Recent Sedimentation in Lake Michigan

J. Great Lakes Res. 17(1):33-50 Internal. Assoc. Great Lakes Res., 1991

RECENT SEDIMENTATION IN LAKE MICHIGAN

Mark H. Hermanson) and Erik R. Christensen2 Department of Civil Engineering and Mechanics and Center for Great Lakes Studies University of Wisconsin - Milwaukee Milwaukee, Wisconsin 53201 ABSTRACT. Sediment cores from 21 locations in Lake Michigan are evaluated to identify sedimentation patterns including accumulation rates, surface layer mixing, focusing, and processes responsible for discontinuous 2lOPb profiles. These patterns were identified based on two different sediment dating models. Accumulation rates vary consistently with basin topography in the north and central areas of the lake, while rates in the southern basin vary widely between sites. Mixing is apparent in all north basin sites but is seldom observed in the south basin and not in the central lake. Storm surges have influenced sediment accumulation, particularly in the south basin where sites are identified that have lost and gained sediment resulting from a storm. In particular, we have found evidence of the 1888, 1905 and, most recently, the 1975 storms. Markers for these storms were not previously reported for Lake Michigan. Storm related slumps and sand layer discontinuities were also identified. An extended period of dry and hot weather during 1913-52 may have caused periodic lower sedimentation rates in the north basin, but not in the south where sediment redistribution by storms has obliterated records. Sediment focusing is a minor process in the north basin, while in the south, the areas of highest sediment accumulation rates are also sites of highest measured focusing, indicating that redistribution is a major variable in south basin sedimentation processes. INDEX WORDS: Radioactive dating, sedimentation rates, lake sediments, Lake Michigan, mathematical models, sediment-water interfaces, sediment distribution.

INTRODUCTION tors has concentrated on the southern basin of Lake Michigan, particularly the southeastern area, where most sediment accumulation takes place. Little attention has been directed to the northern basin in previous investigations in comparison with the south basin because sediment deposition is slower and less spatially variable, as identified in Cahill's (1981) surface sediment survey. Our objective is to evaluate processes that have influenced sedimentation in all of Lake Michigan, with special emphasis on the north basin. These processes are reflected in the 2lOPb and mcs stratigraphic records. Different models are used to date sediments and help us identify sources of stratigraphic discontinuities in 2lOPb profiles. Biological or physical mixing are common sources of these profile breaks in addition to periods of low sedimentation. We also evaluate focusing patterns throughout the lake and relate these to sedimentation patterns.

Measurement of sedimentation variables in lakes has made it possible to identify the processes governing sediment movement and accumulations. Since the first application of 210Pb geochronology to lake sediments in the early 1970s (Krishnaswamy et al. 1971), investigators have devoted considerable effort to using this nuclide for observing sedimentation rates in parts of Lake Michigan (Robbins and Edgington 1975, Edgington and Robbins 1976a). The results of these reports have enabled more specific investigations of fluxes of trace contaminants to sediment (Edgington and Robbins 1976b, Christensen and Bhunia 1986, Christensen and Goetz 1987), resuspension and chemical fluxes (Eadie et al. 1984), and bottom currents and suspended sediment concentrations (Lesht and Hawley 1987). Much of the effort of recent investigaIPresent address: Illinois State Water Survey, 2204 Griffith Dr., Champaign, IL 61820. 2To whom correspondence should be addressed.

33

34

HERMANSON and CHRISTENSEN 89'

88'

50 I ,

46'

I

87'

86'

85'

100

I

kilometers ---,--~-.bo"';--~,--------+- 46'

I 45'

Green Bay

43'

Milwaukee 43'

42'-t--_--J.

88'

--+---+-+--+---+--_ _+42'

87'

86'

85'

areas on the east coast where sedimentation processes are very active. Central basin cores were taken only on the east coast. Core CLM K was collected from a valley between the eastern shore and the ridge that separates the northern and southern basins. Cores CLM Land M were taken from a sloping part of the basin. Cores were collected from the R/V Neeskay with a triple corer dropped to the bottom from about 5 meters above the sediment. We used polybutyrate core liners (Benthos, Inc., North Falmouth, MA) with 6.7 cm inner diameter. Cores were sectioned aboard ship using hydraulic pressure to force a measured portion of the core out the top of the liner where it was sliced off and transferred into 125-mL polyethylene bottles. Samples were stored at 4°C at the end of the cruise. Laboratory Procedures Sediments were oven dried to constant mass at 95°C. We calculated porosity cf> by a gravimetric method assuming a solids density of 2.45 g/cm3, using

m

(1)

+ -dFIG. I. Lake Michigan, showing sampling locations for this study and locations of cores 7I-I-CI and 73-4-C4, designated as 1 and 4, from Edgington and Robbins (1976a). Focusing is indicated by either + (1.1 < Mml M a :::;; 2) or + + (MmiMa > 2). Contributing areas are shown by - (MmiMa < 0.9). In all other cases the stations are neither focusing nor contributing (0.9 :::;; Mml M a :::;; 1.1). No symbols are indicated by stations 1 and 4.

METHODS We collected 21 cores from Lake Michigan during 1984, 12 from stations in the southern basin (SLM), 3 from the eastern side of the central basin (CLM), and 6 from the northern basin (NLM) (Fig. 1, Table 1). In the rougher topography of the northern basin, we sampled deepest areas (stations A, B, C, E), a slope adjacent to these sites (station G), and one shallow mound (station I). In the southern basin, which has a flat topography in comparison with the north, we took samples along two parallel transects between Milwaukee and Benton Harbor, Michigan. These stations include the deepest parts of the southern basin and nearshore

2.45w

where md is mass of dried solids and w is mass of water lost by drying. Density of water is assumed to equal 1 g/cm 3 in this equation. Bulk density p (g/cm 3) of each layer was calculated as 2.45(1-cf», and the mass per surface area (g/cm2) was the product of bulk density and depth interval in the core (cm). The g/cm2 values were cumulated by layer beginning at the top of the core to derive cumulative mass used in place of depth in the core to correct the effects of sediment compaction (Christensen 1982). We measured chronology of the sediment using 137Cs and 21OPb. Automated gamma spectroscopy (662 keY) with a NaI(Tl) detector was used for determination of mcs. 210Pb and its daughter nuclides were separated from sediment into a plating solution by digestion of sediment in 6N HCl with a 209PO internal standard. The 210Pb granddaughter, 2IOPO, was used to quantitate 21OPb. The 209PO and 21OPO were spontaneously deposited onto 2.2 cm diameter copper disks (MacKenzie and Scott 1979). To ensure equilibrium between 210Pb and 2IOPO, there was a 6-month interval between sampling and plating. We analyzed 209PO and 21OPO

RECENT SEDIMENTATION IN LAKE MICHIGAN TABLE 1.

