Southern Lake Michigan Sediments: Changes in Accumulation Rate, Mineralogy, and Organic Content

J. Great Lakes Res., 1980 Internat. Assoc. Great Lakes Res. 6(4): 321-330

SOUTHERN LAKE MICHIGAN SEDIMENTS: CHANGES IN ACCUMULAnON RATE, MINERALOGY, AND ORGANIC CONTENT

David K. Rea, Richard A. Bourbonniere, t and Philip A. Meyers Oceanography Program Department of Atmospheric and Oceanic Science The University of Michigan Ann Arbor, Michigan 48109

ABSTRACT. Data combined from three one-meter gravity cores raised from a single site in southeastern Lake Michigan permit reconstruction of the last 3,550 years of sedimentation at that location. Sediment has been accumulating at about 50 g m-2 y-t through most of the upper 50 em of the core and nearly 900 g m- 2 y-t below that depth. Several of the physical and chemical properties of the sediments change when the sedimentation rates change; more rapidly accumulating sediments have a finer grain size, more carbonate minerals and more inorganic carbon, less organic carbon, and, among the organic fractions, relatively more humin and less fulvic acid. All these rate and abundance changes apparently occurred about 3,300 years ago and seem to be associated with the lowering of lake levels since the Nipissing high stand which ended about 4,000 years ago.

SAMPLES AND ANALYSIS Multiple gravity cores were obtained from a single location (our site SLM 77-26) situated at 42° 20' N, 86° 50' W in 118 meters of water on the southeastern edge of the southern basin of Lake Michigan. These cores, all slightly more than one meter long, consisted of dark gray silt above 58 cm and dark gray clayey silt with thin black bands below that depth. The oldest sediments recovered are about 3,600 years old. We have analyzed three of these cores for: C-14 age dating and stable carbon isotope determinations of the sediment organic matter; porosity, mineralogy, and grain size along the length of a core; and down-core distributions of total organic carbon (TOC), inorganic carbon, and the three humic matter fractions.

INTRODUCTION Changes in the accumulation rate of subaqueous sediments can affect the mineralogical and chemical composition of the sediment. For example, dissolution of carbonate minerals commonly occurs in surficial Great Lakes sediments (Kemp and Dell 1976, Rea and Pigula 1979). More rapid burial of these surface layers retards the dissolution process and results in a higher carbonate content of the deposits. Similarly, organic matter degrades in the oxic surface layer of sediments. Areas having high sediment accumulation rates generally have a more rapid transition from oxic to anoxic conditions and a consequently greater preservation of organic carbon compounds (Gaskell et al. 1975). As some evidence now exists for significant changes in Great Lakes sediment accumulation rates occurring about 3,000 to 4,000 years ago (Graham and Rea 1980), we have taken this opportunity to investigate the magnitude of these rate changes and their effects upon the composition of Lake Michigan sediments.

Radiocarbon Dating Four samples of material from one core were sent to Geochron Laboratories, Cambridge, MA, for C-14 age dating. Obtaining accurate C-14 dates on disseminated organic materials in the Great Lakes sediments is complicated by the signficant but unknown amount of geologically

t Present address: National Water Research Institute, Canada Centre for Inland Waters, Burlington, Ontario L7R 4A6.

