Primary production in Lake La Cruz (Spain) over the last four centuries: reconstruction based on sedimentary signal of photosynthetic pigments

We analysed photosynthetic pigments in annually laminated sediment of meromictic Lake La Cruz, Spain, to cope with the timing and characterisation of primary productivity changes over the last four centuries. The photosynthetic pigments identified included chlorophyll a and b (and its derivatives) and specific carotenoids of different algal groups, such as zeaxanthin, lutein, alloxanthin, diadinoxanthin and diatoxanthin among others. Marker pigments of phototrophic sulfur bacteria were also observed, including bacteriochlorophyll a derivatives, homologue series of bacteriophaeophytins d and bacterial carotenoids okenone and chlorobactene. We investigated the diagenetic processes of pigment alteration in anoxic sediments and the possible implications for paleoproductivity reconstruction and interpretation. The lack of systematic down-core changes in diagenetic indicators suggests that variability of sedimentary pigment concentrations is the result of changes in lake productivity. The lower concentration of algal photosynthetic pigments in the bottom of sediment sequence corresponds to the onset of meromictic conditions. Before that, sediment was not continuously anoxic and the preservation of pigments was reduced. Regarding photosynthetic bacteria, green sulfur bacteria derivatives indicate that population growth was limited until the settled organic matter was sufficient to provide enough sulphide. Moreover, the presence of bacterioviridine, an oxidised derivative of bacteriochlorophyll a, suggest the competitive relation among photosynthetic bacteria. The high relative abundance of zeaxanthin indicates the dominance of picocyanobacteria in the primary productivity over the last centuries. Fluctuations of the pigment signal observed in the sediment sequence during the meromictic period were well correlated with fluctuations of solar activity.     

Analysis of fossil pigments from algae and bacteria in meromictic Lake Fidler, Tasmania, and its application to lake management

Lake Fidler is an ectogenic meromictic lake with a monimolimnion maintained by periodic incursions of brackish water from the lower Gordon River estuary. A dam across the middle reaches of the Gordon River has restricted these incursions of brackish water and meromictic stability has rapidly declined. A palaeolimnological study was carried in order to assess the historical development of meromixis and the impact of the dam on the microbiological communities in the lake. Fossil pigments in a 17 m sediment core were analysed using reverse phase high performance liquid chromatography (rp-HPLC) and mass spectrometry (MS). In addition, taphonomic studies of pigment production, deposition and degradation in the water column and surface sediments were used to identify planktonic and benthic pigment degradation processes and constrain the stratigraphic interpretation. Results comparing the pigment composition of pelagic sediment traps and littoral surface sediments indicated that the core from the centre of the lake would permit a historical reconstruction of planktonic bacterial and algal communities. Marked increases in prokaryotic pigments ca 3500 yr B.P. suggested the possible colonisation of a chemocline by phototrophic bacteria. Further changes in chlorophyll: carotenoid ratios and changes in relative abundances of both chlorophyll a and bacteriochlorophyll c derivatives also indicated that a change in the depositional environment had occurred; possibly due to altered stratification or anoxia. From this we infer the onset of either intermittent or permanent meromixis. Further increases in prokaryotic pigment abundance suggested that the present state of permanent meromixis was firmly established by 2070 ±50 ¹⁴C yr B.P., and diatom analysis confirmed the development of a stable mixolimnion. High resolution studies of the top 10 cm of sediments measured pigments in mean concentrations of 15.1 ng g^-1 with a mean S.D. of only 2.78 indicating little change in pigment abundance since the construction of the dam. Thus, Lake Fidler still retains most of the features of meromixis. However, evidence from nearby Lake Morrison and Sulphide Pool has shown that any further declines in meromictic stability will cause a rapid reversion to holomixis. Palaeolimnological evidence from the early stages of meromictic development of Lake Fidler suggests that such reversion to holomixis may not permanently eliminate all the microbiological communities, and that, given time, they may return and prosper with re-establishment of a suitable chemocline. These studies will guide recommendations for a management strategy to prevent the further decay of meromixis in the Gordon River lakes.     

