Past Environmental Changes Research Articles

Local species assemblages are influenced more by past than current dissimilarities in photosynthetic activity

Most land on Earth has been changed by humans and past changes of land can have lasting influences on current species assemblages. Yet few globally representative studies explicitly consider such influences even though auxiliary data, such as from remote sensing, are readily available. Time series of satellite‐derived data have been commonly used to quantify differences in land‐surface attributes such as vegetation cover, which will among other things be influenced by anthropogenic land conversions and modifications. Here we quantify differences in current and past (up to five years before sampling) vegetation cover, and assess whether such differences differentially influence taxonomic and functional groups of species assemblages between spatial pairs of sites. Specifically, we correlated between‐site dissimilarity in photosynthetic activity of vegetation (the enhanced vegetation index) with the corresponding dissimilarity in local species assemblage composition from a global database using a common metric for both, the Bray–Curtis index. We found that dissimilarity in species assemblage composition was on average more influenced by dissimilarity in past than current photosynthetic activity, and that the influence of past dissimilarity increased when longer time periods were considered. Responses to past dissimilarity in photosynthetic activity also differed among taxonomic groups (plants, invertebrates, amphibians, reptiles, birds and mammals), with reptiles being among the most influenced by more dissimilar past photosynthetic activity. Furthermore, we found that assemblages dominated by smaller and more vegetation‐dependent species tended to be more influenced by dissimilarity in past photosynthetic activity than prey‐dependent species. Overall, our results have implications for studies that investigate species responses to current environmental changes and highlight the importance of past changes continuing to influence local species assemblage composition. We demonstrate how local species assemblages and satellite‐derived data can be linked and provide suggestions for future studies on how to assess the influence of past environmental changes on biodiversity.

Preservation of microbial DNA in marine sediments: insights from extracellular DNA pools.

Marine sediments harbour extracellular DNA (exDNA) not associated with currently living organisms. Including this exDNA in genetic surveys may distort abundance and diversity estimates of living prokaryotic communities. We separately extract exDNA and intracellular DNA (inDNA) from 11 horizons in a 10-m deep sediment core from Aarhus Bay (Denmark) that spans > 9000 years of Holocene sedimentation. We compare depth profiles of bacterial and archaeal 16S rRNA gene compositions to those of macrofaunal activity (bioturbation), sulfate and methane concentrations, sediment age and lithology. Among these variables, bioturbation shows the strongest relationship with the two DNA pools. In bioturbated surface sediments, the majority of Operational Taxonomic Units (OTUs) present in exDNA is absent from inDNA, thus belonging to microorganisms that were not alive at the time of sampling. Below the bioturbation zone, the two DNA pools display a much higher phylogenetic similarity. At all depths, the majority of exDNA and inDNA sequences show highest sequence similarities to sediment microorganisms, a finding that is additionally supported by separate analyses on low- and high-molecular weight exDNA. Our results indicate that in Aarhus Bay the vast majority of prokaryotic exDNA is turned over, thus not contributing to a genetic archive of past environmental change.

Cladoceran assemblage changes across the Eastern United States as recorded in the sediments from the 2007 National Lakes Assessment, USA

Cladoceran zooplankton subfossils have been widely applied as indicators of lake health and used to infer past environmental change. However, our understanding of how the environment controls the abundance and distribution of Cladocera, as recorded in lake sediments, is still developing; only a few large landscape studies have been conducted in temperate North American lakes. Here we examine cladoceran remains from across 72 lakes and through time by analysing sediments that were collected as part of the US Environmental Protection Agency 2007 National Lakes Assessment. Using both taxonomic and functional-trait approaches, we related both taxonomic and functional-trait subfossil cladoceran assemblages to a suite of biological, chemical, and physical variables of lake health by applying Redundancy Analyses and Multivariate Regression Trees. Primary gradients identified as predictors of the cladoceran assemblages were: calcium and pH, depth and measures associated with pelagic and littoral habitat, and climate. Notably, calcium was identified as the primary driver of the Bosminidae, with greater relative abundances of Bosmina longirostris at concentrations ≥3.99 mg L−1, and with Eubosmina spp. associated with lower concentrations. Comparison of surface and pre-industrial sediments on a northern (temperate) lake subset suggested that the primary agents of change were localized, as we did not detect consistent directional shifts in Cladocera. In particular, the quality and complexity of the littoral habitat were associated with changes in key cladoceran groups through time. Overall, our study identified insightful relationships between cladoceran subfossil assemblages and environmental variables across spatial and temporal gradients, which are critical for refining the utility of Cladocera as ecological indicators in paleolimnological studies.