321

322

REA, BOURBONNIERE, and MEYERS

old organic carbon incorporated into the sediments at the time of burial (King, Lineback, and Gross 1976). Eroding glacial tills and Holocene deposits along the lakeshore provide almost all of the fine-grained sediment input into Lake Michigan, and these materials contain old soil horizons and other organic materials which are transported and deposited along with the zeroage organic material. As a result, any single C-14 date represents a maximum age for the sample. This problem can be solved in part by obtaining multiple dates on each core. The age difference between dated samples provides the linear sedimentation rate for that interval. If the age of surface sediments is known or can be assumed to be zero, the age of all levels in the core can be calculated. King et al. (1976) used this approach on two Lake Michigan cores and found generally uniform sedimentation rates on the basis of several dated samples in each core. This technique relies upon the assumption that the relative amount of inert carbon remains unchanged throughout the time of deposition of the core (see discussion in Graham and Rea 1980). If so, the C-14 age of surficial sediment is a direct measurement of the amount of inert carbon present, and that age can be subtracted from all others in the core to give a corrected "true" age. As a test of the assumption that no important changes have occurred in the input character of organic carbon, 1 3 C/ l 2 C ratios were determined in the core sections selected for 1 4 C age dating. Photosynthetic fixation of carbon generally results in an isotopic depletion in 1 3 C of about -20%0 from the inorganic carbon source (Peters, Sweeney, and Kaplan 1978). Bicarbonate is this source in aqueous systems having pH's of 8, such as the oceans and the Great Lakes, whereas atmospheric CO 2 is the source for land plants. The isotopic ratios of bicarbonate and CO 2 are 0%0 and -7 % 0 relative to Pee Dee Belemnite, respectively, and this difference is preserved in organic matter biosynthesized in the two systems. Hence, fluctuations in relative amounts of lakederived versus land-derived organic carbon will be recorded in the 1 3 CI 1 2 C ratios of core sections. Grain Size and Mineralogy Grain size analyses were completed every 5 cm along the core. Samples were taken from the core, placed in 30-mL sample jars with filtered, distilled water and disaggregated by gentle shaking. The

resulting slurry was sieved at 63 microns to separate the sand from the silt plus clay fractions. The sandy material was dried and weighed, and the fine fraction, usually 90 to 95% of the total, was analysed for grain size at 1.0 cp intervals from 4 cp to 10 cp using a Coulter Counter electronic particle size analyzer (Shideler 1976). Median grain sizes determined from these measurements have a precision of ± 0.1 cp. Bulk mineralogical analyses were done by X-J,:ay diffraction every 10 cm along the core. Relative amounts of minerals present were determined using an empirical relationship developed by Dr. Donald M. Peacor of The University of Michigan, Department of Geological Sciences. The amount of a mineral present is determined on a relative basis by comparing peak areas to the area of the 20.86 0 2 () quartz peak. The resulting values are then multiplied by an empirical factor determined by mixing various known percentages of quartz and the mineral in question and compiling a working curve that can be applied to unknown mineral percentages. This method provides a good estimate of the relative amount of minerals present in typical soils and sediments of the Great Lakes region. These include quartz, calcite, dolomite, plagioclase, feldspar, and orthoclase feldspar for which the precision of measurement is ± 10% of the value or better, and the 7 A (chlorite and kaolinite), 10 A (illite), and 14 to 15 A (smectites and vermiculites) clays which have a lower precision, perhaps ± 20% of their value. Porosity and Dry 6ulk Density Porosity determinations were made on a sample interval ranging from 1 cm near the top of the core to 5 cm below 45-cm depth. Water content of the sample was determined by weight loss upon freeze drying. These values were converted to porosity by assuming an average density of the sediment grains, 2.65 g cm- 3 in this case. For a 1 g mass of sediment with a water content of 80%, for example, porosity is calculated as: p = water volume = total volume 0.80 g/1.0 g cm- 3 0.80 g/l.O g cm- + 0.20 g/2.65 g cm-

------~---"'-----3 3

= 0.914.

Dry bulk density, the true dry mass of sediment per unit volume, then is simply:

SOUTHERN LAKE MICHIGAN SEDIMENTS

C-14 AGE IN YEARS B.P.

