effet de la duretC de I'eau le long du profil des sbdiments. Un profil de datations au radiocarbone pour la marne du lac Little, sud de I'Ontario, exhibe une concordance satisfaisante avec les rksultats obtenus indkpendamrnent a l'aide d'une chronologie Ctablie sur 1'Ctude des pollens. Dans certaines conditions dbterminhs, les dCp6ts de marne peuvent fournir des rksultats convenables pour Ctablir la chronologie du Quatemaire. [Traduit par le journal] Can. J. Earth Sci., 20, 599-615 (1983)

Introduction A number of problems have been identified in relation to 14Cdating of lake sediment profiles. The most widely discussed of these are the so-called hardwater effect and the associated problem of reconstruction of the initial 14C activity of a sample (Godwin 1951). Deevey et al. (1954) contributed to the discussion by documenting the extent of the hardwater effect in lake sediments and indicated that apparent ages could overestimate true ages by up to 2000years. Stuiver (1967) suggested a now frequently adopted procedure of correcting for the hardwater effect by using the 14C activity of the present-day surface sediment as the initial activity of all 'Present address: CSIRO Division of Groundwater Research, Private Bag, P.O. Wembley, Western Australia, 6014. 'Present address: Department of Biological Sciences, Simon Eraser University, Burnaby, B.C., Canada V5A lS6.

subsequent samples deeper in the sediment profile. As he indicated, however, a deficiency of this method is that the hardwater effect is not necessarily constant with time and may vary during sediment deposition. Karrow and Anderson (1975) similarly expressed concern that the assumption of a constant geochemical and hydrologic regime throughout the history of a lake was unrealistic. Mott (1975) actually demonstrated variation in the hardwater effect within a single sediment profile, and by using palynological stratigraphy he established that differences exist in the hardwater effect between lakes in a given region. Some workers (e.g., Mott 1975) have suggested that organic material is more suitable for 14C dating than inorganic carbonate as it does not suffer from the hardwater effect. This, however, is not necessarily correct. The schematic diagram of the carbon cycle of a lake shown in Fig. 1 demonstrates that organic material

,

600

CAN. J. EARTH SCI. VOL. 20. 1983

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HYDROLOGIC

PARAMETERS

INORQANIC CARBON

ORGANIC CARBON

CYCLE

CYCLE

---III

Surface Outflow

--+ I

Aquat lc Plants

I

I

I

.

I

I

I

I

II

Sediment

LI-,-------------I Groundwatsr Inflow

FIG. 1 . Schematic diagram of lake carbon budget. Shown from left to right are the relevant hydrologic parameters, the inorganic carbon cycle, and the organic carbon cycle.

derived from totally submerged aquatic plants must obtain its carbon from the total dissolved inorganic carbon of its environment and consequently must also respond to the hardwater effect. Emergent plants that derive their carbon from atmospheric C02 may reasonably be expected to have an initial 14Cactivity of 100% modem carbon (pmc). Samples of fine organic material for dating comprise an unknown proportion of emergent and submergent organic detritus and thus the use of fine organic material for 14C dating my involve problems of the hardwater effect. Shotton (1972) has, for example, documented an apparently constant hardwater error of 1700 years in organic material from late Quaternary sediments in Jutland, Denmark. A further point that has received little attention is the possible presence of detrital organic material or carbonate in samples for dating. It is difficult to recognise detrital organic matter yet the presence of detrital carbonate within an authigenic carbonate sediment can sometimes be detected by microscopic analysis. The presence of quartz grains, dolomite, clay minerals, and weathered carbonate grains may also indicate the

likelihood of detrital carbonates being present. Terasmae (1979) estimated apparent ages of marl samples from a single deposit as 300 years older than the true ages and suggested that the marl, which contained silt, was also likely to contain detrital carbonate. In general it appears that authigenic carbonate derived from direct precipitation of aqueous lake carbonate is not likely to contain detrital carbonate and will be free of this potential source of uncertainty if the carbonate deposit is remote from surface discharge sources into the lake. Such authigenic carbonate will, at the time of formation, reflect the isotopic composition of lake water, which in turn is controlled by the hydrogeologic and geochemical regimes. Variations in these environmental conditions of a lake can thus give rise to variations in the hardwater effect during sediment deposition by affecting the extent to which atmospheric C02 can equilibrate isotopically (14c) with lake dissolved inorganic carbon (DIC).More specifically, if there are variations of lake DIC with time due to relative changes in the amount of surface runoff, precipitation, and groundwater inflow, then there will be an unknown


TURNWET AL.

Trenton and Black River Groups Limestone

a Guelph-Lockport-Amabe Formations Dolomite

Salina Formation Dolomite,Shale

l

Bertie-Akron Formation Shale

0Precambrian

Bois Blanc Formation Limestone

Meaford-Dundas Shale Collingwood & Queenston

0Shales

FIG.2. Location of marl lakes in southern Ontario after Guillet 1969.

correction for the hardwater effect. One may also expect these variations to be reflected in the stable isotope ( 2 ~ 180, 13C) and geochemical composition of the lake water and hence in the sedimentary material, including molluscs. 1 8 0 fractionation between water and carbonate is strongly temperature dependent (=0.4%0 per degree Celsius, O'Neil et al. 1969) and must be considered when interpreting carbonate 180compositions. The temperature dependence of the 13C fractionation is much smaller (0.06%0 per degree Celsius, Ernrich et al. 1970) and may be neglected in the temperature range of lake waters considered here. This information may then be used to qualitatively detect possible variations in the hardwater effect along the sediment profile.