Sedimentation Patterns of Photosynthetic Bacteria Based on Pigment Markers in Meromictic Lake La Cruz (Spain): Paleolimnological Implications

Sedimentation fluxes of photosynthetic pigments and their derivatives, focused on the anoxygenic bacteria populations, were determined in meromictic Lake La Cruz over a three-year period using sediment traps. Bacterial settling occurs in pulses in response to lake dynamics. Two main annual events affect particle settling in Lake La Cruz: (1) Following an event of massive calcium precipitation in summer that caused water whiting, a maximum flux of chlorophyll a and derivatives towards the sediment was registered. Although whiting is accompanied by massive phytoplankton biomass sinking, it was not followed by concomitant photosynthetic bacterial sinking. (2) In contrast, the disruption of the chemocline and the contact of bacterial populations with oxygenated waters during autumn overturn provoked a destruction of the sulfur bacterial plate, causing mass mortality, registered in the settling material as photosynthetic pigments and their derivatives from phototrophic bacteria. Among the bacteriochlorophyll a oxidation products, bacterioviridin (bvir) was detected in high relative abundance in the settled material collected by sediment traps. The much increased flux of bvir after thermal overturn, associated with mass mortality of the purple sulfur bacterium, Amoebobacter purpureus, confirms the suitability of this molecule as a useful paleoindicator of the degree of oxygen exposure experienced by the purple sulfur bacteria population.     

Chemical stratigraphy of recent sediments from a depth gradient in a meromictic lake, Nordbytjernet, SE Norway, in relation to variable external loading and sedimentary fluxes

Cores of recent sediments were sampled along a depth gradient in a 23 m deep kettle lake with stagnant deep waters containing exceptionally high concentrations of dissolved iron and manganese. Sediment cores were taken on two occasions, in 1978 and 1997, before and after an incidence of full circulation. The aims of this study are to see how oxic and anoxic conditions in the water column influence stratigraphy and sediment focusing and, to compare cores from 1979 and 1998 to see how measured element fluxes and external events are reflected in the chemical stratigraphy. Element analyses show characteristic stratigraphic patterns that depend on the ability to undergo redox transformations, sorptive properties and chemical equilibria in the anoxic deep waters and porewaters. In sediments from the oxic part of the lake Al, Cr, Co, Ni, Zn Cu, Cd, and Pb were well correlated. Positive correlations are seen between elements associated with primary production and sulphur. In the anoxic part of the lake most metals were positively correlated with carbonate. Phosphorus correlated positively with iron in sediments from oxic waters and negatively with manganese and iron deep-water sediments. Porewater analyses indicate that recycling from the deep-water sediments was negligible. The stratigraphy of lead agrees with the historic variation in atmospheric input and is used as a chronological marker. Assessed deposition rates agree with measurements in sediment traps. Most elements more than double their rates of deposition towards the deepest point of the lake, while sulphur, manganese and carbonate had maxima around the depth of the redoxcline in the water. Variations in the external loading and variable redox conditions in the deep waters explain variations in the chemical composition of recent sediments.     

Development and application of sedimentary pigments for assessing effects of climatic and environmental changes on subarctic lakes in northern Sweden

A surface-sediment survey of pigments in 100 lakes in the Scandes Mountains, northern Sweden, was combined with a reconstruction of Holocene sedimentary pigments from Lake Seukokjaure to assess the major factors regulating phototrophic communities, and how these controls may have changed during the period from the deglaciation (~9700 cal. years BP) to the present. The study area covers a pronounced gradient of temperature and precipitation, and encompasses the subarctic tree line, an important ecotonal boundary in this region. Lake Seukokjaure is located in a presently treeless basin close to the modern tree line. The spatial survey of sedimentary pigments was analyzed using principle components analysis (PCA) and redundancy analysis (RDA). PCA explained 73–83% of variance in pigment abundance and composition, whereas RDA explained 22–32% of variation in fossil assemblages. Dissolved organic carbon (DOC) content of lake water, sediment δ¹³C, maximum lake depth, elevation and lake-water conductivity were all identified as environmental variables with significant association with pigment abundances in the spatial survey, although phototrophic communities of lakes situated in different vegetation zones (alpine, birch, conifer/birch) were incompletely distinguished by the ordinations. In the RDAs, the primary pigment variability occurred along a production gradient that was correlated negatively to water-column DOC content and δ¹³C signature of sediments. This pattern suggested that the important controls of primary production were light regime and terrestrial supplies of ¹³C-depleted carbon. In contrast, depth, elevation and conductivity were found to be more important for the differentiation of the phototrophic community composition. Application of these spatial survey results to the Holocene sediment record of Lake Seukokjaure demonstrated the importance of DOC for the temporal development of the lake, from an early state of high production to a period of slight oligotrophication. In general, the algal changes were regulated by the interaction of DOC and conductivity, although transitions in the phototrophic community during the late Holocene were less easily interpreted. Terrestrial vegetation development thus appears to be of utmost importance for the regulation of primary production in oligotrophic alpine and subarctic lakes and climate impacts on lakes, whereas other basin-specific factors may control the ontogeny of algal community composition.     