DBD = (1.0 - P) 2.65 g cm- 3 •

Organic Materials Total carbon was determined on whole freezedried sediment and total organic carbon (TOC) on acid-treated sediment with a Hewlett-Packard Model l85-B CHN Analyzer. The procedure gives an average coefficient of variation of 3.7 percent. Inorganic carbon was estimated by difference on the samples analyzed for total carbon and TOe. Sediment humic matter was fractionated into fulvic acid, humic acid, and humin for fourteen samples according to the operational definitions established by soil scientists (Schnitzer and Khan 1972). Freeze-dried carbonate-free sediment is extracted with 0.5 N NaOH by shaking for 24 hours at room temperature under a nitrogen atmosphere. Fulvic acid and humic acid are made soluble by this procedure, and the humin fraction is defined as the residual organic matter which remains associated with the sediment. Fulvic acid is separated from humic acid by acidifying the alkaline solution to cause precipitation of humic acid. The purification of these organic matter fractions was identical to that used on Lake Huron sediments by Bourbonniere and Meyers (1978) and is described in detail by Bourbonniere (1979). SEDIMENTATION IN SOUTHEASTERN LAKE MICHIGAN Although we used three different cores for the several analytical procedures, the data are presented as if they all came from a single core. Core descriptions upon opening match well, so errors arising from this procedure, which assumes that the same depth represents the same sedimentary horizon among the three cores, are minor and are within one 5-cm sample interval. Linear Sedimentation Rates and Mass Accumulation Rates Figure 1 shows an age-depth plot of the sediment at our location. The four radiocarbon dates (Table 1) define a minimum of two linear sedimentation rates (LSR). The rates are approximately 0.14 mm y-l between 3 and 50 cm depth, and very high below 50 cm. The age data permit an infinitely high LSR for this part of the core; for the purposes of discussion, we will assume that

323

1000. 3000. 5000. Ot----r----t-----+-----r ~1.0mm/y, Cs-137

20 (/)

a::: w

0.14 mm/y, C-14 /

l:;j 40 ~

~

W

U

Z

60

::c

6: w a

80 1.5mm/y, estimated

100

FIG. 1. Carbon-14 ages of sediments at the southeastern Lake Michigan core site. Vertical bars denote the dated interval; horizontal bars reflect uncertainty of dates.

TABLE 1. C-l4 and co"ected sediment ages and corbon isotope values at different depths in the southeastern Lake Michigan core, Asterisk (tit) indicates age determined by application of modern sedimentation rates from Cs-137 data (Edgington and Robbins 1975, their station 26). C-13 values are relative to the PDB standard. Sediment age, years before 1977 Corrected for detrital organic carbon

Depth in core 3.0cm 35.5 cm 71.0 cm 106.5 cm

1895 ± 155 4140 ± 165 5400 ± 140 5405 ± 145

30* -26.3 ± 0.2 2275 ± 226 -26.4 ± 0.2 3535 ± 209 -26.6 ± 0.2 3540 ± 212 -26.4 ± 0.2

REA, BOURBONNIERE, and MEYERS

324

the LSR between 50 and 110 cm was an order of magnitude higher than that between 3 and 50 cm, or 1.5 mm y-l. Cs-137 results reported by Edgington and Robbins (1975) give a LSR of 1.0 mm y-l for the top 3 cm of the sediment at this site (Table 1). Linear sedimentation rates, therefore, show high values prior to about 3,300 years ago, a value of 0.14 mm y-l between then and approximately 30 years ago, and a present value of about 1.0 mm y-l . We prefer the interpretation of LSR's given above and on Figure 1 because: 1) it is the simplest possible explanation of the given age data; and 2) more "sophisticated" interpretations do not improve the estimate of the timing of the significant LSR change. It is certainly possible to fit other LSR curves to the four C-14 ages. If one connects points of known age, the large LSR change occurs at the 71-em age of 3,535 ± 209 Y (Table 1), from 0.30 mm y-l above to centimeters per year below. Fitting a smooth curve to the four points would result in a LSR of about 0.20 mm y-l changing to several mm y-l somewhere between 45- and 60-cm depth or at an estimated age of 3,100 to 3,200 years. As it is unlikely that the LSR change was instantaneous, such a curved line may represent the most likely case. It would predict a change in other sedimentary parameters where they do occur and at about the same time as the rate model implied by Figure 1. However, given the precision and accuracy constraints of our age data, formulations of LSR history more complicated than Figure 1 do not improve the inherent value of our information, so we have not used them. Linear sedimentation rates are important information, but they are a measure of the sediment plus interstitial water accumulating at any one place and not always a good estimate of the true amount of material accumulating in terms of mass per unit time. A more useful value for quantitative work is the mass accumulation rate (MAR) which is a measure of the dry mass of material accumulating per unit area and time, for lacustrine work commonly g m- 2 y-l. The use of the MAR allows comparison of true deposition rates among sediments of variable water content; it is calculated by finding the product: LSR (em y-l) X DBD (g em-3 )

=MAR (g em-2 y-l).

