Showing papers in "Journal of Paleolimnology in 1999"

Lacustrine sedimentary organic matter records of Late Quaternary paleoclimates

Abstract: Identification of the sources of organic matter in sedimentary records provides important paleolimnologic information. As the types and abundances of plant life in and around lakes change, the composition and amount of organic matter delivered to lake sediments changes. Despite the extensive early diagenetic losses of organic matter in general and of some of its important biomarker compounds in particular, bulk identifiers of organic matter sources appear to undergo minimal alteration after sedimentation. Age-related changes in the elemental, isotopic, and petrographic compositions of bulk sedimentary organic matter therefore preserve evidence of past environmental changes.

Stable isotope (δ13C and δ15N) signatures of sedimented organic matter as indicators of historic lake trophic state

Abstract: We explored the use of carbon and nitrogen isotopes (δ13C and δ15N) in sedimented organic matter (OM) as proxy indicators of trophic state change in Florida lakes. Stable isotope data from four 210Pb-dated sediment cores were compared stratigraphically with established proxies for historical trophic state (diatom-inferred limnetic total phosphorus, sediment C/N ratio) and indicators of cultural disturbance (sediment total P and 226Ra activity). Diatom-based limnetic total P inferences indicate a transition from oligo-mesotrophy to meso-eutrophy in Clear Lake, and from eutrophy to hypereutrophy in Lakes Parker, Hollingsworth and Griffin. In cores from all four lakes, the carbon isotopic signature of accumulated OM generally tracks trophic state inferences and cultural impact assessments based on other variables. Oldest sediments in the records yield lower diatom-inferred total limnetic P concentrations and display relatively low δ13C values. In the Clear, Hollingsworth and Parker records, diatom-inferred nutrient concentrations increase after ca. AD 1900, and are associated stratigraphically with higher δ13C values in sediment OM. In the Lake Griffin core, both proxies display slight increases before ~1900, but highest values occur over the last ~100 years. As Lakes Clear, Hollingsworth and Parker became increasingly nutrient-enriched over the past century, the δ15N of sedimented organic matter decreased. This reflects, in part, the increasing relative contribution of nitrogen-fixing cyanobacteria to sedimented organic matter as primary productivity increased in these waterbodies. The Lake Griffin core displays a narrow range of both δ13C and δ15N values. Despite the complexity of carbon and nitrogen cycles in lakes, stratigraphic agreement between diatom-inferred changes in limnetic total P and the stable isotope signatures of sedimented OM suggests that δ13C and δ15N reflect shifts in historic lake trophic state.

The carbon cycle and biogeochemical dynamics in lake sediments

Abstract: The concentrations of organic carbon (OC) and CaCO 3 in lake sediments are often inversely related. This relation occurs in surface sediments from different locations in the same lake, surface sediments from different lakes, and with depth in Holocene sediments. Where data on accumulation rates are available, the relation holds for organic carbon and CaCO 3 accumulation rates as well. An increase of several percent OC is accompanied by a decrease of several tens of percent CaCO 3 indicating that the inverse relation is not due to simple dilution of one component by another. It appears from core data that once the OC concentration in the sediments becomes greater than about 12%, the CO 2 produced by decomposition of that OC and production of organic acids lowers the pH of anoxic pore waters enough to dissolve any CaCO 3 that reaches the sediment-water interface. In a lake with a seasonally anoxic hypolimnion, processes in the water column also can produce an inverse relation between OC and CaCO 3 over time. If productivity of the lake increases, the rain rate of OC from the epilimnion increases. Biogenic removal of CO 2 and accompanying increase in pH also may increase the production of CaCO 3 . However, the decomposition of organic matter in the hypolimnion will decrease the pH of the hypolimnion causing greater dissolution of CaCO 3 and therefore a decrease in the rain rate of CaCO 3 to the sediment-water interface.

Relationship between C:N ratios of lake sediments, organic matter sources, and historical deforestation in Lake Pleasant, Massachusetts, USA

Abstract: The C:N ratios of lake sediments may reflect proportions of terrestrial and algal carbon contributing to accumulation of sediment. This possibility was tested in Lake Pleasant, Massachussetts, USA which underwent watershed deforestation in about 1780 A.D. The C:N profile of a 70-cm sediment core clearly reflected deforestation through a rise in C:N ratio caused by an increase in watershed contributions to sedimentary carbon. Spatial variability of C:N in modern surficial sediments is small compared to the change caused by deforestation.

Surface-sediment and epilithic diatom pH calibration sets for remote European mountain lakes (AL:PE Project) and their comparison with the Surface Waters Acidification Programme (SWAP) calibration set