Multi-proxy Reconstruction of Trophic State,Hypolimnetic Anoxia and Phototrophic Sulphur Bacteria Abundance in a Dimictic Lake in Northern Germany over the past 80 Years

During monthly investigations from 1996 to 2000, a hypolimnetic layer of phototrophic sulphur bacteria (Chromatium spp.) were observed in Lake Dudinghausen, a small dimictic lake in northern Germany. This paleolimnological study was initiated to detect if the occurrence of sulphur bacteria was related to cultural eutrophication or reflected natural conditions. Therefore, diatoms, algal pigments, okenone, geochemical proxies, and ²¹⁰Pb and ¹³⁷Cs were used in four sediment cores to investigate historical changes in trophic development, hypolimnetic redox conditions, anoxia and phototrophic sulphur bacteria abundances. Fossil diatoms, pigments, the ratio of chlorophyll derivatives to total carotenoids and the ratio of chlorophyll a to its derivatives suggest two phases of eutrophication coupled with hypolimnetic anoxia over the last ~80 years: a first phase from about 1923–1932 and a second from 1952 to 1982. In the first phase the ratios of Fe–Mn as well as Fe–Ca increased, suggesting seasonal anoxia. However, hypolimnetic anoxia was only weak because low levels of okenone suggest no mass development of sulphur bacteria. In contrast, sulphur bacteria increased during the early stages of the second eutrophication phase, suggesting increased temporal and spatial hypolimnetic anoxia. Surprisingly, the ratios of Fe–Mn as well as Fe–Ca decreased during this time. Possibly Fe, Mn and Ca were equally reduced through the intense anoxia. In the final stage, sulphur bacteria decreased again. As these bacteria need both anoxic conditions and a certain amount of light, the increased nutrient load probably led to low Secchi depth and therefore insufficient light conditions. In more recent years, diatoms and pigments suggest a decrease in nutrient levels. A second mass development of sulphur bacteria occurred, probably due to improved light conditions and continued anoxia in the upper hypolimnion. We conclude that the recent development of phototrophic sulphur bacteria do not represent natural conditions in Lake Dudinghausen. Furthermore, the upper sediments contain a completely new diatom flora that never occurred in older sediments of Lake Dudinghausen. Therefore, nutrient levels may eventually reach natural conditions, however they may not represent biological background reference conditions.     

Recent evolution of Lake Arreo,northern Spain: influences of land use change and climate

We present a high-resolution, multiproxy reconstruction of the depositional history of Lake Arreo, northern Spain, for the last 60 years. We conducted sedimentological, geochemical and diatom analyses in short cores and made a detailed comparison with regional instrumental climate data (1952–2007), limnological monitoring of the lake (1992–2008) and recent land use changes that affect the lake catchment. Chronology is based on “floating” discontinuous varve counts and ¹³⁷Cs and ¹⁴C dates. Four periods were identified in the Lake Arreo recent history: (1) prior to 1963, varved facies intercalated with fine turbidite deposits, and diatom assemblages dominated by Cyclotella taxa indicate predominantly meromictic conditions, (2) from 1964 to 1978, permanent anoxia persisted in bottom waters, as shown by similar facies and diatom assemblages as before, though detrital layers were coarser, (3) from 1979 to 1994, sediment delivery to the lake increased and laminated, clastic facies were deposited, and (4) from 1995 to 2008, dominance of massive facies and an increase in Fragilaria tenera and Achnanthes minutissima reflect relatively lower lake levels, less frequent bottom anoxia with more frequent water column mixing, similar to modern conditions. The period 1952–1979 was a time of meromixis and varved facies deposition, and was characterized by higher rainfall and less intense agricultural pressure in the watershed. There were two short humid periods (1992–1993 and 1996–1998) when monitoring data show more anoxic weeks per year and relatively higher lake levels. Increased cultivation of small landholdings in 1963, and particularly after 1979, caused a large increase in sediment delivery to the lake. The inferred lake evolution is in agreement with monitoring data that suggest a transition from dominantly meromictic conditions prior to 1993–1994 to a predominantly monomictic pattern of circulation since then, particularly after 2000. The synergistic effects of intensive water extraction for irrigation and lower rainfall since 1979, and particularly since 1994, brought the long period of meromictic conditions in Lake Arreo to an end. Water balance and sediment delivery to the lake are dominant factors that control the limnological and mixing conditions in Lake Arreo and they must be considered in management and restoration plans.     