This paper describes a problem-solving approach to , 14C dating of lake sediments. A survey of several marl lakes in southern Ontario (Fig. 2), which are actively precipitating calcium carbonate, was undertaken in order to study their stable isotope (2H, 180, 13C, 14C) and geochemical (major ion) compositions. The object was to determine whether the 180and 13Ccompositions of lake water were in fact related to the 14C activity of lake water and hence whether they could be used to detect variations in the hardwater effect along the sediment profile. As a case study, a profile of 14C activities was obtained from a marl bed in Little Lake, southern Ontario, and variations in the stable isotope composition of the carbonate were used to qualitatively predict variations in the hardwater effect along the


602

CAN. J. EARTH

sediment profile. The independent chronology established by detailed pollen analysis was carried out in order to test the calculated 14C ages.

Methods Coring methods Two adjacent sediment cores were taken from ice cover over a centrally located marl bed in Little Lake following a piston coring method described by Patterson et al. (1978). The cores were recovered intact inside a 7.5 cm aluminum core barrel and returned immediately to the laboratory for processing. Core temperatures were kept below 4°C at all times. Samples for I4C, I3C, and 180analysis on carbonates were taken from the second, longer core (see Turner and Fritz 1983, Fig. 2).

sc:I.

VOL. 20, 1983

samples by treatment with dilute hydrochloric acid. Aqueous DIC samples for 14Canalysis were precipitated in the field as barium carbonate. Carbon dioxide generated from samples was converted to benzene and the 14C activity measured by direct liquid scintillation counting. (c) 180, 'H (water samples) 1 8 0 analyses were performed by the standard C 0 2 equilibration method (Epstein and Mayeda 1953). H ' analyses were performed on hydrogen produced by reduction of H20 over hot (800°C) uranium (Godfrey 1962). Results are reported in the standard 6 notation relative to standard mean ocean water ( s ~ o w ) . Pollen analysis The sediment core was subsampled for pollen analysis at intervals (see Appendix 1) throughout its entire length. Each sample was treated following standard palynological techniques (Faegri and Iversen 1975), and subsequently rinsed through an ethyl alcohol series, tertiary butyl alcohol, stained with safranin-0, and mounted in silicone oil. The pollen was identified and tabulated at 400X magnification and under oil immersion at 1000x magnification for critical determinations. Relative pollen frequencies were calculated on a minimum total of 300 arboreal pollen grains.

Sample preparation for isotopes (a) 13C and 180 Samples of marl for 13cand 1 8 0 analysis were divided into two groups. In the first group a small sample (0.5 g) was taken, treated with 5% sodium hypochlorite to oxidize organic material, washed with distilled water, dried, and screened through a 149 pm screen to remove shell fragments. These samples are denoted as "aliquot" samples in Figs. 11 and 12. In the second group, marl was separated from the molluscs in the remaining bulk sample by wet screening through a 149 pm screen and was again treated with 5% sodium hypochlorite, washed with distilled water, and dried. These samples are Aqueous geochemistry denoted as "bulk" samples in Figs. 11 and 12. Isotope Treatment of samples for aqueous geochemistry is analysis was then carried out on carbon dioxide described in Turner and Fritz (1983). generated by conversion of a small portion of each Results and discussion sample following standard methods (McCrea 1950). This approach was adopted to eliminate spurious effects General aspects of lake geochemistry introduced by sampling procedures. The marl consists The survey aspect of this study involved examinaof a highly uniform grain size carbonate micrite of tion of the aqueous and isotopic geochemistry of approximately 10 pm diameter. Isotope effects of grain eight marl lakes in southern Ontario. Because of the size were not examined. The freshwater mollusc presence of fresh marl deposits these lakes were Valvata sincera was found sufficiently well preserved expected to be actively precipitating CaC03. This was along the entire core to be identified and was used as a tested by carrying out geochemical modeling for each of reference mollusc to eliminate species-related isotope the eight individual lakes and two spring-lake systems effects (Fritz and Poplawski 1974). Molluscs were hand shown in Fig. 2 and Table 1. An aqueous geochemical picked, washed, treated with 5% sodium hypochlorite program was used to model calcite precipitation and solution, washed, and dried before treatment with 100% COz degassing from lake water. Initial lake water phosphoric acid. Aqueous carbonate samples were samples were taken in spring and early summer 1979 precipitated in the field as barium carbonate and reacted and chemical analyses of these waters were used as the as above. 13C and 1 8 0 analyses on carbonates are initial data for the model. Final state water samples were reported in standard 6 notation relative to PDB (Peedee taken in the fall of the same year. The amount of calcite belemnite). precipitated was taken as the observed difference in (b) 14C Ca2+ concentration between the two sampling times. Marl samples were screened to remove organic This amount was then modeled to precipitate from the detritus and mollusc fragments. The less than 149 pm initial water and the change in total DIC,pH, pC02 and fraction was collected, treated with 5% sodium pIAPcalcite(IAP= ion activity product aCa2+ . aco3'-) hypochlorite, washed in distilled water, and dried. compared with the observed values for the final state Residual carbonate material was removed from organic water. The model was also applied to two springs that


TURNER ET AL.

/

40

GUELPH DOLOMITE m w t s L.

FIG.3. Major ion distributions in lake waters.

discharge directly into two of the lakes. Results shown in Table 1 demonstrate the generally good agreement between the lake geochemistry predicted by the calcite precipitation - C 0 2 degassing model and actual final state lake geochemistry. The relation between initial and final states is independent of reaction path. The major ion compositions of the lakes studied are shown in Fig. 3. The waters are clearly dominated by their Ca and DIC compositions with minor influence by Mg. There is a correlation between bedrock type and the Mg content of lake waters. Thus lakes located on the Guelph-Lockport Dolomite (Fig. 2) have higher Mg contents than those located on Trenton Limestone. This is probably due to dolomite dissolution in the drainage basins of lakes located on dolomite bed rock. In relation to the 14Cdating problem, however, no distinct aqueous ionic species could be identified that might provide

information on changes in the hardwater effect along the sediment profile. Hardwater effect and the 13C and 1 8 0 composition of carbonates The hardwater effect for a lake is dependent on the extent to which atmospheric C 0 2 exchanges with the dissolved aqueous carbon reservoir. For a given C 0 2 exchange rate the hardwater effect will also depend on the surface area to volume ratio, the relative hydrologic residence time of water in the lake, and the total DIC of the lake water. The hardwater effect for a lake with a relatively long hydrologic residence time, low total DIC, and a high surface area to volume ratio would be a minimum since under these conditions the opportunity for atmospheric C 0 2 exchange is an optimum. Similarly, for lakes that undergo a high degree of C 0 2