Abstract: A modern diatom-pH calibration data-set consisting of surface-sediment diatom assemblages from 118 lakes and 530 taxa is presented. The AL:PE data-set is from high-altitude or high-latitude lakes in the Alps, Norway, Svalbard, Kola Peninsula, UK, Slovenia, Slovakia, Poland, Portugal, and Spain (pH range = 4.5-8.0; DOC range = 0.2-3.2 mg l-1). In addition, 92 epilithon samples from 22 high-altitude or high-latitude lakes comprise an AL:PE epilithon diatom-pH data-set. Weighted averaging partial least squares regression is used to develop pH-inference models. The AL:PE data-set has a root-mean-square-error of prediction (RMSEP) of 0.33 and a maximum bias of 0.36 pH units and r2 of 0.82, as assessed by leave-one-out cross-validation. The epilithon data-set has, after data-screening and the deletion of one very obvious outlier, a RMSEP of 0.23 and a maximum bias of 0.18 pH units and r2 of 0.88. The 167 sample SWAP diatom-pH data-set from lowland or upland lakes in the UK, Norway, and Sweden has a RMSEP of 0.29 and a maximum bias of 0.23 pH units and r2 of 0.86. The pH optima, as estimated by weighted averaging and Gaussian regression, are compared for the three data-sets (AL:PE, SWAP, AL:PE epilithon). There is a good correspondence between the AL:PE and the AL:PE epilithon optima, but a consistent bias between the AL:PE and SWAP optima, with the SWAP optima being lower than the AL:PE estimates. The predictive performances of the AL:PE and SWAP calibration data-sets are compared using independent test samples and six core sequences, all from high-altitude lakes, one in south-east Siberia and five in eastern Scotland. The results show the importance of using the AL:PE data-set for inferring lake-water pH from diatom assemblages in high-altitude or high latitude lakes with low DOC concentrations.

Environmental history of the Colombian savannas of the Llanos Orientales since the Last Glacial Maximum from lake records El Pinal and Carimagua

Abstract: Late Quaternary environments have been studied by pollen analysis of lake sediments from the savannas of the Colombian Llanos Orientales at 180 m elevation. The pollen record form Laguna El Pinal (4°08′N, 70°23′W), dated by 6 AMS radiocarbon dates, starts at 18,290 14C yr B.P. The record from Laguna Carimagua (4°04′N, 70°14′W), also dated by 6 AMS dates, starts at 8270 14C yr B.P. Both records show a landscape dominated by grassland savanna with only few woody savanna taxa, such as Curatella and Byrsonima, frequent fires, and little occurrence of forest and/or gallery forest along the rivers. The savanna ecosystem at the studied sites was relatively stable during the last 18,000 yrs, but minor changes in floral composition, and in the proportion of savanna/forest, have been recorded. Very little gallery forest and the non permanent lake conditions of Laguna El Pinal reflect the driest period, interpreted to reflect low rainfall rates and long dry seasons during the Last Glacial Maximum until 10,690 14 C yr B.P. During the Late Glacial, Laguna El Pinal was a permanent shallow lake, and changed into a lake with higher water levels during the Holocene, indicating wetter conditions. Expansion of regional gallery forest also started at around 10,690 14C yr B.P. Little vegetational change observed in Laguna Carimagua at 5570 14 C yr B.P., in combination with a simultaneous decrease of savanna observed in previously studied lakes, suggest a change to regional wetter conditions. Thus, the Holocene before 5500 14 C yr B.P. was somewhat drier than the following period until about 3850 14C yr B.P. In both records, Late Holocene lake deposits are incomplete. Shore vegetation of Laguna Carimagua always included a minor contribution of the palms Mauritia and Mauritiella. The marked increase of palms during the last c. 3800 yrs points to increased human impact on the vegetation under the wettest Holocene climate regime.

Environmental Characteristics of Lake Tecocomulco, northern basin of Mexico, for the last 50,000 years

Abstract: Paleoenvironmental studies have documented the late Pleistocene to Holocene evolution of the lakes in the central and southern parts of the basin of Mexico (Texcoco and Chalco). No information was available, however, for the lakes in the north-eastern part of this basin. The north-eastern and the central and southern areas represent, at present, different environmental conditions: an important gradient exists between the dry north and the moister south. To investigate the late Pleistocene to Holocene characteristics of the north-eastern lakes in the basin of Mexico two parallel cores (TA and TB) were drilled at the SE shore of Lake Tecocomulco. Stratigraphy, magnetic properties, granulometry, diatom and pollen analyses performed on these sediments indicate that the lake experienced a series of changes between ca. > 42,000 yr BP and present. Chronological control is given by five radiocarbon determinations. The base of the record is represented by a thick, rhyolitic air-fall tephra that could be older than ca. 50,000 yr BP. After this Plininan event, and until ca. 42,000 yr BP, Lake Tecocomulco was a moderately deep, freshwater lake surrounded by extended pine forests that suggest the presence of cooler and moister conditions than present. Between ca. 42,000 and 37,000 yr BP, the lake became shallower but with important fluctuations and pollen suggests slightly warmer conditions. Between ca. 37,000 and 30,000 yr BP the lake experienced two relatively deep phases separated by a dry interval. A second Plinian eruption, represented in the sequence by a dacitic an air-fall tephra layer dated at 31,000 yr BP, occurred in the area by the end of this dry episode. Between ca. 30,000 and 25,7000 yr BP Tecocomulco was a fresh to slightly alkaline lake with a trend towards lower level. After ca. 25,700 yr BP very low lake levels are inferred, and after ca. 16,000 yr BP the data indicate the presence of a very dry environment that was persistent until the middle Holocene. After 3,500 yr BP lacustrine conditions were re-established and the vegetation cover shows a change towards higher percentages of herbaceous taxa.

