Lake Ecosystem Responses to Holocene Climate Change at the Subarctic Tree-Line in Northern Sweden

A Holocene sediment sequence from Lake Seukokjaure, a subarctic lake at tree-line in northern Sweden, was analyzed to assess major changes in the structure and functioning of the aquatic ecosystem in response to climate change and tree-line dynamics. The compiled multi-proxy data, including sedimentary pigments, diatoms, chironomids, pollen, biogenic silica (BSi), carbon (C), nitrogen (N) elemental and stable-isotope records, and total lake-water organic carbon (TOC) concentration inferred from near-infrared spectroscopy (NIRS), suggest that the Holocene development of Lake Seukokjaure was closely coupled to changes in terrestrial vegetation with associated soil development of the catchment, input of allochthonous organic carbon, and changes in the light regime of the lake. A relatively productive state just after deglaciation around 9700 to 7800 cal years BP was followed by a slight long-term decrease in primary production. The onset of the local tree-line retreat around 3200 cal years BP was accompanied by more diverse and altered chironomid and diatom assemblages and indications of destabilized soils in the catchment by an increase in variability and absolute values of δ¹³C. An abrupt drop in the C/N ratio around 1750 cal years BP was coupled to changes in the internal lake structure, in combination with changes in light and nutrient conditions, resulting in a shift in the phototrophic community from diatom dominance to increased influence of chlorophytes, likely dominated by an aquatic moss community. Thus, this study emphasizes the importance of indirect effects of climate change on tree-line lake ecosystems and complex interactions of in-lake processes during the Holocene.     

Assessing the response of forest productivity to climate extremes in Switzerland using model–data fusion

The response of forest productivity to climate extremes strongly depends on ambient environmental and site conditions. To better understand these relationships at a regional scale, we used nearly 800 observation years from 271 permanent long‐term forest monitoring plots across Switzerland, obtained between 1980 and 2017. We assimilated these data into the 3‐PG forest ecosystem model using Bayesian inference, reducing the bias of model predictions from 14% to 5% for forest stem carbon stocks and from 45% to 9% for stem carbon stock changes. We then estimated the productivity of forests dominated by Picea abies and Fagus sylvatica for the period of 1960–2018, and tested for productivity shifts in response to climate along elevational gradient and in extreme years. Simulated net primary productivity (NPP) decreased with elevation (2.86 ± 0.006 Mg C ha^?1 year^?1 km^?1 for P. abies and 0.93 ± 0.010 Mg C ha^?1 year^?1 km^?1 for F. sylvatica). During warm–dry extremes, simulated NPP for both species increased at higher and decreased at lower elevations, with reductions in NPP of more than 25% for up to 21% of the potential species distribution range in Switzerland. Reduced plant water availability had a stronger effect on NPP than temperature during warm‐dry extremes. Importantly, cold–dry extremes had negative impacts on regional forest NPP comparable to warm–dry extremes. Overall, our calibrated model suggests that the response of forest productivity to climate extremes is more complex than simple shift toward higher elevation. Such robust estimates of NPP are key for increasing our understanding of forests ecosystems carbon dynamics under climate extremes.     

A multi-proxy Late-glacial palaeoenvironmental record from Lake Bled, Slovenia

This study investigates the palaeoecological record (δ¹⁸O, δ¹³C, pollen, plant macrofossils, chironomids and cladocera) at Lake Bled (Slovenia) sedimentary core to better understand the response of terrestrial and aquatic ecosystems to Late-glacial climatic fluctuations. The multi-proxy record suggests that in the Oldest Dryas, the landscape around Lake Bled was rather open, presumably because of the cold and dry climate, with a trend towards wetter conditions, as suggested by an increase in tree pollen as well as chironomid and cladocera faunas typical for well-oxygenated water. Climatic warming at the beginning of the Late-glacial Interstadial at ca. 14,800 cal yr BP is suggested by an increase in the δ¹⁸O value, the appearance of Betula and Larix pollen and macrofossils, and a warmth-adapted chironomid fauna. With further warming at ca. 13,800 cal yr BP, broad-leaved tree taxa (Quercus, Tilia, Ulmus), Artemisia, and Picea increase, whereas chironomid data (Cricotopus B) suggest lowering of lake levels. After 12,800 cal yr BP (and throughout the Younger Dryas), the climate was colder and drier, as indicated by lower δ¹⁸O values, decline of trees, increase of microscopic charcoal, xerophytes and littoral chironomids. A warmer climate, together with the spread of broad-leaved tree taxa and a deeper, more productive lake, mark the onset of the Late-glacial/Holocene transition. These results suggest that terrestrial and aquatic ecosystems at Lake Bled were very dynamic and sensitive to Late-glacial climatic fluctuations. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorised users. Guest editors: K. Buczkó, J. Korponai, J. Padisák & S. W. Starratt Palaeolimnological Proxies as Tools of Environmental Reconstruction in Fresh Water     

