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A 27.15-m-long core from Lake Chapala, west-central Mexico (1520 masl) possesses a continuous 15,000-year record of sediment accumulation with abundant diatoms, pollen, charcoal particles, and organic and inorganic carbon. Analyses of multiple sediment variables revealed the lake was deep, dominated by Stephanodiscus niagarae, and was surrounded by temperate forests of Pinus, Quercus, and Alnus in the late Pleistocene (14,700–12,760 cal yr BP). During the Pleistocene-Holocene transition (12,760–9660 cal yr BP), the lake level dropped and a temperate forest of Pinus and Quercus occupied the watershed and was persistently affected by fire. During the earliest Holocene, Quercus nearly disappeared and Pinus, relatively abundant, declined abruptly. Pollen of dry forests became more common, indicating the onset of drier conditions in the region. During the interval from 7500 to 6500 cal yr BP, more humid climate conditions prevailed and abundant Surirella spp. diatom valves were well preserved. Sediments of the time also contain Stephanodiscus niagarae and Ulnaria acus, suggesting deep-water conditions. Between 6500 and 2000 cal yr BP, the lake experienced higher conductivity and longer water residence times. The Pinus and Quercus forests began to be replaced by open vegetation, with Asteraceae and Poaceae dominating the pollen spectrum. Lake level fluctuated, but overall declined. Between the interval of ca. 2 ky to 0.5 cal yr BP basin conditions remained relatively wet. During the last ~ 0.5 kyr BP to present day, the lake level declined and ionic concentration increased, as inferred from representation of planktonic and benthic saline diatom taxa and TIC increases. Numerous micro-tephras are common in this interval; additionally, frequent fires favored the dominance of Poaceae. Paleolimnological records suggest that after the Last Glacial Maximum, Chapala and other lakes of west-central Mexico maintained relatively high water levels. Some records from the region showed evidence of increased humidity beginning ca. 8200 cal yr BP, a consequence of strength intensification in North American Monsoon, associated with ENSO variability and/or tropical cyclones. The record from the last 0.5 cal yr BP in Lake Chapala is similar to records from other lakes in Central Mexico, as it displays a trend toward drier conditions with a more variable precipitation regime. [ABSTRACT FROM AUTHOR]

Lake sediments are important archives of past climate variability and lake responses to climate. In order to accurately infer past climates, it is necessary to understand, and account for, the ecological processes that affect the record of indicators preserved in lake sediment. This is particularly the case with respect to the concentration of carbon and nitrogen (TOC, TN, and calculated C:N), and the stable isotope composition of organic matter preserved in lake sediments. These are common, yet ambiguous, tracers of environmental change. Ideally, palaeoenvironmental reconstructions using the concentration and isotope composition of organic matter should be grounded in a detailed understanding of the sources of the organic material. This study documents the history and evolution of Blue Lake, an environmentally and culturally important oligotrophic, groundwater window lake on North Stradbroke Island, Queensland, Australia. We utilise organic matter δ13C, TOC, TN, and C:N from a 2.4 m sediment core with a basal age of 7.5 cal kyr BP, to investigate changing organic matter sources as a measure of the climate sensitivity of Blue Lake. This interpretation is supported by data from contemporary algae, aquatic and terrestrial plants, and catchment soils. We show that lake nutrient dynamics drove an increase in algal biomass at 4.2 cal kyr BP. This change coincides with a widely documented intensification of the El Niño-Southern Oscillation, which we infer to have influenced lake nutrient concentrations by reducing groundwater throughflow. Climatic changes resulted in marked changes in lake primary productivity, despite relatively little turnover of the lake diatom flora and catchment vegetation. This suggests that south-east Queensland dune lakes are sensitive to climate changes and helps to refine past and future palaeoclimate research using sediments from these lakes. It also indicates that increased nutrient concentrations in Blue Lake may result from projected changes in 21st Century climate. [ABSTRACT FROM AUTHOR]