Palaeoclimatic implications of isotopic data from modern and early Holocene shells of the freshwater snail Melanoides tuberculata, from lakes in the Ethiopian Rift Valley

Abstract: Carbon and oxygen isotope ratios in the shells of the freshwater snail Melanoides tuberculata yield information on the isotopic composition of the water in which the shell was formed, which in turn relates to climatic conditions prevailing during the snails' life span. Melanoides is particularly important because it is widespread in Quaternary deposits throughout Africa and Asia and is ubiquitous in both fresh and highly evaporated lakes. Whole-shell and incremental growth data were collected from modern and fossil shells from two lakes in the Ethiopian Rift Valley. δ18O values in the modern shells from Lake Awassa are in equilibrium with modern waters, while δ18O values in subfossil shells from the margins of Lake Tilo indicate high rainfall during the early Holocene. Sequential analysis along the growth spiral of the shell provides information on seasonal or shorter-term variability of lake water during the lifetime of the organism.

Environmental changes documented by sedimentation of Lake Yiema in arid China since the late Glaciation

Abstract: In this study, a 6 m long core (16,000 BP) at the center of the dry Lake Yiema, a closed lake of Shiyang River drainage in Minqin Basin of the arid northwestern China, was retrieved to recover the history of climate changes and lake evolution in the area. Five radiocarbon dates on organic matter were obtained. A chronological sequence is established based on these five dates and other dates from nearby sites. Magnetic susceptibility, particle size and chemical composition were analysized for climate proxies. The proxies indicate that a drier climate prevailed in the Shiyang River drainage during the last glacial. Lake Yiema was dry and eolian sand covered most part of the lake basin. During the early and middle Holocene, a moister climate prevailed in the drainage. Climate became dry stepwise with an abrupt transition from one stage to another during the entire Holocene and became driest since about 4,200 BP. Maximum dry climate spells occurred at about 12,000-10,000 BP and after about 4,200 BP. A dry climate event also existed at about 7,600 BP. Periodical sand storms with about 400-yr cycle happened during the middle Holocene. Desiccation processes of the lake started at 4,200 BP, and were accelerated since the last 2,500 yrs by the inflow water diversion for agriculture irrigation. During the past 2,500 yrs, the lake size has been closed associated with the human population, implying that the human impact has been accelerating the lake desiccation superimposed on the natural climate deterioration.

Mass occurance and sporadic distribution of Corynocera ambigua Zetterstedt (Diptera, Chironomidae) in Danish lakes. Neo- and palaeolimnological records

Abstract: The chironomid Corynocera ambigua (Tanytarsini) is commonly reported as a cold-stenothermal species living in shallow lakes in arctic and subarctic regions. In palaeoecological studies of temperate lakes, larval remains of C. ambigua are usually found in late-glacial sediments from the Allerod and Younger Dryas periods, and often in association with subfossil Chara oospores. During a surface sampling program of chironomid head-capsules in 41 Danish temperate lakes, C. ambigua was found to comprise 25% of the chironomid assemblages in two lakes, and was sporadically found in 8 other lakes (0.5-10%). A 70 cm palaeo-stratigraphy from the shallow (max depth 1.2 m) and eutrophic (total phosphorus = 150 μg P l-1) Lake Stigsholm showed that C. ambigua has been abundant in the last 4-5 centuries. At a sediment level of 25 cm (~year 1925, 210Pb dating), C. ambigua began to decrease in frequency while Chironomus plumosus, Procladius sp., Cladotanytarsus gr. mancus and Tanytarsus spp. increased, suggesting an increased nutrient loading and an approach to eutrophic conditions. In 1995 C. ambigua was still very abundant in Lake Stigsholm but in early March 1997 no living larvae were found. An extremely heavy growth of Elodea, Enteromorpha and filamentous algae in the summers of 1995 and 1996, with following degradation in the fall, might have influenced the invertebrate population dynamics. No significant distinguishing characteristics were found for the lakes supporting C. ambigua. Its occurrence in warm (~20°C) Danish lakes brings into question the perception of the species as being cold-stenothermal.

Late Quaternary palaeohydrology of the Konya Basin, Turkey, based on isotope studies of modern hydrology and lacustrine carbonates

Abstract: The Konya plain in south central Anatolia, Turkey, which is now largely dry, was occupied around the time of the Last Glacial Maximum by a fresh-oligosaline lake covering more than 4000 km2 Sediment cores from three residual water bodies (Pinarbasi, Akgol and Suleymanhaci) within the larger Pleistocene lake basin, have been analysed using a multidisciplinary approach The sediment sequences are dated as spanning the last 50 Ka years, although breaks in sedimentation mean that there is only partial chronological overlap between them Carbon and oxygen isotope analyses on lacustrine carbonate from the three cores give contrasting isotope profiles which reflect the different ages and independent hydrological behaviour of different sub-basins through the late Quaternary Distinguishing changes that are regional from local effects is aided by modern isotope hydrology studies and by comparing the carbonate δ13C and δ18O values to diatom and other analyses undertaken on the same cores

Metals and models: Diagenic modelling in freshwater lacustrine sediments

Abstract: This review aims to distill and synthesize the existing information on the use of models to describe and predict the distribution and movement of metals in lacustrine sediments. As such it examines the causes of metal diagenesis, the origin and form of the equations that govern these phenomena, and the predictability or measurability of the parameters that appear in the models. The paper concludes by highlighting some seminal results from modelling studies, including the determination of the factors controlling the formation of surficial Mn-Fe-enriched zone or layers, the substantial contribution possible from metal reduction to organic matter regeneration, the calculation of mixing-corrected metal input histories to lakes, and the prediction of growth rates and morphologies for both deep-sea and lacustrine ferromanganese nodules.