To convert to g m- 2 y-l, this value is multiplied by 104 ; hence:

MAR (g m-2 y-l) = LSR (mm y-l ) X DBD (g em-3 ) X 103 .

MAR values in our core range from a low of about 50 g m- 2 y-l to a maximum of over 1000 g m- 2 y-l with a large increase occurring where the LSR rises abruptly. Downcore Changes in Grain Size and Mineralogy The median grain size in the core decreases from 6.4 ct> at the surface to 7.3 ct> at 45 cm and remains reasonably uniform below that depth (Figure 2). This uniformity of the lower part of the core is emphasized by the position of the dominant whole-phi mode which remains at 7 to 8 ct> for 11 of the 13 samples between 45 and 105 cm. Increased variability in grain size and modal position characterize the upper portion of the core. In addition to the average grain size, we have also determined the weight percent sand in th~se sediments. Sand concentrations do not always "follow" the average grain size values (Figure 2, especially the peak at 15 cm), suggesting occasional nonequilibrium events such as slumping from shallower water or perhaps unusually severe storms. The concentrations of the different mineral groups also vary along the length of the core (Figure 2). Quartz values are low at the surface, increase abruptly at 10 cm (sample interval is 10 cm), and gradually decrease downcore. The carbonate minerals show their lowest values between 10 and 40 cm, increasing both toward the surface and with depth from that interval. Clay minerals comprise approximately 10% of the sediment and are slightly greater in abundance between 10 and 40 cm where the carbonates are less abundant (Figure 2). Among the clays the 7 A and lOA minerals occur in subequal amounts and the 14 to 15 A component is very small. Feldspars comprise a constant 5 to 8% of the sediment; plagioclase comprises half to twothirds of the feldspars. The depth of reduced carbonate abundances coincides with the time of low LSR between 3,300 and 30 years ago. Figure 2 also shows a plot of the dolomite and calcite abundances in our core. These minerals appear to be sensitive indicators of dissolution in Great Lakes sediments (Dell 1972, Dell 1973, Kemp and Dell 1976, Rea and Pigula 1979, Graham

SOUTHERN LAKE MICHIGAN SEDIMENTS AVG. GRAIN SIZE

WT% SAND

CALCITE & DOLOMITE

325

TOTAL MINERALOGY (%)

5. 6. 7. O. 10. 20. 10. 20. 30. 20. 40. 60. 80. 100. Or-----+--¥-----r-----+¥---<-->--;---+--+---+-----<-+---+-::::a----<--I--ooo:t--+----+---+-----<"......---,

20

QUARTZ

(f)

r:r

w w 40

f-

:2

f-

Z

W

0

z

60

I f-

CL W

0

80

100

FIG. 2. Grain size and mineralogy of sediments at the southeastern Lake Michigan core site. Average grain size is in phi (
and Rea 1980), with both lower calcite to total carbonate ratios and lower carbonate contents denoting times or regions of greater dissolution. These values reach their minima in the 10- to 40-cm samples of our core (Figure 2). The relative abundance of minerals in Great Lakes sediment is controlled by the mineralogy of the sediment source and by both hydrodynamic sorting and chemical dissolution during transport. Source mineralogy is the ultimate control and, in general, lacustrine sediments are a direct reflection of that mineralogy, None-the-less, the relative abundance of minerals may change somewhat during transport. Dissolution of calcite and dolomite has been documented in all Great Lakes (Dell 1972, Dell 1973, Kemp and Dell 1976, Rea and Pigula 1979, Graham and Rea 1980). Feldspar is less abundant in the sediments of Lakes Ontario and Erie than in the eroding shoreline bluffs, suggestive of weathering and dissolution during transport (Thomas 1969, Kemp and Dell 1976). In Little Traverse Bay, Lake Michigan, the relative abundance of feldspar in the sediments is approximately twice that in the nearby

