Abstract Abstract--The factors affecting the amounts and types of organic matter in lacustrine sediments are summarized in this review, and synthesis, of published studies. Biota hving in the lake and in its watershed are the sources of the organic compounds initially contributed to the lake system. Microbial reworking of these materials during sinking and early sedimentation markedly diminishes the total amount of organic matter while replacing many of the primary compounds with secondary ones. Much of the organic matter content of sediments is the product of this microbial reprocessing. Various organic matter components of lake sediments nonetheless retain source information and thereby contribute to the paleohmnological record. Carbon/nitrogen ratios of total organic matter reflect original proportions of algal and landderived material. Carbon isotopic compositions indicate the history of lake productivity and carbon recycling. Biomarker compounds provide important information about contributions from different biota. Sterol compositions and chainlength distributions of n-alkanes, n-alkanoic acids, and n-alkanols help distinguish different algal and watershed sources and also record diagenetic alterations. Abstract Stabilization of functional-group-containing biomarkers by conversion into saturated or aromatic hydrocarbons or by incorporation into bound forms improves their preservation and hence record of source information. Lignin components provide important evidence of watershed plant cover, and pigments reflect algal assemblages. The interplay of the factors influencing the organic matter content of lake sediments is illustrated by overviews of sedimentary records of four lake systems--Lake Biwa (Japan), Lake Greifen (Switzerland), Lake Washington (Pacific Northwest), and the Great Lakes (American Midwest). INTRODUCTION Organic matter constitutes a minor but important fraction of lake sediments. It is made up of a complex mixture of lipids, carbohydrates, proteins, and other biochemicals contained in the tissues of living benthic micro-organisms and contributed from the detritus of organisms formerly living in the lake and its watershed. Humic substances are diagenetically formed from these biochemical starting materials and constitute the major part of the complex mixture of organic materials. Significance of sediment organic matter The organic matter content of lake sediments participates in a variety of biochemical and geochemical processes. Benthic animals and microbes depend on organic matter for their nutrition. Because many components of organic matter are relatively easily oxidized forms of reduced carbon, the dissolved oxygen concentration of sediment pore water is controlled largely by the availability of oxidizable organic matter below the sediment/water interface. Furthermore, redox reactions, in which oxidized inorganic species become reduced by interaction with organic matter, are common in sediments. Organic matter is consequently a dynamic biogeochemical component of sediments. Significance of sediment organic matter Organic matter also forms part of the paleolimnologic record preserved in lake sediments. The different types of biota populating a lake and its watershed produce organic matter having distinctive biochemical compositions_ Changes in the community structure of these biota create variations in the amounts and types of organic matter deposited at different times in the history of a lake. During the process of deposition in the lake bottom, organic matter is subject to microbial reworking, with the result that tts original composition becomes altered_ The degree of alteration provides yet more information about past lake environments, particularly the amount of water column mixing that occurred during sediment deposition. Sources of organic matter in lake sediments The particulate detritus of plants that have lived in the waters and on the land surrounding a lake comprises the primary source of organic matter to the bottom deposits. Plants can be divided into two geochemically significant groups on the basis of their biochemical compositions: (1) nonvascular plants that lack woody and cellulosic tissues, such as simple algae, and (2) vascular plants that have these tissues, such as grasses, shrubs, and trees. These latter types of plants exist on land and in the shallow parts of lakes as bottom-rooted, emergent vegetation. The relative contributions from these two plant groups is influenced strongly by lake morphology, watershed topography, and the relative abundances of lake and watershed plants. Organic matter in lake sediments covers the spectrum of being predominantly algal in some lakes to being land-derived in others. Sources of organic matter in lake sediments Bacteria and other microbes in the water and sediment of lakes and the soil of the lake watershed rework and degrade aquatic and land-derived organic matter. Although some bacteria are capable of photosynthesis or chemosynthesis, lake environments generally provide enough organic substrate that the dominant role of bacteria is the heterotrophic decomposition of organic detritus. An important exception exists in strongly stratified lakes where bacteria can become major producers of primary organic matter. Bacteria biosynthesize characteristic forms of organic matter while breaking down the preformed material remaining from the plants and animals of the lake and watershed. Sources of organic matter in lake sediments In addition to these local sources of organic matter, winds commonly transport material, such as pollen, from sources outside the local watershed. The eolian component is typically a small fraction of the total organic mixture, yet it can contain distinctive components potentially useful to paleolimnetic and paleoclimatic studies. Sources of organic matter in lake sediments A budget of organic matter sources to Lake Michigan, a large oligotrophic freshwater lake, gives an illustration of their relative magnitudes. About 90% of the organic carbon in the waters of the lake originates from algal production, 5% is carried to the lake by rivers, and 5% is brought in with air-borne particles and precipitation (Andren and Strand, 1981 )_ These contributions of organic matter are subject to selective degradation as they sink to the lake bottom, and therefore the initial source proportions may not be preserved in the sediment record. Depositional processes affecting organic matter A variety of processes act to alter the character of organic matter in the relatively short time between its origin and its burial in a lake bottom. Foremost among these processes is its degradation during sinking. Sediment trap studies in Lake Michigan show that only 6% of the organic carbon photosynthetically