35

Locations, depths, and lengths of Lake Michigan sediment cores,

1984. Coordinates Latitude Longitude North West

Core NLM A B

C E G I

Water Depth

Core Length

(m)

(em)

44°20'20" 44°28'20" 44°31 '40" 44°40'50" 44°41'40" 44°35'00"

86°38'20" 86°43'25" 86°42'20" 86°45'00" 86°55'00" 87°04'36"

235 250 263 263 216 190

91 84 78 73 28 65

43°32'30" 43°38'20" 43°45'25"

86°53'40" 86°46'00" 86°39'10"

118 111 81

59 61 55

43°02'00" 42°38'00" 42°28'00" 42°22'40" 42°15'30" 42° 13'00" 42°10'40" 42° 19'40" 42°32'00" 42°38'20" 42°46'00" 42°54'00"

87°53'25" 87°32'00" 87°28'20" 87° 17'00" 87°00'00" 86°47'30" 86°32'30" 86°40'35" 86°42'20" 86°57'00" 87°15'00" 87°34'10"

7 88 88 110 100 90 25

42 45 51 60 36 31 12 66 73 84 84 9

CLM K

L M SLM A B

C D E F G H I

J K

L

by alpha spectroscopy using surface barrier detectors. 2lOPO data were corrected to 100070 recovery of internal standard and 100070 counter efficiency. I37CS and 210Pb Dating Data for I37CS enable us to check 2lOPb data by providing independent dates for material fluxes to sediment column. Significant inputs to the atmosphere of mcs, a fission product of 235U, began with atomic weapons testing in 1954, with a peak in 1963 (Pierson 1971). The earliest appearance of mcs in the sediment core should correspond to a 2lOPb dated layer of 1954, and the peak I37CS activity should correspond to 1963, provided that sediment mixing plays a minor role. While only two dates can be determined from mcs, the naturally occurring nuclide 2lOPb has the potential to provide a continuous time scale in the core. Two 2lOPb-based dating models have gained a significant amount of attention, the constant ini-

72

87 160 125 74

tial concentration (CIC) and constant rate of supply (CRS) models (Appleby and Oldfield 1978, Appleby et al. 1979). In the CIC model, the rates of mass sedimentation and unsupported 2lOPb supply are proportional to one another such that the 2lOPb concentration (activity) always has a constant value at the top of the sediment core. In this model, the age t (yr) of a layer of depth is given by: 1

t = -

A

s ) In (_0 s(z)

(2)

where A (0.03114 yr- 1) is the decay constant of 2lOpb, and So and s(z) (dpm/g) are the 210Pb activities at the water-sediment interface and at given depth, z, respectively. In the CRS model, the basic assumption is that the net supply of unsupported 2lOPb is constant despite variations in the mass sedimentation rate. The age of a layer of depth z is here calculated from

HERMANSON and CHRISTENSEN

36

t

=

~

(A~~) )

In

(3)

where A o and A(z) (dpm/cm2) are integrated activities, N

Ao

E pSdZ i=l I

I

(4)

I

N

A(z)

= i =EI(z) pSdz I

I

I

(5)

where p (g/cm 3) is the bulk dry density, Sj (dpm/g) is the 210Pb activity, and DZi (cm) is the thickness of layer L Depth indices i = 1 and N correspond to the top layer and the deepest layer with measurable unsupported 210Pb activity, respectively. The depth index I(z) corresponds to the depth z. The average mass sedimentation rate r (g/cm 2/ yr), was calculated from r

=

A (m 2

-

m l)

In ( ~ ) S2

(6)

where Sl' S2 (dpm/g) are 21°Pb activities in layers of cumulative masses m l and m 2 (g/cm 2), respectively. Layer 1 should be the uppermost layer with negligible mixing, and layer 2 one of the lowest with measurable unsupported 210Pb activity. Where there is a break in the 210Pb profile, layer 2 should be located at the oldest layer above the discontinuity. The criteria used here to identify 21°Pb profile discontinuities are based on the standard deviations of the 210Pb data (Figs. 2-6). For vertical breaks to occur we require that there are at least two consecutive 210Pb activities that are identical within the standard error (e.g., SLM H, 1958) or that the 210Pb curves, defined within the limits of the standard deviations, and based on straight-line segments connecting at least two points both before and after the discontinuity, do not meet there (e.g., NLM B, 1958 and SLM J, 1905-20). For horizontal breaks (low precipitation) to occur we require that it is impossible to draw straight-line segments through three consecutive points within the standard errors when moving into and out of the discontinuity (e.g., NLM A, B). While these criteria seem reasonable, we realize that they may not be entirely adequate to uniquely define discontinuities. Thus there are marginal cases which have been assigned years in parentheses (Figs. 2-6).

Where mixing occurs in the top layers, a given date calculated from Eqs. 2 or 3 would not be represented by a specific depth z, but rather by a distribution of depths through the core with a peak at a most probable date. Little mixing would result in a narrow peak while significant mixing would cause a broad peak. Thus, if mixing occurs, neither CIC nor CRS models would be entirely appropriate. However, it can be argued that the CIC model would better account for the instantaneous transfer of the peClk from a narrow input impulse through a finite layer of rapid mixing because its constant 210Pb activity predicts the same dates for the top and the bottom of the mixing layer. This will be explained later in general terms and in connection with a specific example. In the case of steady-state sedimentation without mixing, the ages calculated from Eqs. (2) and (3) become identical. For steady-state sedimentation with rapid mixing, these equations are no longer valid to calculate a specific age for a given layer. However, they are still useful for the following reasons: (a) the time difference between the CRS and CIC ages below the mixing layer provides a measure of the time resolution of a pollutant profile, (b) the CIC model indicates that true age of the most recent contributions to any layer, (c) the CIC model provides ages that are identical to those obtained from the conventional method based on the assignment of sedimentation rates to log-linear portions of the 210Pb activity vs. depth curves between discontinuities below the mixing zone, and (d) although the CIC and CRS dates would appear to be meaningless in the mixing zone, evaluation of the relationship between the tangents to CIC and CRS depth-age profiles at the sedimentwater interface can be helpful in determining the nature of the most recent sedimentation processes, Le., whether regular sedimentation occurs, or if there is mixing or rapid deposition of material, as during a storm surge. If non-steady-state conditions occur, the CRS model may provide a more accurate age estimate. Further discussion of points (a) through (d) is given below. For the sake of illustration, let us consider a sediment of constant porosity. The concentration c of a tracer is then,

(7)

37

RECENT SEDIMENTATION IN LAKE MICHIGAN

0.6 0

0.7

0.8

Total Cs-137 Activity, dpm/g

Excess Pb-210 Activity, dpm/g

Porosity 0.1 1.0

0.9

II

2 4

II

0.2

1.0 0.4 0.60.8

2.0

10 4.06.08.0

40 60 0

20

10

NLM-A

20

4 6

1907-53

8

8

I,

0

I 2

I

4

U

..c

a

Q)

0

8

0

t-+<,...;...,

'I

NLM-B

(1958)

+

I

6

E

40 0

'h

+

+

6

30

+

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

t-+<

2

"'"

4

......