eroding tills (Rea and Pigula 1979), the opposite of what is observed in the lower Great Lakes. Hence, feldspar concentrations may be controlled by more than just the source-dissolution phenomena that determine calcite and dolomite concentrations. The association of maxima in clay mineral abundances with poorly to very poorly sorted sediment in Lake Erie (Dell, written communication) and Lake Michigan (Rea and Pigula 1979) suggests that the energy of the lake-floor environment also exerts some control on sediment mineralogy. Through geologic time, physical processes on the lake floor will vary in intensity with changing water depth and with changing climate, and the lake sediments will record these changes (Lineback, Dell, and Gross 1979). Significant changes in the chemistry and temperature of the hypolimnion during Holocene time are less likely than changes in either input rate or the vigor of circulation. Lake Superior sediments record continuous carbonate dissolution throughout Holocene time (Farrand 1969, Dell 1972), suggesting that the waters have never been significantly less corrosive

326

REA, BOURBONNIERE, and MEYERS

than now. Conversely, Lake Huron sediments of early to mid Holocene age contain up to 40% carbonate minerals (Graham and Rea 1980), suggesting that the waters were not significantly more corrosive than they are now. Density considerations suggest that bottom-water temperatures will not have changed from their present 4 to 5°C. The evidence at hand, therefore, suggests that the rate of carbonate mineral dissolution has not changed significantly during Holocene time. The rate of accumulation of carbonate minerals in lake sediments depends, as it does in the oceans (Berner 1971), on the balance between the rate of supply and the rate of dissolution. Sediment input is controlled by the rate of shoreline erosion which supplies about 90% of the sediment to Lake Michigan (Monteith and Sonzogni 1976). Erosion rates, in turn, depend largely on lake levels, thus input rates and the abundance of carbonate minerals should reflect lake-level history (Hough 1966, Prest 1970). Information presented in the paper appears to confirm this first-order association. Organic and Inorganic Carbon Essentially no change occurs in the stable carbon isotope ratios (Table 1), indicating no change in source of organic matter over the time period this core represents. Most of this organic matter appears to be land-derived in view of the depletion in 13 e of -26 %0 throughout the sediment. In contrast to the carbon isotope data, the distribution of total organic carbon and inorganic carbon (Figure 3) fluctuates with depth in the core. TOe shows a sharp decrease from a surface value of 2.5% to 1.4% at 9 em. Below this depth, TOe rises slightly and remains essentially constant at about 1.8% down to about 55 em, where it begins to decrease to lower values again. Below 60 em, the values are generally between 1.25 and 1.4%. These data are similar to values of TOe from southern Lake Michigan sediments analysed by Shimp et al. (1971). Inorganic carbon content shows more fluctuation than the TOe data. A subsurface maximum of 3.7% occurs at 4 em, followed by a sharp decrease to 1.6% at 14 em. Between 14 arid 55 em, inorganic carbon fluctuates about a mean value of 2.1 %. Below 55 cm, a fluctuating but nevertheless clear increase with depth occurs, attaining at the bottom the highest concentration of 4.25%.

The changes in the amount of TOe and inorganic carbon occur where the LSR changes (compare Figure 1 and Figure 3). Rapid sedimentation rates in the lower portion of the core caused relatively quick burial and, therefore, enhanced preservation of carbonate minerals. These rapidly accumulating sediments were, however, relatively poor in organic carbon (Figure 3). Between about 3 and 50 cm, where lower inorganic and higher organic carbon values occur, the average LSR was low, but perhaps not constant as suggested by fluctuations in the inorganic carbon concentrations. In the surficial zone of higher LSR, both the inorganic and organic carbon contents increase to relatively high values (Figure 3). Distribution of Organic Carbon in Humic Matter The distribution of organic carbon in the humic matter fractions isolated from core sections were calculated from elemental analyses of these fractions. These results are expressed as a percent of the total isolated organic carbon. Throughout the length of the core humin predominates, fulvic acid is intermediate, and humic acid comprises a minor portion of the organic carbon. These relative abundances agree with those of surficial sediments from Lakes Huron, Erie, and Ontario (Kemp 1973). The distributions of fulvic acidcarbon and humin-carbon in Figure 3 appear as near mirror images as a result of the very low and nearly constant humic acid-carbon content of the sediment. An apparent decrease in fulvic acid-carbon occurs over the top 10 cm of the core with a corresponding increase in the humin-carbon content. The values for the 8- to 9-cm section appear to be anomalous. At this level, much courser black material was mixed with the gray silt. Sediment and the alkaline extract from this section released copious amounts of hydrogen sulfide during acidification; such an occurrence was not noticed for any other section analyzed. These observations indicate that reducing conditions enhance the preservation of fulvic acid. Between 10 and 50 em, the fulvic acid-carbon contribution exhibits a slight decrease, then a larger decrease where the sedimentation rate changes near 50 em and decreases further with depth. The humin-carbon content exhibits opposite trends. It is important to note that these distribution data, since they are expressed as a