Ostracod-based environmental reconstruction over the last 8,400 years of Nam Co Lake on the Tibetan plateau

From a 332-cm long lacustrine core taken at 60 m water depth and 25 surface sediment samples taken at different water depth sites in Nam Co lake (4,718 m a.s.l.) in the middle-south part of the Tibetan Plateau, we identified nine species of ostracods (Crustacea: Ostracoda) belonging to six genera. Using lithological data, auto-ecological information of the recovered taxa and an ostracod-based transfer function for water depth reconstruction, we distinguished three main environmental stages over the past 8,400 years: during Stage I (8,400–6,800 yr BP), the climate changed from warm-humid to cold-humid, and eventually to cold-arid. The water depth of the site was much lower than today and changed from an estimated 50 to 20–30 m. During Stage II (6,800–2,900 yr BP), environmental conditions were again warm-humid, turning into a cold-arid episode. The lake water depth initially stayed much shallower than today, but then gradually deepened to around 50 m. At the earlier period of Stage III (2,900 yr BP–present), the climate became again warm-humid from cold-arid status. There was a cold-dry event between 1,700 and 1,500 yr BP, which intensified afterward while the surface run off weakened. Early in this stage, lake depth decreased slightly, but then it continuously deepened to 60 m. Our results revealed that central Tibet experienced wavily warm toward tendency in early Holocene, a shift from warm-humid to cold-dry conditions in the middle Holocene, and from warm-humid to cold-dry conditions in the late Holocene. They also show that ostracod assemblages are not only indicative of cold-warm conditions, but are also usable to imply the dry–wet status of a lake area by the inferring water depth variations. Finally, this study provides baseline data on (natural) climate change in this mountain region against which to compare global change impacts.     

Gap regeneration and replacement patterns in an old-growth <Emphasis Type="Italic">Fagus</Emphasis>–<Emphasis Type="Italic">Abies</Emphasis> forest of Bosnia–Herzegovina

We examined the influence of small-scale gap disturbances on stand development and tree species coexistence in an old-growth Fagus sylvatica–Abies alba forest in the Dinaric Mountains of Bosnia–Herzegovina. The structure and composition of tree regeneration in gaps were compared to the forest as a whole, and the influence of gap size on the density and composition of regeneration was assessed. Transition probabilities were also calculated from gapfillers in different life stages to examine canopy replacement patterns. The structure and composition of tree regeneration were similar between gaps and the forest as a whole, and there was no relationship between overall regeneration density and gap size, indicating most individuals established prior to gap formation. Likewise, there was no strong evidence of gap-size partitioning for shade tolerant F. sylvatica and A. alba, although less tolerant Acer pseudoplatanus only recruited to taller life stages in larger gaps. Transition probabilities calculated from the seedling and sapling data suggest that most gaps will be captured by F. sylvatica, while probabilities based on pole-sized gapmakers indicate both A. alba and F. sylvatica will be maintained in the canopy. We suggest that gaps primarily play a role in reorganizing advance regeneration, and that coexistence of shade tolerant F. sylvatica and A. alba is more likely related to their differential ability to tolerate shaded understory conditions, particularly during larger life stages, rather than gap-size partitioning.     

