Record of postglacial organic matter delivery and burial in sediments of Lake Ontario

Org. Geochem. Vol. 24, No. 4, pp. 463-472, 1996 Pergamon PII: S0146-6380(96)00041 - 1

Copyright ~3 1996 ElsevierScience Ltd Printed in Great Britain. All rights reserved 0146-6380/96 $15.00 + 0.00

Record of postglacial organic matter delivery and burial in sediments of Lake Ontario J A M E S E. S I L L I M A N , I P H I L I P A. M E Y E R S I and R I C H A R D A. B O U R B O N N I E R E 2 ~Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109-1063, U.S.A. and -'Environment Canada, National Water Research Institute, Burlington, Ontario L7R 4A6, Canada A~tract--A 12 m piston core obtained from Lake Ontario has allowed us to study the accumulation of organic matter in sediments deposited since the retreat of the Laurentian ice sheet about 12,500 years ago. Discrepancies between radiocarbon dates of disseminated organic matter and ostracod shells emphasize the magnitude of the recycling and retention of organic carbon in the Great Lakes. Concentrations of CaCO3 and organic carbon decrease down core due to the presence of fine grained glaciolacustrine clays at the base of the sedimentary sequence. Increases in sediment grain size indicate periods with enhanced fluxes of terrigenous material. C/N ratios, %organic carbon and % CaCO~ vary proportionally with fluctuations in terrigenous input. C/N ratios indicate that lacustrine algae have been the main source of organic matter to Lake Ontario sediments. Organic 6~'C values become heavier at the bottom of the core, suggesting a shift in carbon sources for the bulk organic matter. Anthropogenic effects are well documented in total hydrocarbon and total fatty acid profiles of modern sediment~, whereas postglacial trends representing natural changes of organic matter are relatively undetectable. Sedimentary profiles of terrigenous/aquatic ratios of n-alkane and n-alkanoic acids indicate that early variations in postglacial sedimentation rates may have impacted the preservation of aquatic organic matter. Changes in watershed vegetation and organic matter delivery to Lake Ontario altered these ratios as deglaciation progressed. Organic geochemical properties of modern sediments have recorded how anthropogenic activity has augmented algal productivity in Lake Ontario by increasing nutrient input. Copyright ,~-",1996 Elsevier Science Ltd Key words

lake sediments, carbon isotopes, radiocarbon dating, C/N ratios, n-alkanes, n-alkanoic acids

INTRODUCTION Lake Ontario, one of the Laurentian Great Lakes of North America, is a large fresh water lake that is surprisingly sensitive to environmental changes. The main reason for this sensitivity is that it has the smallest surface area of the five Great Lakes and consequently is more easily impacted by changes in its watershed. Lake Ontario is also the only one in this lake system that receives most of its sediment from tributaries rather than coastal erosion (Kemp and Harper, 1976). Furthermore, it is the final basin in the overall flow pattern of Great Lakes water from Lake Superior via the St. Lawrence River to the ocean. As a result, the sediments of Lake Ontario provide a valuable record of both recent anthropogenic and older postglacial natural environmental changes. Organic matter is an important part of the sedimentary record of environmental changes to this lake. It is introduced to the lake by several pathways. Terrigenous organic matter is carried to Lake Ontario by rivers, eolian transport and rainfall, Algae and bacteria within the epilimnion along with microbes at lower depths and within the sediments contribute to aquatic organic matter. The proportions of these two types of material available for

sedimentation can vary as conditions change, and their amounts that are preserved in the sedimentary record can fluctuate. Such variations are part of the record of environmental change, Both land-derived and lacustrine organic matter have geochemical characteristics that help distinguish between their contributions. Elemental, carbon isotopic, and biomarker distributions have been employed to investigate the effects of the recent European settlement of the watershed and consequent environmental changes on organic matter delivery to the Lake Ontario ecosystem (Schelske and Hodell, 1991; Bourbonniere et al., 1991; Bourbonniere and Meyers, 1996). Our research has used these geochemical parameters to expand the study of sedimentary organic matter and environmental variations to the entire postglacial sedimentary record of Lake Ontario. POSTGLACIAL HISTORY OF LAKE ONTARIO The Laurentian Great Lakes are imprints left in the North American landscape by the massive Laurentide continental glacier of the latest Pleistocene ice age. The postglacial period began after the Laurentian ice sheet had retreated from the Ontario Basin, approximately 12,500 yr ago (Pair 463

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James E. Silliman et al.