We measured stable isotopes (δ18O and δ13C) in Sphagnum cellulose that was extracted from a long peat core drilled in the ombrotrophic Mohos peat bog, Ciomadul Mountain, Romania. The 10-m-long peat profile spans the period from 11,800 cal yr BP to present. The δ18O and δ13C data indicate there were several cooling events and warm periods in the area of the Mohos peat bog during the Holocene. The 8.2-ka cold event, however, was not detected using δ18O and δ13C values. Response of the peat bog to changing environmental conditions was inferred using data on organic matter accumulation, independent of the stable isotope results. All cool periods during the Holocene, whether of short or long duration, were identified as times of reduced organic matter accumulation rate. Similarly, dry periods were also correlated with reduced accumulation rates of organic matter. [ABSTRACT FROM AUTHOR]

Annually laminated lake sediments provide high-resolution and accurate timescales for reliable paleoenvironmental reconstructions. We investigated human activity in a low-human-impact region of Poland, taking advantage of varved sediments from Lake Szurpiły, which span the last 8200 years. Multiple sediment variables indicated that catchment evolution and lacustrine responses, mainly to changes in forest cover, were divided into four phases. Because of sparse or only seasonal occupation of settlements in NE Poland, human impact in the region was insignificant until ca. 939 ± 55 BC (lacustrine phase I). During Phase II (939 ± 55 BC–AD 1392 ± 38), we recorded an increase in human indicators, reflecting the direct influence of a Yatvingian community. Afterwards, between AD 1392 ± 38 and 1770 ± 30 (phase III), permanent settlements and agricultural land use stabilized. The beginning of the last period (phase IV) was clearly identified by all sediment variables, which responded simultaneously to local human activities. Disappearance of laminae from AD 1858 ± 22 until about AD 1997 indicated disturbance of pre-existing, stable depositional conditions, followed by environmental recovery in the last 20 years. Such recovery may have been related to recent socio-economic changes and establishment of a park that reduced human influence on the lake. [ABSTRACT FROM AUTHOR]

Holocene loss-on-ignition (LOI) records were developed for radiocarbon-dated sediment cores from 20 lakes in the Uinta Mountains of northeastern Utah, USA. The hydrogeomorphic setting of each lake was characterized, permitting an analysis of connections between LOI values and lake setting. Median LOI values exhibit a significant negative correlation with lake elevation, with the range of elevations and the maximum slope of the watershed, with the extent of ice/snow and exposed rock in the watershed, and with the volume of through-flowing water. In contrast, LOI is positively correlated with the extent of forest cover and wetlands in the watershed. In addition, forward stepwise linear modeling demonstrates that lake area/lake depth ratio is a significant negative predictor for median LOI, that shoreline complexity is a significant positive predictor for maximum LOI, and the standard deviation of LOI values is greater in lakes surrounded by watersheds with more wetlands and surface water. Some LOI time series contain excursions at finer temporal scales that are recorded synchronously in adjacent lakes, however many excursions appear in only a single record. Similarly, step changes in LOI that are clear in some records are not preserved in the sediments of nearby lakes. Over longer timescales, some LOI records are essentially constant during the Holocene. In contrast, some lakes record steadily rising LOI values, and approximately half of the lakes have complicated LOI records with millennial-scale variability. Lakes with large surface area, large area/depth ratios, large watershed area/lake area ratios, rugged watersheds, and extensive ice/snow cover are more likely to record steady LOI values over time. The degree to which LOI records from neighboring lakes vary emphasizes that lacustrine proxy records reflect a strong filtering effect, whereby the signal of an external forcing is recorded to varying degrees depending on the nature of the lake and surrounding watershed. [ABSTRACT FROM AUTHOR]