Lake evolution in a semi-arid montane environment: Responses to catchment change and hydroclimatic variation

Abstract: Pollen, geochemical and sedimentological data from Sidi Ali, a montane Moroccan lake, provide a 7000 yr record of changes in climate, catchment vegetation and soil erosion intensity. Diatoms, non-silicious algae, macrophyte fossils and ostracods from the same core record the dynamics of the lake ecosystem. Oxygen isotope and trace-element ratios of benthic ostracods appear to be relatively insensitive to climatic variation in this open lake with low water-residence time, but diatom plankton / periphyton (P/L) ratios show lake-level variations that are probably climate controlled. At least two superimposed processes are recorded, but at different timescales: catchment vegetation and soils show long-term changes due to climate and human impact, whereas P/L ratios suggest century-scale oscillations in lake depth. The timing of changes in algal and macrophyte productivity and carbon cycling within the lake broadly corresponds to changes in terrestrial vegetation, suggesting either that lake nutrient status is linked to catchment vegetation and soils, or that both were influenced by climate. The lack of a sensitive and independent (non-biological) climate proxy makes it more difficult to assess the lake''s ecological response to short-term climate variation. Overall, the lake''s evolution has been influenced both by catchment-mediated nutrient flux and by changes in water balance, thus having characteristics in common with both temperate and arid zone lakes.

Taking varves to bits: Scanning electron microscopy in the study of laminated sediments and varves

Abstract: Conventional high resolution studies of varved sediments are able to identify clastic and biogenic laminae, but are often unable to resolve the nature of fine-scale lamination contained therein. This intra-annual signal provides us with the highest potential resolution from the sedimentary record and can be resolved using scanning electron microscopy (SEM). Six case studies from lacustrine and marine settings are presented to illustrate the combination of clastic and biogenic fabric types typically found in laminated sediments. Clastic laminae fabrics include those which originate through grain settling and those which are event deposits. The correct identification of event deposits is essential if varves are to be used chronologically. SEM-based biogenic laminae fabric studies have identified seasonal faunal successions where individual laminae may be less than 100 thick and most recently, deep chlorophyll maxima (DCM) summer diatom floras, providing an insight into seasonal scale processes. High resolution lamina fabric studies can provide a basis for generating records of seasonal and inter-annual variability, thus contributing to our understanding of lacustrine and marine processes and palaeoenvironmental interpretation.

Greigite (Fe3S4) as an indicator of drought – The 1912–1994 sediment magnetic record from White Rock Lake, Dallas, Texas, USA

Abstract: Combined magnetic and geochemical studies were conducted on sediments from White Rock Lake, a reservoir in suburban Dallas (USA), to investigate how land use has affected sediment and water quality since the reservoir was filled in 1912 The chronology of a 167-cm-long core is constrained by the recognition of the pre-reservoir surface and by 137Cs results In the reservoir sediments, magnetic susceptibility (MS) and isothermal remanent magnetization (IRM) are largely carried by detrital titanomagnetite that originally formed in igneous rocks Titanomagnetite and associated hematite are the dominant iron oxides in a sample from the surficial deposit in the watershed but are absent in the underlying Austin Chalk Therefore, these minerals were transported by wind into the watershed After about 1960, systematic decreases in Ti, Fe, and Al suggest diminished input of detrital Fe-Ti oxides from the surficial deposits MS and IRM remain constant over this interval, however, implying compensation by an increase in strongly magnetic material derived from human activity Anthropogenic magnetite in rust and ferrite spherules (from fly ash?) are more common in sediment deposited after about 1970 than before and may account for the constant magnetization despite the implied decrease in detrital Fe-Ti oxides An unexpected finding is the presence of authigenic greigite (Fe3S4), the abundance of which is at least partly controlled by climate Greigite is common in sediments that predate about 1975, with zones of concentration indicated by relatively high IRM/MS High greigite contents in sediment deposited during the early to mid-1950s and during the mid-1930s correspond to several-year periods of below-average precipitation and drought from historical records Relatively long water-residence times in the reservoir during these periods may have led to elevated levels of sulfate available for bacterial sulfate reduction The sulfate was probably derived via the oxidation of pyrite that is common in the underlying Austin Chalk These results provide a basis for the paleoenvironmental interpretation of greigite occurrence in older lake sediments The results also indicate that greigite formed rapidly and imply that it can be preserved in the amounts produced over a short time span (in this lake, only a few years) This finding thus suggests that, in some lacustrine settings, greigite is capable of recording paleomagnetic secular variation

Spatial and interannual variations in sedimentation patterns recorded in nonglacial varved sediments from the Canadian High Arctic

Abstract: Multiple cores taken throughout Nicolay Lake in the Canadian High Arctic contain laminated sediments that are interpreted as varved. Annual sediment accumulation during the last 197 years reveals three major patterns that have important implications for hydroclimatic reconstruction. Widespread sediment dispersal is evident during most years and contrasts with years when anomalous localized and bifurcating patterns of deposition occur. Localized deposition is limited to the centre of the lake and is attributed to turbidity currents that originate on the delta foreslope. In contrast, a bifurcating pattern is produced when the river supplying the delta switches from one to two delta distributaries, resulting in altered proximal accumulation. Principal component analysis reveals additional accumulation anomalies that are sedimentologically indistinct, but constitute important sources of localized variance. The recognition and removal of anomalous deposition patterns is critical for accurate reconstruction of sediment flux from the catchment, particularly in long records where geomorphic conditions may have changed. This study identifies potentially misleading depositional artifacts and a methodology to recognize similar depositional controls in other lakes.