Wet avalanches: long-term evolution in the Western Alps under climate and human forcing

Abstract. Understanding wet avalanche intensity and the role of past environmental changes on wet avalanche occurrence is a main concern especially in the context of a warming climate and accelerated environmental mutations. Avalanches are closely related to fast cryosphere changes and may cause major threats to human society. Here, we used the sedimentary archive of the Alpine Lake Lauvitel (Lac du Lauvitel; western French Alps) to establish the first long-term avalanche record in this Alpine region. For this purpose, we used a novel CT-scan methodology that allows the precise identification of coarse material – from sand to pebble – transported to the lake and embedded within the finer continuous sedimentation. We identified a total of 166 deposits over the last 3300 yr cal. BP. In parallel, a detailed pollen analysis gave an independent record of environmental changes. Based on modern observation, lake monitoring, seismic investigations and sedimentological evidences, coarse material deposits were attributed to wet avalanche events. Our results highlight the effect of vegetation cover on the avalanche hazard while a period of strong frequency increase occurred after 780 yr cal. BP. In Lake Lauvitel, this period corresponds to a major forest clearance induced by the rise of human land use. Climate forcing on the avalanche hazard was investigated before and after the vegetation shift. On a multicentennial scale, wet avalanches preferably occur during periods of larger glacier extent, in which higher winter precipitation probably generates a sufficiently thick snow cover. On a sub-centennial scale, avalanches are more frequent during periods of relative warming, resulting in a destabilization of the same snow cover in spring season. Our results highlight as well the role of forest cover in mitigating wet snow avalanches' occurrence. In the context of predicted warmer temperatures, this study raises the question of whether a wet avalanche hazard increase may be expected in the near future especially at higher altitudes.

Can the subterranean fauna be used as proxy for past environmental changes? – the example of the Carpathians cave fauna

Carpathian Mountains were one of the main refuge areas during the climate changes of the Pleistocene and the Holocene in Europe and one of the richest regions in the world in subterranean (caves and associated habitats) endemic species. Nevertheless, the Carpathian Mountains subterranean fauna importance is underestimated especially due to dispersed information on its diversity and the scarcity of molecular studies in the area. Here, we present a first general view of the cave fauna hotspot represented by the Romanian Carpathians and the geological and historical processes that shaped the patterns of subterranean distribution and diversity at regional scale. The Carpathians are an amalgam of various geological units with complex paleogeographical evolution that is reflected in completely different species assemblages dominated by unit specific fauna groups. Phylogeography of Coleoptera and environmental parameters are adding to the general view at regional scale and offer additional explanation for this exceptional subterranean diversification in a non-Mediterranean region. We also use the example of the Carpathians cave fauna as proxy for past environmental changes in the area. Troglobionts are endemic on small areas and by studying their present distributions and phylogeny, past processes of landscape evolution on the surface can be better understood.

Quantifying Sediment Transport Dynamics on Alluvial Fans From Spatial and Temporal Changes in Grain Size, Death Valley, California

