A modified method for biogenic silica determination

We describe a modified method for the determination of biogenic silica (BSi) in lacustrine sediment samples with BSi concentrations >0.5% SiO₂ (detection limit). To speed up the leaching procedure (48 samples per day), we suggest the use of Teflon autoclaves that can be left unattended (no stirring of the solution) for 3 h at 100°C in an oven. We suggest a volume correction by Na measurement to account for volume errors during pipetting off a hot solution. We also propose to carry out a correction for minerogenic silica by the simultaneous determination of Al in the leachate. Analytical precision and accuracy of the proposed method are 2.4% (relative standard deviation) and 70–110% recovery (relative to the mean of four international reference sediment samples), respectively. Additionally, we demonstrate that pre-treatment of the samples with H₂O₂ and/or HCl prior to alkaline leaching is not necessary for all samples. Due to the great diversity of lacustrine and marine sediments, however, the pre-treatment procedure should be tailored for samples from different sites.     

The effects of organic removal treatment on the integrity of δ<Superscript>18</Superscript>O measurements from biogenic silica

Prior to oxygen isotope analysis of biogenic silica by fluorination techniques, sediments must be cleaned of all organic compounds. Despite the expanding volume of research utilising oxygen isotope ratios in biogenic silica in palaeoclimatology, very little is known about the effects of different standard preparation methods for the removal of organic matter on the isotope values. Here we compare a number of methods for the removal of organic matter in order to assess their effectiveness and influence on subsequent isotope measurements. Not all the methods described here effectively removed organic matter, the best treatments were H₂O₂ and ignition in air at 550°C. Ignition in air at 950°C, and heating in vacuum at 250 and 450°C results in marked alteration of isotope values, whereas use of hydrogen peroxide (H₂O₂), nitric acid (HNO₃) and plasma ashing have little or no effect, within the bounds of analytical error. Ignition in air at 550°C was the most effective technique, and induced no alteration of δ¹⁸O_silica, but practical constraints may limit the application of this method routinely. Our findings indicate that considerable caution is required when preparing sediment for isotope analysis of biogenic silica.     

Uranium and Thorium Isotope Distribution in an Offshore Bottom Sediment Core of the Selenga Delta, Lake Baikal, Siberia

Distribution of uranium and thorium isotopes in a short sediment core obtained offshore of the Selenga Delta in Lake Baikal, Siberia, was investigated to establish their sedimentary behaviors and to look for a linkage to paleoenvironmental changes. The sediments were composed of dominantly fine detrital materials (70–85%) and a relatively high sedimentation rate (ca. 0.03 cm y⁻¹). The depth profile of ²³⁸U content in bulk sediment samples showed a large variation of 70–123 Bq kg⁻¹, while ²³²Th profile showed a relatively narrow range from 36 to 56 Bq kg⁻¹. The observed ²³⁴U/²³⁸U activity ratios revealed a marked disequilibrium ranging from 1.53 to 1.84 with a mean value of 1.71 ± 0.07, demonstrating the presence of 50–80% authigenic ²³⁸U in the bulk sediments. The distribution of this authigenic ²³⁸U did not display any clear correlation with variations in sediment composition (organic, carbonate, Bio-SiO₂ and mineral contents) including grain size median. The profile of terrigenous ²³⁸U showed a relatively similar pattern to that of ²³²Th. Results of sequential leaching indicate that ²³⁸U in Fe–Mn oxyhydroxides fractions were responsible for the distribution of authigenic ²³⁸U rather than in Bio-SiO₂ fraction. The distribution of authigenic ²³⁸U in the bottom sediments may be explained by the fluctuation of U adsorption capacity on particles including organic matter and Fe–Mn oxyhydroxides before they entered the lake. This study highlights the potential use of authigenic and terrigenous U (Th) signatures in sediments to trace the behavior of U (Th) and to reconstruct environmental (e.g., hydrological) changes in the lake catchment area.     