Development of paleolimnological inference models for pH, total nitrogen and specific conductivity based on planktonic diatoms

Abstract: Canonical correspondence analysis (CCA was used to explore and identify statistically significant relationships between the distributions of planktonic diatoms and the physical and chemical properties of 50 Connecticut lakes. Six variables (pH, total nitrogen, calcium, sulfate, potassium and chlorophyll- a concentrations) were found to be significantly correlated with either or both of the first two extracted axes. The pH and calcium concentration, and to a lesser extent total nitrogen concentrations, were the most important variables controlling the distributions of planktonic diatoms in this suite of lakes. Paleolimnological inference models were developed for pH, total nitrogen (TN) and specific conductivity. Weighted averaging with (WAtol) and without (WA) tolerance downweighting, with and without bootstrap resampling techniques, and using either classical or inverse deshrinking methods were used to develop inference models for each variable. The pH and TN yielded sufficiently high λ1/λ2 ratios and a highly significant first (constrained) axis when entered as single variables in both constrained and partially constrained CCA analyses, supporting the idea that reliable inference models could be developed for these variables. The r2 and RMSE of prediction values ranged from 0.73 to 0.86 and 0.37 to 0.6, respectively for pH, and from 0.4 to 0.64 and 59 μg/l to 95 μg/l, respectively for TN. Inference models for specific conductivity also yielded significant goodness-of-fit statistics. However, because specific conductivity was removed from the CCA analysis due to its high variance inflation factor and did not yield a significant relationship when entered as the sole variable in a partial constrained CCA, inference models for this variable will probably not yield any additional environmental information. The use of only planktonic diatoms in construction of inference models is discussed.

8000-year history of palaeoproductivity in a large boreal lake

Abstract: An 8000-year record of palaeoproductivity, based on the chemical and chironomid stratigraphies from Lake Paijanne, S. Finland, was assessed with respect to known morphometric, climatic and anthropogenic events. A gradual trend of dystrophication and an associated decrease in aquatic production was detected during the Holocene, with the following exceptions: (1) high diatom and chironomid production around 8000-6000 cal yr BP, (2) eutrophication around 2000 cal yr BP, and (3) an anthropogenic signal during the last few decades. The changes in chironomid assemblages, before the past few decades, have mainly been shifts in concentration, but not in species composition. Variation in chironomid production was mainly explained by the accumulations of biogenic silicon, carbon and organic matter. Nutrient availability seems to be important in controlling biogenic silicon, which we use to infer past diatom production. The high production ca. 8000-6000 cal yr BP and the fluctuation in chironomid influx after ca. 2000 cal yr BP, however, were probably caused by the proposed warm/dry and cold/wet conditions during these times, respectively. These results highlight the sensitivity of boreal shield lake ecosystems to climatic forcing. In contrast, the pronounced change in the morphometry of the basin around 7000 cal yr BP had little effect on the trophic state of the lake. The human-induced trophic change during the past few decades has affected the Lake Paijanne ecosystem to an extent never experienced before during the last 8000-years.

Cladoceran and diatom evidence of lake-level fluctuations from a Finnish lake and the effect of acquatic-moss layers on microfossil assemblages

Abstract: We studied Holocene lake-level fluctuations from a small lake, Iso Lehmalampi, southern Finland, utilizing cladoceran and diatom analyses. We report data from a sediment core (A) taken from the deepest part of the lake (8.1 m) where two layers of moss, mixed with gyttja, were found. These layers were formed in situ during the early Holocene (1. ca. 8100-7900, 2. ca. 7300 BP). Lake-level fluctuations were inferred also from another core C, which did not have moss layers. According to the ratio of planktonic/littoral Cladocera, the water level was high around 9000 BP and started to fall before 8000 BP. The lowering continued until 7000 BP and the moss layers were formed during this lowering. Water level was high again ca. 6000 BP and lowered towards ca. 4000 BP. The late Holocene is characterized by several rapid fluctuations of lake-level. The ratio of planktonic/littoral Cladocera and the diatom species composition in core A showed drastic changes between the moss layers and the non-moss gyttja sections of the core. We suggest that they reflect changes in sedimentary facies between the local moss environment and the pelagic bottom. Thus, cores which contain moss layers may lead to erroneous interpretations of lake-level fluctuations.