and Rodrigues, 1993). The present-day forms of the Great Lakes differ from the several stages that existed at different times during the last glacial and early deglacial periods as land areas isostatically rebounded and drainage routes opened and closed. Lake Agassiz, a large periglacial lake that existed northwest of Lake Superior, formerly connected the Great Lakes system to the Mississippi River. Prior to 11,500-11,000 yr ago, the majority of the Laurentide ice sheet meltwater drained through Lake Agassiz and the Mississippi River enroute to the Gulf of Mexico (Teller, 1990). During the same time period, water levels in the still-depressed Ontario Basin dropped rapidly to sea level and allowed marine waters to invade via the St. Lawrence River to form the Champlain Sea (Pair and Rodrigues, 1993). Glaciomarine clays were deposited during this part of the postglacial history of Lake Ontario. Broecker et al. (1989) used oxygen isotope and radiocarbon data collected from planktonic foraminifera in the Gulf of Mexico to identify decreases in meltwater flow through the Mississippi River between 11,200 and 10,000 yr ago. By this time melting had caused sufficient retreat of the Laurentian ice sheet so as to expose water channels leading from Lake Agassiz to Lake Superior. Such conditions diverted the drainage of meltwater from the Mississippi River to the St. Lawrence River Valley by allowing water to flow eastward through the Great Lakes (Broecker et al., 1989). Broecker et al. (1989) also suggest that meltwater was routed back to the Mississippi River through Lake Agassiz around

10,000 yr ago, due to the Marquette glacial advance, before being rediverted through the St. Lawrence River to finally establish the modern flow pattern of water from the Great Lakes to the North Atlantic Ocean.

METHODS

Sampling and dating

Sediments of Station E30 in the Rochester Basin of Lake Ontario (Fig. 1) were sampled by coring in 1981. A 12 m piston core, a 1.5 m Benthos gravity core, and a 40 cm box core were obtained. The piston core was vertically extruded and cut into alternating 3 and 6 cm sections. The Benthos core was also vertically extruded and cut into I cm intervals to 20 cm, 2 cm intervals from 20 to 50 cm and 5 cm intervals beyond 50 cm. Subcores from the box corer were similarly sectioned. All sections were sliced aboard ship, frozen immediately and then freezedried within three months of collection. By comparisons of core contents, we estimated that the upper 110 cm of sediment had been lost during piston coring. We have adjusted all piston core depth intervals for this lost material, and we have combined the piston core and Benthos core data to give a continuous and complete record of postglacial organic matter accumulation in the Rochester Basin. The Benthos core and companion subcores from the box core were dated by the 2"~Pbmethods that are routinely used by Robbins and co-workers on Great

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Fig. I. Site E30 in the Rochester Basin of Lake Ontario. Sediment samples were obtained at this 233 m-deep location by box coring, gravity coring, and piston coring in 1981. Inset shows the relationship of Lake Ontario (outlined) to the other four Great Lakes of North America.

Record of postglacial organic matter delivery and burial Lakes cores (e.g. Robbins and Edgington, 1975). The excess 2~°Pb contents of the sediments were applied to a steady-state mixing model and sediment accumulation rates were determined (e.g. Schelske et al., 1988). These cores exhibit sediment mixing due to bioturbation, which indicates that the bottom waters have remained oxygenated. The mixing integrates the accumulation of a single year over a multi-year depth. The temporal resolution of near-surface sediments from Station E30 is ca. I 1 yr (Eisenreich et al., 1989). Sections of the piston core sediments have been ~4C dated by two different procedures. Traditional radiocarbon dating of disseminated organic matter was performed at the University of Waterloo, Ontario (R. Drimmie, pers. commun.). In addition, ostracod shells from the sediments were ~4C dated by accelerator mass spectrometry (AMS) at the University of Arizona. Analytical strategy We have employed a variety of analytical determinations to assess various aspects of delivery and burial of organic matter in sediments of Lake Ontario. Calcium carbonate abundances can help identify changes in sources of lake sediments. CaCO~ originates from both algal production and erosion of limestone bluffs and glacial tills surrounding Lake Ontario. Total organic carbon (TOC) concentration is the summation of inputs from aquatic production, riverine transport and atmospheric deposition. Carbon/nitrogen (C/N) ratios of organic material can be used to distinguish between aquatic and terrestrial sources. C/N ratios less than 10 are characteristic of lacustrine algae, whereas vascular land plants have C/N ratios greater than 20 (Meyers and lshiwatari, 1993; Meyers, 1994). Organic 6~3C values are the same in Great Lakes algae as in land plants and are therefore not useful in identifying sources of organic matter in lakes (Meyers and lshiwatari, 1993), but they are sensitive to changes in algal productivity (Schelske and Hodelk 1991; Hollander et al., 1992). The types and concentrations of geolipids (i.e. hydrocarbons and fatty acids) in sediments record the effects from both source inputs and selective diagenesis during and after sedimentation (Meyers and Ishiwatari, 1993). Hydrocarbons are relatively robust fractions of geolipids that tend to resist biodegradation. As a result, they retain characteristics that reflect their origin. Fatty acids are much more reactive in aquatic settings, due to the presence of functional groups, and can therefore be useful to distinguish between source changes and diagenesis (Meyers and Ishiwatari, 1993). Terrigenous/aquatic ratios of geolipids can often clarify trends observed for concentrations of total geolipid fractions. C27, C;~, and C3~ sedimentary n-alkanes are characteristic of contributions from land plant waxes, whereas C,5, C~7, and C~9 n-alkanes indicate algal input (Eglinton and Hamilton, 1963). C24, C_,~,,and C_,~n-alkanoic acids are representative