How information about sediment transport processes is transmitted to the sedimentary record remains a complex problem for the interpretation of fluvial stratigraphy. Alluvial fan deposits represent the condensed archive of sediment transport, which is at least partly controlled by tectonics and climate. For three coupled catchment‐fan systems in northern Death Valley, California, we measure grain size across 12 well‐preserved Holocene and late‐Pleistocene surfaces, mapped in detail from field observations and remote sensing. Our results show that fan surfaces correlated to the late Pleistocene are, on average, 30–50% coarser than active or Holocene fan surfaces. We adopt a self‐similar form of grain size distribution based on the observed stability of the ratio between mean grain size and standard deviation downstream. Using statistical analysis, we show that fan surface grain size distributions are self‐similar. We derive a relative mobility function using our self‐similar grain size distributions, which describes the relative probability of a given grain size being transported. We show that the largest mobile grain sizes are between 20 and 35 mm, a value that varies over time and is clearly lower in the Holocene than in the Pleistocene; a change we suggest is due to a drier climate in the Holocene. These results support recent findings that alluvial fan sedimentology can record past environmental change and that these landscapes are potentially sensitive to climatic change over a glacial‐interglacial cycle. We demonstrate that the self‐similarity methodology offers a means to explore changes in relative mobility of grain sizes from preserved fluvial deposits.

Temporal changes in the relationship between tree‐ring growth and net primary production in northern Japan: a novel approach to the estimation of seasonal photosynthate allocation to the stem

Tree-ring (TR) observations provide important data on long-term forest dynamics and their underlying ecophysiological mechanisms. To elucidate the seasonal link between photosynthetic carbon acquisition and TR growth, we analyzed the correlation between observed TR data (carbon sink) and model-estimated net primary production (NPP; carbon source). Temporal trends of the TR–NPP correlation over the last century were also analyzed to identify influences of past climate changes. We used TR data from Picea glehnii at seven sites on Hokkaido Island, Japan, which were obtained from the International Tree-Ring Data Bank. At each site, NPP was estimated using the Vegetation Integrative Simulator for Trace gases model, which was driven by long-term (1900–2010) meteorological data. Site-mean tree-ring width index (TRWI) chronologies were analyzed to reveal any relationship with the current or previous year’s annual or monthly NPP. We found moderate to strong correlations between TRWIs and model-estimated monthly NPP from April to June, especially in June of the current year, but no clear spatial trend was observed. During the twentieth century, the TRWI–NPP correlation increased for February, March, April, and July NPP of the current year and for October NPP of the previous year. Ecophysiologically, the period from April to June corresponds to the season when tree cambial cells are formed in the study area. Our findings suggest that photosynthate produced during this cambial growth season is allocated to stem growth and that this source allocation season has become longer due to past environmental changes.

Recent ecological change in ancient lakes

AbstractAncient lakes are among the best archivists of past environmental change, having experienced more than one full glacial cycle, a wide range of climatic conditions, tectonic events, and long association with human settlements. These lakes not only record long histories of environmental variation and human activity in their sediments, but also harbor very high levels of biodiversity and endemism. Yet, ancient lakes are faced with a familiar suite of anthropogenic threats, which may degrade the unusual properties that make them especially valuable to science and society. In all ancient lakes for which data exist, significant warming of surface waters has occurred, with a broad range of consequences. Eutrophication threatens both native species assemblages and regional economies reliant on clean surface water, fisheries, and tourism. Where sewage contributes nutrients and heavy metals, one can anticipate the occurrence of less understood emerging contaminants, such as pharmaceuticals, personal care products, and microplastics that negatively affect lake biota and water quality. Human populations continue to increase in most of the ancient lakes’ watersheds, which will exacerbate these concerns. Further, human alterations of hydrology, including those produced through climate change, have altered lake levels. Co‐occurring with these impacts have been intentional and unintentional species introductions, altering biodiversity. Given that the distinctive character of each ancient lake is strongly linked to age, there may be few options to remediate losses of species or other ecosystem damage associated with modern ecological change, heightening the imperative for understanding these systems.

Late glacial and early Holocene climate and environmental changes in the eastern Baltic area inferred from sediment C/N ratio