Diatom stratigraphy in five offshore sediment cores from the northwestern Baltic proper implying large scale circulation changes during the last 8500 years

Abstract: Five Holocene sediment cores from the northwestern Baltic proper were analysed for lithology, siliceous microfossil assemblages and geochemical parameters. The data indicate that surface water salinity and redox conditions below the halocline have changed drastically at least four times since the Baltic Sea changed from a fresh water lake (the Ancylus Lake) to a semi-enclosed brackish water sea (the Litorina Sea) c. 8500 yrs BP. These variations appear to be mainly effects of changes in water depth at the thresholds of inlet areas. Based on these changes, and earlier studies of the shoreline displacement in the inlet areas, we propose a tentative model for changes of large scale water circulation in the Baltic Sea during the last c. 8500 yrs. At the transition from fresh to brackish water 8500 14C yrs BP, upwelling of nutrient rich bottom water started to occur, causing a slight increase in primary production. Diatom assemblages in sediments indicate a slow rise in surface water salinity during this period. At 7000-6500 14C yrs BP, surface water salinity and primary production simultaneously increased, as anoxic bottom conditions were established at depth below the halocline. We suggest that high primary production was caused by increased input of oceanic water, leading to increased upwelling of nutrient rich bottom water. At the anoxic bottoms laminated sediments formed until 5000-4500 14C yrs BP. This period (c. 7000-4500 14C BP) was contemporaneous with the post-glacial transgression maximum in Oresund, and we suggest it represents the most saline phase of the Baltic Sea post-glacial history. Due to a regression in Oresund starting 4500 14C yrs BP, upwelling decreased and the halocline was lowered, resulting in decreased primary productivity and hence oxic deep water conditions. The diatom assemblages of the sediments indicate a lowering of salinity at the beginning of this period. We suggest that the second period of anoxic bottom conditions c. 2000-1500 14C yrs BP was caused by a change of dominating inflows from the Oresund to the Belt Sea. This resulted in decreased salinity of the inflowing water which did not penetrate to the deepest parts of the basin as frequently as before. The diatom record indicates both a second lowering of salinity and a change in the large scale water circulation at the beginning of this period.

Anthropogenic changes in the sediment composition of Lake Gości& aogon;ż (central Poland), during the last 330 yrs*

Abstract: According to historical sources, the development of settlement around Lake Gości& aogon;z during the last 330 yrs was intense at two time periods: the second half of the 1700's, and from ca. 1880 until 1944. The small farms were then abandoned, following which the lake surroundings were planted with forest trees. The presented study of human influence on the lake ecosystem and surrounding vegetation has been based on analyses of general sediment composition and its chemistry, pollen, Cyanobacteria, Chlorophyceae, Rotatoria, Cladocera, and a preliminary diatom survey. The history of human impact has been divided into four phases: 1. Phase of small local hamlets (before ca. 1770): The human impact was rather moderate then, but the cultivation of Canabis sativa, Secale cereale and later of Fagopyrum is evidenced from that time. 2. Phase of ‘Hollandii’ settlement (ca. 1770-1863). Its influence is indicated first by the recession of deciduous wood (Corylus, Carpinus) stands, which triggered drastic drop of calcium in sediments. The development of rural economy in the area, including, an extension of agriculture (Secale and other cereals, crucifers, potatoes), and animal breeding based partly on grazing in the forest, is evidenced only after 1820. 3. Phase of German colonization (1863-1944): In the early periods (before 1910) the pollen spectra do not document any essential change in the type of farming, however, a serious disturbance of the lake ecosystem and sediment chemistry is evidenced by the blooms of Araphidinae diatoms and Tetraedron minimum, a maximum frequency of Bosmina longirostris, disturbances of the regular spring blooms of Chrysophyceae, appearance of vivianite, distinct maxima of organic matter, potassium, and iron concentration in sediments and an increase of the sedimentation rate. An intensification of agricultural activities commenced around 1910; woods, including also pinewoods, were then heavily devastated, and farming extended on poor soils, what was symptomatic for the general poverty of population. Coincidently in the lake, Centriceae showed blooms, Araphidinae diatoms and Tetraedron minimum developed, and the content of potassium, iron and phosphorus increased substantially, indicating altogether rising eutrophication. 4. Phase of restoration of the natural landscape (after 1945): The farm degradation from ca. 1944 is very weakly expressed in pollen data, which show a substantial fall of farming indicators from 1953/6 only, when the whole area was used for forest plantation. It was accompanied by a certainly spontaneous development of Betula and Alnus woods, this process progressing till recent time. The gradual extinction of farming activity near Lake Gościąz was accompanied by abrupt changes in the lake ecosystem, expressed by the restored blooms of Chrysophyceae expansion of planktonic Cladocera, rapid decline of phosphorus and extinction of vivianite from sediments. As documented by the drop of Cu/Zn ratio, lake hypolimnion has been weakly oxidized since 1949, what was probably responsible for the drop of Fe and Mn content in sediments. Increasing strength of overturns affected preservation of laminae in sediments, which almost completely disappeared after 1966.

Temporal and spatial trends of lake acidity in northern Sweden

Abstract: Changes in lake-water pH, alkalinity and colour were inferred from diatoms in surface sediment samples and sediment samples from pre-industrial times from 118 northern Swedish lakes This palaeolimnological study does not support the hypothesis that there is a large-scale modern acidification in the two northernmost counties of Sweden; pH had decreased significantly in eight lakes, while five had a significant increase Partial least-squares regression of changes in water chemistry in relation to catchment characteristics was performed to evaluate the causes of the acidity status Furthermore, temporal trends were assessed from long sediment cores from five acidic lakes The results suggest that the presently acid lakes have faced a long-term acidification trend over several thousand years due to soil-forming processes and vegetation development However, due to the acid sensitivity of the region, future acidification trends in northern Swedish lakes should be carefully observed and assessed

Late Glacial to Early Holocene environmental adjustment in the Mediterranean semi-arid zone of the Salines playa-lake (Alacant, Spain)

Abstract: The transition from the Late Glacial to the Early Holocene in the endorheic Salines sequence, which is characterized cyclical sedimentation, occurs between 5.50 and 2.85 m depth. From 5.50-3.50 m depth the cycles are composed of a centimetre alternation of layers of dolomitic marls and gypsarenites and from 3.50-2.85 m depth by the alternation of calcitic marls and calcarenites.