Artemia parthenogenetica in Lake Hayward, Western Australia. II. Feeding biology in a shallow, seasonally stratified, hypersaline lake

Primary production in Lake Hayward, Western Australia, is dominated by benthic microbial communities, with limited planktonic primary production. This study investigated the question of howArtemia, commonly regarded as simple, obligate, non-selective filter feeders, were able to survive in this system. Bacteria (heterotrophic and autotrophic, filamentous and unicellular) were the major components in the diet of theArtemia in Lake Hayward. These bacteria were derived from bacterial aggregates in the water column and also from benthic mat material (both still attached to the substrate and from pieces floating in the water column). Benthic diatoms were a substantial dietary component of animals living in the unstratified shallow regions. Photosynthetic eukaryotic nanoplankton comprised a minor component of the diet of thisArtemia population. Gut contents of a large number of animals and the results of a simple laboratory test indicated that these animals utilise substrate-bound food resources. The results of the present study raises the question of the ecological significance of surface grazing by brine shrimps in other shallow, benthos dominated saline systems.     

The palaeolimnology of Lake Fidler, a meromictic lake in south-west Tasmania and the significance of recent human impact

Three meromictic lakes in the World Heritage Area of south-west Tasmania possess unusual microbiological communities. Their meromixis is maintained by periodic incursions of brackish water from the nearby Gordon River which, in its lower reaches, is a salt-wedge estuary. In 1977 the construction of a dam in the middle reaches of the river restricted penetration of the salt-wedge and meromixis rapidly declined in all three lakes. A palaeolimnological study was carried out on one of the lakes, Lake Fidler, firstly to determine the history of meromixis and its associated microbiological communities, and secondly to assess whether the recent and rapid decline of meromixis is inconsistent with natural rates of development of the Gordon River meromictic lakes. One part of this study included the analysis of the stratigraphy of fossil diatoms from a 17-metre sediment core dating back 8000 yrs. Detrended Correspondence Analysis and Analog Matching were used to compare diatom species assemblages in core samples with diatom samples from a reference dataset consisting of a selection of lake and river sites in the lower Gordon River valley. Five distinct stratigraphic zones were identified in the core. These zones indicated specific stages in the development of the Gordon River lakes from river backwaters to ectogenically-maintained meromictic lakes which will, finally, become terrestrialised by encroaching rainforest. The onset of a stratified water column was identified by the emergence of a dominant freshwater algal flora which suggested that the lake had developed a mixolimnion and become meromictic ca. 2070 ± 50 ¹⁴C yrs ago. In the context of this long history of meromixis, the rapid demise in meromictic stability following construction of the dam is judged to be inconsistent with natural rates of development. The palaeolimnological studies, of which this paper is one part, prompt recommendations for a management strategy to prevent the further decay of these meromictic lakes in the World Heritage Area.     

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

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).     

Phototrophic activity and redox condition in Lake Hamana, Japan, indicated by sedimentary photosynthetic pigments and molybdenum over the last ∼250 years