Experimental shading alters leaf litter breakdown in streams of contrasting riparian canopy cover

1. Headwater stream ecosystems are primarily heterotrophic, with allochthonous organic matter being the dominant energy. However, sunlight indirectly influences ecosystem structure and functioning, affecting microbial and invertebrate consumers and, ultimately, leaf litter breakdown. We tested the effects of artificial shading on litter breakdown rates in an open‐canopy stream (high ambient light) and a closed‐canopy stream (low ambient light). We further examined the responses of invertebrate shredders and aquatic hyphomycetes to shading to disentangle the underlying effects of light availability on litter breakdown. 2. Litter breakdown was substantially slower for both fast‐decomposing (alder, Alnus glutinosa) and slow‐decomposing (beech, Fagus sylvatica) leaf litters in artificially shaded stream reaches relative to control (no artificial shading) reaches, regardless of stream type (open or closed canopy). 3. Shredder densities were higher on A. glutinosa than on F. sylvatica litter, and shading had a greater effect on reducing shredder densities associated with A. glutinosa than those associated with F. sylvatica litter in both stream types. Fungal biomass was also negatively affected by shading. Results suggest that the effects of light availability on litter breakdown rates are mediated by resource quality and consumer density. 4. Results from feeding experiments, where A. glutinosa litter incubated under ambient light or artificial shade was offered to the shredder Gammarus fossarum, suggest that experimental shading and riparian canopy openness influenced litter palatability interactively. Rates of litter consumption by G. fossarum were decreased by experimental shading in the open‐canopy stream only. 5. The results suggest that even small variations in light availability in streams can mediate substantial within‐stream heterogeneity in litter breakdown. This study provides further evidence that changes in riparian vegetation, and thus light availability, influence organic matter processing in heterotrophic stream ecosystems through multiple trophic levels.     

Rapid sympatric ecological differentiation of crater lake cichlid fishes within historic times

Background  

After a volcano erupts, a lake may form in the cooled crater and become an isolated aquatic ecosystem. This makes fishes in crater lakes informative for understanding sympatric evolution and ecological diversification in barren environments. From a geological and limnological perspective, such research offers insight about the process of crater lake ecosystem establishment and speciation. In the present study we use genetic and coalescence approaches to infer the colonization history of Midas cichlid fishes (Amphilophus cf. citrinellus) that inhabit a very young crater lake in Nicaragua-the ca. 1800 year-old Lake Apoyeque. This lake holds two sympatric, endemic morphs of Midas cichlid: one with large, hypertrophied lips (~20% of the total population) and another with thin lips. Here we test the associated ecological, morphological and genetic diversification of these two morphs and their potential to represent incipient speciation.     

A multi-proxy reconstruction and comparison of Holocene palaeoenvironmental changes in the Alvor and Alcantarilha estuaries (southern Portugal)

The present study investigates the history of two estuaries on the Algarve coast, southern Portugal, through a multi-proxy approach that links sedimentology, geochemistry, palynology, microfaunal analyses and radiocarbon dating. These analyses provide an overview of the development of these estuaries over the last ～7500 years. Palynological data reveal climate-driven vegetational shifts over the whole period. Microfauna, which is composed predominantly of foraminifers and ostracods, provides evidence for periods of marine and brackish water conditions, observed in both estuaries. Whereas the sediment record from the Alvor estuary shows a clear, nearly continuous silting-up sequence, the development of the Alcantarilha estuary is characterised by fluvial sedimentation, replaced by a marine environment at about 7300 yr cal BP. Marine transgression is indicated by marine-brackish foraminifera and ostracod assemblages, while upper-marsh pollen decreased and wetland communities increased at the same time. A continuous shallowing of the estuaries followed between ～6700 yr cal BP and ～5700 yr cal BP in Alvor and 4500 yr cal BP in Alcantarilha. This was interrupted by a distinct high-energy event, possibly a storm or tsunami, between 6400 and 5800 yr cal BP in Alvor; the event was not recorded in the Alcantarilha estuary. Afterwards, the silting-up restarted with clear similarities to the pre-event situation and continued until ～3500/2000 yr cal BP. This process was influenced by the build-up of sand barriers in front of the estuaries. This is also documented by a higher percentage of upper-marsh vegetation. The core from Alcantarilha estuary shows another layer of high-energy deposits, with mainly marine foraminifers and ostracods after 4500 yr cal BP. The youngest parts of the two records are characterised by decreasing tidal influence and increasing anthropogenic impact, indicated by increasing values of cultivated plants and other human-induced changes of vegetation cover.     