Reconstructing past eutrophication trends from diatoms and biogenic silica in the sediment and the pelagic zone of Lake Constance, Germany

Abstract: In Lake Constance, phosphorus concentrations and the seasonal development of phytoplankton communities in water samples from the pelagic zone were regularly recorded since the 1950's. Before the 1950's, there were occasional investigations of plankton communities since 1896. We compared these data with the sedimentary record in two sediment cores. Then, the eutrophication history of Lake Constance was inferred from diatoms. The record of biogenic silica in the cores is discussed with respect to diatom biomass increase. Diatom assemblages in the sediment cores precisely reflected the pelagic diatom development for the period 1971--1992. Both sediment cores and the water samples have a high interannual variability of diatom assemblages. Below a sediment depth of 27 cm (AD 1920), more than 50% of the diatoms were partly corroded, and we limited the reconstruction of trophic state changes to the interval of 1920--1993. Oligotrophic conditions of Lake Constance were indicated by the dominance of various Cyclotella taxa from 1920 to 1940. Since 1939/1940, increasing abundance of it Tabellaria fenestrata showed oligotrophic to mesotrophic conditions. Between 1953 and 1956, increasing Stephanodiscus hantzschii and disappearing Cyclotella indicated advanced eutrophication and total phosphorus values ranged between 8--10 mg m-3 during turnover in late winter. Further eutrophication was shown by disappearing T. fenestrata and increasing S. minutulus in 1963. Maximum TP concentrations of 87 mg m-3 occurred in 1979/80 and was accompanied by increasing abundances of Aulacoseira granulata. From 1986 to 1992, reoccurrence of Tabellaria fenestrata and Cyclotella indicate some recovery of Lake Constance. Biogenic silica and diatom abundances were similar among cores but indicate a 3--4 fold increase of diatom biomass only. This was far below the estimate of biomass increase from sedimentary pigment data (25 fold) and the estimate of phytoplankton data from the literature (70 fold).

Sedimentation patterns of diatoms in Lake Holzmaar, Germany - (on the transfer of climate signals to biogenic silica oxygen isotope proxies)

Abstract: The seasonal sedimentation pattern of diatom valves in Lake Holzmaar was investigated during 1995 by deploying sediment traps at three different lake depths. According to the sedimentation pattern, the major reproduction zone of diatoms was restricted to the upper 6 m of the water body. The population growth started late in April and blooms of Cyclotella cf. comensis Grun., which dominates the plankton diatoms, and Fragilaria crotonensis Kitton were collected in traps during June and September, and July, respectively. During summer, the seasonal sedimentation pattern of each taxon, as collected in the upper traps, was reflected in the concentrations in the lowest trap. However, in May and from September onwards, the community composition in the lowest trap and augmented trapping rates suggest both sediment focusing and resuspension of bottom sediments.

Image analysis as a method to quantify sediment components

Abstract: Varved lake sediments, with their annual to seasonal resolution, have a high potential for inferring past environmental and climatic conditions. To fully utilize the information present in varved records, high-resolution analyses, which often are time-consuming and difficult to perform, are desirable. The investigation reported here aims at (i) developing image analysis as a method for estimating annual accumulation rates of sediment components such as minerogenic matter, organic matter and biogenic silica, and (ii) assessing the relative importance of these components for changes in varve thickness. Image analysis was used to digitize the grey-scale variations and to measure the varve thickness of 540 varves (476-1015 AD) from Lake Kassjon in northern Sweden. From the 35 cm long digitized sediment sequence, 108 consecutive five-year samples were cut out quantitatively, and relationships between grey-scale variations and sediment dry mass and individual sediment components were assessed. There is a strong correlation between corrected grey-scale (i.e. the product of grey-scale and varve thickness) and the dry mass accumulation rate (r = 0.90, p < 0.001). With a stepwise multiple regression a significant model (R2 = 0.81) between corrected grey-scale and the accumulation rates of minerogenic matter (r = 0.90, p < 0.001) and biogenic silica (r 0.26, p < 0.012) was obtained. Considering the minor contribution and weak significance of biogenic silica, image analysis can be used as a fast and non-destructive method to infer past annual accumulation rates of dry mass and minerogenic matter in Kassibn. The model of the relationship between changes in varve thickness, and water content and accumulation rates of sediment components has little predictive power (R2 = 0.45). The result shows that the varve thickness in Kassjon, at least during the period 476-1015 AD, is not determined by a single sediment component but partly depends on interactions between the major sediment components.