We analyzed photosynthetic pigments, total organic carbon (TOC), biogenic silica, and Mo, a redox-sensitive element, in ²¹⁰Pb-dated sediment cores to reconstruct the historical changes in primary productivity and anoxia in the central basin of Lake Hamana, a brackish lake in Shizuoka Prefecture, Japan, over the last 250 years. The algal photosynthetic pigments we analyzed included chlorophyll a (and its derivatives), chlorophyll b (and its derivatives), and carotenoids such as -carotene, lutein, zeaxanthin, diatoxanthin, fucoxanthin, alloxanthin, and -carotene. Marker pigments for phototrophic sulfur bacteria were also recorded, including okenone and bacteriopheophytin a, originating from Chromatium (a genus of purple sulfur bacteria), and isorenieratene and bacteriochlorophylls e ₁, e ₂ and e ₃ (and corresponding bacteriopheophytins) from brown Chlorobium (a brown-colored group of green sulfur bacteria). The occurrence of these pigments throughout the length of all cores indicates that the anoxia in Lake Hamana has existed over at least the last 250 years. The indicators related to primary productivity – TOC and pigments of aerobic and anaerobic phototrophs – and an indicator of anoxia, Mo, increased after 1860, indicating that productivity had increased in both the oxic and anoxic (sulfidic) zones. The depth profiles of the indicators in the sediment cores showed that among phototrophic sulfur bacteria, Chromatium preferentially increased relative to brown Chlorobium when the lake productivity was high, and hence high anoxia existed in the lake. This can be explained by a shallowing of the oxic/anoxic boundary zone due to changes in temporal and/or spatial extents of seasonal anoxia, which made the light intensity in the upper anoxic zone high enough for Chromatium to grow. The upper Chromatium layer may absorb the wavelengths of light that favor the growth of brown Chlorobium in the water column, resulting in a relative decrease in brown Chlorobium. During the 1950s, the trends among the indicators changed significantly. This change is attributed to the construction of training walls, built to direct tidal currents into the lake, on the Imagire-guchi Channel, the sole inlet of seawater to the lake, during 1954–1956, and the resultant increases of seawater intrusion and lake salinity. A decrease in okenone and bacteriopheophytin a, or in okenone/isorenieratene ratio, after 1960 accompanying a decrease in Mo, is attributed to a deepening of the anoxic zone, where the light intensity became too low for growth of Chromatium, more light-demanding than the brown Chlorobium. A decrease of zeaxanthin (cyanobacteria) after 1960 relative to lutein (green algae) and diatoxanthin (diatoms and dinoflagellates) indicated a change in algal assemblage, presumably due to the increased salinity. Principal component analysis with a data set of total algal carotenoids, okenone, isorenieratene, and Mo also suggested that a major change occurred around the 1950s.     

The formation of flocculated clay laminae in the sediments of a meromictic lake

Sediments of Lake Fidler, a meromictic lake in south-west Tasmania, contain distinctive laminae. In order to determine their composition and formation, these laminae were studied using a combination of X- ray analysis, scanning electron microscopy, X-ray diffraction and vibrational spectrometry. Results indicated that the laminae were composed of clay originating from the adjacent Gordon River estuary. The clay was also found as part of the general sediment matrix of the core. The evidence indicates that the laminae are formed during incursions of brackish water from the adjacent Gordon River estuary into Lake Fidler which cause the flocculation of clay minerals in the water column. These clay minerals then sink rapidly to the sediments to form laminae. Although the formation of these laminae pre-dates meromixis, their mode of formation corroborates the theory that meromixis is maintained ectogenically by periodic inflows of brackish water replenishing monimolimnetic salts.     

Sediment indicators of meromixis: comparison of laminations, diatoms, and sediment chemistry in Brownie Lake, Minneapolis, USA

Meromixis has several powerful effects on lakes, yet there is no single definitive sediment indicator of meromixis. In this study three sediment indicators of meromixis were compared in Brownie Lake, Minneapolis, Minnesota, a small eutrophic lake that became meromictic around 1925. The results show that in Brownie the onset of laminations and changes in the iron to manganese ratio most likely occurred before the development of permanently anoxic bottom water and that changes in the diatom assemblage occurred later, most likely only when meromixis was well developed.This is the 11th in a series of papers published in this special AMQUA issue. These papers were presented at the 1994 meeting of the American Quaternary Association held 19–22 June, 1994, at the University of Minnesota, Minneapolis, Minnesota, USA. Dr Linda C. K. Shane served as guest editor for these papers.     

Abundance,biomass and composition of spring ice algal and phytoplankton communities of the Laptev Sea(Arctic)