465

of land plant waxes, flowers and pollen (Rieley et al., 1991). The shorter chain C~2, C~4, and C,6 n-alkanoic acids are found in all plants, but are more prevalent in algae (Cranwell et al., 1987). Terrigenous/aquatic ratios of hydrocarbons (HC) and fatty acids (FA) used in this study are defined below. Terrigenous/Aquatic Ratio (TAR,c) = (C27 + C2,, + C3,)/(C,~ + C , + C,~) Terrigenous/Aquatic Ratio (TARFA) = (C24+ C2~+ C28)/(C,_, + C,4 + C,~) Even though all of these geochemical parameters provide information on sources and sedimentary conditions pertaining to organic matter, they can be misleading if treated individually. By correlating them to each other, details concerning the delivery. burial and nature of organic material are more clearly defined. Analytical procedures CaCO, concentrations. Freeze-dried samples were analyzed for calcium carbonate using the carbonate bomb technique of Miiller and Gastner (1971). Samples were reacted with 3N HCI, and the volume of CO, released was measured and compared to the volumes released from known amounts of pure CaCO~ to determine the percentage in each sample. The carbonate-free residue remaining after acid treatment was collected, rinsed, and dried for use in elemental CHN analyses and organic carbon isotopic determinations. Organic carbon and nitrogen concentrations. Amounts of organic carbon and residual nitrogen in carbonate free residues from the carbonate bomb procedure were measured with a Carlo Erba EA1108 CHNS-O analyzer. This procedure involves heating the sample at 1020C and measuring the combustion products by gas chromatography (Verardo et al., 1990). Known amounts of sulfanilamide (CoH~N,O_,S) were used to calibrate the instrument and to calculate the quantities of C and N released from the samples. Total organic carbon (TOC) concentrations were then calculated on a whole-sediment basis, adjusting for the carbonate concentrations determined from the bomb technique. C/N ratios were calculated on an atomic basis. Organic carbon isotopic values. Isotopic contents of organic matter were determined from analyses carried out in the Stable Isotope Laboratory at The University of Michigan. The " C f C ratios of the residual carbon were determined with a Finnigan Delta S mass spectrometer calibrated with the NBS-21 (graphite) standard. Combustion of the carbonate-free organic matter was performed at 80ffC in sealed Vycor tubes in the presence of CuO and Cu. Data are corrected for t70 and are expressed in conventional 6~C notation relative to the PDB standard.

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Lipid extraction and analysis. Geolipids were Soxhlet-extracted with a 1:1 mix of toluene and methanol, partitioned, saponified and methylated (Leenheer et al., 1984). Saturated hydrocarbon and fatty acid methyl ester fractions were isolated on alumina over silica chromatography columns, and they were analyzed with a Carlo-Erba 4160 gas chromatograph equipped with an on-column injection and a 30 m fused silica capillary column coated with SE30. Quantification was achieved using internal standards added before extraction, and FID response factors were determined using quantitative mixtures of known compounds. Data have been corrected for the small amounts of procedural contaminants determined by blank analyses.

RESULTS AND DISCUSSION

Organic matter record in postglacial sediments Our determinations of several geochemical parameters in postglacial sediments from Lake Ontario have allowed us to interpret paleoenvironmental conditions that governed the delivery and burial of organic material within these sediments during the past 12,500 yr. Radiocarbon dating. Radiocarbon dating of Lake Ontario sediments dramatically shows the magnitude of carbon recycling in the Great Lakes. Anomalously old ages of postglacial sediments were obtained from dating disseminated organic matter from piston core samples (Table l). We believe that the base of the sedimentary sequence corresponds to the beginning of the deglacial period, which occurred around 12,500 yr ago. Dates based on organic matter, however, exceed 20,000 yr even belbre a depth of 800 cm is reached. This is because Lake Ontario, like the other Great Lakes (Rea and Colman, 1995), recycles and retains carbon within its waters and sediments. The result is a mixing of old and new carbon. When fresh organic carbon is incorporated into the sediments of Lake Ontario, it combines with older pre-existing sedimentary carbon as well as old carbon received Table I. Comparison of unadjusted radiocarbon dates from disseminated organic matter and ostracod shells in piston core LO-81-E30 from the Rochester Basin of Lake Ontario. Sediment depths have been corrected for the loss of the upper 110 cm during coring Depth (cm) 113-119 212-218 316-322 415-421 421-424 521-524 569-572 632-645 669-681 681~b87 763-769 899-905 908-911 * AMS dates