6

1920 - 47

8 10

10

1888

12

12

0

0

I

2

II

4

+ + +

NLM-C

I

6

4 6

8

8

0

0

I I

2

+<>-t-< 't'

2

......

NLM-E

,I

't<

>1<'"

4

4

I'

6

6

}

8 10 0.6

2

0.7

0.8

0.9

8

1.0 0.1

0.2

0.4 0.6 0.8 1.0

2.0

4.0 6.08.0 10

20

40 60 0

10

20

30

10 40

FIG. 2. Porosity, lIOPb profile and pertinent dates, and 137Cs profile for cores NLM A, B, C, and E.

where Co (g/cm 3 ) is the concentration at the sediment-water interface and v (cm/yr) the rate of sedimentation. From Eq. (2), we find for the age t CiC of the CIC model,

For the CRS model we obtain the age t CRS from Eq. (3),

1 >.: In (1 +

Z

Zm -

(8)

) z+>.: (9)

38

HERMANSON and CHRISTENSEN Total Cs-137 Activity, dpm/g

Excess Pb-21 0 Activity, dpm/g

Porosity

0.1 10 20 30 40 0.6 0.7 0.8 0.9 1.0 0.2 2.0 4.0 6.08.010 o~..J....---I._~-+L-..J...._L-L...l..l-......J_-I.......J....!...L_..J--YL-!-...I----I.:r-.L-..., 0

NLM/

2 4

6

+

+

2 4 6

8

8

oL-_L~.l-=.--:::-::::t====---------.----I----:-~:----to

~

2

E

()

-

..c

a. Q)

o

4

- t* ::.:=t- ~

6

2

4

I

8

6

NLM-I

8

ol----~-+--------------____,r____l--_w_-____iO 2

2

4

4

6

6

CLM-L

8

8

ol-----l.---r---+----------------,----If-------;Jc;-----IO 2

2 1913 - 56

4

4

6

6

CLM-M

8 10 0.6

0.7

0.8

0.9

1.0 0.1

0.2

0.4 0.6 0.8 1.0

2.0

4.06.08.0 10

20

40 60

8

0

10

20

30

10 40

FIG. 3. Porosity, 2IOPb profile and pertinent dates, and 137Cs profile for cores NLM G and I, and CLM Land M.

Thus, the time difference, L:, t by: 1

>;: 1n (1 +

Z

v)

tCRS - t CIC ' is given

0 ~ Z ~ Zm

the time resolution of the sediment record of a pollutant can be seen from the following expression for the sediment concentration originating from a spike of tracer at t = 0,

Zm - Z+>;: .:1t

(10) (11)

Note that L:, t is constant below the mixing zone, making the CIC and CRS curves parallel, and that the value approaches zm/v when AZ m < < v. The relationship of L:, t below the mixed zone to

The significance of L:, t is thus the time it takes for the pulse in Eq. (11) to decline to a fraction exp (- L:,tv/zm) of its maximum value. For AZm < < v, this fraction is 36.8070. Regarding point (b), it is clear that material

39

RECENT SEDIMENTATION IN LAKE MICHIGAN

06

07

08

Total Cs-137 Activity, dpm/g

Excess Pb-21 0 Activity, dpm/g

Porosity 09

0.1 10

1.0 02040608

10 20406080

20

40 60

o

t

I

6

II I

8 10

Sand Layer No Excess Pb-21 0

,~,

+

21-

40

o ~

~

~

30

2 4

6

1878-1919 - ---

---~

8 1o

+

II

20

+

~

II

4

10

,I

II

2

0

12

SLM-C

14

41-

\

o

E u

..c

I

1\

a.

4

o

6

++

+

2

I

(\)

o -

-4

~-I

6

SLM-D

8

8 0

I I

2

I

4

o

t

+

"t

-+---<

I

2 4

6

6

SLM-E

8

8 0

I I

21-

1

I

I

I

4161-

I

~I Sand Layer No Excess Pb-21 0 ____

- -~

I 8 10 0.6

2

II 0.8 0.9

1.0 0.1

0.2

0.4 0.6 0.8 1.0

2

+

4

~8~2~ ~~ _

6

SLM-F

I

07

o

++

I

2.0

406.08.0 10

8 I

20

40 60

I

0

10

I

20

30

10 40

FIG. 4. Porosity, 2IOPb profile and pertinent dates, and J37Cs profile for cores SLM C, D, E, and F.

added in a given year will reside in the mixing layer according to an exponential die-away function, Eq. (11), such that some of it will enter the historical layer immediately, and reach a given depth according to the CIC model with a given rate of sedimentation. This is then the most recent contribution to that layer. As to point (c), it may be seen from Eqs. (2) and (6) that applying the CIC method is in fact equivalent to estimating ages from average sedimentation rates below the mixed zone.

Finally, with regard to point (d), we shall demonstrate later that similar slopes for the CRS and CIC curves at the sediment-water interface indicates regular, finite sedimentation, while different slopes indicate mixing or rapid sedimentation during a storm surge. Whether one or the other of the latter two phenomena occurs must be determined from other information. In the above discussion of mixing, the simple mixing model of Berger and Heath (1968) was adopted. This model includes rapid mixing in a

40

HERMANSON and CHRISTENSEN Porosity 0.6 0

0.7

0.8

0.9

0.1 1.0

I

2 4

Total Cs-137 Activity, dpm/g

Excess Pb-21 0 Activity, dpm/g 0.2

1.0 0.4 0.60.8

2.0

10 4.06.08.0

40 60

10

0

20

30

2

t

SLM-H

+

4

+

6

II 10

6

+ +

8

40 0

8 10

[I

12

12

I

14

14

16

16

18

18

E

U

..c:. +-'

a. Q)

0

0

0

SLM-I

II I II

2 4

, I I'

6

J

2

+

4 6

I

III

8

8

0

0

2

2

4

+

SLM-J

+

4

6

6

8

8

10

10 1905 - 20 12

12

14

14 16 0.6

0.7

0.8

0.9

1.0 0.1

0.2

0.4 0.60.8 1.0

2.0

4.06.08.0 10

20

40 60

0

10

20

30

16 40

FIG. 5. Porosity, 2JOPb profile and pertinent dates, and 137cs profile for cores SLM H, I, and J.