0.77

0.77 0.77

1.73

1.36 1.34

(6/16/793. 0.97 To Inglesby Lake (9/22/79) 0.89 Predicted 0.87

0.16

0.18 0.16

0.35

0.34 0.35

0.05 0.05

0.74 0.84

0.03 0.03

0.05

0.81 0.90

0.66 0.62

0.03

0.10 0.09

0.09

0.25 0.25

0.25

0.25 0.22

0.22

Na

0.84

0.90

0.20 0.22

1.37

1.11 1.14

0.22

0.77 0.77

1.36 1.37

1.71

0.77

Mg

1.54

Ca

Hepworth Lake (6/30/79) 1.38 To Hepworth Lake (lO/lS/?S) 0.61 Predicted 0.61 Inglesby Lake

Predicted

(6/30/79) To Francis Lake (10/18/79)

Predicted Francis Lake

(9/22/79)

To Dry Lake

(6/17/79)

Dry Lake

(5 12/79] To Blue M e (9/18/79) Predicted Blue Lake Spring (7/17/79) To Blue Lake (9/18/79) Predicted

Blue Lake

Lake

0.04 0.03

0.03

0.02 0.02

0.02

0.01 0.01

0.01

0.02 0.02

0.02

0.04 0.04

0.05

0.04 0.04

0.04

K

0.19 0.15

0.15

0.10 0.10

0.09

0.11 0.10

0.10

0.20 0.17

0.17

0.21 0.21

0.21

0.21 0.21

0.21

SO4

0.18 0.18

0.18

0.07 0.05

0.05

0.07 0.05

0.05

0.10 0.08

0.08

0.39 0.39

0.41

0.39 0.37

0.37

C1

2.11 2.08

2.28

2.67 2.72

4.33

2.75 2.75

4.33

2.21 2.35

3.46

3.75 3.54

4.28

3.75 3.54

3.87

Alk

2.15 2.12

2.28

2.68 2.73

4.30

2.82 2.82

4.23

2.29 2.44

3.44

3.84 3.62

4.63

3.84 3.62

3.86

CT

7.99 8.01

8.21

8.22 8.22

8.18

7.92 7.94

8.43

7.76 7.73

8.18

7.83 7.86

7.53

7.83 7.86

8.30

pH

2.94 2.96

3.09

3.07 3.09

2.88

2.77 2.78

3.07

2.68 2.62

2.89

2.57 2.57

2.22

2.57 2.57

3.04

-log pCOz

8.33 8.33

7.97

8.21 8.21

7.51

8.46 8.46

7.41

8.43 8.43

7.61

8.05 8.05

8.35

8.05 8.05

7.65

calcite

~ I A P

15.3 15.3

24.1

12.0 12.0

22.5

12.0 12.0

21.0

16.50 16.50

23.5

20.0 20.0

6.4

20.0 20.0

8.8

Temp. ("C)

Mass 6I3C(%0) calcite (PDB) (rnMol/kg)

TABLE1. Results of modeling calcite precipitation and COz degassing from nine lake and two spring-lake systems during 1979 Flux Coz (rnMol/kg)


0.84

0.78 0.84

0.37

0.25 0.37

0.20

0.20 0.20

0.83 0.82

2.17

1.09 1.11

1.21

1.09 1.08

0.73 0.79

1.00 0.98

1.73

0.79

0.22 0.24

1.09 1.09

1.24

0.24

Mg

1.22

Ca

0.08 0.07

0.07

0.23 0.10

0.10

0.04 0.04

0.04

0.48 0.47

0.47

0.43 0.42

0.42

Na

0.01 0.01

0.01

0.03 0.02

0.02

0.02 0.02

0.02

0.03 0.03

0.03

0.03 0.03

0.03

K

0.10 0.10

0.10

0.17 0.18

0.18

0.13 0.11

0.11

0.15 0.15

0.15

0.14 0.12

0.12

SO4

0.10 0.07

0.07

0.28 0.09

0.09

0.06 0.03

0.03

0.70 0.71

0.71

0.53 0.49

0.49

C1

2.34 2.36

2.59

2.21 2.65

4.74

2.99 3.09

4.94

2.91 2.88

3.508

2.28 2.38

2.63

Alk

2.41 2.43

2.66

2.28 2.73

6.68

3.13 3.24

4.88

3.19 3.16

3.402

2.30 2.40

2.59

CT

7.85 7.86

8.03

7.83 7.82

6.82

7.72 7.77

8.23

7.54 7.76

8.50

8.05 7.97

8.32

pH

2.76 2.76

2.85

2.76 2.70

1.46

2.54 2.56

3.00

2.41 2.60

3.24

2.96 2.82

3.14

-log pCOl

8.33 8.33

8.05

8.39 8.39

8.79

8.47 8.47

7.62

8.76 8.56

7.49

8.14 8.14

7.71

calcite

~ I A P

15.0 15.0

24.1

15.3 15.3

9.3

12.0 12.0

22.0

2.0 2.0

18.4

17.0 17.0

24.5

Temp. ("C)

-3.6

-6.1

-7.3

-11.8

-5.5

-7.2

-0.5

-5.91

-4.5

-4.5

0.08 8.47

2.90

0.73

0.002

0.06

(mMol1kg)

co2

Flux

~ I A P(calcite) =

0.13

1.05

0.91

0.24

0.13

Mass 613c(%o) calcite (PDB) (mMol/kg)

Data show the initial and final observed water chemistries and the results of modeling calcite precipitation from the initial chemistry. NOTES: taken as calcite saturation at 25'C.