TOC 5,750 + 310 6.160 + 230 8.040 +_ 230 10.940 + 210

Ostracod shells*

8.660 + 105 9.250 + 100 10,950 + 140 10,970 + 460 15,880 + 1,390 11,035 + 105 20.570 :k 2,440 > 36,000 12,800 _ 135

from degraded terrigenous organic matter brought in by rivers. All of these different types of carbon contribute to the overall carbon supply in Lake Ontario. Therefore, radiocarbon dating of disseminated organic matter in Lake Ontario sediments results in misleading ages that are often substantially greater than the actual sediment age. To contribute towards the resolution of this ~4C dating problem, ostracod shells throughout the piston core were radiocarbon dated. As evident in Table 1, these ages correlate much closer to the glacial history of the basin sediments. This is because the dissolved bicarbonate (HCOD, from which the calcite shells are formed, is made up of mostly atmospheric carbon. Even though the ostracod dates are more reliable than those ages based on disseminated organic matter, they are still affected by the recycling and retention of carbon through the "hard-water effect". The dissolved bicarbonate reservoir in the Great Lakes is not only composed of dissolved atmospheric CO2, but also of old carbon derived from the dissolution of limestone and dolomite from Paleozoic rocks present in the lake basins (Rea and Colman, 1995). In addition, old carbon can be supplied by CO2 produced from the oxidation of allochthonous soil organic matter. Rea and Colman (1995) calculated the hard-water effect to be 257 + 56 yr for Lake Michigan and 438 + 45 yr for Lake Huron. The hard-water effect for Lake Ontario has not been determined, but may be greater than that of Lake Huron because of the eastward flow pattern of the Great Lakes. The ostracod ages in Table 1 have not been adjusted for the old carbon and, consequently, should be considered maximum values. To further complicate the radiocarbon dating of Lake Ontario sediments, tree ring chronologies indicate that atmospheric '4C production has not been constant during the Holocene. Cosmogenic variations have perturbed the '4C record in the past, especially during the 10,000-8,000 yr bp (year before present) interval (Becker and Kromer, 1993). As a result, ~4C dates during this time period become less precise. Despite these various complications in radiocarbon dating, the ostracod dates are a marked improvement over the bulk organic matter t4C ages. Furthermore, the discrepancy between the ostracod and organic matter radiocarbon dates shows that Lake Ontario sediments contain a larger proportion of detrital organic carbon than is present as dissolved inorganic carbon in the lake water. Using the data from Table 1 and equations presented in Faure (1986, p. 389), we calculated that the proportion of detrital material varies between 28 and 47% of sedimentary TOC in Lake Ontario. Since the lake is enclosed by land, unlike oceans, it receives important contributions of old organic carbon from decaying terrigenous material. Unlike most smaller lakes, Lake Ontario has a long settling distance through which organic

Record of postglacial organic matter delivery and burial material sinks. As a result, deposition takes longer and allows for more degradation of fresh material to take place before organic matter is incorporated into its sediments, thereby magnifying the proportional significance of detrital carbon. Sediment texture. Grain size in the bottom 2 m of the piston core hovers about a minimum value of 0.3 lam and rises to a maximum of 2 lam between 450 and 200 cm (Fig. 2). This sediment grain size curve agrees well with the postglacial history of Lake Ontario. As the Laurentide ice sheet scraped its way to the Ontario Basin, it brought along with it allochthonous material of terrigenous origin composed mainly of rock flour. Upon melting of the glacier, these fine grained sediments were deposited as glaciolacustrine clays at the base of the postglacial sedimentary sequence of the Lake Ontario Basin. Grain size increases up core after the start of glacial melting. Between 400 and 350 cm ( ~ 8,000 yr bp) glaciers had long disappeared from the surrounding landscape, but sediments continue to coarsen up core to almost 200 cm. This may indicate lower lake levels and/or a wetter climate during this time span. The factors controlling these two processes are independent for Lake Ontario: lake levels are controlled mostly by local isostatic rebound of drainage channels whereas climate is controlled by hemispheric or regional atmospheric conditions. Climate simulations conducted by Webb et al. (1987) relate July radiation levels to annual precipitation-evaporation (P-E) ratios for postglacial periods. These models suggest that July radiation values decreased by 3% from 9,000 to 6,000 yr bp, which would lead to a slight increase in annual ~ E and a wetter climate. The increasing trend in grain size up core could therefore record intensified river flow to Lake