formed in the surface water of the southern part of this lake reaches the sediment surface 100 m below (Table 1) . Nearly 85% of the carbon is oxidized before leaving the epilimnion, that part of the water column above the thermocline (Eadie et al., 1984) _ In more shallow depths, organic matter would have shorter sinking times and consequently shorter exposures to oxidation in the water column. Shallowwater lakes consequently usually have sediments richer in organic matter than found in deep lakes like the Laurentian Great Lakes_ Once reaching the bottom, organic matter continues to be subject to oxidative alteration and destruction. Resuspension of sedimented organic matter can be substantial (e.g. 75 g Corg/m2yr at a depth of 100 m in Lake Michigan (Table 1) , or nearly 10 times the annual flux of organic carbon to the bottom; Eadie et al., 1984; Meyers et al., 1984a; Meyers and Eadie, 1993) . Organic matter so resuspended is re-exposed to oxidation in the water column. Resuspension is usually greater in large, well-mixed lakes than in small lakes. Depositional processes affecting organic matter Bioturbation, or biological mixing, of surface sediments also prolongs the exposure of organic matter to oxidation and adds to this the degradation due to the nutritional needs of benthic fauna. The depth of bioturbation in the Great Lakes, as an example, averages about 10 cm (Robbins and Edgington, 1975) . By analogy to marine studies (Cobler and Dymond, 1980) , about 75% of the organic carbon reaching the surface of bottom sediments is destroyed in the bioturbated layer. Where seasonal or permanent anoxic bottom-water conditions exist, bioturbation is diminished or absent, because benthic animals required dissolved oxygen for respiration. Many temperatezone lakes experience periods of anoxia during summer thermal stratification, and the sediments of these lakes typically have higher concentrations of organic matter than do permanently oxygenated lakes. Varved sediments, in particular, indicate little or no resuspension and the absence of bioturbation_ Once buried below the zone of bioturbation, organic matter is subject to alterations by anaerobic bacteria. The waters of most lakes contain insufficient dissolved sulfate to make sulfate reduction an important process. Instead, methanogenic bacteria continue degradational processes if the sedimented organic matter contains a sufficient fraction of labile materials to sustain microbial activity. In general, components from aquatic sources are more sensitive to the bacterial degradative processes than are materials from land sources. Organic matter in lacustrine sediments consequently often has a large and relatively unreactive fraction of land-derived organic residues. Depositional processes affecting organic matter Less reactive forms of organic matter become more dominant as the more reactive forms are utilized by microbes and by deposit-feeding animals. For example, humic substances and lipids are less reactive than amino compounds and carbohydrates, and therefore become larger proportions of the organic matter with depth in sediments of Lakes Ontario, Erie, and Huron (Kemp and Johnston, 1979) . In heavily reworked sediment, only the most resistant forms of the original organic mixture remain. Throughout this reprocessing, residues of microbially synthesized organic matter are added to the mixture, some to be modified in turn or destroyed, but others to be preserved_ Depositional processes affecting organic matter The final result of these multiple processes is that the organic matter in sediments is often markedly different from that which is produced by the biota in and around the lake. Furthermore, the amount of organic matter buried in subsurface sediments is a small fraction of that originally introduced into the surface waters. INDICATORS OF SOURCES AND ALTERATIONS OF TOTAL ORGANIC MATI'ER IN LAKE SEDIMENTS Organic carbon stable isotopes Most photosynthetic plants incorporate carbon into organic matter using the C3 Calvin-Benson pathway which preferentially incorporates ~2C into organic matter, producing a shift of about -20%0 from the carbon isotope ratio of the inorganic carbon pool. Some plants use the C4 Hatch-Slack pathway, and others, mostly succulents, utilize the CAM (crassulacean acid metabolism) pathway. The isotope shift for the C4 pathway is -8 to -12%0; that of the CAM pathway can vary from -10 to -20%0. Organic matter produced from atmospheric CO2 by land plants using the C3 pathway consequently has an average 6 ~3C (PDB) value of ca-28%0 and by those using the C4 pathway ca-14%0 (cf. O'Leary, 1988) . Lake-derived organic matter is normally isotopically indistinguishable from organic matter from the surrounding watershed (Table 2) , except under special conditions as seen later. Organic carbon stable isotopes The existence of the several isotopic signatures of photosynthetically fixed organic carbon would seem to offer opportunities to trace sources of organic matter in lacustrine food webs and to lake sediments. In the sedimentary organic matter of most lakes, however, the differences originally present in the total carbon isotopic contents of specific flora are lost because of food web integration of the different carbon sources. For example, photosynthesizers living in Fayetteville Green Lake, New York, have 6"C (PDB) values ranging from -15 to -41%0. Despite this range among available food sources, most animals in the lake have isotope signatures between -25 and -30%0, and bottom sediments have a 6 t3C value of -28%0 (Fry, 1986) , which is close to the isotopic compositions of organic matter from other lakes ( Table 2) . Organic carbon stable isotopes Microbial reworking of organic matter during early diagenesis can potentially modify its bulk carbon isotopic content. Organic matter is a mixture of different types of compounds which deviate differently in isotopic content from the inorganic carbon pool. Amino acids of marine plankton, for example, are depleted in 13C by -17%0 on average, whereas the lipid fraction has a 6~3C value of -28%0 (Degens, 1969) . Selective loss of more reactive fractions of the total organic matter may create a diagenetic shift in the isotope ratio which will mask the original source signature. Laboratory decomposition of the C4 marsh grass, Spartina alterniflora, produced progressive losses of isotopically heavy organic material which created an extrapolated isotope shift in bulk organic carbon from -13 to -17%0 (Benner et al., 1987) . An equivalent shift to lighter carbon isotope values has Hatcher et al. (1982 Hatcher et al. ( , 1984 . Organic carbon stable isotopes been documented in an organic-carbon-rich sediment core from Mangrove Lake, Bermuda (Hatcher et al., 1982) . The 6 =3C ratio