We assessed the utility of using the sediment total organic carbon/total nitrogen (C/N) ratio as an indicator of paleoclimate changes in the eastern Baltic area during the late glacial and early Holocene. The C/N ratio in sediments from Lake Lielais Svētiņu, eastern Latvia, was compared with other sediment variables that are used as proxies of past climate and environment. Analysis revealed that although the organic matter (OM) content in late glacial sediments was extremely low, the C/N ratio captured information about OM origin, and fluctuations in the ratio tracked climate oscillations. The C/N ratio was significantly positively correlated with pollen-inferred mean summer temperature. Therefore, C/N ratio was lower under colder conditions, indicating a predominantly phytoplankton origin of OM, and was higher during warmer conditions, when there was more vegetation around the lake. A strong positive correlation between C/N ratio and the paleopigment beta carotene suggested that elevated phytoplankton production resulted from higher nutrient availability that was controlled largely by the input of terrestrial OM to the lake during warmer climate episodes. Thus, C/N ratio was a good indicator of paleoclimate changes, at least for the late glacial period, when generally cold conditions prevailed. This study also demonstrates the power of multi-proxy paleolimnological analyses for investigating past environmental changes in lakes and their watersheds.

The record of postglacial environmental changes of the southern Baltic coastal zone in the sequence of fossil soils

PurposeThe studies aimed to reconstruct major phases of litho-morpho-pedogenic processes and past environmental changes in the Baltic coastal zone near Poddąbie (54°37,070′ N, 16°57,897′ E) based on physical, chemical, pollen, and fossil Cladocera analysis of the polycyclic pedocomplex.Materials and methodsThe polycyclic pedocomplex under study included seven fossil soils developed from the Early Holocene until modern times from various materials. The soils were sampled and analyzed using standard procedures in soil science to determine their physical and chemical properties. Pollen and fossil Cladocera analysis was also carried out in the examined profile. Radiocarbon dates of selected samples were also specified.Results and discussionThe studied polycyclic pedocomplex has been developed from materials of varied origin, including glaciofluvial, fluvial, aeolian, and biogenic deposits of the lake. According to the results of pollen and fossil Cladocera analysis and radiocarbon dating, this body of water has evolved from the beginning of Preboreal period to the half of Boreal. The studied soils represent various reference groups and advancement of development. They constitute relics of former geomorphological and soil-forming processes, which have taken place over the last 9500 years, including periglacial processes, wildfires, Baltic Sea shoreline changes, and aeolian activity. The last stage of the coastal relief dynamics was conditioned by the fire which took place about 1472–1637 AD. The lack of plant cover and intensive abrasion due to direct vicinity of the coastline has caused the deposition of aeolian material, which is the parent material of nutrient-poor initial soils.ConclusionsThe analysis of fossil soil development with the use of specialized research methods allowed for reconstructing the dynamics of past geomorphological processes and natural environment changes in the study area. The studied polycyclic pedocomplex constitutes a record of environmental changes taking place in the central part of the Polish Baltic coastal zone from the Early Holocene to the present day. The studied soil sequence is located within a land depression developed in place of the former water reservoir created as a result of permafrost degradation, and filled with Holocene deposits.

Radiocarbon Wiggle Matching on Laminated Sediments Delivers High-Precision Chronologies

ABSTRACTHigh-resolution sediment chronologies with the best possible time control are essential for comparing palaeoecological studies with independent high-precision climatic, archaeological or historic data in order to disentangle causes and effects of past environmental, ecological and societal change. We present two varved lake sediment sequences from Moossee and Burgäschisee (Swiss Plateau) that have chronologies developed with Bayesian models and radiocarbon (14C) dating of terrestrial plant macrofossils extracted from sediment samples with constant age ranges. We illustrate the potential of high-resolution14C dating for the construction of robust, high-precision sediment chronologies. The mean 2σ age uncertainties were reduced to±19 cal yr for Moossee and to±54 cal yr for Burgäschisee over the entire period of 3000 cal yr, while 2σ uncertainties of only±13 cal yr and±18 cal yr respectively, were achieved for shorter time intervals. These precisions are better than or comparable to those of previous varve studies. Our results imply that a sophisticated subsampling strategy and a careful selection of short-lived and well-defined terrestrial plant remains are crucial to avoid outlying14C ages. A direct linkage between palaeoeological studies with dendrochronologically dated, local archaeological sites as well as a precise comparison with high-resolution climate proxy data have become feasible.

Development of 15 nuclear EST microsatellite markers for the paleoendemic conifer Pherosphaera hookeriana (Podocarpaceae).