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Ontario from wetter climate conditions. However, Anderson and Lewis (1985) and Larsen (1987) interpreted drowned beach profile records as evidence for lower lake levels of Lake Ontario during the same time period. Both processes (intensification of river flow and lower lake levels) would have delivered coarser sediments and greater amounts of terrigenous material to the deep-basin site of our cores. At around 200 cm depth in the piston core, river flow declined and/or lake level rose to cause grain size decreases. CaCO~ and organic carbon concentrations. The fine grained glaciolacustrine clays at the bottom of the piston core should be low in organic carbon and CaCO+ since they are primarily composed of crushed rock. This supposition is confirmed by the TOC and CaCO~ concentration profiles in Fig. 2, which are low at the base of the core and rise to maxima between 450 and 200 cm. The concentrations mimic the grain size curve for postglacial sediments. Enhanced delivery of terrigenous sedimentary components is suggested by increases in grain size and is paralleled by increases in TOC concentration (Fig. 3a). This correlation suggests that the grain size increase records a period of wetter climate, intensified land runoff, greater transport of nutrients to Lake Ontario, and augmented aquatic productivity. Organic carbon concentrations in the piston core reach a maximum of 2.2% and then drop sharply to around I% at approximately 200 cm. This decrease corresponds to a drop in sediment grain size (Fig. 2). A decline in fluvial input to the lake would also reduce nutrient transport and diminish primary productivity. This in turn would decrease the organic carbon flux to the sediments. As can also be seen in Fig. 2, the TOC concentration in the Benthos core rises to 5% in the most recent sediments as the impact

C a C O 3 (%)

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t Fig. 2. Grain size; concentrations of CaCO and organic carbon, and orgamc matter C/N ratios in postglacial sediment samples from piston core LO-81-E30 from the Rochester Basin of Lake Ontario. Unadjusted radiocarbon ages are from AMS dating of ostracod shells.

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Fig. 3. Cross plots showing direct correlations between (a) grain size and TOC concentration, (b) grain size and CaCO~ concentration, (c) concentrations of TOC and CaCO3, and (d) grain size and organic matter C/N ratios for postglacial sediments from piston core LO-81-E30 from the Rochester Basin of Lake Ontario. of anthropogenic delivery of nutrients to Lake Ontario increases. The changes in CaCO3 concentration are interesting and indicate the importance of biogenic sources. Like TOC, elevated concentrations of CaCO3 correlate with increased grain size (Fig. 3b). CaCO, input is most likely affected by various sources including bluff erosion, but calcite-secreting microorganisms seem to be its major source. This is suggested by the cross plot between %TOC and %CaCO3 contained in Fig. 3c. Schelske and Hodell (1991) also relate sedimentary CaCO~ to primary production from their 8~C, TOC, and historical phosphorus loading data collected from a neighboring core site in Lake Ontario. The set of peak CaCO~ values between 450 and 400 cm in Fig. 2 ( ~ 25% CaCO3) correlates to the sediment grain size peak at the same depth. The set of high values ( ~ 17% CaCO3) between 300 and 250 cm corresponds to a broader set of maximum grain size peaks. The pulse of increased terrigenous input between 350 and 250 cm, implied by the grain size increase, may have had a longer duration than the earlier terrigenous input episode between 450 and 400 cm. The longer interval of terrigenous input would have resulted in a greater delivery of allochthonous sediments to Lake Ontario that could have diluted the CaCO~ sediment signal and caused the CaCO3 concentration to decline. CaCO~ concentrations then diminish in younger sediments due to decreased nutrient levels as a result of reduced terrigenous input indicated by a drop in sediment grain size.