of total organic carbon changes from -17%o in surface sediments to -21%o in sediments below 5 m, a depth corresponding to ca 4000 yr ago, as organic carbon concentrations increase from 20 to 28% (Fig. 1 ). This shift apparently is caused at least partly by selective loss of the isotopically heavy carbohydrate fraction of total organic matter , but changes in the proportion of land-derived and marine-derived organic matter may also contribute to it. Organic carbon isotope ratios do not seem to display diagenetic shifts in lake sediments having more typical carbon concentrations. In a core from Lake Michigan, for example, 6"C values remained at -26%0 from modern to 3500-yr old sediments in which organic carbonconcentrations varied between 1 and 3% (Rea et al., 1980) . If the possibility of diagenetic effects can be determined, then differences between ~ J~C values of sediments deposited at different times can provide paleoenvironmental information. The history of productivity enhancement of Lake Ontario is recorded by a shift in organic carbon 613C values from -27%0 in sediments deposited in the mid 1800s to -25%0 in sediments from 1970 to 1980 (Schelske and Hodeli, 1991) . Periods of dessication of Walker Lake, Nevada, created shifts in the overall carbon isotopic contents of this saline lake which are preserved as heavier organic matter 6~3C values in sediments from 2 and 5 Kya (Benson et al., 1991) . Similarly, Talbot and Johannessen (1992) have documented organic carbon 6'3C shifts to heavier values in sediments of Lake Bosumtwi, Ghana, that were deposited under the drier conditions that evidently existed near the end of the last glacial maximum 25-10 Kya. Organic matter synthesized by aquatic organisms records times when the waters of both Walker Lake and Lake Bosumtwi were concentrated by evaporation and consequently saltier. Episodes of extensive recycling of organic matter within the water column can produce algal organic matter which is isotopically light (e.g Rau, 1978) , again illustrating some of the potential paleolimnologic value of organic carbon isotopes. Nitrogen stable isotopes The nitrogen isotope contents of sediment organic matter have not been widely studied by organic geochemists, yet important paleolimnological information about organic matter source and diagenesis can be obtained from ~ JSN measurements. Inorganic nitrogen reservoirs available to land plants and aquatic plants differ in their isotopic contents_ Dissolved nitrate has an isotope ratio of +7 to + 10%0, whereas atmospheric molecular nitrogen has a 6 ~SN of about 0%00 (cf. Peters et al., 1978) . This difference is preserved in the isotopic contents of plankton (c515N of + 8%0) and C3 land plants (5 ~SN of + 1%o) and has been used to trace food chain relationships in a coastal marine marsh by Peterson et al. (1985) . Nitrogen stable isotopes The few investigations of the nitrogen isotope content of lake sediments show that the source information derived from the difference between atmospheric and dissolved forms of nitrogen appears to be preserved. The potential isotopic effects expected from preferential degradation of nitrogencontaining organic substances are minor or absent. In sediments from lakes Superior and Ontario, Nriagu (1976, 1977) found that areas receiving a large proportion of land-sourced organic matter have lighter 6 ~SN values than areas where aquatic organic matter is dominant (+3.7%0 vs +4.9%,0). Significant amounts of downcore degradation of nitrogenous matter occurred relative to total organic matter at all locations in these lakes, increasing the potential for diagenetic isotope shifts. Nevertheless, no differences are evident between di tSN ratios in surface sediments and those at the bottom of half-meter cores. Most of the decomposition of organic matter that might cause nitrogen isotope shifts evidently occurs during sinking and prior to sedimentation in the Great Lakes. Meyers and Eadie (1993) note a change of -1 to -29/00 in the 6 tSN values of sinking particulate organic matter between the near-surface and near-bottom of Lake Michigan. This shift is consistent with postulated microbial reproeessing of the sinking organic material. Nitrogen stable isotopes The nitrogen isotope record from Lake Bosumtwi contains major changes to heavier organic ~ tSN values in sediments interpreted to have been deposited during periods of dry glacial-age climate. Talbot and Johannessen (1992) suggest that evaporative losses of isotopically light ammonium nitrogen from the relatively saline and alkaline lake waters occurred during these periods. Land runoff would have been diminished in these intervals, hence the 615N excursions reflect aquatic production of organic matter from a modified nitrogen reservoir. Organic matter C /N ratio The presence or absence of cellulose in the plant sources of organic matter to lakes influences the C/N ratios of sediments. As shown by the examples in Table 3 , nonvascular aquatic plants have low C/N ratios, typically between 4 and 10, whereas vascular land plants, which contain cellulose, have C/N ratios of 20 and greater. Lakes for which the contribution of organic matter from vascular land plants is small relative to water-column production, exemplified by Walker Lake and Lake Michigan, show lower C/N ratios in their sediments than do lakes receiving important amounts of vascular plant debris, such as Mangrove Lake and Lake Bosumtwi (Table 3) . Ratios of 13-14 for surface sediments of the latter two lakes suggest a mixture of non-vascular and vascular contributions, which is expected for most lakes. Organic matter C /N ratio Selective degradation of organic matter components during early diagenesis can modify elemental compositions and hence C/N ratios of organic matter in sediments. C/N ratios of modern wood samples are consistently higher than those of wood that had been buried in sediments (Table 3) . A similar change, although much more subtle, accompanies deposition of organic matter. Settling particles in Lake Michigan have a C/N value of 9; the ratio in resuspended bottom sediments is 8. A decrease in C/N ratios has also been observed in soils (e.g., Sollins et al_, 1984) , where it involves the microbial immobilization of nitrogenous material accompanied by the remineralization of carbon. These changes in the elemental composition of sedimentary organic matter are not commonly large enough, however, to erase the original C/N difference between vascular and non-vascular plants. Organic matter C /N ratio Burial in lake bottoms appears to stabilize organic matter C/N ratios to further diagenetic alteration. The C/N values of Quaternary sediment samples from the upper 10 m of a core from Mangrove Lake vary irregularly between 11 and 17 ( Fig. 1) . This variation