Late Holocene storm-trajectory changes inferred from the oxygen isotope composition of lake diatoms,south Alaska

The oxygen isotope ratios of diatoms (δ¹⁸O_diatom), and the oxygen and hydrogen isotope ratios of lake water (δW) of lakes in south Alaska provide insight into past changes in atmospheric circulation. Lake water was collected from 31 lakes along an elevation transect and diatoms were isolated from lake sediment from one lake (Mica Lake) in south Alaska. In general, δW values from coastal lakes overlap the global meteoric water line (GMWL). δW values from interior lakes do not lie on the GMWL; they fall on a local evaporation line trajectory suggesting source isotopes are depleted with respect to maritime lakes. Sediment cores were recovered from 58 m depth in Mica Lake (60.96° N, 148.15° W; 100 m asl), an evaporation-insensitive lake in the western Prince William Sound. Thirteen calibrated ¹⁴C ages on terrestrial macrofossil samples were used to construct an age-depth model for core MC-2, which spans 9910 cal years. Diatoms from 46, 0.5-cm-thick samples were isolated and analyzed for their oxygen isotope ratios. The analyses employed a newly designed, stepwise fluorination technique, which uses a CO₂ laser-ablation system, coupled to a mass spectrometer, and has an external reproducibility of ±0.2‰. δ¹⁸O_diatom values from Mica Lake sediment range between 25.2 and 29.8‰. δ¹⁸O_diatom values are relatively uniform between 9.6 and 2.6 ka, but exhibit a four-fold increase in variability since 2.6 ka. High-resolution sampling and analyses of the top 100 cm of our lake cores suggest large climate variability during the last 2000 years. The 20th century shows a +4.0‰ increase of δ¹⁸O_diatom values. Shifts of δ¹⁸O_diatom values are likely not related to changes in diatom taxa or dissolution effects. Late Holocene excursions to lower δ¹⁸O_diatom values suggest a reduction of south-to-north storm trajectories delivered by meridional flow, which likely corresponds to prolonged intervals when the Aleutian Low pressure system weakened. Comparisons with isotope records of precipitation (δP) from the region support the storm-track hypothesis, and add to evidence for variability in North Pacific atmospheric circulation during the Holocene.

Charcoal morphotypes in lake sediments from British Columbia (Canada): an assessment of their utility for the reconstruction of past fire and precipitation

Quantitative analysis of variations in morphological types of charcoal were undertaken in sediment cores from three lakes on the Interior Plateau (BC, Canada) over the period AD 1919–2000. Seven distinct morphological types of charcoal were identified based on particle shape and structural features and were compared with seasonal precipitation and recorded area burned within 20 km-radius of study lakes. Fragile-type charcoal fragments, termed type M, displayed significant relationships to recorded area burned in sediment cores from Prosser (r ² = 0.5; p = 0.0001) and Opatcho (r ² = 0.2; p = 0.02) lakes. However, nonsignificant correlations (p > 0.05) were found between total charcoal and area burned. Robust and highly elongated morphotypes C and F were correlated to recorded spring precipitation (r ² = 0.5; p = 0.002) in Opatcho Lake. Charcoal from a sediment core from Big Lake, the lake with the largest watershed, was significantly but inversely related to past fires (r ² = 0.44; p = 0.0003), suggesting important contributions from secondary transportation and deposition. Models were developed to infer relative area burned and precipitation for the study lakes. Our results suggest that charcoal morphotypes are related to the biogeoclimatic and lake watershed characteristics. This study also suggest that charcoal morphotypes can provide insights on past fire and climate, which was not possible based on traditional analysis of total charcoal.     