layer of finite thickness and no mIxmg below. Although more sophisticated models including local mixing (Officer and Lynch 1982, Christensen and Bhunia 1986) may be more appropriate, we believe that this simple model is quite adequate to illustrate the principles discussed here. Note also that non-local mixing effects have been demonstrated in laboratory experiments (Krezoski and Robbins 1985). However, no actual lake data have

been shown yet to require the implementation of non-local mixing effects (Fukumori and Christensen 1989). We used I37CS activities to evaluate sediment focusing by calculating a ratio of the activity observed in the sediment column to expected atmospheric input (Christensen and Bhunia 1986). The I37CS activity measured in the sediment, Mm , is calculated as

41

RECENT SEDIMENTATION IN LAKE MICHIGAN Porosity 0.6

0.7

0.8

Total Cs-137 Activity, dpm/g

Excess Pb-210 Activity, dpm/g 0.1 1.0

0.9

0.2

1.0 0.4 0.60.8

2.0

10 4.06.08.0

20

40 60

0

10

20

30

40

0r-........---J._~--tL--.l.-_.L.-.L....J....L..--l_.....L.........I.....l....I.._...L-_L"l---1f-...L-......I.....,....J---,O 2

i

4

(1975)

II

8

E

+~

SLM-K

6

+

10

2

+

4 6

+

+

8 10

<.> - 12 ..s:::

-g-

0

12

14

14

16

16

18

18 1888 - 1905

20

20

22

22

24

24

;'6

26

28

28

30 0.6

0.7

0.8

0.9

1.0 0.1

0.2

0.4 0.60.8 1.0

2.0

4.06.08.0 10

20

40 60

0

10

20

30

30 40

FIG. 6. Porosity, lIOPb profile and pertinent dates, and 137Cs profile for core SLM K.

N

M

ill

= i=l 1: psLlz I

I

(12)

I

where Si (dpm/g) is measured 137CS actIvIty. The atmospheric input, M., is calculated by N

M a = i= 1:1F exp (-At) I

I

(13)

where F j (dpm/cm 2) is atmospheric input in year i, A = decay constant for 137CS, t i is the number of years the nuclide deposited in year i has been in the sediment. F i was calculated from 90Sr inputs at Argonne, IL, for SLM; Green Bay, WI, for NLM; and an average of the two sites for CLM. We converted 90Sr data from HASL-329 (Health and Safety Laboratory 1977) to 137CS using a 137CS:90Sr ratio of 1.5, considered to be a valid fallout ratio of these isotopes worldwide (Volchok et al. 1970). A Mm/M a ratio approximately equal to 1 shows that all atmospheric 137CS appears in the sediment.

A value less than 1 implies that sediment particles are moved away from the site by turbulence and transported ("focused") elsewhere where the ratio is greater than 1. Identification of sediment focusing is strengthened by measuring two nuclide fluxes. Fluxes of 137CS and 2lOPb should be correlated at a given site if focusing or redistribution of material is occurring. Under quasi steady-state conditions, the 2lOPb flux 1 to the sediment may be written as 0

N

A .1:. PiSjLlzi. 1=

I

(14)

where A (yr- I ) is the decay constant for 2lOpb, Si (dpm/g) is the 2lOPb activity, and the other symbols have the same meaning as in Eq. 5.

42

HERMANSON and CHRISTENSEN TABLE 2.

Lake Michigan sediment core parameters (1984).

Mass Sedimentation Rate (g/cm 2/yr)

Focusing Factor (Mm/M a )

I

0.0136 0.0137 0.0099 0.0133 0.0095 0.0046

0.995 1.41 1.05 0.983 0.560 0.323

L M

0.0173 0.0205

0.661 0.961

0.737 0.710

C

0.0135 0.0101 0.0109 0.0172 0.0738 0.0143 0.0330 0.0495

0.979 0.705 0.213 0.909 2.94 0.738 1.13 2.81

0.947 0.514 0.199 0.673 2.28 0.872 1.46 2.97

Core NLM A

B C E G

210Pb Flux Focusing Category

10

(dpm/cm2/yr) 0.931 1.39 0.964 1.07 0.626 0.324

+

CLM SLM 0 E

F H I 1

K

++ + ++

RESULTS AND DISCUSSION Sedimentation Rates and 210Pb Fluxes Results of the measurements of porosity, 21OPb, and mcs for all cores in Table 1 that could be analyzed are plotted in Figures 2-6. Cores that could not be dated included CLM K and SLM G which were from nondepositional areas (Cahill 1981) characterized by a thin (:5 1 cm) layer of recent deposit over glaciolacustrine clay. Also, cores SLM A, B, and L were excluded from the analysis because of their very low sedimentation rate. Although several of the cores exhibit discontinuities in the porosity and 210Pb profiles, to be discussed later, it is still possible to calculate average mass sedimentation rates according to Eq. 6. The results of these calculations along with estimates of 210Pb fluxes and focusing factors appear in Table 2. Corresponding values of 210Pb fluxes and mass sedimentation rates are shown in Figure 7. It is clear from this plot that the highest 210Pb fluxes and sedimentation rates occur for stations J, K, and H in southern Lake Michigan. These stations, and NLM B, are also the only ones with a focusing factor significantly greater than 1 (Table 2). Note

3.5

r--------~------r--___,

3

...>.

~ E

2.5 Sl ...-H

u

..........

E

Q.

2

"U

x ~ 1.5

o N

o

o

I

0

NL"'-B NL"'-E NLM-C 0

.D

a..

SlM-J

" .... SlM-C - NLM-A Sl... -I Cl"'-L ClM - M

-

8..9_ SLM-D

0.5

-

SlM-F

NL"'-I SlM-E

o

oL------'---------'------'----------J

o

0.02

0.04

0.06

0.08

Mass sedimentation rate, g/cm 2 /yr

FIG. 7. Mass sedimentation rate vs. 2IOPb flux for Lake Michigan sediment cores collected for this study. The regression line was plotted from all points except SLM H. The correlation coefficient = 0.875.