resembles a similar downcore C/N profile from Lake Yunoko, Japan, which Ishiwatari et al., (1977) interpreted as changes in the source mixture. Indication of preservation of the C/N source signal Talbot and Johannessen (1992) PHILIP A. MEYERS and RYOSnl ISmWATARI was also found by Ertel and Hedges (1985) , who demonstrated that vascular plant debris isolated from a sediment sample having a bulk C/N of 15 retains the elevated CfN values characteristic of cellulosic plants. The sediment ratio indicates mixing of vascular and nonvascular plant debris rather than diagenetic alteration. Talbot and Johannessen (1992) similarly conclude that selective diagenesis has had no discernible effect on organic matter C/N ratios in sediments as old as 27,500 yr in Lake Bosumtwi. Judicious interpretation of C/N ratios can yield information about past sources of organic matter to lake sediments. Ishiwatari and Uzaki (1987) , for example, reported a sharp transition in atomic C/N values from an average of 9 to an average of 13 at the 250 m level in a drilled core of sediment from Lake Biwa, Japan. They interpreted this change as a shift in proportions of organic matter source from algal dominance in younger sediments to land plant dominance in deeper sediments. This interpretation is consistent with a change in sediment types from fine-grained deep-water sediment to shallow-water sands and gravels at the 250m depth. Another example is from the sediments of Pleistocene Lake Karewa, India. Several marked increases to values of 10 or more occur in C/N ratios that typically remain below 4 in sediments deposited between 2.4 Myr and 400 Kyr (Krishnamurthy et al., 1986) . Concomitant lighter 613C values indicate that land plant contributions to sediment organic matter increased at these times. DIAGENESIS OF HUM1C SUBSTANCES Humic substances are operationally divided into fulvic acid (soluble in base and acid), humic acid (soluble in base but not acid), and humin (insoluble in base and acid). Between 60 and 70% of organic matter in young lacustrine sediments is constituted by humic substances; this percentage rises to over 90% in older sediments (Ishiwatari, 1985) . In lithified sediments, the term "kerogen" is commonly used to describe the non-soluble fraction of organic matter; this material is equivalent to the humin of younger, non-lithified sediments. Because of this operational and probably geochemical equivalence, the term "protokerogen" has been used by some workers instead of humin. DIAGENESIS OF HUM1C SUBSTANCES The geochemical compounds grouped as humic substances are complex and diverse. As a consequence, studies of this organic matter fraction have traditionally employed general characterizations, such as elemental and spectroscopic analyses. These procedures show that humic substances, themselves products of diagenetic alteration of biogenic organic matter, evidently continue to undergo limited diagenesis in lake sediments. Bourbonniere and Meyers (1978) used infrared and visible spectroscopy to explore the possible condensation of fulvic acid into humic acid in a half-meter-long core of Lake Huron Fig. 2 . Fulvic acid, humic acid, and humin C/N values and proportional contributions to total organic carbon in a core of Lake Michigan sediment. Sediment age is from modern to 3500 yr. B.P_ The downcore decrease in organic carbon concentration is mostly from diagenetic losses of fulvic and humic acid concentrations, producing a proportional increase in the relative contribution of humin. Atomic C/N ratios are not affected by the diagenetic changes. Core location, sedimentology, and dating are described by Rea et al. (1980) and Bourbonniere (1979) _ sediment. They found no evidence for this conversion. Furthermore, overall diagenetic effects are minor, inasmuch as H/C ratios of the fulvic acid (ca 2.2) and the humic acid (ca 1.6) do not change downcorc. The H/C values greater than one reflect the aliphatic nature of the Lake Huron humic substances and a probable origin from algal or bacterial orgamc matter, as opposed to vascular plant material. The 550-yr period represented in this core of Lake Huron sediment is inadequate for major diagenetic effects to appear in fulvic and humic acids. Over multi-thousand-year periods of time, the effects of diagenesis of humic substances become evident in lake sediments. The humm fraction increases from 65% of the total humics at the surface to 85% of the total at the bottom of a 1 m core of Lake Michigan sediment (Fig. 2) , a depth corresponding to ca 3500 yr ago (Reaet al., 1980) . Some of this increase may arise from conversion of fulvic and humic acids to humin, but the overall decrease in total sediment organic carbon indicates that diagenetic loss of organic matter continues throughout the core and that preferential degradation of fulvic and humic acids occurs. Despite evidence of organic matter degradation in this core, the 6~3C ratio of total organic carbon does not shift from -26.3%0 (Reaet al., 1980 )_ The C/N ratios of both fulvic and humic acids remain ca 7 (Fig. 2) and suggest a common origin from aquatic or bacterial sources. In contrast, the humin fraction has a C/N ratio of ca 15. Humin typically has an atomic C/N value of 14-16 (Ishiwatari, 1985) , and this ratio suggests an important component of cellulosic land plant material for humin. DIAGENESIS OF HUM1C SUBSTANCES Comparisons of samples of wood from modern trees and of wood that has been buried for thousands of years in sediments have been used as proxies for the kinds of chemical changes that can occur in humic substances (e.g., Meyers et al., 1980a; Hedges et al., 1985; Spiker and Hatcher, 1987) . A consistent observation of such studies is that cellulose, made up of aliphatic monomeric units, decreases in abundance in relation to lignin, which is a polyaromatic macromolecule. Solid-state J3C nuclear magnetic resonance spectra have revealed that a trend from predominantly aliphatic character to predominantly aromatic character occurs in the humin fraction of progressively older sediments from Mangrove Lake, Bermuda. In this case, humic substances convert to less reactive, more aromatic material in a period of about 10 Kyr (Hatcher et al., 1982) , largely through preferential destruction of their aliphatic components. LIPID BIOMARKERS AND THEIR ALTERATIONS The molecular composition of the lipid fraction of sediment organic matter provides particularly useful information about the sources and diagenetic alterations of organic matter. Lipids are operationally defined as that fraction of organic matter which can be isolated by extraction with an organic solvent. The term "geolipid" is sometimes used to describe the materials obtained from solvent extraction of sediments in recognition of the