Elemental (C,N, H and P) and stable isotope (δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C) signatures in sediments from Zeekoevlei,South Africa: a record of human intervention in the lake

We used elemental carbon, nitrogen, phosphorus and hydrogen ratios (C/N, N/P and H/C) with total organic carbon (TOC) and total phosphorus (TP) as well as stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) to investigate the source and depositional conditions of organic matter in sediments from Zeekoevlei, the largest freshwater lake in South Africa. Typical C/N (10–12), H/C ratios (≥1.7) and δ¹³C_organic values (−22 to −19‰) together with the increase in TOC concentration indicate elevated primary productivity in lower middle (18–22 cm) and top (0–8 cm) sections of the sediment cores. Seepage of nutrients from a nearby waste water treatment plant, rapid urbanization and heavily fertilized farming in the catchments are responsible for the increased productivity. Consistent with this, measured δ¹⁵N_organic values (∼11‰) indicate increased raw sewage input towards the top-section of the core. Although cyanobacterial blooms are evident from the low δ¹⁵N values (∼3‰) in mid-section of the core, they did not outnumber the phytoplankton population. Low N/P ratio (∼0) and high TP (100–2,200 mg l⁻¹) support cyanobacterial growth under N limited condition, and insignificant input of macrophytes towards the organic matter pool. Dredging in 1983, caused sub-aerial exposure of the suspended and surface sediments, and affected organic matter preservation in the upper mid-section (12–14 cm) of the core.     

The δ18O in the Guliya shallow ice core during the prevailing summer monsoons and sea-air interactions

The correlations of the δ¹⁸O_max in the shallow ice core from the Guliya ice cap on the Tibetan Plateau with the global sea surface temperatures (SST) and height at the 500 hPa over the Northern Hemisphere were analyzed. The correlated regions on oceans that have a significant influence on the δ¹⁸O_max in the Guliya ice core are all located in ocean currents, or convergent regions of currents. They are the eastern Equatorial Pacific, the Northern Pacific Current, the Hot Pool in the eastern Indian Ocean, the Mozambique Current, the Northern Atlantic Current, the Canary Current and the Atlantic Equatorial Current. The δ¹⁸O_max in the Guliya ice core has negative correlations with the SST located in the lower latitudes, and positive correlations with the SST in the middle latitudes. The correlated areas at the 500 hPa that have a great impact on the δ¹⁸O_max are located in the subtropical highs over the mid-low-latitude oceans and the long-wave trough area over Balkhash Lake, where there are marked negative correlations between the heights in those areas and the δ¹⁸O_max. The influencing mechanism is displayed by the diversity of the vapor origins transported to the Guliya region. The strengths of the European ridge and the ridge over Baikal Lake have notable positive correlations with the δ¹⁸O_max. The two systems indirectly influence the vapor transportation towards the Guliya region by the adjustment of long-wave trough and ridge.     

Mid- to late-Holocene climate variability and anthropogenic impacts: multi-proxy evidence from Lake Bliden,Denmark

We conducted multi-proxy geochemical analyses (including measurements of organic carbon, nitrogen and sulphur stable isotope composition, and carbonate carbon and oxygen isotope composition) on a 13.5 m sediment core from Lake Bliden, Denmark, which provide a record of shifting hydrological conditions for the past 6,700 years. The early part of the stratigraphic record (6,700–5,740 cal year BP) was wet, based on δ¹⁸O_carb and lithology, and corresponds to the Holocene Thermal Maximum. Shifts in primarily δ¹⁸O_carb indicate dry conditions prevailed from 5,740 to 2,800 cal year BP, although this was interrupted by very wet conditions from 5,300 to 5,150, 4,300 to 4,050 and 3,700 to 3,450 cal year BP. The timing of the latter two moist intervals is consistent with other Scandinavian paleoclimatic records. Dry conditions at Lake Bliden between 3,450 and 2,800 cal year BP is consistent with other paleolimnological records from southern Sweden but contrasts with records in central Sweden, possibly suggesting a more northerly trajectory of prevailing westerlies carrying moisture from the North Atlantic at this time. Overall, fluctuating moisture conditions at Lake Bliden appear to be strongly linked to changing sea surface temperatures in the Greenland, Iceland and Norwegian seas. After 2,800 cal year BP, sedimentology, magnetic susceptibility, δ¹³C_ORG, δ¹³C_carb and δ¹⁸O_carb indicate a major reduction on water level, which caused the depositional setting at the coring site to shift from the profundal to littoral zone. The Roman Warm Period (2,200–1,500 cal year BP) appears dry based on enriched δ¹⁸O_carb values. Possible effects of human disturbance in the watershed after 820 cal year BP complicate attempts to interpret the stratigraphic record although tentative interpretation of the δ¹⁸O_carb, magnetic susceptibility, δ¹³C_ORG, δ¹³C_carb and δ¹⁸O_carb records suggest that the Medieval Warm Period was dry and the Little Ice Age was wet.     