C / N ratios and 6~C,,,~,,,,, values. Both lacustrine and terrigenous organic matter possess distinctive elemental characteristics. C/N ratios of postglacial organic matter from Lake Ontario range between 4 and 9. Since the C/N ratios are all less than 10, lacustrine algae must have been the primary source of organic matter (e.g. Meyers, 1994) throughout the past 12,500 yr. The C/N ratios increase from ~ 4 at the base of the core to ~ 8 at about 200 cm and then decrease sharply in younger sediments (Fig. 2). Meyers and Ishiwatari (1993) describe similar trends between fresh and sedimented organic matter in Lake Michigan. The variation could be due to selective degradation of organic matter components during diagenesis. However, the excursion to elevated C/N ratios towards 200 cm and then the decline to 113 cm may actually reflect changes in the types of contributions of organic matter. Sediment grain size data follows this same pattern (Fig. 2). Larger grain sizes correspond to elevated C/N ratios (Fig. 3d). Increasing grain size suggests a shift towards a larger proportion of terrigenous sediments and organic material being transported to Lake Ontario, even though lacustrine algae remain the dominant source of organic matter. Above 200 cm, C/N ratios decrease as terrigenous input diminishes. 8'3C values of piston core organic matter vary between - 25.5 %0 and - 28.0 %0 (Table 2). These values fall within the range of 8'sC signatures characteristic of both lacustrine algae and Cs land plants (Meyers, 1994). Most of the core samples at the base of the core have low C/N ratios, suggesting an algal origin of their organic matter, and also have slightly heavier 8~C values. The fact that this organic material has radiocarbon ages > 20,000 yr (Table 1) must mean that it is a blend of contemporaneous organic carbon and much older recycled organic carbon. Therefore, its properties should reflect the combined effect of these different sources. The subtle shift to heavier organic carbon isotopes at the base of the piston core suggests that contributions of organic matter were different during this period of sediment Table 2. TOC concentrations, organic C/residual N atomic ratios. and organic ~h~C (PDB) values in sediments from piston core LO-81-E30 from the Rochester Basin of Lake Ontario. Sediment depths have been corrected for the loss of the upper 110 cm during coring Depth (cm) II0-113 128--131 207 210 313 316 412 415 557 560 623 626 669-672 757-760 896-899 996-999 1112-1115

T O C (%)

C,,N ratio

8"C

1.38 1.27 1.19 1.65 1.89 0.73 0.68 0.92 0.55 0.23 0.26 0.23

6,5 6.4 6.3 7.8 7.4 4.9 4.5 5.6 4.3 3.9 4.9 3.9

- 26.12 - 26.71 - 26.77 27.34 - 26.96 - 27.20 - 27.98 - 27.25 - 27.67 - 26.66 - 25.8 I - 26.28

Record of postglacial organic matter delivery and burial accumulation than in subsequent times. Possible changes include: (1) heavier 8~3C values in aquatic organic material due to lower atmospheric pCO2 during the latest Pleistocene ( ~ 12,000 yr bp) and (2) lighter ~5~C signatures from older recycled organic matter obtained from the glacial erosion of Paleozoic bedrock. Hydrocarbon and Jatt)' acM concentrations. The TOC-normalized THC (total hydrocarbon) and T F A (total fatty acid) concentrations show little change in the piston core (Fig. 4). In contrast, both THC and T F A concentrations increase markedly at the top of the Benthos core because of human effects on primary production in Lake Ontario. In these profiles, anthropogenic effects in modern sediments are easily distinguished from the less dramatic natural effects on organic matter present in the postglacial sediments. Total concentrations of all n-alkane and n-alkanoic acid components present in the sediments seem to be altered more significantly by anthropogenic activities as opposed to natural processes. n-Alkane and n-alkanoic acid terrigenous/aquatic ratios. Terrigenous/aquatic ratios of n-alkanes (TARHc) and n-alkanoic acids (TARHc) in Lake Ontario postglacial sediments vary much more with sediment depth than the concentrations of these geolipids (Fig. 4). Even though terrigenous organic matter is commonly enriched in n-alkanes relative to algal material (Meyers and lshiwatari, 1993), TARHc values remain effective in identifying changes in the proportions of terrigenous vs aquatic contributions of hydrocarbons. The effects of diagenesis, however, may selectively alter n-alkane and n-alkanoic acid concentrations and thereby contribute to variations in the TAR values.

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Shorter chain aquatic n-alkanes and n-alkanoic acids are more susceptible to degradation than their longer chain terrigenous counterparts (Haddad et al., 1992; Meyers and Eadie, 1993). The TARHc values are low from the bottom of the piston core to about 600-500 cm, which is an interval of elevated periglacial sediment accumulation. High sedimentation rates may explain the low TAR,c and TARvA values within this depth range due to increased preservation of algal n-alkanes and n-alkanoic acids. The n-alkanoic acid curve is not as smooth as the n-alkane profile, reflecting the greater sensitivity of n-alkanoic acids to preservational variations in sedimentary settings (Ho and Meyers, 1994). Between 600 and 400 cm, both ratios increase significantly. This interval corresponds to the time period between l 1,000 and 8,000 yr ago. Climate models and pollen data suggest that abrupt changes in glacial ice volume, atmospheric warming trends, and drastic changes in vegetation occurred during this time interval (Jacobson et al., 1987; Kutzbach, 1987; Webb et al., 1987). Terrigenous/aquatic ratios of n-alkanes and n-alkanoic acids are sensitive to contributions from higher plants to sedimentary organic matter. Therefore, changes in watershed vegetation resulting in increased contributions from tree and shrub litter to Lake Ontario sedimentary organic material would cause TARHc and TARvA values to rise significantly. Enhanced amounts of runoff during a period of wetter climate would also add to the observed sharp increases in TAR values for n-alkanes and n-alkanoic acids between 600 and 400 cm. Terrigenous/aquatic ratios of both n-alkanes and n-alkanoic acids decrease from their maxima between