differences that exist between the molecular compositions of unaltered biological matter and the contents of geological samples. In older, lithified sediments where the distinction between unaltered and altered lipids becomes greater, the term "bitumen" is typically employed. LIPID BIOMARKERS AND THEIR ALTERATIONS Lipid extraction of lake sediments yields hydrocarbons and the hydrocarbon-like carboxylic acids, alcohols, ketones, aldehydes, and related compounds commonly having l0 or more carbon atoms. Many of these compounds have specific biological origins; all of them reflect enzymatic control on their molecular structures. Retention of their biological heritage earns the name "biological marker", commonly abbreviated to "biomarker", for many of these compounds. This characteristic makes the lipid fraction especially important to geochemical studies, even though it typically constitutes only a few percent of the total sediment organic matter. LIPID BIOMARKERS AND THEIR ALTERATIONS Several extensive reviews of the lipid biomarker contents of young, non-lithified sediments are available (e.g., Barnes and Barnes, 1978; Cranwell, 1982; Johns, 1986; Miiller, 1987) . We have selected examples of studies that illustrate the responses of geolipid biomarker compounds to paleolimnetic changes and to diagenetic alterations. The biomarker alterations can be used as proxies for the general processes affecting all forms of lacustrine organic matter. Alteration of lipids during sedimentation Studies of settling particles intercepted by sediment traps indicate that lipids undergo substantial amounts of degradative alteration during sinking to the bottoms of lakes, so that the geolipid compositions of sediments differ from their source compositions. The relative contributions of alkanoic acids and of alkanols become smaller, and distributions change, in settling sediment collected at progressively deeper depths in Lake Michigan (Meyers et al., 1980b (Meyers et al., , 1984a ). Alteration of lipids during sedimentation The n-Cl6 and n-Cls acids and alcohols from algal material become less dominant, whereas the evenchain C~-C30 components from the waxy coatings of land plants become proportionally more important. Fatty acid decomposition rates calculated from sediment trap compositions show that n-C~6 degrades ca l0 times faster than n-C30 (Meyers and Eadie, 1993) . Similarly, Kawamura et al. (1987) found that distributions of n-alkanes in sunficial sediments from Lake Haruna, Japan, were depleted in n-C~7, an algal biomarker, relative to the hydrocarbon contents of sediment trap material in this lake. Aquatic lipid matter is evidently preferentially degraded during the sinking of particulate material, probably because of the relative freshness of this material. In contrast, land-derived organic matter has been microbially reworked before arrival in the lake, and only its most durable lipid components survive to become part of the sedimentary record. Alteration of lipids during sedimentation Comparison of the lipid compositions of aquatic organisms with those of underlying sediments shows degradation trends similar to those found in sediment trap studies. Robinson et al., (1984a) analyzed the lipid contents of bacteria, rotifers, and protozoans from Priest Pot, a eutrophic lake in the English Lake District. They found that shorter carbon-chain n-alkanols degrade faster than their longer homologs and that sterols having more than two double bonds do not survive sinking to the lake bottom. Alteration of lipid compositions continues in the sediments of this lake, most intensely in the surface layer. In particular, solvent extractable lipids become less abundant, and "bound" lipids, those that can be isolated only after acid or base hydrolysis of sediment samples, increase. In several lakes, evidence of microbial oxidation of n-alkanes, the least reactive type of lipid, has been found in the appearance of sedimentary n-alkan-2-ones having chain-length distributions similar to those of their precursor hydrocarbons Rieley et al., 1991a) . Preferential degradation of algal components ultimately biases the geolipid record of organic matter sources retained in the sediments of most lakes. For example, Goossens et al. (1989) showed that land plant lipids dominate the biomarker content of modern sediment from Lake Vechten, The Netherlands, although carbon budgets indicate that land contributions comprise a small part of the total organic carbon input to the lake system. Changes in geolipid sources vs selective diagenesis Geolipid compositions reflect the combined effect of original source inputs and selective diagenesis during and after sedimentation. Within the range of geolipid molecular structures, some are less likely to be altered by diagenesis than are others. Comparisons between easily altered and less easily altered molecules help to distinguish diagenetic changes from source changes. Hydrocarbons are generally not as sensitive to diagenetic alteration as are oxygen-containing lipids. Within the n-alkane fraction of hydrocarbons, longer-chain-length molecules are less degradable than are their shorter-chain-length counterparts and can preserve evidence of source changes. As an illustration, the dominant n-alkane has shifted from n-C27 in pre-1900 sediments to n-C31 in modem sediments of Priest Pot, England, as marsh and shore grasses replaced trees as the major source of land-derived organic matter . In contrast to the long-chain n-alkanes, the shortchain components from the algal contributions in this productive lake experience substantial diagenetic losses. Changes in geolipid sources vs selective diagenesis The relative contributions of organic matter from lake and watershed biota effect the biomarker compositions of lake sediments, n-Alkanoic acid distributions in sediments of eutrophic Esthwaite Water and Blelham Tam, England, give similar bimodal patterns having maxima at C16 and C24 (Cranwell, 1974) , representing combinations of aquatic and land-plant sources. In contrast, the distribution found in the Carbon Number Fig. 3 . Chain-length distributtons of n-alkanes, n-alkanols, and n-alkanoic acids in surficial sediments from four Japanese lakes. Proportions of the components in each lipid fraction are given as percentage of the total fraction. Long chain-length components originate from land plants, whereas short components are produced by aquatic organisms. The productivities of the lakes increase progressively from oligotrophic Lake Motosu to eutrophic Lake Suwa, and the relative proportions of long-to-short components decrease as lake productivities increase. Data are from Kawamura and Ishiwatari (1985) . Changes in geolipid sources vs selective diagenesis sediments of oligotrophic Lake Grasmere has