A record of vegetation dynamics and lake level changes from Lake Emakat,northern Tanzania,during the last c. 1200 years

Analyses of down-core variations in pollen and charcoal in two short cores of lake sediment and wood samples taken from the in situ remains of Nuxia congesta from Lake Emakat, a hydrologically-closed volcanic crater lake occupying the Empakaai Crater in northern Tanzania, have generated evidence of past vegetation change and lake level fluctuations. Eight AMS radiocarbon (¹⁴C) dates on bulk samples of lake sediment provide a chronological framework for the two cores and indicate that the sediment record analysed incorporates the last c. 1200 years. The in situ remains of a Nuxia congesta tree, now standing in deep water, were dated with three additional AMS ¹⁴C dates, suggesting tree growth within the interval ∼1500–1670 AD. Down-core variations in pollen from terrestrial taxa, particularly the montane forest trees Hagenia abyssinica and Nuxia congesta, indicate a broad period of generally more arid conditions in the catchment to c. 1200 AD and at a prolonged period between c. 1420 and 1680 AD. Variations in pollen from plants in lake margin vegetation indicate low lake levels, presumably as a result of reduced effective precipitation, contemporary with indications of relatively dry conditions mentioned above, but also during the late 18th and the late 19th centuries. The presence of charcoal throughout both cores indicates the frequent occurrence of vegetation fires. An increase in burning, evident in the charcoal data and dated to the early to mid second millennium AD, could relate to an expansion of human population levels and agricultural activity in the region.     

The efficiency of kerosene flotation for extraction of chironomid head capsules from lake sediments samples

Kerosene, a grade mineral oil, is commonly used to extract beetles from sediment. Here, the use of kerosene to extract chironomid head capsules was tested on 10 samples from sediment of different lakes, with different organic matter content as measured by loss on ignition, and estimated ages. Our results revealed that this flotation tool is very effective in extracting either full or half chironomid head capsules. The mean extraction efficiency was 89.3 ± 8.0% with an estimated relative abundance error ranging from −1% to 1% for 46 of the 57 identified taxa. Larger chironomids (400–500 μm width), which are often full of sediment particles, have the highest relative abundance error, with a maximum of 4.3% for Corynocera oliveri-type. A canonical correspondence analysis showed that, despite this small bias, samples retrieved with the kerosene flotation do not differ from the whole sample assemblages. These results give us confidence in the use of this flotation technique for chironomid sample preparation.     