Premise of the StudyNuclear microsatellite markers were developed for population genetic analysis of the threatened paleoendemic conifer Pherosphaera hookeriana (Podocarpaceae).Methods and ResultsFifteen variable loci were identified showing one to 13 alleles per population, with seven loci displaying at least four alleles in all populations, and the average number of alleles per locus ranging from 4.80 to 5.93 per population. Levels of observed heterozygosity per locus varied from 0.00 to 0.91, while average heterozygosity across all loci varied from 0.54 to 0.63 between populations. All loci also amplified in the endangered congener P. fitzgeraldii, but only five of the loci had more than one allele.ConclusionsThese 15 loci are the first microsatellite markers developed in the genus Pherosphaera. These loci will be useful for investigating the species' extant genetic diversity and structure, the impact of past environmental change, and the significance of asexual reproduction.

Conservation of the Natura 2000 Areas in the Context of Environmental Changes in Past and Present: a Case from the Polish Carpathians Geoheritage

The Ecological Network Natura 2000 project has been implemented in Poland since 2004, based on the Act on Nature Conservation. The Natura 2000 areas are often distinguished by particular values of the geological heritage, and sometimes also of cultural heritage. According to the premises of the programme, natural habitat should be identified by the conditions of occurrence and mutual relationships between the abiotic and biotic elements. Despite this assumption, in many cases, their valuation and description of the abiotic features is too cursory, and sometimes, it is even lacking. The valley of the Tarnawka River in the Polish Carpathians is the selected example of a Natura 2000 area. The presented characteristic of this area constitutes a supplement to its importance in the field of geological and cultural heritage. Also documented was the history of causes and changes of the natural environment dating back to the early Middle Ages. The physiographic characteristic of the area and the results of analysing the past transformations of its environment provide very rich material. It should be useful and used not only to supplement the importance of the area but also to conduct the comparative evaluation of the current and potential changes and threats caused by natural processes but, above all, by human activities.

Past environmental changes during the Late Holocene sea-level fall (last 2.7 Ka) at Bahía Samborombón, NE coastal plain (Argentina)

Here we present a reconstruction of vegetation history and environmental changes at the central zone of Bahía Samborombón during the Late Holocene sea-level regressive phase (last 2.7 Ka) corresponding to MIS 1. This study is an integrative approach based on the combination of geomorphological and micropaleontological (pollen, non-pollen palynomorphs and foraminifera) proxy data. We found agreement among the environmental inferences from each proxy, which clearly show the evolution of the estuarine saltmarsh from an intertidal environment under significant marine influence towards a supratidal brackish-freshwater environment as sea-level was falling. The evidence of this evolution is (1) coastal progradation encompassing at least 5.7 km as revealed by relict shorelines. Saltmarsh elevation is another evidenced as a result of accretion due to fine sediments accumulation of at least 1.2 m thick. We posit based on these values, significant progradation and accumulation processes. (2) Plant succession, characterized by the replacement of low saltmarsh vegetation by that typical of high saltmarsh; which then is associated to brackish-freshwaters plant communities in shallow water-bodies environments. (3) A gradual decrease of dinocysts and foraminifera assemblages, the latter absent during the last millennium indicating a decrease in marine influence and the consequent lower salinity values. Bahía Samborombón coastal plains became a regional scale character during the last 2.7 Ka due to its surface increase, turning into one of the most extensive southeastern-South America saltmarshes, currently constituting a biosphere reserve.

Unveiling the History of a Peculiar Weevil-Plant Interaction in South America: A Phylogeographic Approach to Hydnorobius hydnorae (Belidae) Associated with Prosopanche americana (Aristolochiaceae)