Diatom assemblages recovered from a Colorado River delta core in northern Patagonia, Argentina were analyzed in order to assess past environmental conditions. A total of 35 samples were selected from a 172-cm core extracted 37 km from the mouth (39°36′19.6″S; 62°29′26.1″W). One-hundred and thirty-eight taxa were recognized and grouped according to life forms. Two diatom zones were identified by cluster analysis. At the base of the sequence, the Diatom Zone I (DZI; 4132 ± 35 − 2919 ± 27 14C yr BP), consisted of clays, and was characterized by assemblages dominated by aerophilous and benthic taxa and chrysophyte stomatocysts, which led to inferences of a sedimentary environment corresponding to a pond experiencing dry periods. The upper section (DZII) was dominated by fine sands and silts encompassing the last ~ 150 yr with abundant planktonic and tychoplanktonic diatoms. Benthic diatoms were abruptly replaced by planktonic forms in this zone indicating a shift to deeper waters. These results characterize the meandering dynamics of a deltaic system. During the Mid-Holocene, more arid periods would have favored the deactivation of meanders and the formation of riverine and oxbow wetlands. In Late-Holocene and historical times, more humid conditions and the hydrological system across the floodplain reactivated the paleochannel. Today, the dominant diatom species are brackish/freshwater fragilaroids. A non-multidimensional scaling analysis showed a lack of analogy between fossil and modern samples. The change in diatom floras in recent historical times was attributed to anthropogenic disturbances, a consequence of the regulation of the river flow. This regulation is evidenced by less discharge, morphological modifications in the floodplain and increased salinity in the last decade. [ABSTRACT FROM AUTHOR]

Reliable estimates of Holocene temperatures are important for understanding past climate dynamics, the response of biota to climate change, and validating climate models. Chironomids in lake sediment cores are used widely to quantify past summer temperatures, for which high-latitude and/or highaltitude lakes, remote from human influence, are usually considered appropriate. Temperature inferences from lowland lakes are likely influenced by other variables, specifically eutrophication and industrial pollution, but their reliability has never been tested. We used a Norwegian chironomid-based transfer function (r2 = 0.91; RMSEP = 1.01 °C) to infer mean July air temperature over the last 200 years, using chironomid assemblages in a core collected from a polluted, nutrient-enriched lake at Speke Hall, Liverpool, England. The chironomidinferred temperatures correlate significantly with the local instrumental temperature record and follow long-term national temperature trends. These results show that chironomids can be used to produce reliable estimates of past mean July air temperature, even when other variables have also influenced the composition of the chironomid community. These findings underline the value of chironomids as sensitive and reliable quantitative proxies for summer temperature. [ABSTRACT FROM AUTHOR]

Lake Uddelermeer (The Netherlands) is characterized by turbid conditions and annual blooms of toxic cyanobacteria, which are supposed to be the result of increased agricultural activity in the twentieth century AD. We applied a combination of classic palaeoecological proxies and novel geochemical proxies to the Holocene sediment record of Lake Uddelermeer (The Netherlands) in order to reconstruct the natural variability of the lake ecosystem and to identify the drivers of the change to the turbid conditions that currently characterize this lake. We show that the lake ecosystem was characterized by a mix of aquatic macrophytes and abundant phytoplankton between 11,500 and 6000 cal year BP. A transition to a lake ecosystem with clear-water conditions and relatively high abundances of ‘isoetids’ coincides with the first signs of human impact on the landscape around Lake Uddelermeer during the Early Neolithic (ca. 6000 cal year BP). An abrupt and dramatic ecosystem shift can be seen at ca. 1030 cal year BP when increases in the abundance of algal microfossils and concentrations of sedimentary pigments indicate a transition to a turbid phytoplankton-dominated state. Finally, a strong increase in concentrations of plant and faecal biomarkers is observed around 1950 AD. Canonical Correspondence Analysis suggests that reconstructed lake ecosystem changes are best explained by environmental drivers that show long-term gradual changes (sediment age, water depth). These combined results document the long-term anthropogenic impact on the ecosystem of Lake Uddelermeer and provide evidence for pre-Industrial Era signs of eutrophication. [ABSTRACT FROM AUTHOR]