Genomic signatures of divergent selection and speciation patterns in a ‘natural experiment’, the young parallel radiations of Nicaraguan crater lake cichlid fishes

Divergent selection is the main driving force in sympatric ecological speciation and may also play a strong role in divergence between allopatric populations. Characterizing the genome‐wide impact of divergent selection often constitutes a first step in unravelling the genetic bases underlying adaptation and ecological speciation. The Midas cichlid fish (Amphilophus citrinellus) species complex in Nicaragua is a powerful system for studying evolutionary processes. Independent colonizations of isolated young crater lakes by Midas cichlid populations from the older and great lakes of Nicaragua resulted in the repeated evolution of adaptive radiations by intralacustrine sympatric speciation. In this study we performed genome scans on two repeated radiations of crater lake species and their great lake source populations (1030 polymorphic AFLPs, n ～ 30 individuals per species). We detected regions under divergent selection (0.3% in the crater lake Xiloá flock and 1.7% in the older crater lake Apoyo radiation) that might be responsible for the sympatric diversifications. We find no evidence that the same genomic regions have been involved in the repeated evolution of parallel adaptations across crater lake flocks. However, there is some genetic parallelism apparent (seven out of 51 crater lake to great lake outlier loci are shared; 13.7%) that is associated with the allopatric divergence of both crater lake flocks. Interestingly, our results suggest that the number of outlier loci involved in sympatric and allopatric divergence increases over time. A phylogeny based on the AFLP data clearly supports the monophyly of both crater lake species flocks and indicates a parallel branching order with a primary split along the limnetic‐benthic axis in both radiations.     

A multiproxy approach to inferring Holocene paleobotanical changes linked to sea-level variation,paleosalinity levels,and shallow lake alternative states in Negra Lagoon,SE Uruguay

A multiproxy analysis of diatoms, chrysophyte cysts, opal phytoliths, and palynomorphs was undertaken in Negra Lagoon, to decipher Holocene changes in paleobotanical proxies associated with sea-level changes and paleolimnological conditions. Before the Holocene transgression (7000 yr BP), a terrestrial system was inferred as no aquatic palynomorphs or biogenic silica remains were recorded. During the sea-level maximum (5200 yr BP), marine/brackish conditions were established as indicated by diatoms, Chenopodiaceae, and a high content of cysts of Peridinioideae. The catchment consisted of grasslands and wetlands as indicated by the opal phytolith data. The Holocene transgression was followed by a sea-level decrease, which led to the onset of brackish/freshwater conditions as inferred from the co-dominance of freshwater and marine/brackish diatoms. This is also supported not only by the concomitant increase in non-siliceous freshwater microalgae and emergent macrophytes, but also by the presence of Myriophyllum. As sea level continued to decrease during late Holocene, a freshwater system was observed because of the complete separation from the ocean. Chrysophyte cysts were consistently recorded and brackish diatoms exhibited reduced abundances. The observed limnological changes are consistent with the alternative states hypothesis of Scheffer’s model (1998). That is, the clear water phases were explained because of high macrophyte abundances, low phytoplankton frequencies, high cyst to diatom ratios, and increase in frequencies of benthic diatom species. Conversely, turbid phases were likely to occur when low cyst to diatom ratios together with increase in planktonic diatoms and decrease in macrophytes values were detected. This highlights the importance of this approach to detect long-term changes in shallow lake alternative states.     

Modelling the spread of Fagus sylvatica and Picea abies in southern Scandinavia during the late Holocene

Aim To test the hypothesis that dispersal characteristics alone can explain the past migration patterns of Fagus sylvatica and Picea abies observed in southern Scandinavia. Location Scandinavia, Europe. Methods The spreading dynamics of both species were analysed using a quantitative data–model comparison approach. Pollen data recording the arrival of the two species at 24 small forest‐hollow sites distributed across the study area were compared with simulated arrival times. The simulations were based on diffusive spread combined with long‐distance dispersal events. By systematically applying different parameter combinations yielding the desired colonization speeds we could identify values for the long‐distance dispersal component that minimized deviations from the observed arrival times. Results According to the minimization process, the optimal spreading rates were 100 m year^?1 for F. sylvatica and 250 m year^?1 for P. abies. Simulated dispersal alone could adequately explain the wave‐like spread of P. abies but failed to explain the scattered establishment pattern observed for F. sylvatica in Scandinavia. At the fine scale of stand establishment, local microclimatic conditions or site disturbance might be more important. The estimated spreading rates are high because the species colonized Scandinavia from different geographic directions and the rates slowed when their ranges overlapped. We present new estimates for the distance and frequency of long‐distance dispersal events for our modelled species. Main conclusions Our analyses suggest that the late Holocene spread of P. abies in Scandinavia was fairly rapid and was limited only by biological processes of dispersal, while that of F. sylvatica was limited by other factors probably controlled by site properties. Picea abies has maintained a rapid and constant rate of spread throughout at least the last 4000 years, despite significant changes in climate. There is uncertainty about the precise relationship between P. abies and climate in Scandinavia, so future distributions are not easy to forecast. For F. sylvatica in Scandinavia, site quality appears to have been a limiting factor, so future land use is likely to dictate its future distribution dynamics in combination with climatic factors.     