Louise Lake (30/6/79) To Louise Lake (t0/18/79) Predicted Marl Lake spring (10/18/79) To Marl Lake (9/20/79) Predicted Raven Lake (6117179) To Raven Lake (9120179) Predicted

Little Lake (5/18/79) To Little Lake (12/5/79) Predicted

Predicted

Julian Lake (6/17/79) To Julim Lake (9/22/79)

Lake

TABLE1. (Concluded)



606

CAN. J. EARTH SCI. VOL. 20, 1983

FIG.4. 8I3c composition of lake DIC distributed according to carbonate hardness.

exchange, the heavy isotope ('H and 180) enrichment due to evaporation may also be expected to be significant. Thus the relative hydrological residence times for lakes in a given region may as a first approximation be estimated by their seasonal variation in stable isotope composition. An estimate of the extent to which a lake water has reached isotopic equilibrium with atmospheric C 0 2may be obtained from the 13ccomposition of DIC.With an atmospheric C 0 2 613C of -7.0%0 and an equilibrium fractionation between C 0 2 and H C 0 3 of - 8.3%0 (Turner 1982) isotopic equilibrium between atmospheric C02 and lake DIC would lead to a 613C composition of 2.0%0for lake DIC at a pH of 8 .O. The degree of approach to isotopic equilibrium is indicated by the approach of 613C DIC to this equilibrium value. Thus we may expect to see varying degrees of correlation between the 14C activity of lake water and the degree of enrichment in the stable isotopes 2 ~ 180, , and 13C for lakes in a given region. Furthermore, the variations in 6180 and 813C DIC of lake waters are

+

recorded in the isotopic composition of precipitated carbonates. Thus, for example, a marl that forms in isotopic equilibrium with atmospheric C 0 2 should have a 6I3C = + 3 to +5%0. This isotopic translation suggests the possibility of detecting relative variations in the hardwater effect along a sediment profile by measuring the stable isotope composition of the precipitated carbonate. The above points will be illustrated in the following by reference to isotopic variations in the lakes of the present study. Firstly, Fig. 4 shows a distribution of lake DIC 13C compositions according to carbonate hardness. The lower line represents molar equivalence of ca2+ and HC03- concentration for lake water. A trend can be seen of increasing 13C composition with decreasing carbonate hardness and is due to the fact that, for a given amount of atmospheric C 0 2 exchange, lakes of lower carbonate hardness will exhibit a greater shift towards positive values. The upper line illustrates a similar trend for lakes with significant Mg2+ content. On the basis of

607


TURNER ET AL.

-13 0

-12.0

-11.0

-9.0

-10.0

-8.0

-7.0

- 6.0

18

60

%OSMOW

FIG. 5. 82H-8180 relations for lakes in this study during the summer-fall of 1979.

this it is possible to identify Little and Inglesby Lakes as having a relatively larger shift toward positive 613c values, enhanced 14C activity, and hence a diminished hardwater effect. Similarly, Blue Lake, which has a high carbonate hardness and relatively low 13c composition (- 11.1 to -6.5%0), may be expected to have a more significant hardwater effect. Secondly, for lakes with relatively long hydrologic residence times, significant evaporative enrichment of ~ 180) is expected. the heavy isotopes of water ( 2 and Samples for 2~ and 1 8 0 analysis were taken from nine lakes during spring and summer of 1979 and the data are shown in Fig. 5. From these data it is once again possible to identify both Inglesby and Little Lakes as having undergone significant evaporative enrichment of 2~ and 1 8 0 , indicating a relatively long hydrologic residence time for these lakes. On the other hand, Blue Lake exhibits little isotopic enrichment, indicating an active hydrologic regime with rapid throughflow of groundwater. This is supported by the stable isotopic composition of a spring that discharges directly into Blue Lake (Fig. 5), which, as it does not differ significantly from the lake water, indicates that lake water has little opportunity to undergo evaporative enrichment during its residence in the lake. The degree of evaporative concentration of 2~ and 180 and exchange of C 0 2 depends also on the surface area to volume ratio of a lake; however, this does not affect our arguments in relation to the correlation between the 14C aGivity of lake water and the degree of enrichment in 2H, 1 8 0 , and 13C.

Thus from the stable isotope data and to some extent direct hydrologic observations it is possible to identify Blue Lake as an example of a lake with a limited degree of isotopic exchange with the atmosphere in terms of both C02 exchange and evaporation. Little and Inglesby Lakes are, on the other hand, at the opposite extreme and appear to interact significantly with the atmosphere in these terms. We may draw two predictions from the above discussion. Firstly, Blue Lake is likely to have a depressed 14Cactivity and hence carbonate precipitating from its waters would have a more marked hardwater effect, whereas in the case of Little and Inglesby lakes the 14C activity of their DIC is likely to be closer to equilibrium with atmospheric C 0 2 and thus precipitated carbonate will exhibit a less marked hardwater effect. Secondly, applying these ideas to a single lake and sediment profile, it ought to be possible to use the stable isotope composition of precipitated carbonate (or molluscs, see Fritz and Poplawski 1974) to qualitatively identify variations in the hardwater effect along a sediment rofile. In particular, covariant shifts in the "0 and ' C compositions are strongly indicative of the extent to which lake waters have evaporated and equilibrated with atmospheric C02. In the situation where photosynthetic activity plays a significant role in the carbon cycle of a lake only the 13C composition of precipitated carbonate should be affected (McKenzie, in press). The predictions are born out by the 14Cdata shown in Figs. 6, 7, and 8, which show the following.