43

RECENT SEDIMENTATION IN LAKE MICHIGAN that stations NLM Band SLM J are from the deepest areas of the northern and southern basins, respectively. The high sedimentation rate at SLM H (0.0738 g/cm2) is consistent with a value of 0.0938 g/cm2 found by Robbins and Edgington (1975) for their core 29 about 14 km southsoutheast of SLM H. Relatively low sedimentation rates in southern Lake Michigan were found at SLM D and E which both are close to the transitional zone of the depositional area (Cahill 1981). Sand was found throughout core SLM E and in certain zones of SLM C and F (Fig. 4). The presence of sand is a sign of high hydrodynamic energy associated with waves or currents. Average sedimentation rates and 210Pb fluxes were found in the remaining southern cores, SLM C, F, and I, the cores from the central basin CLM Land M and about half of the northern cores, i.e., NLM A, B, and E. NLM B has a relatively high 21°Pb flux, and NLM G and I have below average 210Pb fluxes and mass sedimentation rates. It is perhaps significant that the latter cores are from a relatively steep slope (G) and from the area around a mound surrounded with steep slopes (I). Only one core from northern Lake Michigan was investigated by Robbins and Edgington (1975). Their core, 103, located less than 1 km east of NLM B, had a sedimentation rate of 0.0147 g/ cm 2/yr compared with the value of 0.0137g/cm2/yr for NLM B (Table 2). The sedimentation pattern in Lake Michigan can be characterized as follows. In the southern basin a counterclockwise gyre acquiring sediment material from the rivers, e.g., St. Joseph River, redistribution, and aerosols, deposits much of its load in the deep part (SLM J, K), and in the southeastern part (SLM H) of the basin. Other depositional areas of the lake are receiving a more uniform input of particles from aerosols and rivers. 210Pb Profile Discontinuities

Several of the 210Pb profiles in Figures 2-6 show departures from smooth curves. These discontinuities can be related to storm events (NLM B; SLM J, K, H), to deposition of sand (SLM C, E; SLM F), to slumping (NLM G, SLM D), or to periods of low sediment input (NLM A, B; CLM M). In order to identify processes causing the discontinuities, it is desirable to determine the dates when they occur. The dates of sediment layers calculated by the CIC and CRS models are plotted as a function of depth in Figures 8-12. Two years obtained

O..-~-----------------r---,

NLM-A

---- ..

(Ie

S

~

0

Prone

CRS Profile

---

_. .... -_

•........

.....

, Cs-137

NLM-B t: 0

C
E D
5

..

10 0

NLM-C

.S

:5

5

.-_.,- .

Q.


0

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

..........

10 0

NLM-E

5 10 1800

1825

1850

1875

1900

1925

1950

1975

2000

FIG. 8. Age-depth profiles for CIC and CRS models applied to lIOPb data. 137Cs dates are shown with bars corresponding to 1954 and 1963.

from 137CS profiles, 1954 and 1963, are also shown in these figures. The dates determined from the CIC and CRS models differ appreciably for all cores except CLM Land M, and SLM D, E, and for parts of SLM J. The differences in the top layers result from mixing (cores NLM A, B, C, E, and I, and SLM C and I).

o 5

NLM-G

----

---.-

CIC Profile CRS Profile

........

Cs-137

---.-.--_ ...•. fr-m:M=I~==~::::::~~~ NLM-I ... ..

..•.

§

0

~

5

E

ol-----'------'-----'----'-----'-----'----:-----'--:::r----l

.S .c

5

~

_

~--

I I

CLM-L

O-Ol----'------'-----'----'-----'-------'------'-=::Jr----l

~

CLM-M

1800

1825

•...

1850

........-.,....--

~

1875

190C

1925

1950

1975

2000

FIG. 9. Age-depth profiles for CIC and CRS models applied to lIOPb data. 137Cs dates are shown with bars corresponding to 1954.

44

HERMANSON and CHRISTENSEN 0,-------------------.,.---,

SLM-C

--

CIC Profile

E

10

~.

0

c

U

5 ~ 0

Ul

.S:

:S

a.

, ,

E

,

...;

~

SLM-D

5

I

c

---.---

10

E

..

ii

15

Ul

c

SLM-E

Cs-137

£a.

20

o

25



5 0

SLM-F



0

CIC Profile CRS Profile

u

Cs -137

.e·-



f



E

SLM-K

._-.-_.

CRS Profile

.. ..

'

30 '--_-'-_---'-_ _- ' - _ - ' - _ - - - '_ _ 1800 1825 1850 1875 ,900 1925 ,950

~__'____'__-.J

5

.--

10 L..-_-'-_---'_ _- ' - _ - - - '_ _-'--_---'-_ _--'--_--' 1800 '825 1850 1875 1900 ! 925 1950 1975 2000

FIG. 10. Age-depth profiles for CIC and CRS models applied to 2lOPb data. I37CS dates are shown with bars corresponding to 1954 and 1963.

It is shown by different slopes of the CIC and CRS curves in the uppermost layers where the CIC dates are nearly constant but the CRS dates become steadily older with depth. This is demonstrated in Figure 13 where CRS and CIC curves have been drawn for the idealized core NLM E. The CRS and CIC curves in the mixing layer are not meaningful by themselves, but are useful only as a means of

1975

2000

FIG. 12. Age-depth profiles of CIC and CRS models applied to 2lOPb data. 137Cs dates are shown with bars corresponding to 1954 and 1963.

characterizing the most recent sedimentation processes. A rapid sedimentation rate such as that coming from a storm (Edgington and Robbins 1976a, Rob-

60

(a)

(b)

Year Before Present

Concentration

45.4 40

Tracer

0.5

22.7" 20

1.0 I

Mixing

Layer I

\

------------

0.----------

SLM-H 5

CIC Profile

.-.-

\

E

....

g.

c

; I

15

C~-

I

.s:

g-

~

4

I ,) ,.

I

'.

I I

.-

•.........- . 10 0

• •- • • •

r---'-----'---'-----'----'-------'-----'--.----\

SLM-J

o

10

e·--

15 1800

1825

1850

1875

1900

1925

'950

45.4f---c,~~---"'>. 044

Year Before Present

-.--'.~.I~

L •. •. '

12.4 yr

----------------~~\

r---~

20

I--S-L-'M-_-I:;-'------'--...

:S

3

Cl

i: ...;

\

1\

.r=

10

E

\

GIG

()

CRS Profile

---

2

GRS

1975

2000

FIG. 11. Age-depth profiles of CIC and CRS models applied to 2lOPb data. I37CS dates are shown with bars corresponding to 1954 and 1963.

FIG. 13. Application of CRS and CIC dates to an idealized core with constant porosity and rapid mixing in finite layer (NLM E). Part (a): CRS and CIC dates as a function of depth. Part (b): Progression into the sediment ofa pulse of tracer originating from an input spike applied at the indicated times, 0, 22.7, and 45.4 years at or prior to the present time t = O. These times coincide with the CIC dates. The pulses are preceded by pulses started at a time f:,t earlier, i.e., at the CRS dates and which still contribute 44% of their maximum value to the total tracer concentration at the given depth. The following parameters were used. Sedimentation rate r = 0.0133 g/cm 1 /yr, average sediment velocity v = 0.066 cm/yr, and mixing depth Zm = 1 em.