Past and future evolution of Abies alba forests in Europe – comparison of a dynamic vegetation model with palaeo data and observations

Information on how species distributions and ecosystem services are impacted by anthropogenic climate change is important for adaptation planning. Palaeo data suggest that Abies alba formed forests under significantly warmer‐than‐present conditions in Europe and might be a native substitute for widespread drought‐sensitive temperate and boreal tree species such as beech (Fagus sylvatica) and spruce (Picea abies) under future global warming conditions. Here, we combine pollen and macrofossil data, modern observations, and results from transient simulations with the LPX‐Bern dynamic global vegetation model to assess past and future distributions of A. alba in Europe. LPX‐Bern is forced with climate anomalies from a run over the past 21 000 years with the Community Earth System Model, modern climatology, and with 21st‐century multimodel ensemble results for the high‐emission RCP8.5 and the stringent mitigation RCP2.6 pathway. The simulated distribution for present climate encompasses the modern range of A. alba, with the model exceeding the present distribution in north‐western and southern Europe. Mid‐Holocene pollen data and model results agree for southern Europe, suggesting that at present, human impacts suppress the distribution in southern Europe. Pollen and model results both show range expansion starting during the Bølling–Allerød warm period, interrupted by the Younger Dryas cold, and resuming during the Holocene. The distribution of A. alba expands to the north‐east in all future scenarios, whereas the potential (currently unrealized) range would be substantially reduced in southern Europe under RCP8.5. A. alba maintains its current range in central Europe despite competition by other thermophilous tree species. Our combined palaeoecological and model evidence suggest that A. alba may ensure important ecosystem services including stand and slope stability, infrastructure protection, and carbon sequestration under significantly warmer‐than‐present conditions in central Europe.     

Mid to Late Holocene palaeoenvironment of Lake Eastern Juyanze (north-western China) based on ostracods and stable isotopes

An exposed section of nearly 13 m thick was investigated in the ancient depression of Lake Eastern Juyanze, north-eastern China. The succession of lacustrine and aeolian deposits covers a period from about 5400 to 2700 cal. a BP as indicated by four radiocarbon ages of macro-plant debris. Fifteen ostracod taxa were identified of which Limnocythere inopinata and Darwinula stevensoni are by far the most abundant species. Stable isotope (O, C) data of ostracod calcite as well as ostracod species abundances and X-ray diffraction results of bulk sediment samples were used to reconstruct the history of Lake Eastern Juyanze. Highest lake levels (water depth ∼10 m) occurred between 5100 and 4100 cal. a BP and were succeeded by intermediate and low levels until about 3150 cal. a BP. Three short-term events of complete desiccation were recorded between 3150 and 2900 cal. a BP. The lake experienced relatively high levels again for some decades at about 3000 cal. a BP and for at least two centuries after 2900 cal. a BP. According to the ostracod and gastropod record and the high abundance of a large diatom (Campylodiscus clypeus) in the lake sediments oligohaline to slightly mesohaline conditions were reconstructed for most of the lake periods (4-6 g/l). Geochemical and palaeontological records of Lake Eastern Juyanze reveal the retreat of the Asian monsoon after China’s Hypsithermal culminating in climate instability at about 3000 cal. a BP.     