Shallow lake trophic status linked to late Holocene climate and human impacts

Lake Mattamuskeet, North Carolina, USA is a large (162 km²) and shallow (mean depth = 1 m) coastal lake, which was significantly modified to support agricultural activities following European settlement in 1850. Paleolimnological proxies measured on a 400-cm sediment core collected from Lake Mattamuskeet reveal shifts in organic matter input and primary producer community structure in response to climatic and human impacts on the lake during the late Holocene. Stratigraphic changes in organic matter content, nutrients, metals, lignin phenols and photosynthetic pigments were used to divide the sediment core into three intervals. Interval I includes sediment deposited between A.D. 360–1584 and indicates a clear-water, sand-bottom state with low algal abundance. In addition, the lake catchment area experienced two significant fires during this interval that were recorded as charcoal layers in the core around A.D. 360 and A.D. 1435 (calibrated ¹⁴C AMS dates). Trophic structure changed with the onset of Interval II (A.D. 1584–1860) when total algal abundance increased, and the primary producer community was comprised primarily of diatoms, chrysophytes, cryptophytes and cyanobacteria. During this interval there was also an increase in terrestrial organic material input into the lake as well as a shift in plant type from woody gymnosperms to non-woody angiosperms as determined from lignin data. Sediment deposited in Lake Mattamuskeet following European settlement (Interval III, A.D. 1860-present) suggests a dramatic increase in organic-matter deposition, metals, primary-producer abundance and the onset of cyanobacterial dominance. Sedimentary evidence indicates that shallow-water primary producers can respond rapidly to climate change and human development.     

A model for inferring dissolved organic carbon (DOC) in lakewater from visible-near-infrared spectroscopy (VNIRS) measures in lake sediment

We developed an inference model to infer dissolved organic carbon (DOC) in lakewater from lake sediments using visible-near-infrared spectroscopy (VNIRS). The inference model used surface sediment samples collected from 160 Arctic Canada lakes, covering broad latitudinal (60–83°N), longitudinal (71–138°W) and environmental gradients, with a DOC range of 0.6–39.6 mg L⁻¹. The model was applied to Holocene lake sediment cores from Sweden and Canada and our inferences are compared to results from previous multiproxy paleolimnological investigations at these two sites. The inferred Swedish and Canadian DOC profiles are compared, respectively, to inferences from a Swedish-based VNIRS-total organic carbon (TOC) model and a Canadian-based diatom-inferred (Di-DOC) model from the same sediment records. The 5-component Partial Least Squares (PLS) model yields a cross-validated (CV) R_CV² R_{CV}^{2}  = 0.61 and a root mean squared error of prediction (RMSEP _CV ) = 4.4 mg L⁻¹ (11% of DOC gradient). The trends inferred for the two lakes were remarkably similar to the VNIRS-TOC and the Di-DOC inferred profiles and consistent with the other paleolimnological proxies, although absolute values differed. Differences in the calibration set gradients and lack of analogous VNIRS signatures in the modern datasets may explain this discrepancy. Our results corroborate previous geographically independent studies on the potential of using VNIRS to reconstruct past trends in lakewater DOC concentrations rapidly.     

The need to correct for the Suess effect in the application of δ<Superscript>13</Superscript>C in sediment of autotrophic Lake Tanganyika,as a productivity proxy in the Anthropocene

The change in dissolved inorganic δ¹³C in the ocean resulting from the change in δ¹³C in atmospheric CO₂ owing to anthropogenic activities (the Suess effect) is well known. The need to correct for the Suess effect when applying δ¹³C in organic matter in lacustrine sediment deposited during the anthropocene as a productivity proxy, is widely although not universally acknowledged. This paper reviews conceptions about the Suess effect in lacustrine δ¹³C_org and methods to adjust for the Suess effect when δ¹³C_org is used to infer recent changes in aquatic productivity. Lake Tanganyika is used as an example to illustrate the necessity of the correction. When the Suess effect is not considered, interpretations of sediment core data can result that are opposite to those achieved with the correction applied, as is here shown in Lake Tanganyika and in other lakes. A new method to correct for the Suess effect is provided which has the advantage of being applicable to data for a larger period (1700–2000) than methods currently available. In addition, Lake Tanganyika is shown to be a net sink for CO₂.     