<正> Abundance,biomass and composition of the ice algal and phytoplank-ton communities were investigated in the southeastern Laptev Sea in spring 1999.Diatoms dominated the algal communities and pennate diatoms dominated the dia-tom population.12 dominant algal species occurred within sea ice and underlyingwater column,including Fragilariopsis oceanica,F.cylindrus,Nitzschiafrigida,N.promare,Achnanthes taeniata,Nitzschia neofrigida,Naviculapelagica,N.vanhoef fenii,N.septentrionalis,Melosira arctica,Clindrothecaclosterium and Pyrarnimonas sp.The algal abundance of bottom 10 cm sea icevaried between 14.6 and 1562.2×10~4 ceils l~(-1)with an average of 639.0×10~4cells l~(-1),and the algal biomass ranged from 7.89 to 2093.5μg C l~(-1)with an av-erage of 886.9μg C l~(-1),which were generally one order of magnitude higherthan those of sub-bottom ice and two orders of magnitude higher than those ofunderlying surface water.The integrated algal abundance and biomass of lower-most 20 cm ice column were averagely 7.7 and 12.2 times as those of upper 20 mwater column,respectively,suggesting that the ice algae might play an importantrole in maintaining the coastal marine ecosystem before the thawing of sea ice.Icealgae influenced the phytoplankton community of the underlying water column.However,the“seeding”of ice algae for phytoplankton bloom was negligible be-cause of the iow phytoplankton biomass within the underlying water column.     

Geochemical constraints on the Palaeocene–Miocene evolution of eastern Azerbaijan,with implications for the South Caspian basin and eastern Paratethys

Fine‐grained Palaeogene–early Neogene strata of the South Caspian basin, specifically the Oligocene–Lower Miocene Maikop Series, are responsible for the bulk of hydrocarbon generation in the region. Despite the magnitude of oil and gas currently attributed to the source interval offshore, geochemical evaluation of 376 outcrop samples from the northern edge of the Kura basin (onshore eastern Azerbaijan) indicates that depositional conditions in these proximal strata along the basin margins were dominantly oxic to mildly suboxic/anoxic throughout three major depositional stages: the Palaeocene–Eocene, Oligocene–early Middle Miocene and late Middle–Late Miocene. Palaeocene–Eocene samples have low average total organic carbon (TOC) values (0.3%), with higher total inorganic carbon (TIC) values (average=2.6%), extremely low sulphur content (0.2%) and relatively high detrital input as indicated by Fe/Al and Ti/Al ratios. C–S–Fe associations, along with relatively lower concentrations of redox‐sensitive trace elements (e.g. V, Ni, Mo, U) indicate dominantly oxic environments of deposition during much of the Palaeocene–Eocene. A pronounced geochemical shift occurred near the Eocene–Oligocene boundary, and continued through the Early Miocene. Specifically, this interval is characterized by a distinct increase in TOC (ranging from 0.1 to 6.3% with an average of 1.5%), C–S–Fe associations that reveal an abrupt relative increase of carbon and sulphur with respect to iron‐dominated Palaeocene–Eocene samples, and higher concentrations of redox‐sensitive trace metals. These changes suggest that a shift away from unrestricted marine conditions and towards more variable salinity conditions occurred coincident with the initial collision of the Arabian plate and partial closure of the Paratethys ocean. Despite periodic basin restriction, the majority of Upper Eocene–Lower Miocene strata in the northern Kura basin record oxic to slightly dysoxic conditions.     

A rock magnetic record from Lama Lake, Taymyr Peninsula, northern Central Siberia

An 11 m long sediment core ftorn Lama Lake, Northern Siberia, has been subjected to intense sedimentological, geochemical and rock magnetic analyses. According to a palynologic investigation the recovered sediments cover the whole Holocene and the late Pleistocene reaching back to about 17 ka. IRM acquisition experiments, hysteresis loop and back field as well as thermomagnetic measurements revealed magnetite in the pseudo-single domain range as the only remanence carrier. Sharp rock magnetic boundaries occur at 20 and 745 cm sub-bottom depth that are clearly linked to shifts in the median grain size of the magnetite. These boundaries are close to the present boundaries that bracket an anoxic zone between the subrecent and a late Pleistocene oxic section of the sediments. Within the anoxic section, magnetites are characterized by significantly larger median grain sizes but within a very narrow grain size range. The shift from fine grained magnetite within the oxic sediments to coarse grained magnetite is interpreted as the result of dissolution of the finest magnetite grains within the anoxic sediments. A significant shift of the Ti/Fe-ratio of the bulk sediment at a sub-bottom depth of 735 cm does not correspond to thermomagnetic properties, i.e. Curie-temperatures do not follow the variable Ti-content of the sediment.     