RECENT SEDIMENTATION IN LAKE MICHIGAN

bins et af. 1978, Evans et af. 1981) could have a similar effect as mixing on the two curves. However, since there were no major storms in the early 1980s, and since storm surges, as will be shown below, are not reflected in several of these cores, there is definite evidence of mixing in the above cores where the CIC and CRS curves have different slopes at the surface. Regarding the choice of dating model, CIC vs. CRS, we make the following comments. By plotting 21°Pb activity vs. cumulative mass, it is seen that most profiles show nearly log-linear portions outside the 210Pb profile discontinuities. This is an indication that the CIC model is nearly correct. However, there is also substantial evidence for the appropriateness of the CRS model (Appleby and Oldfield 1978, Appleby et af. 1979). In addition, if the CIC model were correct during storm events we would expect a constant CIC date, i.e., a vertical line during the event. However, from Figures 11 and 12, we see several cases of younger CIC dates at deeper layers for SLM Hand K. This means that the particle flux is increased relatively more than the 210Pb flux which is a characteristic feature of the CRS model. Also, the CIC model is clearly unacceptable in the top layers of cores NLM B, E and SLM I where the dates are younger than that of the sampling year (1984). Another factor to consider is that mixing in the upper layers will introduce a dating uncertainty equal to the time difference between the CRS and the CIC curves below the mixing layer. Finally, in choosing between the two dating models, known dates of storm events or dry periods should be taken into account. As a result of all of these considerations, we find that not a single one of these dating models is generally applicable, but that the actual date of an event in the sediment record is somewhere between the CIC and CRS dates. For storm events we used such values, and in all other cases, the CIC dates. For slumping or deposition of sand layers, the CRS model is clearly not applicable. Also, the CIC model would not be valid for a sand layer itself. A period of low precipitation and resulting low sedimentation is reflected through a lower slope of the dating curves vs. time, as in NLM B during the time period from 1920 to 1947 (Fig. 8). Slumping is shown in cores NLM G, I (Fig. 9) and SLM I (Fig. 11). Deposition of a sand layer during storm surges, which occurs for cores CLM C and F (Fig. 10), is shown by unusually old CIC dates for the layer caused by near zero unsup-

45

ported 210Pb activity (Fig. 4). A summary of discontinuities and their probable causes is given in Table 3. These features are explained in more detail below. Mixing

Mixing of surface layers of sediment is usually attributed to biological activity (Robbins et af. 1977, Krezoski et af. 1978). From Figures 8-12, mixing is apparent in most north basin cores, but only appears in 2 of 11 south basin cores and not in either core from the central basin. The effect of mixing is loss of dating resolution. However, a storm event occurs at a specific date. Let us assume that there is mixing in a surface layer and that steady-state conditions prevail, except for the occurrence of a storm. In the case of a major storm surge such as the 1888 event shown in core NLM B (Fig. 2), where a large amount of material was deposited within a relatively short time, a point on the CIC curve indicates then the specific date of that event. Note, however, that the CIC date will only be correct if the 210Pb activity of the deposited material was sufficiently diluted to be the same as the one found in the mixing layer in the absence of recent storms. From the piece wise log-linear features of the 210Pb curve around and after the 1888 event, this appears to be the case. Points on the CIC curve that are not associated with specific storm events do not, as described above, represent material of one common age, but rather a mixture of ages. However, the age of the most recent additions is identical with the CIC age of a given point, assuming steady-state conditions. This is confirmed by the fact that the 1954 brackets from I37CS measurements generally intersect the CIC curves of Figures 8-12. The number of years of dating uncertainty lost listed in Table 3 is the average difference between the CRS and the CIC dates below the mixing zone where slopes of the CRS and CIC depth-time lines become similar. From Figure 13 (a), the dating resolution for the idealized core NLM E is 12.4 yr. The meaning of a CIC date is the date of the most recent input from a spike of tracer, and that of a CRS data, the entry date of an earlier spike contributing 440,10 of its maximum value. Most of the other cores of Table 2 have either smaller mixing depths Zm or larger sedimentation rates v such that AZ m < < v meaning that the above fraction is closer to 37%.

HERMANSON and CHRISTENSEN

46

TABLE 3. Summary of discontinuities in mpb profiles of Lake Michigan sediment cores (1984) caused by storms, dry periods and mixing. Buried Storm Events or Discontinuities

Core

Lower Sedimentation Rates (dry periods)

Number of Years of Dating Uncertainty Because of Surface Mixing

1907-1953 1920-1947

7 14 8 8 <2 7

NLM A B C E G I

1888, (1958) Slump, 1895 (1905)

CLM L M

1913-1956

3 <2

SLM C D E F H I J K

1878-1919 sand layer 1892-1944 sand layer 1888-1905, (1940), 1958, (1975) 1905-20, (1940), 1975 1888-1905, 1945 (1940?), (1975)

The progression with depth of the tracer pulse, Eq. (11), coming from an input spike of tracer is shown in Figure 13 (b). The number of years of dating resolution lost based on the actual core data (Table 3) shows where mixing has its greatest influence. Note, in particular, that for SLM J and K, there appears to be little mixing although the near constant 210Pb activities in the top suggest that mixing is present (Figs. 5 and 6). This apparent mixing is attributed to storm surges. Storm Surges Storms of varying intensity, documented with meteorologic and oceanographic data, occurred on the Great Lakes in 1905, 1913, 1916, 1918, 1919, 1940, 1952, 1958, and 1975 (Donn 1959, Murty and Polavarapu 1975, Evans et al. 1981, Lewis 1987). Popular literature on Great Lakes ship wrecks, such as Ratigan (1980), has been used to identify storms for the periods before the beginning of weather data collection in the late 19th century (Murty and Polavarapu 1975). Our NLM B core (Fig. 2) shows two vertical 210Pb discontinuities corresponding to possibly

6 <2 <2 <2 <2 10 <2 <2

1958 and to 1888. These breaks could have resulted from storms on the lakes in 1958 (Lewis 1987) and in 1888, when 91 ships sailing on the Great Lakes were damaged or sunk during a storm (Ratigan 1980). Core NLM G, collected from a steeply sloping part of the north basin, contains an overlap in the 210Pb profile characteristic of a slump (Fig. 3). Similar slumps are also found in NLM I (Fig. 3) and SLM I (Fig. 5). These slumps are probably caused by storms. SLM C and F (Fig. 4) both have missing 210Pb data that correspond to sand layers. Approximate dates of these layers suggest that the sand may have been deposited during turbulent weather. At SLM C, a storm in the late 1800s or early 1900s may have been the cause, while a SLM F, any or all of the storms occurring between 1888 and 1940 may be responsible. The SLM H 210Pb profile shows four discontinuities that can be related to storm dates (Fig. 5). The break corresponding to 1975 may be evidence of the storm resulting in the loss of the ore carrier Edmund Fitzgerald on Lake Superior (Lewis 1987, Ratigan 1980). The high sedimentation rate and