Comparison of radiocarbon and OSL dating methods for a Late Quaternary sediment core from Lake Ulaan,Mongolia

Both radiocarbon and optically stimulated luminescence (OSL) dating methods were applied to test their suitability for establishing a chronology of arid-zone lacustrine sediments using a 5.88-m-long core drilled from Lake Ulaan, southern Mongolia. Although the radiocarbon and OSL ages agree in some samples, the radiocarbon ages are older than the corresponding OSL ages at the 550-cm depth horizon (late Pleistocene) and in the 100–300-cm interval (early to late Holocene). In the early to late Holocene, radiocarbon ages are consistently older than OSL ages by 4,100–5,800 years, and in the late Pleistocene by 2,700–3,000 years. Grain-size analysis of early to late Holocene sediments and one late Pleistocene sediment sample (550-cm depth) indicates that eolian processes were the dominant sediment-transport mechanism. Also, two late Pleistocene sediments samples (from 400- to 500-cm depths) are interpreted to have been deposited by both eolian and glaciofluvial processes. Accordingly, the radiocarbon ages that were older than the corresponding OSL ages during the Holocene seem to have been a consequence of the influx of ¹⁴C-deficient carbon delivered from adjacent soils and Paleozoic carbonate rocks by the westerly winds, a process that is also active today. In addition to the input of old reworked carbon by eolian processes, the late Pleistocene sediments were also influenced by old carbon delivered by deglacial meltwater. The results of this study suggest that when eolian sediment transport is suspected, especially in lakes of arid environments, the OSL dating method is superior to the radiocarbon dating method, as it eliminates a common ‘old-carbon’ error problem.     

Climate variability of southern Chile since the Last Glacial Maximum: a continuous sedimentological record from Lago Puyehue (40°S)

This paper presents a multi-proxy climate record of an 11 m long core collected in Lago Puyehue (southern Chile, 40°S) and extending back to 18,000 cal yr BP. The multi-proxy analyses include sedimentology, mineralogy, grain size, geochemistry, loss-on-ignition, magnetic susceptibility and radiocarbon dating. Results demonstrate that sediment grain size is positively correlated with the biogenic sediment content and can be used as a proxy for lake paleoproductivity. On the other hand, the magnetic susceptibility signal is correlated with the aluminium and titanium concentrations and can be used as a proxy for the terrigenous supply. Temporal variations of sediment composition evidence that, since the Last Glacial Maximum, the Chilean Lake District was characterized by three abrupt climate changes superimposed on a long-term climate evolution. These rapid climate changes are: (1) an abrupt warming at the end of the Last Glacial Maximum at 17,300 cal yr BP; (2) a 13,100–12,300 cal yr BP cold event, ending rapidly and interpreted as the local counterpart of the Younger Dryas cold period, and (3) a 3,400–2,900 cal yr BP climatic instability synchronous with a period of low solar activity. The timing of the 13,100–12,300 cold event is compared with similar records in both hemispheres and demonstrates that this southern hemisphere climate change precedes the northern hemisphere Younger Dryas cold period by 500 to 1,000 years. This is the third in a series of eight papers published in this special issue dedicated to the 17,900 year multi-proxy lacustrine record of Lago Puyehue, Chilean Lake District. The papers in this special issue were collected by M. De Batist, N. Fagel, M.-F. Loutre and E. Chapron.     

Sedimentary facies and climate control on formation of vivianite and siderite microconcretions in sediments of Lake Baikal, Siberia

Authigenic vivianite and siderite microconcretions were found, respectively, in hemipelagic and deltaic facies of 600-m-long BDP-98 sediment section from Lake Baikal. Textural investigations of these microconcretions show that they are typically <1 mm in size, irregular in shape and composed of aggregated crystallites. Dissimilar orientation of vivianite and siderite crystallites suggests formation at different depths in the sediment; up to tens of centimeters for vivianite and tens of meters for siderite. Chemical analyses of both the vivianite and the siderite indicate cation composition characterized by minor amounts of Mn, Ca and Mn apart from the dominating Fe. Rather limited and distinctive carbon isotopic composition of the siderite, with δ ¹³C_VPDB values between about +13 and +16‰, implies formation of the mineral in the methanogenic zone of diagenesis. Isotopic composition of oxygen in the siderite (δ ¹⁸O_VPDB values between about −10 and -11‰ ) is consistent with crystallization temperature at about 10–30°C and water δ ¹⁸O_SMOW values between about −10 and -16‰ . The distribution of the authigenic minerals in the section suggests changes in both sedimentary facies and climate, where vivianite formation was controlled by hemipelagic depositional conditions during the Pliocene and Quaternary, whereas siderite reflects impact of deltaic conditions during the Miocene.     