We studied high-resolution stratigraphy of phosphorus (P) forms in the Holocene sediments of large shallow Lake Peipsi (Estonia/Russia) in order to evaluate the lake ecosystem response to environmental changes and track the lake's eutrophication history. We distinguished four main periods in the history of Lake Peipsi, each having likely different factors responsible for the distribution pattern of P fractions in the sediment record. We suggest that in the oldest period, from ca. 10.4 up to 7.3 cal ka BP, the sediment composition was mainly determined by rising water level, the second period dated 7.3-2.3 cal ka BP was governed mainly by stable hydrology and P loading, while the third period between 2.3 and 0 cal ka BP was primarily influenced by emerging anthropogenic impact. The sediments from the last period since 1950 are subject of ongoing diagenetic processes but still reflect rapid eutrophication of the lake. Comparison of the results with periods derived from other sediment proxies proved the usability of P fractions stratigraphy in reconstruction of the development of lakes. [ABSTRACT FROM AUTHOR]

Hypoxia in freshwater systems is currently spreading globally and putting water quality, biodiversity and other ecosystem services at risk. Such adverse effects are of particular concern in permanently stratified meromictic lakes. Yet little is known about when and how meromixis and hypoxia became established (or vanished) prior to anthropogenic impacts, or how human activities such as deforestation, erosion and nutrient cycling affected the mixing regimes of lakes. We used calibrated hyperspectral imaging (HSI) data in the visible and near infrared range from a fresh, varved sediment core taken in Lake Jaczno, NE Poland, to map sedimentary pigments at very high resolution (sub-varve scale) over the past 1700 years. HSI-inferred bacteriopheophytin a (bphe a, produced by anoxygenic phototrophic bacteria) serves as a proxy for meromixis, whereas HSI-inferred green pigments (chlorophyll a and diagenetic products) can be used as estimators of aquatic productivity. Meromixis was established and vanished long before significant human disturbance in the catchment was observed in the late eleventh century AD. Under pre-anthropogenic conditions, however, meromixis was interrupted frequently, and the lake mixing regime flickered between dimixis and meromixis. During two periods with intense deforestation and soil erosion in the catchment, characterised by sedimentary facies rich in clay and charcoal (AD 1070-1255 and AD 1670-1710), the lake was mostly dimictic and better oxygenated than in periods with relative stability and a presumably closed forest around the lake, i.e. without human disturbances. After ca. AD 1960, meromixis became established quasi-permanently as a result of eutrophication. The persistent meromixis of the last ~60 years is unusual with respect to the record of the last 1700 years. [ABSTRACT FROM AUTHOR]

Prediction of future Arctic climate and environmental changes, as well as associated ice-sheet behavior, requires placing present-day warming and reduced ice extent into a long-term context. Here we present a record of Holocene climate and glacier fluctuations inferred from the paleolimnology of small lakes near Istorvet ice cap in East Greenland. Calibrated radiocarbon dates of organic remains indicate deglaciation of the region before ~10,500 years BP, after which time the ice cap receded rapidly to a position similar to or less extensive than present, and lake sediments shifted from glacio-lacustrine clay to relatively organic-rich gyttja. The lack of glacio-lacustrine sediments throughout most of the record suggests that the ice cap was similar to or smaller than present throughout most of the Holocene. This restricted ice extent suggests that climate was similar to or warmer than present, in keeping with other records from Greenland that indicate a warm early and middle Holocene. Middle Holocene magnetic susceptibility oscillations, with a ~200-year frequency in one of the lakes, may relate to solar influence on local catchment processes. Following thousands of years of restricted extent, Istorvet ice cap advanced to within 365 m of its late Holocene limit at ~AD 1150. Variability in the timing of glacial and climate fluctuations, as well as of sediment organic content changes among East Greenland lacustrine records, may be a consequence of local factors, such as elevation, continentality, water depth, turbidity, and seabirds, and highlights the need for a detailed spatial array of datasets to address questions about Holocene climate change. [ABSTRACT FROM AUTHOR]