Interspecific interactions take place over both long and short time-frames. However, it is not completely understood if the interacting-partners persisted, migrated, or expanded in concert with Quaternary climate and landscape changes. We aim to understand whether there is concordance between the specialist weevil Hydnorobius hydnorae and its parasitic host plant, Prosopanche americana in space and time. We aim to determine whether Prosopanche had already established its range, and Hydnorobius later actively colonized this rare resource; or, if both host plant and herbivore expanded their range concomitantly. We performed population genetic, phylogeographic and Bayesian diffusion analysis of Cytochrome B sequences from 18 weevil localities and used paleodistribution models to infer host plant dispersal patterns. We found strong but uneven population structure across the range for H. hydnorae with weak signals of population growth, and haplotype network structure and SAMOVA groupings closely following biogeographic region boundaries. The ancestral areas for both Hydnorobius and Prosopanche are reconstructed in San Luis province within the Chaco Biogeographic province. Our results indicate a long trajectory of host-tracking through space and time, where the weevil has expanded its geographic range following its host plant, without significant demographic growth. We explore the past environmental changes that could underlie the boundaries between locality groups. We suggest that geographic dispersal without population growth in Hydnorobius could be enabled by the scarcity of the host plant itself, allowing for slow expansion rates and stable populations, with no need for significant demographic growth pulses to support range expansion.

Flotsam samples can help explain the δ13C and δ15N values of invertebrate resting stages in lake sediment

The stable isotopic composition of chitinous remains of Cladocera (water fleas) and freshwater Bryozoa (moss animals) preserved in lake sediment records can provide supporting insights into past environmental and ecosystem changes in lakes. Here we explore whether analyses of these remains isolated from lake flotsam can provide information on the driving variables affecting the isotopic composition of these remains. We collected flotsam in 53 lakes and found enough material in 33 lakes to measure the stable carbon and nitrogen isotope ratios (expressed as δ13C and δ15N values, respectively) of resting stages. These values were compared with lake characteristics, water chemistry measurements, and the isotopic composition of sedimentary organic matter (SOM) in the lakes. Mean δ13C values of cladoceran ephippia and SOM were correlated and both were also negatively correlated with deep water methane concentrations and indicators of lake stratification. This supports the findings of previous studies that methane-derived carbon can provide a significant proportion of carbon entering planktonic food webs. Mean δ15N values of bryozoan statoblasts and SOM were correlated, suggesting that both reflect the δ15N values of phytoplankton. Our results provide information on how environmental variables in lakes can influence the δ13C and δ15N values in resting stages, but flotsam samples can also potentially be used to assess seasonal stable isotope variability of resting stages. Both types of information are important to improve palaeoenvironmental interpretations of stable isotope records based on these remains in lake sediments.

Climatic history of Ny-Alesund region, Svalbard, over the last 19,000 yr: Insights from quartz grain microtexture and magnetic susceptibility

Twenty sediment samples of a 1 m deep trench were analyzed to decipher the past environmental changes in and around Ny-Alesund, Svalbard, Arctic Region. The quartz grain microtexture reveals predominant glacial activities in the top 40 cm of the section, while the middle 40–55 cm part represents some aeolian activities along with glacial signatures. The bottommost part, in addition to glacial markers, exhibits some aqueous evidences as well. The lithology shows medium-grained sand in the upper part and coarse-grained sand with occasional shell pieces in the lower part. Angular gravels (2 mm–12 mm) are present throughout with increasing size from top to bottom. On the basis of above observations and 14C AMS dates, it can be summarized that after the Last Glacial Maximum (LGM), the pre-Holocene period shows rapid glacial retreat, followed by a warmer period during the early Holocene. Mid- and late Holocene is marked by a predominantly glacial environment characterized by melt water streams originating from the glaciers and flowing into the fjord. Magnetic susceptibility studies have also been attempted and four alternate stages of colder and warmer phases have been established. Though some similarities among the different climatic phases are discernible between the quartz grain microtexture and magnetic susceptibility studies, they are not completely compatible, which is probably due to their different responses to the climatic variations.