Spatial variability in surface sediment composition characterised by near-infrared (NIR) reflectance spectroscopy

Abstract: Near-infrared (NIR) spectral data were obtained from 165 surface sediment samples from a northern Swedish humic, mesotrophic lake (0.5 km2). The NIR spectra, together with data on water depth and loss-on-ignition (LOI), allow an evaluation of the source of spatial variance in sediment characteristics. The results show that water depth and organic matter (LOI) account for 20 and 16%, respectively, of the variance in the NIR-absorbance data. More importantly, the spatial variance in the spectral data suggest that NIR analysis of lake sediments mainly reflect sediment properties that cannot simply be explained by depth or amount of organic matter. The influence of inlets and land-use (e.g. clear-cutting) on sediment characteristics was more pronounced in the spectral data than would be expected from the LOI data. This is explained by differences in the chemical composition of the organic matter, as revealed by NIR spectroscopy, rather than in the amount of organic matter or depth-related sedimentation properties (e.g. particle size or density). This initial attempt to characterise sediment properties using NIR suggests that NIR analysis might become a valuable complementary tool to traditional sediment characterisation.

Differences in the eutrophication dynamics of Ammersee and Starnberger See (Southern Germany), reflected by the diatom succession in varve-dated sediments

Abstract: Diatom assemblages in the annually laminated sediments of two neighboring, pre-alpine lakes (Ammersee and Starnberger See, Southern Germany) were analyzed and compared year by year. Within both varve records of the last decades, the same 4-phase-succession of planktonic diatom species is preserved, consisting of 1. a dominance of Cyclotella species 2. assemblages of Fragilaria crotonensis, Asterionella formosa and Stephanodiscus minutulus 3. a mass bloom of Aulacoseira islandica, and 4. a dominance of Stephanodiscus species. This diatom sequence is considered as a model of a 'basic species sequence of eutrophication'. Time lags and differences in the duration of the phases between both lakes show evidence of an earlier start for nutrient loading in Starnberger See than in Ammersee, and a faster development to a higher trophic state level in the latter. The different reactions of the lakes are attributed to hydrological differences such as the existence or lack of a major tributary, the size of the catchment area, and the water residence time. The reconstruction of the eutrophication dynamics appears to be reliably hindcast by the inference of total phosphorus (TP) concentrations using weighted averaging regression and calibration techniques. Remarkable differences in the occurrence of some diatoms in both stratigraphies, which are not related to the general changes of the 'basic species sequence of eutrophication' model, may be due to the epilimnetic silica content (e.g. Fragilaria crotonensis) or competitive weaknesses (e.g. Tabellaria flocculosa).

Sedimentological evidence for environmental changes since 34,000 years BP from Lago di Mezzano, central Italy

Abstract: Sediment cores recovered from Lago di Mezzano, central Italy, were petrographically and geochemically (dry densitity, total organic carbon) investigated. A floating chronology was established with sedimentation rates derived from varve thickness measurements, and this chronology was both supported and extrapolated with calibrated AMS-14C-datings. The profile has a length of 29.7 m and comprises a total of 34,000 years. Late Pleistocene sediments consist of minerogenic-organic mud with few benthic diatoms and an organic carbon content of 2%, thus suggesting a high allochthonous input. The onset of the Late Glacial at 14,580 cal BP is documented by a lithologic change to more organic-dominated sedimentation. The Younger Dryas cold event is recorded between 12,650 and 11,400 cal BP and exhibits higher dry densities and minerogenic input. These dates agree with records from other lacustrine archives in Europe and the Greenland ice cores. The early Holocene comprises a laminated organic diatom gyttja deposited at a time of climatic amelioration and increased primary productivity. The establishment of an anoxic hypolimnion enhanced the varve and organic matter preservation. Since 3700 varve years BP the sedimentation pattern has been strongly influenced by human impact, as documented by the increase in minerogenic sediments and turbidites as well as higher sedimention rates. The onset of this influence coincides with a Bronze Age settlement at the lake shore.

A multi-proxy trophic state reconstruction for shallow Orange Lake, Florida, USA: Possible influence of macrophytes on limnetic nutrient concentrations.

Abstract: We retrieved four sediment cores from shallow, eutrophic, macrophyte-dominated Orange Lake (A = 51.4 km2, zmax <5 m, zmean < 2 m), north-central Florida, USA. The 210Pb-dated profiles were used to evaluate spatial and temporal patterns of bulk sediment and nutrient accumulation in the limnetic zone and to infer historical changes in lake trophic state. Bulk density, organic matter, total carbon, total nitrogen, total phosphorus and non-apatite inorganic phosphorus (NAIP) concentrations displayed stratigraphic similarities among three of four cores, as did accumulation rates of bulk sediment, organic matter and nutrients. Accumulation rates were slower at the fourth site. Nutrients showed generally increasing rates of accumulation since the turn of the century. Percentages of periphytic diatom taxa increased progressively in the cores after ~ 1930. Diatom-inferred limnetic total P trends were similar among profiles. Eutrophic conditions were inferred for the period prior to the turn of the century. The lake was hypereutrophic in the early decades of the 1900s, but inferred limnetic total P values declined after ~ 1930. Declining inferred limnetic total P trends for the last 60--70 years were accompanied by concomitant increases in accumulation rates of total P and NAIP on the lake bottom. Several lines of evidence suggest that after ~ 1930, phosphorus entering Orange Lake was increasingly utilized by submersed macrophytes. Paleolimnological records from Orange Lake highlight the importance of using multiple sediment variables to infer past trophic state and suggest that aquatic macrophytes can play a role in regulating water-column nutrient concentrations in shallow, warm-temperate lakes.