Diatoms, organic matter and magnetic susceptibility in a 10-m-long sediment sequence from coastal Lake Lilaste, Latvia, were analysed to evaluate Holocene environmental changes related to past sea-water intrusions. Lake Lilaste is located ~1 km from the present sea coast in an area with a low uplift rate and a threshold altitude of 0.5 m a.s.l. It was thus considered to be an appropriate site to study the influence of past sea level fluctuations on the lake and its sediments. Variations in diatom community composition, along with sediment lithostratigraphy, show that a shallow, nutrient-rich freshwater lake existed there during the early Holocene. The first brackish-water diatoms appeared concurrent with a sea level rise ca. 8700 ± 50 cal a BP, but long-term, intermittent inputs of brackish water were observed between 6700 ± 40 and 4200 ± 80 cal a BP. During those time spans, diatoms indicate increased nutrient concentrations and high conductivity, a consequence of occasional mixing of brackish and freshwater that promoted biological productivity. Lilaste was isolated from the sea at 4200 ± 80 cal a BP, after which a stable freshwater environment, dominated by planktonic diatoms such as Aulacoseira ambigua, A. granulata, A. islandica and A. subarctica, was established. At 400 ± 50 cal a BP, planktonic diatoms were gradually replaced by Fragilaria spp., indicating the beginning of anthropogenic impact. The reconstructed relative water-level curve from the lake coincides with the eustatic sea level curve from 6800 ± 40 cal a BP onwards. There was a distinct increase in abundance of brackish-water diatoms when the sea level reached the threshold of Lilaste, which at that time was probably about 3 m lower than the present sea level. According to radiocarbon-dated shifts in the diatom community composition, the Litorina Sea transgression was a long-lasting event (ca. 2200 years) in the southern part of the Gulf of Riga, where the land uplift rate was near zero. It culminated more than 1000 years later than at other sites with higher uplift, in the northern part of the Baltic Sea. [ABSTRACT FROM AUTHOR]

Abbott Lake (Monterey County, California) is one of only a few natural, permanent lakes located in coastal central California. As a result, it represents an important site for assessing spatiotemporal patterns of past hydroclimatic variability. Combined sedimentologic and isotopic analyses are used to describe 1400 years of hydrologic variability within the lake and its drainage basin. Specifically, changes in sand content, percent total organic matter, and δO values suggest changes in paleo-runoff, depositional environment, and relative δO values, respectively. Nine AMS C dates on discrete organic material provide age control with a basal age estimate of ~2400 cal years BP. However, we focus on the most recent 1400 years where the age model is most robust. The results indicate that coastal central California was relatively dry (less runoff) during the Medieval Climatic Anomaly (MCA) and variably wet (more runoff) during the Little Ice Age (LIA). These results are compared to Zaca Lake (Kirby et al. 2014) and Lake Elsinore (Kirby et al. 2010), both located south of Abbott Lake, and to flood event layers from Santa Barbara Basin (Hendy et al. 2013). All three sites, including SBB's flood event history, indicate a variably wet LIA and dry MCA; however, the absolute timing of wet and dry episodes is more congruent between Zaca and Elsinore than to Abbott Lake suggesting real spatiotemporal variability along the southern to central California coast. Finally, Abbott Lake is compared to Pacific Decadal Oscillation and Pacific sea surface temperature reconstructions. These comparisons suggest a Pacific-sourced control on late Holocene hydroclimates along coastal central California. [ABSTRACT FROM AUTHOR]

Understanding the response of carbon (C) accumulation to past climate changes can provide useful insights for predicting the fate of C in a future, warmer world. Here, we present data from three well-dated peat cores that reveal the history of wetland C accumulation in the Sanjiang Plain (China), and its links to Holocene climate and environmental changes. Regional C accumulation was largely governed by monsoon-driven depositional conditions. Before ~4400 cal year BP, the strong summer monsoon favored development of lakes with relatively slow and stable C accumulation rates, ~20 g C m year. Thereafter, with the sharp decline in summer monsoon precipitation, peatlands began to form at the expense of the paleolakes, leading to a roughly sixfold increase in the C accumulation rate. This was a consequence of the higher primary production in peatlands compared to lakes. During the interval from 4400 to 500 cal year BP, the gradually increasing C accumulation rate responded to the decreasing strength of the summer monsoon. The decline in C accumulation rate over the past 500 years is attributed to both the strengthened summer monsoon and intensified human influences. [ABSTRACT FROM AUTHOR]