Spatial variability of chironomid death assemblages in the surface sediments of a fluctuating tropical lake (Lake Naivasha,Kenya)

Studies addressing within-lake variability of fossil chironomid assemblages are very few, and all deal with hydrologically stable temperate lakes where the question of spatial integration mostly relates to the mixing of faunal assemblages associated with shallow, warm-water habitat and those associated with deeper, cold-water habitat. Here we study within-lake variability of surface-sediment chironomid assemblages in the fairly large (∼100–170 km² since 1983) and shallow (Z _max = 5–8 m) fluctuating tropical lake basin of Lake Naivasha, Kenya, and compare the patterns observed with those in two smaller adjacent basins, one similarly shallow (Lake Oloidien, 5.1–5.7 km2, 5–8 m), the other deep and stratified (Crescent Island Crater, 1.9 km², 14–17 m). Chironomid assemblages were analysed in core-top samples and surface sediments along inshore to offshore transects, and how well individual samples represented the total (basin-wide mean) subfossil assemblage was considered both in terms of taxon richness and taxon percent composition. Within-lake variability of subfossil chironomid concentrations (with generally higher absolute values in nearshore samples) could be explained by effects of sediment winnowing and focusing, whereas between-lake variability reflected their relative susceptibility to wind-driven sediment disturbance or bottom anoxia. In all study lakes, but most significantly in lakes Naivasha and Oloidien, species distribution in the subfossil chironomid assemblages showed a strong nearshore to offshore gradient, which in these shallow lakes, reflects the dominant control of substrate and food quality on species distribution in the living community. Particularly in the larger basins, nearshore samples better represented the total lake assemblage than offshore samples, because the former always contained a component of mud-dwelling species whereas the latter often lacked a component of macrophyte-dwelling species. Our results show that although sedimentation dynamics in the shallow, wind-stressed Lake Naivasha is dominated by frequent resuspension and random sediment redistribution, the near- to offshore gradient in chironomid habitat remains imprinted on subfossil assemblages. We conclude that also in shallow fluctuating lakes, given sufficient size, incomplete pre-burial spatial integration of habitat-specific chironomid assemblages can be exploited for within-lake calibration of environmental gradients.     

Paleolimnological evidence of Holocene climatic changes in Lake Blanca, southern Uruguay

Paleolimnological data are presented on Holocene climatic changes in Lake Blanca, a small (0.6 km²), coastal fresh waterbody in southern Uruguay. Using a sediment core that extended to 7,300 year BP, analyses of pooid, panicoid and chloridoid phytoliths (Poaceae) allowed us to identify 16 Opal Phytoliths Association Zones (OPAZ). Both temperature and humidity indices were calculated from relative percentages of opal phytoliths and the climatic trends were inferred. During the early Holocene, prior to the first Holocene marine transgression (∼ ∼8,000–7,000 yr BP), cool-arid climatic conditions were inferred. After the first Holocene transgression, predominance of cool-humid conditions was registered until about 4,000 yr BP. A transition from cool-semiarid/arid to alternate warm-temperate and humid-very-humid conditions was detected for ∼∼4,000–1,000 yr BP. Increases in trophic state were observed because of such a climatic amelioration. After ∼ ∼1,000 yr BP, a shift from warm to temperate and cool temperatures was inferred. Such a cooling episode was assigned to a “LIA” which took place by the end of the 18th century and the beginning of the 19th century. A decrease in trophic state was observed during this climatic deterioration. After such a little ice age, present temperate humid conditions were established. The humidity indices calculated from relative percentages of opal phytoliths for the last ∼ ∼70 years were compared to annual rainfall data. Such a comparison showed that opal phytoliths are reliable indicators of humidity.     