P



I

I

Blue Lake

Limnaea decamp; Sphaerium simile Surface Marl Gyraulus parvus

M

A

14c Activity;

103pmc

J

J

A 1979

S

A 103 pmc (Spring)

O Surface Marl: 40pmc

FIG.6. Isotope data for Blue Lake: 6I3cvalues of lake DIC during the summer-fall of 1979, 613c of surface marl and molluscs, and 14C activities of lake DIC and surface marl (laboratory numbers: WAT 541, WAT 655, and WAT 578, respectively).

(1) The 14cactivity of Blue Lake water (103 pmc) is relatively depressed. This point is further illustrated in Fig. 6: the 14C activity of Blue Lake water is not enriched in 14C relative to the spring that recharges it, indicating little opportunity for atmospheric C 0 2 exchange. (2) The 13C and 180composition of both carbonate and molluscs is determined by the 13C and ''0 of the aqueous carbonate reservoir (see also Fritz and Poplawski 1974). (3) Lack of 14C equilibrium between DiC and surface carbonate samples could be due to a sampling problem with surface sediments where the surface sample actually incorporates some carbonate containing prebomb levels of 14cand consequently would calculate a finite apparent age. Bioturbation may also be partly responsible for the lower 14C activity of sediment surface carbonates relative to the lake DIC. (4) Little and Inglesby lake waters have relatively high 14C activities and the stable isotope compositions of marl and molluscs are enriched relative to those of Blue Lake.

The Little Lake core The following hypotheses were subsequently tested on cores taken from Little Lake: (1) that it is possible to obtain reliable dates on authigenic carbonate that has

precipitated from an aqueous environment where detrital carbonates are likely to be absent; (2) that variations in the hardwater effect along the sediment profile during sediment deposition may be detected by variations in the stable isotope composition of the carbonate or molluscs within the sediment profile. Microscopic analysis of marl samples from the Little Lake core showed that detrital carbonates were absent. X-ray diffraction patterns of the insoluble residue of samples revealed only trace quantities of detrital quartz and gypsum. 14C chronology Two sets of 14C ages are shown in Fig. 9. The unadjusted set was calculated assuming the initial 14C activity of the material to be 100 pmc, i.e., that there is no hardwater effect in Little Lake. From the 14C activity of present-day Little Lake water (124 pmc) this assumption is justified. However, in the discussion below evidence is drawn from stable isotope data that suggests that the hardwater effect for Little Lake has not remained constant throughout its depositional history. For comparison, adjusted ages were calculated following the suggestion of Stuiver (1967) that the apparent age of the surface sample should be used to correct all subsequent samples for the hardwater effect. Ages adjusted in this manner give apparent ages that are

TURNER ET AL. 1

1

1

1


Little Lake -6

-5

-4

'

.Sphaerium

0

simrle

,..

Valvata sincera Helisoma anceps

-

+I

+2

.Gyraulus

-

parvus

WSurtace Marl L

I

M

J

J

A S 1979

O

N

A

A 122prnc

14cActivity;

D

125prnc

Surface Marl : 90pmc

FIG.7.Isotope data for Little Lake: S13Cvalues of lake DIC during summer-fall of 1979, 8l3C of surface marl and molluscs,

and

I4cactivities of lake DIC and surface marl (laboratory numbers: WAT 573, WAT 653, and WAT 600, respectively).

I

lnglesby Lake

L imnaea decamp; Sphaerium simile

Gyraulus parvus Surface Marl

Helisoma anceps

M

J

J

A 14cActivity;

123 prnc

A 1979

S

O

A 113pmc

Surface Marl : 78prnc

FIG.8. Isotope data for Inglesby Lake: 8% values of lake DIC during the summer-fall of 1979, S13cvalues of surface marl and molluscs, and I4cactivities of lake DIC and surface marl (laboratory numbers: WAT 558, WAT 663, and WAT 601, respectively).


610 I 4 C ACTIVITY


% modern

S ' 3 ~ ( ~ ~ UNADJUSTED ~ ) % o O f Iota'

AGE

ADJUSTED AGE

l 4 C Sample

828-

POLLEN ZONE

INFERRED "C DATE FROM

(cm)

POLLEN STRATIGRAPHY (years B P )

-

Rumex P ~ n eDecllne 3d Rise

160 300.400

3c HemlockHardwood Zone

-

I 3000

-

4800

-

Minvmum

3s

111

0 0

Surface-

68-65

-

1

116-12a

3b

Hemlock W

SAMPLING INTERVAL

191.200

230-236

Hardwood 291-301

-

Zone 7000

2 Pine Zone

-21 7

990f30L9170f300

910 300-

1 348-354 305

10 800

FIG.9. Reduced pollen diagram for Little Lake core (see Fig. 10) and calculated radiocarbon dates on marl samples (laboratory numbers: WAT 600, WAT 716, WAT 707, WAT 682, WAT 683, WAT 708, WAT 684, and WAT 657, respectively from the core top).