RECENT SEDIMENTATION IN LAKE MICHIGAN apparent absence of mixing makes the 1975 break visible. It cannot be ruled out that the 1975 storm surge break in fact is caused by mixing. However, because of the similarity of slopes of the CIC and CRS curves near the sediment-water interface (Fig. 11), and because the 1975 storm also appears to be reflected in cores SLM J and K, we believe that there is significant evidence for the storm surge interpretation. Likely influences of the 1958 and 1940 storms are also apparent in SLM H. The oldest discontinuity in the core has occurred as a result of the 1888 storm. SLM H is about 6 km northwest of core 74-1-Cl (Fig. 1), collected in 1974 and analyzed by Edgington and Robbins (l976a). Their 210Pb profile shows three vertical discontinuities, two of them associated with the 1958 and 1940 storms. The core does not extend beyond 1940, so it is not possible to corroborate our 1888-1905 profile break with observations from their core. Edgington and Robbins (l976a) also reported on core 73-4-C4 collected about 25 km to the northeast of SLM H, showing profile breaks that correspond to 1958, 1940, and 1915 storms. Cores SLM J and K have two and three 210Pb profile discontinuities, respectively. At the surface of both cores, there appears to be a deep mixing zone. However, the characteristic divergence of CIC and CRS dating profiles at the surface for cores with surface mixing is not apparent at J (Fig. 11) or K (Fig. 12). Hence, the roughly vertical 210 Pb profiles at the surfaces of J and K are not caused by mixing. The CIC and CRS profiles diverge below the surface, corresponding to the 1975 storm. A second break occurred in the mid-1940s at K, probably a result of the 1940 storm. The earliest discontinuity at SLM J is a result of one or more of the storms between 1905 and 1919. At SLM K, the earliest break occurred in the late 19th century corresponding to storms in 1888 or 1905. Low Precipitation Cores NLM A and B (Fig. 2) and CLM M (Fig. 3) show horizontal 210Pb profile discontinuities. This feature is similar to a discontinuity found in Lake Erie core M32 (Robbins et al. 1978) collected from 64 meters, which is much shallower than our NLM cores. Thus, in contrast to the case for core M32, it is unlikely that weather-induced turbulence in the water column is the cause of the horizontal discontinuities in our cores. Our hypothesis is that the horizontal discontinu-

47

ities represent periods of low sediment accumulation resulting from below normal amounts of riverine particle input to the lake caused by low stream discharge. Low stream flows are caused by dry (Skaggs and Brown 1987) and hot weather. Hot weather has a compounding effect on low precipitation in creating low stream inputs to the lake because of the increased evaporation and following reduction of stream flow. We checked precipitation records (U.S. Department of Agriculture 1922, U.S. Department of Commerce 1920 through 1987) for points in the northern Lake Michigan basin, including Green Bay and Manitowoc, Wisconsin, and Ludington, Michigan, which have long, continuous precipitation records. Some of the driest and hottest years between 1885 and 1985 correspond with periods of low sediment input to the northern basin. NLM A shows low sediment input corresponding to the 1907-1953 period, while NLM B shows such a period from 1920-1947. The low input at CLM M occurred between 1913 and 1956. These broadly defined periods are centered on the early 1930s, when precipitation amounts in the northern Lake Michigan basin and elsewhere in North America were much lower than normal. The lower precipitation during this period is also reflected in the lowest 10-year lake levels since recording started in 1860 (Ristic 1980). This period of hot, dry weather is revealed in reduced rates of sediment accumulation in the northern basin and CLM M. The absence of the dry weather phenomenon in the southern basin may result from different patterns of sediment accumulation in the north and south basins. While the north is influenced by input of particles, the south is affected by particle load and by redistribution processes, particularly those responsible for movement of material to the southeastern side of the lake. Hence, the reduction in particle inputs to the south may be masked in the stratigraphic record by continuing redistribution processes. We are not aware of other reports of the effects of low precipitation on sedimentation rates. Thus, even though we have found the effect in three out of sixteen cores, it is clear that our findings are subject to future confirmation. Focusing Patterns Focusing has been defined as movement of sediment toward deeper parts of a lake usually result-

48

HERMANSON and CHRISTENSEN

ing from periodic turbulence such as overturn (Likens and Davis 1975). In addition, the influence of storms on movement of sediment from one site to another is also interpreted as focusing. In our analyses, only sediment that has focused since the onset of I37CS fallout in the early 1950s is considered. The following analysis is based on the I37CS focusing factor Mm/M a • However, as can be seen from Table 2, there is a strong correlation between this factor and the 210Pb flux so that the analysis applies to 210Pb as well. Focusing data are grouped into four categories based on Mm/M a ratios (Table 2). The first category is Mm/M a < 0.9, representing sites where sediment is being contributed, shown on Figure 1 as "-". Second is 0.9 < Mm/M a < 1.1 where there is no contributing or focusing. Third is 1.1 < Mm/M a < 2, where there is moderate focusing (marked as "+" on Figure 1), and fourth is Mm/M a > 2, the major focusing zones, marked as "+ +". In the northern basin, cores G and I are in contributing zones. Core NLM B, collected in the area of the deepest sounding in the lake, is the only north basin area we sampled that shows significant focusing. Here the Mm/M a ratio shows 41070 more sediment accumulating than if there were no focusing. The other deep sites in the northern basin cores, C and E, show no focusing. The low level of contributing and focusing in the north basin shows that it has a relatively inactive sediment environment. CLM cores Land M are located on a basin slope similar in nature to NLM G, but only L has an M m/ M a value characteristic of a contributing site. CLM M is neither a contributing nor a focusing site. Contributing and focusing are more dynamic in the southern basin than in the north. Of cores SLM C, D, E, F, and I only C and F are not contributing locations, while J is a focusing area and Hand K are major focusing sites. Robbins and Edgington's (1975) core 29, located to the south-southeast of H, is also a major focusing zone, with an Mm/M a of 2.70, calculated from their data, about the same as our observation at H. Cores SLM Hand 29 were from shallower water than any SLM cores except A and G, but show more focusing, again apparently resulting from movement of sediment into the area by a counterclockwise gyre in the southern basin, possibly aided by shore erosion and by input from the St. Joseph River. The two deeper sites, SLM J and K, are both focusing zones, as expected for deep areas, but the larger value at K and H