Fourier transform infrared spectroscopy,a new method for rapid determination of total organic and inorganic carbon and biogenic silica concentration in lake sediments

We demonstrate the use of Fourier transform infrared spectroscopy (FTIRS) to make quantitative measures of total organic carbon (TOC), total inorganic carbon (TIC) and biogenic silica (BSi) concentrations in sediment. FTIRS is a fast and cost-effective technique and only small sediment samples are needed (0.01 g). Statistically significant models were developed using sediment samples from northern Sweden and were applied to sediment records from Sweden, northeast Siberia and Macedonia. The correlation between FTIRS-inferred values and amounts of biogeochemical constituents assessed conventionally varied between r = 0.84–0.99 for TOC, r = 0.85–0.99 for TIC, and r = 0.68–0.94 for BSi. Because FTIR spectra contain information on a large number of both inorganic and organic components, there is great potential for FTIRS to become an important tool in paleolimnology.     

Records of environmental and climatic changes during the late Holocene from Svalbard: palaeolimnology of Kongressvatnet

A multi-core, multidisciplinary palaeolimnological study of the partially varved sediment of a deep, meromictic, arctic lake, Kongressvatnet (Svalbard, Western Spitsbergen), provides a record of environmental and climatic changes during last ca. 1800 years. The chronology of sedimentation was established using several dating techniques (¹³⁷Cs, ²¹⁰Pb, varve counts, palaeomagnetic correlation). A multiproxy record of palaeolimnological variability was compiled based on sedimentation rates, magnetic properties, varve thickness, organic matter, geochemistry, pigments from algal and photosynthetic bacteria, mineralogy and biological assemblages (diatoms, Cladocera). The major features recognised in our master core K99-3 include a shift in sediment source and supply (magnetic measurements, geochemistry) probably caused by glaciological changes in the catchment around 38–32 cm core depth (AD 700–820). Additional environmental changes are inferred at 20–18, 8–4.5 and 3–2 cm (AD ca. 1160–1255; 1715–1880; 1940–1963, respectively). During the past ca. 120 years a prominent sedimentological change from brownish-grey, partly laminated silt-clay (varves) to black organic-rich deposits was observed. From AD 1350 to AD1880 the sediment is comprised of a continuous sequence of varves, whereas the earlier sediments are mostly homogeneous with only a few short intercalated laminated sections between AD 860 and 1350. Sedimentation and accumulation rates increased during the last 30 years (modern warming). Pigment concentrations are very low in the lower ca. 32 cm of the core (AD 820) probably because of the high turbidity high energy environment. The high sulphur content in the uppermost 32 cm of sediment has given rise to two horizontally stratified populations of sulphur anaerobic photosynthetic bacteria, as inferred from their specific carotenoids. These bacteria populations are much more abundant during the Little Ice Age (LIA) than during warmer periods (e.g., during the Medieval Warm Period and 20th century). Diatoms are lacking from the core base up to 18 cm (ca. AD 1255); at this level, species indicative of mesotrophic water are present, whereas from 17 cm to the top of the core, oligotrophic taxa such as Staurosira construens/S. pinnata complex dominate, indicating extended ice coverage and more oligotrophic waters during the LIA. The concentration of Cladocera subfossil remains (dominated by Chydorus) are relatively high in the deepest sections (54–32 cm), whereas the upper 32 cm are characterized by a very low concentration of remains, possibly because of the strongly anoxic conditions, and in this upper sediment section rotifer resting eggs become prevalent. We interpret these changes as responses to climate forcing through its impact on glacial melt water, lake ice cover duration and mainly redox conditions in deep water. The observed changes suggest that at least some of our recorded changes may parallel the Greenland Ice core, although our study added more details about the inferred climatic changes. Further aspects are discussed, such as catchment processes, glacial activity, duration of the Medieval Warm Period, the Little Ice Age, local human activity, and limnology.