America (Bernabo and Webb 1977). More locally this transition is dated between 10 300 and 10 800 years BP (Barnett 1971; Sreenivasa 1973; McAndrews 1972; Karrow et al. 1975; Mott and Farley-Gill 1978). An unadjusted age of 9990 + 300 years BP is obtained from gyttja at the base (385 cm) of the Little Lake core (Fig. 9), which appears slightly younger than the above dates for the spruce-pine transition. Pine forests were dominant by 9000years BP and began to delcine by 8000 years BP (Bernabo and Webb 1977). Baerg (1978) has a radiocarbon date of 7226 + 144 years BP at what appears to be the top of the pine zone from a bog 4 km north of Little Lake. McAndrews (1972) estimated an age of 7000 years BP for the end of the pine zone at Crawford Lake. This date is bracketed by unadjusted dates at 291-301 and 348-354cm on carbonate from Little Lake and confirms an age of approximately 7000 years BP as the end of the pine zone. In relation to this, dates ranging from 7600 to 7900 years BP for this event from Pollen stratigraphy The detailed pollen record of the Little Lake sedi- other lakes in southern Ontario (McAndrews 1981) ments (Fig. 10; Appendix 1) appears similar to pollen appear to be rather too old. Pollen of deciduous tree taxa characterize zone 3. diagrams obtained for other small lake basins throughout the central Great Lakes region. To provide some This zone is intempted by a decline of Tsuga between palynostratigraphic comparison, the pollen zone desig- 160 and 210cm, which is denoted as zone 3b on the nations consistently identified by McAndrews (1972, pollen profile. Davis (1976) states that the decline in 1973, 1981) in southern Ontario are adopted for the hemlock occurred simultaneously at 4800years BP throughout its range, and hemlock reappears in the Little Lake diagram. Pollen zones 2, 3, and 4 are evident from the Little pollen record about 2000yea~slater. A similar age in Lake profile. The ealier zone 2 shows a preponderence southern Ontario may be inferred from the pollen of Pinus pollen. The small peak of Picea at the base may diagram at Maplehurst Lake (Mott and Farley-Gill reflect the end of the zone 1 Picea period. The transition 1978) and Found Lake (McAndrews 1981). The onset of from spruce to pine has been confidently dated between the hemlock decline is placed rather closer to 4000 -+ 10 000 and 11 000 years BP throughout eastern North 200years BP by the Little Lake core. The discussion

generally too young when compared with pollen chronology. Both sets of 14C ages shown in Fig. 9 follow a logical chronological sequence along the profile, and age inversions are absent. Also shown in Fig. 9 are the major pollen zones taken from Fig. 10. The core bottom dates on carbonate and gyttja are stratigraphically consistent and the unadjusted age of the gyttja sample of 9990 + 300 years BP agrees reasonably well with other estimates of the spruceipine transition as indicated below. These points generate confidence that, with the possible exception of 14C sample from 56-62 cm and the surface sample, the unadjusted ages are valid. From this point it is necessary then to apply only minor qualitative modifications to the unadjusted ages based on the pollen and stable isotope data as described in the following sections.


TURNER ET AL.


612

CAN. J. EARTH SCI. VOL. 20. 1983

Manlrnum Hardwater Effect

40 20

50

-

100 Q

tzo -

160 180 0 5 200 140

a

220

-

X

Minimum Hardwater

-

8

.t

................

Effect

,,:..Y ...... :*... .p........ '.'.'.+ .,., . ...... ......... ................ 1": :A:.:.. ....... .......................... ....... ......................... ...... ...... ......... .... .... ,......... . ..0.:.. +':. .......1[ ......... . ...... ..... ................ . ..... .........~..... ................. ....... &................ .:.:.:.:.< ..:.: .".*..' .........:.:.:. '.'.. ....... ............... ....... ........ ..::A::::::::.:.:.:.:,, ................ ........ ........ .'.'.>..'.'.'.'.'*'.'.'. ................ ............................. ........ .......... +. .". .,.. *:.:.>:*. ............................ ................. ................ ................. ....... ................. ....... ....... ........,......... .:.+:~:::::k:::::)t:j:i:; .................. ,

'

'

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+.

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'

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240

300 320 340 360 Molluscs: 260

2so

*

........ ....... ....... ....... ....... ...... .....

1

Gursulus Dwvu$^":'9::'-.

.............................. ................ ............... ............................ .......*...-* ......... ..:.:.k.:.:.:.+.: ......... ........ ........ .......

wart:

FIG. 11. 813C values of marl and molluscs.

below, based on stable isotope data, suggests that 14C sample 230-236 cm may in fact have an anomalously old apparent age, but in any case the onset of the hemlock minimum, zone 3b, occurs closer to 4000 years BP at Little Lake. However, a date of 5710 + 135 years BP on marl sediments from Van Nostrand Lake (McAndrews 1973) appears to be too old. The return of hemlock at the begining of zone 3c has not been dated elsewhere, but if Davis's estimate of 2000 years for the duration of the hemlock minimum is considered, an age of about 3000 years BP may be inferred for the beginning of zone 3c. The unadjusted 14C date for sample 116-128 cm justifies this conclusion. An abrupt increase in Pinus strobus pollen occurs under zone 3d at 100-80cm. Varve counts from Crawford Lake indicate a date of 300-400 years BP (McAndrews 1972) for this event. The apparent age at 56-65 cm of 2740 + 120 years BP for the pine rise in the Little Lake core is clearly inconsistent with both the pollen chronology and the uncorrected date at 116- 128 cm of 2750 + 100 years BP. This date is discussed more fully below.

The Little Lake pollen diagram lacks the characteristic zone 4 peak in Ambrosia, which may not have been sampled. However, it does show an increase in the pollen of the European weed species Rumex, which begins to increase at 40 cm, concomitant with a decrease in Pinus strobus pollen. The trends expressed by Rumex and Pinus strobus have been interpreted to indicate forest clearance by European man. A date of 160 years BP has been obtained from varve counts for the appearance of Rumex at Crawford Lake (Boyko-Diakonow 1979). Stable isotopes 13Cand 180 Two general observations can be made regarding the stable isotope data shown in Figs. 11 and 12: (1) the marked degree of convai-iance in 8l3C for both marl and molluscs in Fig. 11 and in 6180 for marl and moluscs in Fig. 12; (2) a significant degree of covariance between 613C and 6180 values, especially between samples 50-53 and 99-101 cm. The coupling between trends suggests that the factors affecting the stable isotope composition of lake waters at the time of carbonate

613

TURNER ET AL.


Maximum Hardwater Effect

-

8

Minimum Hardwater m+ Effect

FIG. 12. 8180values of marl and molluscs.

precipitation result in shifts in the same direction (enrichment or depletion) for both 613C and 6180, and also indicates that shifts in 6180 of carbonates is due more to variation in 6180 of lake water than temperature-dependent fractionation effects. Thus, for example, the shift to more negative values in both 613C and 6180 compositions of marl and mollusc components between 50-53 and 99-101 cm could indicate a shorter residence time for lake water during this sedimentation period and be due to a change in the hydrologic regime of the lake. As discussed above, these covariant shifts are not thought to be due to variations in the lake's productivity.