327

SOUTHERN LAKE MICHIGAN SEDIMENTS

ORG-C

1.

2.

INORGANIC-C (%)

1.5

2.5

DIST. OF ORGANIC-C

3.5

20.

40.

60.

(%) 80.

Or----+----+~v--r---+-+----tv--+--__tT~--+----+-r_t-----+--_+,~f---+--+-__t

100.

20 (f)

0::: W I-

w 40 ~

I-

Z W U

z

60

:r

I-

0W

0

80

FULVIC ACID [!J HUMIC ACID (!) HUMIN ~

100

FIG. 3. Carbon content in weight-percent of sediments at the southeastern Lake Michigan core site. The three organic-carbon fractions are given in percent of organic-carbon and sum to 100%.

percent of the isolated organic carbon, are independent of the total organic carbon content. Thus, it appears that changes in sedimentation rate affect not only the amount but also the type of organic matter preserved in the sediment. Mass Accumulation Rates The MAR for anyone sediment component can be calculated by multiplying the total MAR by the weight-percent of that component in the sediment. The mass accumulation rates for sediment, inorganic carbon, and organic carbon (Figure 4) are determined largely by the LSR values. Where linear sedimentation rates are low, the mass accumulation rates for sediment, inor-

ganic carbon, and TOC are virtually constant with depth. In the lower part of the core where the high LSR occurs, the MAR values for total sediment and inorganic carbon show increases with depth. However, the MAR of organic carbon in the lower part of the core does not increase but fluctuates about the value of about 13 g m-2 y-l.

DISCUSSION Significant geological and geochemical parameters of sediments from southern Lake Michigan vary with rates of accumulation. This strongly implies that not only some aspects of the geology but also some of the organic geochemistry of these

REA, BOURBONNIERE, and MEYERS

328

MASS ACCUMULATION RATES (G/SQ.M-YR) SEDIMENT

o

250

500

INORGANIC-C

750

o

15

30

TOT

o

ORGANIC-C

5

10

15

Ot---+-.,.,..-+--+------i~-+-----+--t__Q'l_-+-+---+---+-_r_-~r_-+-----+--+--_;

20 U1

n:: w w

~

40

~ ~

Z

W

U

z 60 I ~

0.... W 0

80

100

FIG. 4. Mass accumulation rates of the total sediment and the organic and inorganic carbon fractions at the south-eastern Lake Michigan core site.

materials are process-controlled rather than sourcecontrolled. Between 50- and 60-cm in depth, the total mass accumulation rate increases downcore by an order of magnitude. At the same time, the proportions of organic carbon in the three fractions of humic matter shift in favor of humin, although the total amount (in weight-percent) of organic carbon decreases. Similarly, the relative amount of dolomite plus calcite increases downcore. In the top few centimeters of the core where the LSR increases from 0.14 mm y-l to 1.0 mm y-l at the top, the amount of carbonate minerals preserved doubles (Figure 2). Humin,

however, appears to decrease in these surficial materials, a change with LSR in the opposite sense from that at 50 em. We interpret these rate-related changes in the organic fractions and in carbonate abundances as being dependent upon how rapidly burial removes these materials from the sediment-water interface and the zone of bioturbation. During times of slow deposition, the sedimentary organic material remains in the oxidized surficial layer for perhaps hundreds of years rather than for tens of years. In this case, oxidative degradation of the organic materials can progress much farther.