a single maximum at C26, indicative of a dominance of landplant wax lipid input. Furthermore, n-alkane distributions provide an approximate record of watershed plant types. In watersheds where grasses dominate, n-C3~ is the major sediment n-alkane, whereas n-C:7 and n-C29 are more abundant in lake sediments where deciduous trees dominate, (Cranwell, 1973a) . Measurements of the carbon isotopic contents of individual n-alkanes isolated from sediments and from leaf waxes allows source identifications to be more specific. Changes in geolipid sources vs selective diagenesis Willow leaves appear to be the major source of the n-C25 to n-C29 hydrocarbons present in Ellesmere Lake, England (Rieley et al., 1991b) . Selective microbial utilization and replacement of biomarker components affects geolipid distributions in lake sediments. The n-alkane distributions of sediments in oligotrophic lakes Motosu and Biwa, Japan, are dominated by the C29, Cjj, and C33 components diagnostic of land-plant waxes ( Fig. 3 ; Kawamura and Ishiwatari, 1985) . The n-alkanol distributions from these two low-productivity lakes contain dominant-to-large proportions of long-chain, plant-wax components as well. The n-alkanolc distributions, however, are not similarly dominated by long-chain, land plant components. The dominant C~6 n-alkanoic acid in these distributions is ubiquitous in the biosphere; it can be found in land plants, in aquatic algae, and in bacteria and other microbes. Its dominance in these sediments, where both the n-alkanes and the n-alkanols record major contributions of land-plant lipids, indicates that microbial utilization and resynthesis of the more-reactive fatty acids has occurred. The sediment from eutrophic Lake Suwa contains a notable amount of the C17 n-alkane diagnostic of algal production, and the n-alkanol and n-alkanoic acid fractions are dominated by C~6 components. The relatively small contributions of longchain acids and alcohols indicates that contnbutions of land-plant lipids are small in Lake Suwa compared to the algal contributions. Changes in geolipid sources vs selective diagenesis Some lipid compounds appear to be particularly resistant to diagenetic alteration in lake sediments. For example, Cranweli (1985) reported the presence of C37-C39 unsaturated ketones in the sediments of three lakes, but their absence in those of three others. These compounds have been found in marine locations, where they evidently can retain their molecular integrity in sediments as old as Eocene (Marlowe et al., 1984) . Although the specific source of these ketones has not been identified in the lacustrine settings, it is probably from prymneslophyte algae by analogy to the marine source. Changes in geolipid sources vs selective diagenesis Wax esters are also evidently resistant to dlagenetic alteration. Fukushima and Ishiwatari (1984) compared the molecular distributions of the component fatty acids and alcohols comprising the wax esters m surface sediments of three lakes in Japan. Those from oligotrophic Lake Motosu are dominated by the C24 n-alkanoic acids and n-alkanols characteristic of land plant epicuticular waxes, whereas those from eutrophic Lake Shoji contain a predominance of the n-C]6 components expected from aquatic plants and few of the long-chain, land-plant materials. The sediment from mesotrophic Lake Haruna displays an intermediate wax ester composition, having the n-C~6 acid and the n-C24 alkanol as the major contributions to thetr respective distributions, The wax esters, because of their low solubilities and presumably low susceptibilities to microbial utilization, provide improved records of the original proportions of algal and watershed sources of sedimentary lipids. Influence of sediment grain size on geolipid compositions The relationship between sediment grain size and geolipid compositions was investigated by Thompson and Eglinton (1978) , who isolated five sediment size fractions from Rostherne Mere, England. The lake is eutrophic, and bottom water anoxia limits benthic reworking of organic matter, n-Alkane distributions nonetheless contain small amounts of algal components and instead are dominated by n-C:zT, n-C29, and n-C~j, as shown by long/short ratios significantly greater than one (Table 4 ). The n-alkanoic acids, however, are nearly equally made up of algal and landplant components, giving long/short ratios of close to one. Relatively high carbon preference indices (CPI) indicate the biogenic characters of both the hydrocarbons and the acids are well preserved. Microbial reprocessing of organic matter has evidently degraded many of the short-chain n-alkanes and has probably replaced much of the original fatty acids with microbial acids. The degree to which these alterations have taken place vary with the sediment size-fractions. In general, land-plant lipid character and total extractable lipid concentrations decrease as particle size dtminishes. This pattern is consistent with the decrease in abundance of intact plant detritus and accompanying decrease in C/N ratios in the sediment fractions_ The trend towards decreasing concentrations is dramatically reversed in the clay-sized particles, which have the highest Ioadings of n-alkanes and of fatty acids. The larger surface area per unit weight and the likelihood of a greater abundance of clay minerals probably provide enhanced sorptive capacities for this size fraction. Influence of sediment grain size on geolipid compositions Two other size-related changes in the lipid compositions are progressive decreases in the ratio of individual n-alkanes to the unresolved complex mixture (UCM) of hydrocarbons and in the proportion of unsaturated n-Ct~ acids to their saturated analog. These changes, combined with the decreases in CPI values, suggest that the lipid contents of the finer-sized particles of sediment have experienced more microbial reworking than those of the coarser particles. Finersized particles settle more slowly to lake bottoms than coarser ones, increasing the exposure time of their organic contents to such attack. Diminished aliphatic/ UCM ratios, in particular, from the finer-sized particles indicate the presence of microbially degraded petroleum residues, presumably from run-off of lubricating oil from the roadways in the watershed (Thompson and Eglinton, 1978) , although some amount of eolian contribution is also possible for the smaller particles. Similarly, Meyers and Takeuchi (1979) found that finer-grain-sized sediments generally contained higher concentrations of extractable aliphatic, aromatic and UCM hydrocarbons and of extractable fatty acids than did coarser sediments in Lake Huron. In addition, the size of the UCM component of sediment hydrocarbons decreased with distance from the mouth of