Climatic change could also have brought about a change in the 1 8 0 composition of precipitation and the lake hydrologic regime. In terms of variation of the hardwater effect this would result in a decrease in the 14c activity of lake water leading to a more significant hardwater effect. Thus 14C sample 56-62 cm may be expected to have a more significant hardwater effect than sample 116-128 cm and consequently give an anomalously old apparent age. This supported by the pollen chronology (Fig. lo), which indicates an anomalously old unadjusted age for 14Csample 56-62 cm. The sample also appears anomalously old relative to I4C sample 1 16- 128 cm.


614

CAN. I. EARTH SCI.

A further significant trend in both the 13cand '*o compositions of marl and molluscs occurs in the depth range 170-240cm, with minimum 613C and 6180 values at 230-240 cm. This suggests that 14C sample 230-236cm may also have an enhanced hardwater effect and therefore appear anomalously old. In fact the unadjusted age is in reasonable chronological sequence with other dates. Despite this, the strength of the stable isotope trend between 170 and 250cm forces us to regard the 14C date at 230-236cm and possibly 191200cm as too old. Consequently we suggest that the boundary between pollen zones 3a and 3b should be brought forward to approximately 4000years BP at Little Lake. Below 230-232 cm a general trend toward more enriched I3C and 1 8 0 compositions occurs, suggesting a declining hardwater effect for these deeper samples. Uncorrected ages are in good chronological sequence and are entirely consistent with the pollen chronology. The gyttja sample at 385 cm gives an uncorrected age of 9990 -+ 300years BP, which is consistent with the I4C chronology of the carbonate samples and the pollen chronology. Overall, two unadjusted dates on carbonate material appear anomalous. (1) The first is the surface sample, which is clearly not in 14cequilibrium with the lake water (Fig. 7). The sample appears to contain pre-bomb carbon and thus its 14Cactivity should not be used for a constant correction factor for the hardwater effect. This point is further emphasized by the surface water 14C activity of 122- 125 pmc, which is close to equilibrium with present-day atmospheric C 0 2 activity in the region and justifies calculating unadjusted 14C ages. (2) The 14C sample at 56-65 cm gives an uncorrected date of 2740 + 120 years BP, which is inconsistent with both the date of 2750 + 120years BP at 116-128 cm and the pollen chronology. Stable isotope data also suggest that this sample was subject to a relatively large hardwater effect and has an anomalously old apparent age.

VOL.

20, 1983

exercised in interpretation of 14C activities since both types of material may be subject to hardwater effects.

Acknowledgements The authors wish to thank Dr. E. J. Reardon for assistance with geochemical modeling of lake waters. Thanks are also due to Ms. Jean Berry, Ms. Diane Simpson, and Ms. Heidi Flatt for their careful isotope and geochemical analyses. Mr. and Mrs. George Clemens are thanked for their permission to sample Blue Lake on several occasions during this work. The research was supported by grants from the Natural Sciences and Engineering Research Council of Canada to Professors Fritz and Karrow. BAERG, B. 1978. Pollen analysis of a post glacial peat bog in

Wellington County, Ontario. B.Sc. thesis, Sir Wilfred Laurier University, Waterloo, Ont., 42 p. BARNETT, C. E. 1971. Geochronology and postglacial forest history of the Lake Medad area, Hamilton, Ontario. B.Sc. thesis, Brock University, St. Catharines, Ont., 25 p. BERNABO, J. C., and WEBB,T., III. (1977). Changing patterns in the Holocene pollen record of northeastern North America: a mapped summary. Quaternary Research, 8, pp. 64-96. BOYKO-DIAKONOW, M. 1979. The laminated sediments of Crawford Lake, southern Ontario, Canada. In Moraines and varves. Edited by C. Schluchter. A. A. Balkema, Rotterdam, The Netherlands, pp. 303-307. DAVIS,M. B. 1976. Outbreaks of forest pathogens in Quaternary history. Proceedings, 4th International Conference on Palynology, Lucknow, India, Vol. 3, pp. 216-227. DEEVEY, E. S., GROSS,M. S., HUTCHINSON, G. E., and KRAYBILL, H. L. 1954. The natural 14C contents of materials from hardwater lakes. Proceedings of the National Academy of Sciences of the United States of America, 40, pp. 285-288. EMRICH, K., EHHALT, D. H., and VOGEL, J. C. 1970. Carbon isotope fractionation during the precipitation of calcium carbonate. Earth and Planetary Science Letters, 8, pp. 363-371. EPSTEIN, S., and MAYEDA, T. K. 1953. Variations in the 180/160 ratio in natural waters. Geochimica et CosmoConclusions chimica Acta, 4, pp. 213-224. The conclusions may be summarized as follows. FAEGRI,K., and IVERSEN, I. 1975. Textbook of pollen (1) 14C dating of authigenic freshwater lake carbonanalysis. 3rd ed. Munksgaard, Copenhagen, Denmark, ates is reliable provided (a) the carbonate is precipitated 295 h. directly from fresh water and is a major component of FRITZ,P., and POPLAWSKI, S. 1974. 180and 13Cin the shells of freshwater molluscs and their environment. Earth and the sediment; and (b) detrital carbonates are absent as Planetary Science Letters, 24, pp. 91-98. determined by microscopic examination. J. D. 1962. The deuterium content of hydrous (2) Stable isotope data on marl or molluscs provide GODFREY, minerals from east central Nevada and Yosemite National valuable information on variations in the hardwater Park. Geochimica et Cosmochimica Acta, 26, pp. 1215effect along the sediment profile and are also a useful 1245. guide in identifying potentially anomalous 14Cdates. GODWIN,H. 1951. Comments on radiocarbon dating for Authigenic marl deposits appeai to be suitable matesamples from the British Isles. American Journal of rial for establishing Quaternary chronology provided the Science, 249, pp. 301-307. above criteria are met. Carbonates are as suitable for 14C GUILLET, G. R. 1969. Marl in Ontario. OntarioDepartment of dating as organic material; however, care must be Mines, Industrial Mineral Report 28, 137 p.