shows that the deepest part of the basin at J is not the area of greatest intensity of focusing. It is apparent from our south basin cores that areas where storm surges have had the most notable effects are also the sites of greatest sedimentation rate and sediment focusing. Thus, focusing in southern Lake Michigan is not only a continuous process governed by processes indigenous to the lake, but is also one driven by turbulence resulting from the major storms. CONCLUSIONS The present investigation has not only confirmed previous observations of the effect of storm surges on sedimentation in the Great Lakes, but also has provided further information on the influence of climate on sedimentation rates, on appropriate dating models (CIC or CRS), on the extent of sediment mixing, and on the sedimentation pattern in Lake Michigan. Large deposits of sediment during storm surges were found only in the four cores with focusing factors significantly greater than 1 (NLM B, SLM H, J, K). Therefore focusing is not only a continuous process, but also one driven by sporadic, major turbulence. Another manifestation of storms is slumping found in cores NLM G and I, and SLM I where older sediment lies over newer material. These slumps were probably caused by the storms. High winds and wave action also appear to have caused deposition of sand layers in cores SLM C and F during the time periods of 1878-1919 and 1892-1944, respectively. Regarding the actual storms reflected in the sediments, we have with various degrees of confidence found the same ones that have been reported previously for the Great Lakes, i.e., those in 1888, 1905-1920, 1940, and 1958 (Edgington and Robbins 1976a, Robbins et al. 1978). Of these storms, only those in 1915, 1940, and 1958 were previously observed to have left markers in Lake Michigan sediments (Edgington and Robbins 1976a). In addition, we have found evidence of the 1975 storm in cores SLM H, J, and K. A comparison of the CIC and CRS 210Pb dating models shows that under steady-state conditions, the CIC model may be most appropriate for cores influenced by mixing. There is also a significant component of the CIC model in cases of focusing, since not only particles but also 210Pb is being redistributed on the lake bottom. However, because

RECENT SEDIMENTATION IN LAKE MICHIGAN

some CIC dates are going forward in time with increasing depth and because of the actual dates of storms and dry periods, the CRS model should be included. The actual dates are between the dates obtained from the two models. Separation of the CIC and CRS dates in the upper sediment layers provides evidence of mixing when, as occurs here, there have been no major storms in recent years, or if a core is not affected by storms. We found mixing in most northern cores (NLM A, B, C, E, and I) and two southern cores (SLM C and I). All these cores are characterized by a relatively low mass sedimentation rate (r < 0.0143 g/cm 2 /yr) and, with exception of NLM I, a relatively high 2lOPb flux (10 > 0.87 dpm/cm 2/yr; Fig. 7). We have observed lower sedimentation rates during the 1930s for two cores in northern (NLM A, B) and one in central (CLM M) Lake Michigan. This time period corresponds to an extended period of dry and hot weather. It appears that the low sedimentation rate is caused by low inputs of particles from rivers during these extreme conditions. Similar periods of low rate of sedimentation were not observed in the south where the sedimentation pattern is more dependent on redistribution of particles rather than direct riverine input. ACKNOWLEDGMENTS The authors appreciate the help of Dr. Fred Nurnburger, Michigan State Climatologist, for providing Michigan precipitation data. Don Nelson, P. D. Anderson, and R. W. Paddock helped with I37CS counting at Argonne National Laboratory. We thank B. Boudreau and L. Benninger for reviewing the manuscript. This work was supported by the V.S. Environmental Protection Agency under assistance agreement R810419, and by the V.S. National Science Foundation Grant No. CES-8701184. The opinions stated are those of the authors and no official endorsements should be inferred. REFERENCES Appleby, P. G., and Oldfield, F. 1978. The calculation of 1ead-21O dated assuming a constant rate of supply of unsupported Pb-21O to the sediments. Catena 5:1-8. _ _ _ _ , Oldfield, F., Thompson, R., and Hottunen, P. 1979. Pb-21O dating of annually laminated lake sediments from Finland. Nature 280:53-55. Berger, W. H., and Heath, G. R. 1968. Vertical mixing in pelagic sediments. J. Mar. Res. 26:134-143.

49

Cahill, R. A. 1981. Geochemistry of Recent Lake Michigan Sediments. Champaign: State Geological Survey Division, Illinois Institute of Natural Resources, Circular 517. Christensen, E. R. 1982. A model for radionuclides in sediments influenced by mixing and compaction. J. Geophys. Res. 87 (Cl):566-572. _ _ _ _ , and Bhunia, P. K. 1986. Modeling radiotracers in sediments: Comparison with observations in Lakes Huron and Michigan. J. Geophys. Res. 9IC:8559-8571. _ _ _ _ , and Goetz, R. H. 1987. Historical fluxes of particle-bound pollutants from deconvolved sedimentary records. Environ. Sci. Technol. 21: 1088-1095. Donn, W. L. 1959. The Great Lakes storm surge of May 5, 1952. J. Geophys. Res. 64:191-198. Eadie, B. J., Chambers, R. L., Gardner, W. S., and Bell, G. L. 1984. Sediment trap studies in Lake Michigan: resuspension and chemical fluxes in the southern basin. J. Great Lakes Res. 10:307-321. Edgington, D. N., and Robbins, J. A. 1976a. Patterns of deposition of natural and fallout radionuclides in the sediments of Lake Michigan and their relation to limno10gical processes. In Nriagu, J. 0., ed. Environmental Biogeochemistry, Vol. 2. Ann Arbor: Ann Arbor Science. pp. 705-729. _ _ _ _ , and Robbins, J. A. 1976b. Records of lead deposition in Lake Michigan sediments since 1800. Environ. Sci. Technol. 10:266-274. Evans, J. E., Johnson, T. c., Alexander, E. c., Lively, R. S., and Eisenreich, S. J. 1981. Sedimentation rates and depositional processes in Lake Superior from 2lOPb chronology. J. Great Lakes Res. 7:299-310. Fukumori, E., and Christensen, E. R. 1989. Modeling 137Cs in lake sediments. Earth and Planet. Sci. Lett. Submitted. Health and Safety Laboratory, Environmental Quarterly. 1977. Final Tabulation of Monthly 90Sr Fal/out Data: 1954-1976. HASL-329. New York: Energy Research and Development Administration. Krezoski, J. R., and Robbins, J. A. 1985. Vertical distribution of feeding and particle-selective transport of 137Cs in lake sediments by lumbriculid oligochaetes. J. Geophys. Res. 90 (C6):11,999-12,006. _ _ _ _ , Mozley, S. C. and Robbins, J. A. 1978. Influence of benthic macroinvertebrates on mixing of profundal sediments in Lake Huron. Limnol. Oceanogr. 23:1011-1016. Krishnaswamy, S., Lal, D., Martin, J. M., and Meybeck, M. 1971. Geochronology of lake sediments. Earth Planet. Sci. Lett. 11 :407-414. Lesht, B. M., and Hawley, N. 1987. Near-bottom currents and suspended sediment concentration in southeastern Lake Michigan. J. Great Lakes Res. 13:375-386.

50

HERMANSON and CHRISTENSEN

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