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Fig. 4. Total concentrations of hydrocarbons (THC) and fatty acids (TFA) normalized to TOC concentrations and terrigenous/aquatic ratios of n-alkanes (TAR,c) and n-alkanoic acids (TARv^) in sediments from piston core LO-81-E30 from the Rochester Basin of Lake Ontario. Unadjusted radiocarbon ages are from AMS dating of ostracod shells.

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J a m e s E. S i l l i m a n et al.

600 and 300 cm to the top of the piston core. The initial decrease could be due to the dilution of land n-alkanes and n-alkanoic acids by elevated aquatic inputs derived from high levels of algal productivity. Once terrigenous inputs start to decline at ~ 200 cm, as indicated by decreases in sediment grain size and C/N ratios (Fig. 2), diminished fluxes of land-plant lipid matter help to lower the TAR.c and TAR~A values. The n-alkane and n-alkanoic acid terrigenous/ aquatic ratios and concentrations are quite dissimilar in the upper 150 cm of sediment (Fig. 4). This is probably due to several important differences in behavior between n-alkanes and n-alkanoic acids in depositional settings, which is supported by the inverse relationship in TAR values shown in Fig. 5. T A R . c values generally increase towards the top of the sediment. Large fluctuations exist in this trend, however, and they are more significant than those present in THC concentrations (Fig. 4). These variations suggest changes in the delivery of hydrocarbons. In upper sediments, TARHc values and THC concentrations change dramatically in opposite directions. This pattern is probably due to enhanced aquatic production during times of increased terrigenous nutrient input, which causes high THC concentrations and dilutes the terrigenous signal to produce low TAR,c values. T F A concentrations generally rise from the bottom of the Benthos core to the top, whereas TARFA values decline to near-surface sediments. From around 25 cm to the surface, TARvA values increase sharply. The near-modern conditions of enhanced productivity may impact sediment contents of the relatively reactive n-alkanoic acids. As organic material settles through the water column to the bottom of the lake, preferential degradation of shorter chain aquatic 3.0

!

t

I

2.5-

L~ 2.0 -r

n~ 1.5

1.0-

0.5

0.5

I

I

1.0

1.5

I 2.0 TARFA

2.5

3.0

Fig. 5. Cross plot exhibiting the inverse relationship between terrigenous/aquatic ratios of n-alkanes (TAR.c) and n-alkanoic acids (TAR~^) in sediments from Benthos core LO-81-E30 from the Rochester Basin of Lake Ontario.

n-alkanoic acids is likely to have already commenced (Meyers and Eadie, 1993). This would cause TARFA values to be high in the surface sediments. Once deposited, organic matter would be attacked by sediment microbes to continue the degradation process. In doing so, such microbes can synthesize secondary fatty acids from the primary organic matter to generate shorter chain aquatic n-alkanoic acids (Kawamura et al., 1987). The secondary acids would lower TARFA values in sub-surface sediments and yield the down core pattern seen in Fig. 4. Organic matter record since European settlement

European settlers arrived in the Lake Ontario region in the early 17th century and initiated a succession of environmental changes that has accelerated in recent years. The watershed has been dominated in turn by pristine forests, agriculture, and finally an industrial and urban/suburban culture. These changes have impacted both delivery of land-derived organic matter to the lake and production of algal organic matter within the lake. The box core samples provide a high resolution record of the last two centuries of sedimentation that augments the combined Benthos and piston core postglacial record. Organic carbon accumulation. The watershed region of Lake Ontario underwent deforestation between 1820 and 1850 (Schelske et al., 1983). Conversion of formerly forested areas to farm fields increased soil weathering and the release of soil nutrients. The enhanced fluxes of elastic material to Lake Ontario, including phosphorus, is responsible for the increase in primary productivity that was gradual until 1950. Urbanization after 1950 magnified nutrient delivery and greatly bolstered productivity of the lake, tripling organic carbon concentrations in the sediments by 1975 (Fig. 1; Bourbonniere and Meyers, 1996). Hy~h'ocarbons. Hydrocarbons degrade about onethird as fast as total organic matter during sedimentation (Meyers and Eadie, 1993). THC concentrations in recent sediments of Lake Ontario are variable with depth (Bourbonniere and Meyers, 1996). Because TOC concentrations are less variable, fluctuations in THC concentrations must indicate changes in hydrocarbon content of the organic material supplied at different times to the sediments. Hydrocarbons tend to increase in Lake Ontario sediments starting around 1930, and their compositions suggest larger inputs of plant-wax components delivered from watershed regions (Bourbonniere and Meyers, 1996). Fat O' acids. Significant variations in T F A concentrations appear in sediments deposited after 1940 (Bourbonniere and Meyers, 1996). Increases in T F A concentrations tend to correlate with increases in organic carbon concentrations which, in turn, correspond to enhanced primary productivity due to larger inputs of soil and anthropogenic nutrients.