615

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T. W. 1975. Palynological KARROW, P. F., and ANDERSON, study of lake sediment profiles from southwestern New Brunswick: Discussion. Canadian Journal of Earth Sciences, 12, pp. 1808- 1812. KARROW,P. F., ANDERSON, T. W., CLARKE,A. H., DELORME, L. D., and SREENIVASA, M. R. 1975. Stratigraphy, paleontology and age of Lake Algonquin sediments in southwestern Ontario, Canada. Quaternary Research, 5, pp. 49-87. MCANDREWS, J. H. 1972. Pollen analyses of the sediments of Lake Ontario. Section 8. 24th International Geological Congress, Montreal, P.Q., pp. 223-227. 1973. Pollen analyses of the sediments of the Great Lakes, North America. Proceedings, 3rd International PalynologicalConference, Moscow, U. S.S .R., pp. 76-80. 1981. Late Quaternary climate of Ontario: temperature trends from the fossil pollen record. In Quaternary paleoclimate. Edited by W. C. Mahaney. Geo Abstracts, Norwich, England, pp. 319-333. MCCREA,J. M. 1950. On the isotope chemistry of carbonates and a paleotemperature scale. Journal of Chemical Physics, 18, pp. 849-858. MCKENZIE, J. A. In press. Carbon-13 cycle in Lake Greiffen: a model for restricted ocean basins. Cretaceous Research. MOTT,R. J. 1975. Palynological studies of lake sediment profiles from southwestern New Brunswick. Canadian Journal of Earth Sciences, 12, pp. 273-288. L. D. 1978. A lateMOTT, R. J., and FARLEY-GILL, Quaternary pollen profile from Woodstock, Ontario. Canadian Journal of Earth Sciences, 15, pp. 1101-1 l l l . O'NEIL,J. R., CLAYTON, R. N., and MAYEDA, T. K. 1969. Oxygen isotope fractionation in divalent metal carbonates. Journal of Chemical Physics, 51, pp. 5547-5558. PATTERSON, R. J., FRAPE,S. K., DYKES,L. S., and MCLEOD, R. A. 1978. A coring and squeezing technique for the detailed study of subsurface water chemistry. Canadian Journal of Earth Sciences, 15, pp. 162- 169. SHOTTON, F. W. 1972. An example of hard-water error in radiocarbon dating of vegetable matter. Nature, 240, pp. 460-461. SREENIVASA, B. A. 1973. Paleoecological studies of Sunfish Lake and its environs. Ph.D. thesis, University of Waterloo, Waterloo, Ont., 184 p. STUIVER,M. 1967. Origin and extent of atmospheric I4C variations during the past 1000 years. In Radioactive dating and methods of low level counting. International Atomic Energy Agency, (IAEA),Vienna, pp. 27-40. J. 1979. Radiocarbon dating and palynology of TERASMAE, glacial Lake Nippising deposits at Wasaga Beach, Ontario. Journal of Great Lakes Research, 5, pp. 292-300. TURNER,J. V. 1982. Kinetic isotope fractionation of carbon- 13 during calcium carbonate precipitation. Geochimica et Cosmochimica Acta, 46, pp. 1183-1 191.

TURNER, J. V., and FRITZ,P. 1983. Enriched I3Ccomposition of interstitial waters in sediments of a freshwater lake. Canadian Journal of Earth Sciences, 20, this issue.

Appendix 1 Pollen and spore types less than 1 %, unless otherwise indicated, not included in the pollen diagram:

10 c m Ericaceae, 28 c m 40 c m 50 c m 80 c m 100 cm 110 cm 130 cm 140 cm 160 c m 180 c m 190 cm 200 cm 210 c m 220 c m 240 c m 250 c m 260 c m 280 c m 290 c m 300 c m 310 c m 325 c m 340 cm 350 c m 360 c m 380 c m

Chenopodiaceae, Polygonum, Potamogeton, Dryopteris-type, unknown Cornus, Rosaceae, Chenopodiaceae, Potamogeton (1.5%), Dryopteris-type, unknown Gramineae, Rosaceae, Thalictrum, unknown Dryopteris-type Gramineae, Lycopodium, Dryopteris-type, unknown Nuphar, unknown Gramineae, Tubuliflorae, Pteridium, unknown Thalictrum, Potamogeton, Nuphar, unknown Plantago, Thalictrum, Pteridium, Dryopteristype Gramineae, Caryophyllaceae, Polygonurn, Potarnogeton, Pteridium, unknown Chenopodiaceae, Posaceae, Potamogeton, unknown Gramineae, Tubuliflorae, Thalictrum, Nuphar, unknown Thalictrum, Potamogeton, Nuphar, Sparganium-type, Dryopteris-type, unknown Ranunculus, Lycopodium, unknown Labiatae, Rosaceae, Gramineae, Potamogeton, Pteridium, Dryopteris-type, unknown Chenopodiaceae, Polygonum, Nymphaea, Lycopodium, unknown (1.4%) Dryopteris-type, unknown Tubuliflorae, Chenopodiaceae, Grarnineae, Ly-, copodium, Dryopteris-type, unknown Lycopodium, unknown Equisetum, unknown Cornus, Potamogeton, Gramineae, unknown Grarnineae, Rosaceae, unknown Gramineae (l.5%),unknown (1.2%) Gramineae, unknown Myrica Lycopodium Chenopodiaceae, Pteridium, unknown