SOUTHERN LAKE MICHIGAN SEDIMENTS Carbonate minerals also remain in contact with corrosive lake waters for much longer periods, thus reducing preservation. The total amount of organic carbon and the amounts of the essentially insoluble minerals present appear to be source-controlled. Prior fo 3,300 years ago, the percent of organics in the sediment was somewhat less than since that time, a result of dilution by the large influx of inorganic material. The MAR of organic materials, however, increases by an order of magnitude below about 50 em. Relative abundances of quartz, clays, and feldspars probably reflect the immediate source materials for the sediments. Of these minerals, only plagioclase feldspars may undergo "chemical weathering" within the lake system (Kemp and Dell 1976). The sediment grain size depends both upon the size of the source material and the vigor of the transporting processes in a fairly well understood manner. SYNTHESIS The time of the large reduction in LSR, about 3,300 years ago, is shortly after the end of the Nipissing high lake levels (Prest 1970). Higher lake levels would enhance bluff and shoreline erosion, and a retreat from the highstand would markedly reduce the sediment input to the lake. During the period of low sedimentation rates, the accumulating materials spend a much longer time at the sediment-water interface or in the oxidized surface layer which is subject to bioturbation. Both the organic material and the carbonate minerals in this zone remain subject to chemical attack for long periods of time. Sedimentation rates increased at least 30 years ago to their present rate of about 1.0 mm y-l . This recent increase is not apparent elsewhere in Lake Michigan (Robbins and Edgington 1975, King et ai. 1976). It may occur in Lakes Erie and Ontario (Kemp et ai. 1974; Kemp and Harper 1976; Robbins, Edgington, and Kemp 1978) but probably not in Huron and Superior (Bruland et ai. 1975, Kemp and Harper 1977). ACKNOWLEDGMENTS This paper has been reviewed by J. O. Nriagu, Gordon Fraser, William F. Kean, C. F. M. Lewis, and David L. Gross who all provided helpful comments and criticisms. We thank William M. Sackett of the University of South Florida for

329

kindly providing stable carbon isotope measurements. Portions of this study were supported by a grant from the Scott Turner Fund administered by the Department of Geological Sciences of The University of Michigan and by a Sigma Xi· Grant-in-Aid of Research. Acknowledgment is also made to the donors of the Petroleum Research Fund administered by the American Chemical Society for partial support. REFERENCES Berner, R. A. 1971. Principles of Chemical Sedimentology. McGraw-Hill, New York. Bourbonniere, R. A. 1979. Geochemistry of Humic Matter in Holocene Great Lakes Sediments. unpub. Ph.D. Thesis, Univ. of Michigan, Ann Arbor. Bourbonniere, R. A., and Meyers, P. A. 1978. Characterization of sedimentary humic matter by elemental and spectroscopic methods. Can. Jour. Spectroscopy 22: 3541. Bruland, K. W., Koide, M., Bowser, C., Maher, L. J., and Goldberg, E. D. 1975. Lead-210 and pollen geochronologies on Lake Superior sediments. Quat. Res. 5:89-98. Dell, C. I. 1972. The origin and characteristics of Lake Superior sediments. In Proc. 15th Con! Great Lakes Res., pp. 361-370. Internat. Assoc. Great Lakes Res. _ _ _ _ 1973. A special mechanism for varve formation in a glacial lake. J. Sed. Petrol. 43:838-840. Edgington, D. N., and Robbins, J. A. 1975. The behavior of plutonium and other long-lived radionuc1ides in Lake Michigan. In Impacts of Nuclear Release into the Aquatic Environment, pp. 245-260. InternaL Atomic Energy Agency, Vienna. Farrand, W. R. 1969. The Quaternary history of Lake Superior. In Proc. 12th Can! Great Lakes Res., pp. 181197. Internat. Assoc. Great Lakes Res. Gaskell, S. J., Morris, R. J., Eglinton, G., and Calvert, S. E. 1975. The geochemistry of a recent marine sediment off northwest Africa. An assessment of source of input and early diagenesis. Deep-Sea Res. 22 :777-789. Graham, E. J., and Rea, D. K. 1980. Grain size and mineralogy of sediment cores from western Lake Huron. J. Great Lakes Res. 6:129-140. Hough, J. L. 1966. Correlation of glacial lake stages in the Huron-Erie and Michigan Basins. Jour. Geol. 74:62-77. Kemp, A. L. W: 1973. Preliminary information on the nature of organic matter in the surface sediments of Lakes Huron, Erie, and Ontario. In Proc. Symp. Hydrogeochemistry Biogeochemistry. pp. 4048. E. Ingerson (Ed.), Clark Co., Washington, D. C. Kemp, A. L. W., and Dell, C. I. 1976. A preliminary comparison of the composition of bluffs and sediments from Lakes Ontario and Erie. Can. Jour. Earth Sci. 13: 1070-1081. Kemp. A. L. W., and Harper, N. S. 1976. Sedimentation