the major river in Saginaw Bay of Lake Huron, verifying its principal origin from urban run-off. Alkane source records in lake sediments Because of their low susceptibility to mtcrobiai degradation compared with other types of organic matter, saturated hydrocarbons can record many aspects of the depositional history of organic matter Washington in urban Seattle and Lake Quinault in rural Washington. Gas chromatographic traces illustrate hydrocarbon distributions in three of these core samples. Carbon numbers of ,-alkanes are indicated on the traces; UCM represents the unresolved complex mixture of hydrocarbons underlying the individual aliphatic hydrocarbon peaks. Most of the increase in Lake Washington hydrocarbon concentrations since ca 1900 is due to the large addition of petroleum residues, indicated by the UCM, to a low background of land plant n-alkanes, typified by the Lake Quinalt hydrocarbon contents. Core sediment ages shown are from 2L°Pb dating. Data are from Wakeham (1976) . Alkane source records in lake sediments Fig. 5. Downcore concentrations of total fatty acids and of representative components of the total acids in parts per million of Lake Suwa, Japan, sediment. Ratios of total fatty acid carbon to total organic matter are given as FAC/TOC. Carbon preference indices of total alkanoic acids (CPIr), of higher-molecular-weight acids (CPIH), and of lower-molecular-weight acids (CPIL) are shown. Composition Diversity Indices (CDI) of fatty acids at different core depths are presented. Major shifts in all parameters at a sediment depth of 20 cm record modem eutrophication of Lake Suwa. Microbial reprocessing of fatty acids in surface sediments is also evident from the distribution of CL5 acids. Data are from Matsuda and Koyama (1977a, b) . Alkane source records in lake sediments sotm:es in lake sediments. One such example compares hydrocarbon contents of sediment cores from lakes in the State of Washington. Sediments of Lake Quinalt, which is surrounded by a forested and virtually pristine watershed, contain low concentrations of hydrocarbons which are dominated by the C27 n-alkane diagnostic of tree waxes (Fig. 4; Wakeham, 1976) . No significant difference exists in the n-alkane contents of sediments deposited in the early 1800s and in the mid-1970s. In contrast, modern sediments of Lake Washington, which is surrounded by the city of Seattle and its suburbs, have hydrocarbon contents dominated by the unresolved complex mixture (UCM) indicative of petroleum residues (Fig. 4) . Anomalously old radiocarbon ages of 15-20Kyr for the aliphatic hydrocarbon extracts of modern sediments confirm large proportions of fossil hydrocarbons (Wakeham and Carpenter, 1976) . Furthermore the contribution of short-chain-length n-alkanes derived from algae is equal to the land-plant n-alkane contributions, suggesting eutrophication of the waters of Lake Washington. In progressively older sediments, the contributions of UCM and short-chain hydrocarbons diminish, until in sediments deposited in the mid1800s the concentrations and distributions resemble those of Lake Quinalt sediments (Fig. 4) . Three sources of hydrocarbons can be identified in these sediments: (1) a natural, low contribution of landplant n-alkanes as found throughout the sedimentary records of both lakes, (2) an input of petroleum residues from urban run-off, which first appears in the Lake Washington record around 1900 and becomes dominant in modern sediments, (3) algal input of Ct7 and Cl9 n-alkanes which is usually small in comparison to the land-plant contributions, probably because of selective diagenetic degradation. It becomes large only in the sediments of Lake Washington which were deposited starting ca 1950, when residential population growth led to a two-decade period of eutrophication of the lake and large increases in the algal production of n-C~7. Fatty acid preservation and degradation in lake sediments Fatty acids are more sensitive to degradation and modification than most types of biogenic lipids. Some fatty acids arc more susceptible to diagenesis than others; comparisons between different componcnts can therefore help to distinguish diagenetic effects from source changes. Fatty acid preservation and degradation in lake sediments Thc results of a study by Matsuda and Koyama (1977a, b) illustrate some of the effects of diagencsis and of source change on the fatty acid contents of sediments. Concentrations of total fatty acids and of total organic carbon decrease rapidly over the top 25 cm of a 150-cm-long core from Lake Suwa, Japan (Fig. 5) . The ratio of fatty acid carbon to total organic carbon also decreases, showing that fatty acids are degraded more readily than total organic matter. Long-chain components, represented by 24:0, 26:0, and 28:0 (number of carbon atoms:number of double bonds) change little with depth, but shortchain acids, such as 15:0, 16:0, and 18:0, decrease markedly. The degradation rates of these latter acids have been estimated in coastal marine sediments to be 6-7 times faster than for the long-chain acids (Haddad et al., 1992) . Shorter-chain unsaturated acids, such as 16:1 and 18: 1, have higher concentrations than saturated acids of the same carbon chain length in surlicial Lake Suwa sediment, but disappear faster than their saturated analogs. In contrast, the concentration of the 24:1 acid appears to experience little change with depth (Fig. 5) . Fatty acid preservation and degradation in lake sediments Fatty acids in lake sediments typically originate from several sources. The long-chain components in Lake Suwa sediments are probably from land plants and as such are the non-reactive survivors of transport of land-derived debris to the lake bottom. The shorterchain components, particularly the alkanoic acids, are products of algal lipidsynthesis in the lake. These lipids are easily metabolized by sediment microbes. The n-15:0 and ante/so-15:0 fatty acids arc indicators of microbial biomass (cf. Cranwell, 1973b) , and they represent in situ production of secondary lipids at the expense of primary organic matter. Most of the concentration changes occur in the top 25 cm of these sediments, corresponding to ca 60 yr (Matsuda and Koyama, 1977a) . Although the decrease in the contribution of fatty acids to total organic matter (FAC/ TOC in Fig. 5 ) is compelling evidence for diagenetic losses of these lipids, it is also probable that there have been source changes as a result of the documented increasing algal productivity of Lake Suwa over this time. The lower concentrations of the C~6 and C,B acids in deeper sediments may arise jointly from formerly lower production and from diagenetic removal. The similar pattern for the Cls acids may reflect the presence of larger microbial populations