Limnological and Climatic Environments at Upper Klamath Lake, Oregon during the past 45 000 years

Upper Klamath Lake, in south-central Oregon, contains long sediment records with well-preserved diatoms and lithological variations that reflect climate-induced limnological changes. These sediment archives complement and extend high resolution terrestrial records along a north–south transect that includes areas influenced by the Aleutian Low and Subtropical High, which control both marine and continental climates in the western United States. The longest and oldest core collected in this study came from the southwest margin of the lake at Caledonia Marsh, and was dated by radiocarbon and tephrochronology to an age of about 45 ka. Paleolimnological interpretations of this core, based upon geochemical and diatom analyses, have been augmented by data from a short core collected from open water environments at nearby Howards Bay and from a 9-m core extending to 15 ka raised from the center of the northwestern part of Upper Klamath Lake. Pre- and full-glacial intervals of the Caledonia Marsh core are characterized and dominated by lithic detrital material. Planktic diatom taxa characteristic of cold-water habitats (Aulacoseira subarctica and A. islandica) alternate with warm-water planktic diatoms (A. ambigua) between 45 and 23 ka, documenting climate changes at millennial scales during oxygen isotope stage (OIS) 3. The full-glacial interval contains mostly cold-water planktic, benthic, and reworked Pliocene lacustrine diatoms (from the surrounding Yonna Formation) that document shallow water conditions in a cold, windy environment. After 15 ka, diatom productivity increased. Organic carbon and biogenic silica became significant sediment components and diatoms that live in the lake today, indicative of warm, eutrophic water, became prominent. Lake levels fell during the mid-Holocene and marsh environments extended over the core site. This interval is characterized by high levels of organic carbon from emergent aquatic vegetation (Scirpus) and by the Mazama ash (7.55 ka), generated by the eruption that created nearby Crater Lake. For a brief time the ash increased the salinity of Upper Klamath Lake. High concentrations of molybdenum, arsenic, and vanadium indicate that Caledonia Marsh was anoxic from about 7 to 5 ka. After the mid-Holocene, shallow, but open-water environments returned to the core site. The sediments became dominated (>80%) by biogenic silica. The open-water cores show analogous but less extreme limnological and climatic changes more typical of mid-lake environments. Millennial-scale lake and climate changes during OIS 3 at Upper Klamath Lake contrast with a similar record of variation at Owens Lake, about 750 km south. When Upper Klamath Lake experienced cold-climate episodes during OIS 3, Owens Lake had warm but wet episodes; the reverse occurred during warmer intervals at Upper Klamath Lake. Such climatic alternations apparently reflect the variable position and strength of the Aleutian Low during the mid-Wisconsin.     

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.     

The comparative limnology of high arctic, coastal, meromictic lakes

Five important features appear in rough order from the surface downwards in physical and chemical profiles from high arctic coastal meromictic lakes. These features are: (1) a supersaturated oxygen maximum, (2) the center of the oxycline, (3) a thermal maximum, (4) a major absorption maximum, and (5) an anoxic stratum. The depth of the absorption maxima and the top of the anoxic strata are both statistically correlated to light penetration and to each other. The depth of the thermal maximum also shows a statistical correlation to light penetration among lakes with a relatively shallow chemocline. The temperature of the thermal maximum appears to be maintained by inputs of light energy while the oxygen maximum is maintained to a large extent by photosynthesis. Thus, these major features are all influenced by light penetration. With the exception of the supersaturated oxygen maximum, all of the above features are dependent for their existence upon the primary chemocline stabilizing the water column. Apparently, in at least some lakes, a near surface secondary chemocline or cool (ca. 4°C) secondary inverse thermocline will enhance the stability of the water column above the primary chemocline sufficiently to allow a supersaturated oxygen maximum to develop in this region. However, the supersaturated oxygen maximum can extend into the primary chemocline, and in highly transparent Sophia Lake (Cornwallis Island, N.W.T.) this feature extends below the primary chemocline.Where the chemocline is found below depths with adequate illumination, features other than the supersaturated oxygen maximum should be found in deeper water as well, or they should be eliminated from the profiles. Thus, where the chemocline is relatively shallow, the depth of features like the thermal maximum or anoxic strata are related most closely to light penetration, but where chemoclines are deep, as in Lake Tuborg (Ellesmere Island, N.W.T.), the depth of the chemocline determines the depth of the oxycline, thermal maximum, absorption maximum and anoxic stratum.This is the second in a series of papers published in this issue on the Taconite Inlet Lakes Project. These papers were collected by Dr R. S. Bradley.