Double the dates and go for Bayes — Impacts of model choice, dating density and quality on chronologies

Reliable chronologies are essential for most Quaternary studies, but little is known about how age-depth model choice, as well as dating density and quality, affect the precision and accuracy of chronologies. A meta-analysis suggests that most existing late-Quaternary studies contain fewer than one date per millennium, and provide millennial-scale precision at best. We use existing and simulated sediment cores to estimate what dating density and quality are required to obtain accurate chronologies at a desired precision. For many sites, a doubling in dating density would significantly improve chronologies and thus their value for reconstructing and interpreting past environmental changes. Commonly used classical age-depth models stop becoming more precise after a minimum dating density is reached, but the precision of Bayesian age-depth models which take advantage of chronological ordering continues to improve with more dates. Our simulations show that classical age-depth models severely underestimate uncertainty and are inaccurate at low dating densities, and also perform poorly at high dating densities. On the other hand, Bayesian age-depth models provide more realistic precision estimates, including at low to average dating densities, and are much more robust against dating scatter and outliers. Indeed, Bayesian age-depth models outperform classical ones at all tested dating densities, qualities and time-scales. We recommend that chronologies should be produced using Bayesian age-depth models taking into account chronological ordering and based on a minimum of 2 dates per millennium.

Reevaluation of carbonate concentration and oxygen isotope records from Lake Qinghai, the northeastern Tibetan Plateau

Lake Qinghai is the largest lake on the Tibetan Plateau, and it is also one of the important sites for studying global environmental changes. Over the past 30 years, many studies have used oxygen isotope of authigenic carbonates from the lake as the proxy to infer past environmental and climate changes on the Plateau. However, debate on interpretations of isotopic data and their environmental implications still exist, largely due to the complex arid environment settings and multiple sources/species for carbonate minerals within the lake. In this study, we systematically analyze δ18O values in different-type carbonates collected [i.e. bulk carbonates, ostracode shells, Chara encrustations, and fine-grain (<63 μm) carbonate minerals] from modern lake sediments and surrounding soils, as well as the down-core δ18O values of bulk/fine-grain carbonates since the Last Glacial Maximum. Together with previously published δ18O records from ostracode shells, we try to re-evaluate the controlling factors of variations in lacustrine carbonate δ18O data and to infer environmental changes on the northeastern Tibetan Plateau since the Last Glacial Maximum. Our results show that the lake depth, or the size of the water body, is an important factor to influence the lake water and carbonate δ18O values. A shallow and small lake would be more easily influenced by precipitation δ18O which is characterized by negative values at Lake Qinghai region, while a deep and large lake would be better to reflect environmental changes such as the precipitation-evaporation balance. The “lake volume” effect might be an explanation for the negative carbonate δ18O values during the early Holocene, which was likely caused by an increased influence of negative δ18O values in precipitation and glacial melt water under a small and shallow water body. The δ18O values of ostracode shells and bulk carbonates show similar variations since both of them are dominated by lake water oxygen isotopic composition, but they still have distinct geochemical information. The isotopic differences between ostracode and bulk carbonates probably reflect the temperature differences between the surface and the bottom of lake water. In addition, the δ18O values of evaporative induced carbonates may correlate with carbonate contents, while those of Chara encrustations do not show any correlation with carbonate contents. Our results suggest that special caution would be necessary when using lacustrine δ18O values of authigenic carbonates to infer past hydrological and climate changes in an arid environment.

Indicator value of oribatid mites in determining past permafrost dynamics in northern European sub‐Arctic peatlands

Permafrost dynamics play an important role in the surface hydrology and carbon balance of northern peatlands. Plant macrofossil analysis with radiocarbon dating has been widely used in detecting past permafrost dynamics in peatlands; however, there is a lack of permafrost‐specific plant indicator species, which makes it challenging to determine the exact timing of historical permafrost aggradation. We investigated the indicator value of oribatid mites in determining past permafrost dynamics in sub‐Arctic peatlands. Analyses of subfossil oribatid mite assemblages of Holocene peat profiles from two mires, one in northern Finland and one in northeastern European Russia, were carried out and interpreted using modern calibration data from the same study areas. The results were compared with previously published reconstructions of permafrost history based on plant macrofossil analyses from the same locations. The results suggest that the oribatid mites Carabodes labyrinthicus, Chamobates borealis and Neoribates aurantiacus are promising indicator species to detect past permafrost occurrence in peatlands. In addition, N. aurantiacus is clearly associated with the presence of lichens, which is particularly useful because lichen remains are rarely preserved in peat deposits. Results are in accordance with earlier studies showing that oribatid mites are useful indicators of past environmental change.