in response to larger amounts of readily metabolized organic matter in younger sediments than in older deposits. Matsuda and Koyama (1977b) examined the possibility that source changes as well as diagenetic changes impact the downcore fatty acid composition of Lake Suwa sediments. They compared the sediment contents to those of potential source materials, and they present several measures of source vs alteration effects. The Carbon Preference Index (CPI), devised by Bray and Evans (1961) to indicate the degree of diagenetic alteration of n-alkanes, can be modified for similar application to n-alkanoic acids. This index is essentially a numerical representation of how much of the original biological chainlcngth specificity is preserved in geological lipids. In fresh lipid material, odd-numbered carbon chains dominate hydrocarbon compositions and even-numbered chains dominate compositions of fatty acids and alcohols. This specificity diminishes as diagenesis proceeds. Matsuda and Koyama (1977b) modified the Carbon Preference Index (Fig. 5) to test for the presence of source changes as well as diagenetic effects. They separated fatty acids into their lower molecular weight (Cn-C~s) and higher molecular weight (C22-C32) groups as CPIL and CPIH, respectively. In the lipid content of most plants, CPIL values are high, ranging between 12 and ca 100; CPIn values are not as high, being between 0.9 and 8, because of the presence of long-chain odd-carbon acids in some land plants. Bacteria generally have low CPI values, and therefore bacterial reworking of sediment organic matter results in lowered CPI values. Fatty acid preservation and degradation in lake sediments The downcore CPI profiles in the sediments of Lake Suwa are variable (Fig. 5) , and they are lower than CPI values of potential source materials. The variability indicates that a combination of source changes and of changes in the degree of microbial reworking is involved. The index for all n-alkanoic acids, CPIT, is similar to CPI, in the upper ca 20 cm of sediment and becomes more like that of CPI, in sediments lower than 25 cm in the core. The generally lower CPI values below 25 cm imply that microbial resynthesis of fatty acids has been important in erasing the original compositions. Much of this resynthesis evidently occurred in the upper part of this core. Fatty acid preservation and degradation in lake sediments A Composition Diversity Index (CDI) was also employed by Matsuda and Koyama (1977b) to evaluate the possibility of changes in sources of fatty acid inputs to the Lake Suwa sediments. The downcore CDI values are shown in Fig. 5 . Higher values below ca 20 cm indicate a greater diversity of sources. Matsuda and Koyama (1977b) interpreted this pattern to reflect the addition of microbial contributions to the sediment contents. This interpretation is consistent with the downcore CPIs, yet both patterns could also arise from increased algal production in modern times. Higher concentrations of C~6 0 between 75 and 100 cm in the core accompany higher percent FAC/ TOC, higher CPIs, and lower CDIs (Fig. 5) . These parameters suggest substantial preservation of the record of fatty acid contributions to the deeper sediments of this lake. On this basis, the CPI and CDI trends may record eutrophication as well as microbial inputs. Original fatty acid source indicators appear not to have been totally replaced by microbial components in this example, probably because of the relatively rapid sedimentation rate in Lake Suwa (ca 4 mm/yr: Matsuda and Koyama, 1977a) . Fatty acid preservation and degradation in lake sediments Unsaturated fatty acids are generally more susceptible to alteration in sediments than are saturated acids of the same chainlengths. Unsaturated n-C~6 and n-C~s acids are major constituents of the iipids of freshwater algae, and they are rapidly degraded by microbial attack (Cranwell, 1976) . In a dated core of sediment from Lake Haruna, Japan, ratios of C~6 ,, C~B ~, C,s 2, and C~s 3 to their corresponding alkanoic acids decrease by a factor of 10 in the upper 8 crn of sediment. Little unsaturated acid remained below this depth, corresponding to 120 yr of accumulation, and consequently the ratios decreased much more slowly (Kawamura et al., 1980) . The change with depth found in Lake Haruna sediments is similar to the unsaturated-to-saturated change illustrated by Lake Suwa sediments (Fig. 5 ) and found in sediments of Lake Huron (Meyers et al., 1980c) . The existence of carbon-carbon double bonds evidently facilitates microbial utilization of these lipids and enhances their degradability. Fatty acid preservation and degradation in lake sediments Variations in factors such as sedimentation rates and degree of lake water oxygenation impact the preservation of the relatively reactive fatty acids incorporated in sediments laid down at different times. Mendoza et al. (1987) reported fatty acid compositions of a core of sediment from Lake Leman, Switzerland, which illustrate the effects of changing sedimentation rates. Concentrations of lower-molecular-weight and higher-molecular-weight acids begin to decrease in the expected diagenetic pattern over the top meter of this core, but they increase in sediments between 2.5 and 3.5 m deep, presumably because of an interlude of higher sedimentation rate. Unsaturated n-C~6 and Rapid burial in lake sediments can dramatically enhance preservation of fatty adds. Concentrations of total fatty acids increase nearly tenfold with depth and the proportions of unsaturated acids more than double over the top 25 cm of sediments in Pyramid Lake, Nevada, (Fig. 6) . The cause of these exceptional patterns is befieved to be slumping of deltaic sediments as a result of agricultural lowering of lake level in the early 1900s, thereby protecting surface sediments from the aerobic bottom waters of this lake (Meyers et al., 1980d) . Source changes at this time can be ruled out, inasmuch as total hydrocarbon concentrations show no enhanced inputs of lipids over this sediment depth range (Fig. 6) . Near the bottom of this core, however, a change to a larger proportion of terrigenous lipids and to better overall lipid preservation occurring at a depth corresponding to 1200-1400 AD is indicated by n-alkane and fatty acid compositions. This is a further illustration that changes in depositional parameters can be preserved in the lipid contents of sediments, even by relatively reactive components like fatty acids. Sources and diagenesis of sterols in lake sediments Sterols
{'paper_id': '36874753', 'title': 'Lacustrine organic geochemistry--an overview of indicators of organic matter sources and diagenesis in lake sediments', 's2_url': 'https://api.semanticscholar.org/CorpusID:36874753', 'source': 'S2ORC'}