Contrasting acclimation abilities of two dominant boreal conifers to elevated CO2 and temperature

High latitude forests will experience large changes in temperature and CO₂ concentrations this century. We evaluated the effects of future climate conditions on 2 dominant boreal tree species, Pinus sylvestris L. and Picea abies (L.) H. Karst, exposing seedlings to 3 seasons of ambient (430 ppm) or elevated CO₂ (750 ppm) and ambient temperatures, a + 4 °C warming or a + 8 °C warming. Pinus sylvestris responded positively to warming: seedlings developed a larger canopy, maintained high net CO₂ assimilation rates (A_net), and acclimated dark respiration (R_dark). In contrast, carbon fluxes in Picea abies were negatively impacted by warming: maximum rates of A_net decreased, electron transport was redirected to alternative electron acceptors, and thermal acclimation of R_dark was weak. Elevated CO₂ tended to exacerbate these effects in warm‐grown Picea abies, and by the end of the experiment Picea abies from the +8 °C, high CO₂ treatment produced fewer buds than they had 3 years earlier. Treatments had little effect on leaf and wood anatomy. Our results highlight that species within the same plant functional type may show opposite responses to warming and imply that Picea abies may be particularly vulnerable to warming due to low plasticity in photosynthetic and respiratory metabolism.     

Schistosomiasis in Lake Mala?i Villages

Historically, open shorelines of Lake Malaŵi were free from schistosome, Schistosoma haematobium, transmission, but this changed in the mid-1980s, possibly as a result of over-fishing reducing density of molluscivore fishes. Very little information is available on schistosome infections among people in lake-shore communities and therefore we decided to summarise data collected from 1998 to 2007. Detailed knowledge of the transmission patterns is essential to design a holistic approach to schistosomiasis control involving the public health, fisheries and tourism sectors. On Nankumba Peninsula, in the southern part of the lake, inhabitants of villages located along the shores of Lake Malaŵi have higher prevalence of S. haematobium infection than those living in inland villages. Overall prevalence (all age classes combined) of urinary schistosomiasis in 1998/1999 ranged from 10.2% to 26.4% in inland villages and from 21.0% to 72.7% in lakeshore villages; for school children prevalence of infection ranged from 15.3% to 57.1% in inland schools and from 56.2% to 94.0% in lakeshore schools. Inhabitants on the islands, Chizumulu and Likoma, also have lower prevalence of infection than those living in lakeshore villages on Nankumba Peninsula. This increased prevalence in lakeshore villages is not necessarily linked to transmission taking place in the lake itself, but could also be due to the presence of more numerous typical inland transmission sites (e.g., streams, ponds) being close to the lake. Temporal data witness of intense transmission in some lakeshore villages with 30–40% of children cleared from infection becoming reinfected 12 months later (also lakeshore village). The level of S. mansoni infection is low in the lakeshore communities. Findings are discussed in relation to fishing in the lake.     

Quantifying the effects of drought on abrupt growth decreases of major tree species in Switzerland

Drought entails important effects on tree physiology, which may result in short‐ to long‐term radial growth decreases. While the majority of studies have focused on annual drought‐related variability of growth, relatively little is known about sustained growth decreases following drought years. We apply a statistical framework to identify climatic factors that induce abrupt growth decreases and may eventually result in tree mortality. We used tree‐ring data from almost 500 standing dead trees and 200 living trees in eight sites of the Swiss network of strict forest reserves, including four of the most important Central European tree species (Abies alba, Picea abies, Fagus sylvatica and Quercus spp.). First, to assess short‐term growth responses to drought under various climate and site conditions, we calculated correlations and linear mixed‐effects models between ring‐width indices (RWIs) and drought based on the Standardized Precipitation Evapotranspiration Index (SPEI). Second, to quantify drought effects on abrupt growth decreases, we applied distributed lag nonlinear models (DLNMs), which account for both delayed effects and the nonlinear relationship between the SPEI and the occurrence of abrupt growth decreases. Positive correlations between RWIs and the SPEI indicated short‐term growth responses of all species, particularly at arid sites. Results of the DLNMs revealed species‐specific growth responses to drought. For Quercus spp., abrupt growth decreases were more likely to occur several years following severe drought, whereas for P. abies, A. alba, and F. sylvatica abrupt growth decreases started frequently immediately in the drought year. We conclude that the statistical framework allows for quantifying the effects of drought intensity on the probability of abrupt growth decreases, which ultimately contributes to an improved understanding of climate impacts on forest community dynamics.     