Record of postglacial organic matter delivery and burial TARvA values indicate much greater land-plant contributions than TAR.c values of the same sediment intervals. Since fatty acids are more degradable than hydrocarbons, Bourbonniere and Meyers (1996) suggest that the TARvA values probably overestimate terrigenous inputs due to preferential alteration of algal n-alkanoic acids during diagenesis. Nonetheless, TFA contents indicate greater proportions of terrigenous input in more recent sediments. However, the large fluctuations present in the TFA concentrations of the box core samples may also be due to variations in sediment bioturbation that would affect the preservation of reactive compounds like fatty acids (Bourbonniere and Meyers, 1996). The various organic matter parameters (%TOC, THC and TFA concentrations, and TAR values) illustrate how anthropogenic activity has enhanced organic matter delivery and productivity in Lake Ontario. These results mimic those obtained from Greifensee, Switzerland by Giger et al. 1980) and Hollander et al. (19921.

CONCLUSIONS The sedimentary record of a 12 m ptston core from the Rochester Basin has revealed a series of natural environmental changes in the Great Lakes that date back roughly 12,500 yr. After the latest glacial--interglacial climate transition, the Laurentian ice sheet retreated out of the lake basins. The retention of detrital organic carbon in the sediments of the Great Lakes is significant (28-47%). Since this involves the mixing of old carbon with new carbon, radiocarbon dates of disseminated organic matter predate glacial retreat up to 10,000 yr. Although radiocarbon ages have been refined by dating ostracod shells, they are still offset by several centuries due to the hard-water effect. Because of the postglacial dynamics in this part of North America (i.e. glacial rebound, major climate changes, glacial recession and changes in lake drainage), geochemical evidence of the Younger Dryas cooling episode (11,500-10,500 yr bp) is concealed. Also, uncertainties of ~4C dates could be on the magnitude of 500 yr or more and inhibit high enough resolution to detect the Younger Dryas. which only lasted 1000 yr. Glaciolacustrine clays at the bottom of the core were identified by their distinctively fine grain size and low TOC and CaCO~ concentrations. High fluxes of land-derived material during the postglacial history of Lake Ontario sedimentation correlate with increased sediment grain size. Increases in CaCO3 and TOC concentrations and C/N ratios are linked to periods of enhanced terrigenous input. C/N ratios indicate that lacustrine algae have been the dominant source of sedimentary organic matter since the end of the last glacial period. Slightly heavier carbon

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isotopic signatures of organic matter found at the base of the piston core suggest a change in the sources of carbon to these sediments. THC and TFA profiles clearly distinguish anthropogenic effects in recent sediments from impacts of natural postglacial sedimentary processes. Terrigenous/aquatic ratios of n-alkanes and n-alkanoic acids in postglacial sediments reveal that possible increases in sedimentation rates may have improved the preservation of aquatic organic matter in the lower portion of the piston core. These same ratios seem to be primarily affected by changes in watershed vegetation and organic matter delivery to Lake Ontario in the upper piston core section. Geochemical compositions of recent sediments from the Rochester Basin in Lake Ontario have recorded the impact of anthropogenic activity on the lake's ecosystem. Increases in total hydrocarbon and fatty acid concentrations as well as terrigenous/ aquatic n-alkane ratios indicate enhanced terrigenous input to the modern sediments of Lake Ontario from urbanization of its watershed regions. Such activity has contributed to the high productivity of Lake Ontario as shown by increases to 4-5% organic carbon in surface sediments. Increased algal productivity is supported by heavier organic ~5~3Cvalues in these modern sediments. Acknowledgements --We thank the donors of the Petroleum

Research Fund, administered by the American Chemical Society, for partial support of this research. Radiocarbon dating of ostracod shells was provided by the University of Arizona Accelerator Mass Spectrometry Facility with support from NSF Grant EAR 92-03883. We also thank the crew of the CSS Limnos for expert assistance in obtaining the cores. Coring and lipid analyses were supported by resources from Environment Canada. Special thanks are given to A. J. Tomasek, S. M. Havach, B. Treem, T. Mayer and J. M. Stewart for their valuable analytical support during the project. We are grateful for the helpful comments provided by Y. Huang upon reviewing this manuscript.

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