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REA, BOURBONNIERE, and MEYERS

rates and a sediment budget for Lake Ontario. J. Great Lakes Res. 2:324-340. and 1977. Sedimentation rates in Lake Huron and Georgian Bay. J. Great Lakes Res. 3:215-220. Kemp, A. 1. W., Anderson, T. W., Thomas, R. 1., and Mudrochova, A. 1974. Sedimentation rates and recent sediment history of Lakes Ontario, Erie, and Huron. J. Sed. Petrol. 44:207-218. King, J. E., Lineback, J. A., and Gross, D. 1. 1976.Palynology and Sedimentology of Holocene Deposits in Southern Lake Michigan. Illinois State Geological Survey Circular 496. Lineback, J. A., Dell, C. I., and Gross, D. 1. 1979. Glacial and post-glacial sediments in Lakes Superior and Michigan. Geol. Soc. Am. Bull. 90:781-791. Monteith, T. J., and Sonzogni, W. C. 1976. U. S. Great Lakes Shoreline Erosion Loadings. Great Lakes Basin Comm., Ann Arbor. Peters, K. E., Sweeney, R. E., and Kaplan, I. R. 1978. Correlation of carbon and nitrogen stable isotope ratios in sedimentary organic matter. Limnol. Oceanogr. 23 :598-604. Prest, V. K. 1970. Quaternary geology of Canada. In Geology and Economic Minerals of Canada, pp. 675764. R. J. W. Douglas (Ed.), Department of Mines and

Resources, Economic Geology Report 1. Rea, D. K., and Pigula, J. D. 1979. Mineralogy and distribution of fine-grained sediments in Little Traverse Bay, Lake Michigan. J. Great Lakes Res. 5:170-176. Robbins, 1. A., and Edgington, D. N. 1975. Determination of recent sedimentation rates in Lake Michigan using Pb-2l 0 and Cs-137. Geochim. Cosmochim. Acta. 39: 285-304. Robbins, J. A., Edgington, D. N., and Kemp, A. 1. W. 1978. Comparative 2 10 Pb, 137CS, and pollen geochronologies of sediments from Lakes Ontario and Erie. Quat. Res. 10:256-278. Schnitzer, M., and Khan, S. 1972. Humic Substances in the Environment. Marcel Dekker, New York. Shideler, G. L. 1976. A comparison of electronic particle counting and pipette techniques in routine mud analysis. J. Sed. Petrol. 46: 1017-1025. Shimp, N. P., Schleicher, J. A., Ruch, R. R., Heck, D. B., and Leland, H. V. 1971. Trace element and organic carbon accumulation in the most recent sediments of southern Lake Michigan. Env. Geol. Note #41. Illinois State Geo1 Survey, Urbana. Thomas, R. 1., 1969. The qualitative distribution of feldspars in surficial bottom sediments from Lake Ontario. In Proc. 12th Conf Great Lakes Res., pp. 364-379. Internat. Assoc. Great Lakes Res.