Paleoenvironmental changes in northwest Mongolia during the last 27?kyr inferred from organic components in the Lake Hovsgol sediment core record

We studied organic components in the X106 sediment core (length 130.3 cm, water depth 236 m, 50°53′01″N, 100°21′22″E) from Lake Hovsgol to elucidate the biological production, source of organic components, and paleoenvironmental and paleolimnological changes during the last 27 kyr in northwest Mongolia. Total organic carbon (TOC) contents (0.20–0.70%) in the core of the last glacial period increased dramatically and attained 3.16–5.85% in the postglacial period (Holocene), together with the increase of the contribution of terrestrial organic matter. Biological production (both terrestrial and aquatic production) based on the TOC contents in the Holocene was 14 times higher than that in the last glacial period. The B?lling-Aller?d warm period and Younger Dryas cool period were both observed at depths of 55–50 cm (ca. 15–13 cal kyr BP) and 50–45 cm (ca. 13–11 cal kyr BP), respectively. We propose here a terrestrial/aquatic index (TAI) for organic matter in lake sediments. The TAI values suggest that terrestrial organic matter in the bottom of the core was less than 10%, increased to 48% in the B?lling-Aller?d warm period, decreased abruptly to 20% in the Younger Dryas cool period, and again increased to 30–40% in the Holocene. Normal-C₃₁ alkane (a biomarker of herbaceous land plants) and n-C₁₈ alkanoic acid (marker of plankton) decreased from the last glacial period to the Holocene, whereas n-C₂₃ alkane and n-C₂₂ alkanoic acid (a marker of higher vascular plants) increased from the last glacial period to the Holocene. Scarce herbaceous plant vegetation, such as Artemisia spp. of the lake basin in the last glacial period, changed into an abundance of higher woody plant vegetation (e.g., Pinus spp., Betula spp. and/or Larix spp.) in the Holocene. Stanol/sterol ratios suggest that relatively high oxygen tension of the lake bottom in ca. 27–22 cal kyr BP decreased from this age to the present, though benthic organisms are still abundant.     

Biogeochemical indicators of environmental changes from 50 Ka to 10 Ka in a humid region of the Brazilian Amazon

We present a geochemical record of a 113.6-cm sediment core (LPT V) from Lagoa da Pata, which is located in the forested upper Rio Negro basin. The record reveals significant changes in the environmental history of Amazonia during the late Quaternary. The results of biogeochemical analyses revealed three hydrological and climatic regimes from 50,000 to 10,000 cal yr BP. The first phase, between 50,000 and 26,300 cal yr BP, was characterized by a relatively wet climate as suggested by relatively high total organic carbon (TOC) and chlorophyll derivate concentrations, indicating high productivity linked to a high lake level. A decrease of the TOC and chlorophyll derivate accumulation rates between 43,100 cal yr BP and 26,300 cal yr BP marks a decrease in the productivity linked to a reduced lake level, indicating a decrease in moisture at the end of this phase. The second phase, between 26,300 and 15,300 cal yr BP, was characterized by a decrease in productivity, reaching a minimum at 21,950 cal yr BP, as indicated by a minima in sedimentary chlorophyll and TOC accumulation rates. Values of δ¹³C increased by 5‰ in relation to the preceding phase, indicating an influence of the C₄ organic matter. High iron concentrations and accumulation rates, related to intense erosion of the lateritic crust in the watershed, were observed. All of the observations indicate a dry phase during this period. A third phase, from approximately 15,300 to 10,000 cal yr BP, was characterized by an increase in lacustrine productivity, as shown by an increased in TOC and chlorophyll derivate concentrations and accumulation rates. These increases likely correspond to a lake level rise due to a wetter climate.     

Utilisation préhistorique de la technique minière d’abattage au feu dans le district cuprifère de Cabrières (Hérault)

Fire-setting to open up mines has been used on hard rock since prehistoric times. In the copper-mining district of Cabrières, the existence of metre-sized spherical or sub-spherical cavities, sometimes spaced along the same vertical in an ore seam, has usually been ascribed to this method 〚12〛, 〚13〛, 〚20〛. Two AMS ¹⁴C dating of micro-charcoal found in dolomite and of burnt ore breccias related to extraction in these cavities give the ages 3830 ± 40 BP, cal BC 2340–2130 and 3900 ± 40 BP, cal BC 2480–2280, which is the first evidence in France of the use of fire-setting in prehistoric mines.