The grain-size distribution of the sediments of closed lake basins is sensitive to lake-level changes and thus to changes in regional climate. Deep-water areas of lakes potentially yield high resolution, continuous records of sedimentation and lake-level changes. In contrast, the marginal areas of lake basins accumulate sediment in a wave-dominated, high-energy environment and may be more sensitive to lake-level changes than deep-water environments, but they might also be more affected by sedimentary hiatuses. Here, we present grain-size data from two sections of exposed nearshore sediments of Dali Lake, North China, and compare them with previously published results from a sediment core from the lake center. We used the grain size-standard deviation method to distinguish various grains-size components of the nearshore sediments, and compared the results with those from surface sediments from various depths in order to investigate past lake-level changes. Combining the grain-size results with a radiocarbon chronology, we defined four lake-level stages during the Holocene: (1) An intermediate lake level from early Holocene to 10.0 cal ka BP. (2) A high lake level from 10.0 to 6.6 cal ka BP. (3) A decline to an intermediate lake level from 6.6 to 1.0 cal ka BP. (4) An abrupt fall to a low lake level from 1.0 cal ka BP to the present when the marginal section was covered with eolian sand. Our results indicate that the total amplitude of lake-level variation during the Holocene was greater than 45 m. This record of lake-level change is in good agreement with previous results obtained from the lake center, and it indicates that the grain-size standard deviation method may be well suited for lake-level reconstruction from nearshore sediments. Moreover, the marginal sections provide evidence of an abrupt short-lived lake-level decline more clearly than the deep-water core. [ABSTRACT FROM AUTHOR]

The molecular biomarker composition of two sediment cores from Sanabria Lake (NW Iberian Peninsula) and a survey of modern plants in the watershed provide a reconstruction of past vegetation and landscape dynamics since deglaciation. During a proglacial stage in Lake Sanabria (prior to 14.7 cal ka BP), very low biomarker concentration and carbon preference index (CPI) values ~1 suggest that the n-alkanes could have derived from eroded ancient sediment sources or older organic matter with high degree of maturity. During the Late glacial (14.7-11.7 cal ka BP) and the Holocene (last 11.7 cal ka BP) intervals with higher biomarker and triterpenoid concentrations (high % nC , nC alkanes), higher CPI and average carbon length (ACL), and lower P (proportion of aquatic plants) are indicative of major contribution of vascular land plants from a more forested watershed (e.g. Mid Holocene period 7.0-4.0 cal ka BP). Lower biomarker concentrations (high % nC alkanes), CPI and ACL values responded to short phases with decreased allochthonous contribution into the lake that correspond to centennial-scale periods of regional forest decline (e.g. 4-3 ka BP, Roman deforestation after 2.0 ka, and some phases of the LIA, seventeenth-nineteenth centuries). Human activities in the watershed were significant during early medieval times (1.3-1.0 cal ka BP) and since 1960 CE, in both cases associated with relatively higher productivity stages in the lake (lower biomarker and triterpenoid concentrations, high % nC and % nC respectively, lower ACL and CPI values and higher P). The lipid composition of Sanabria Lake sediments indicates a major allochthonous (watershed-derived) contribution to the organic matter budget since deglaciation, and a dominant oligotrophic status during the lake history. The study constrains the climate and anthropogenic forcings and watershed versus lake sources in organic matter accumulation processes and helps to design conservation and management policies in mountain, oligotrophic lakes. [ABSTRACT FROM AUTHOR]

Although bottom water hypoxia (O < 2 mg L) is presently widespread in the Baltic Sea coastal zone, there is a lack of insight into past changes in bottom water oxygen in these areas on timescales of millennia, and the possible driving factors. Here, we present a sediment-based environmental reconstruction of surface water productivity, salinity and bottom water oxygen for the past 5400 years at Gåsfjärden, a coastal site in SE Sweden. As proxies, we use dinoflagellate cysts, benthic foraminifera, organic carbon (C), biogenic silica (BSi), C/N, C/P, Ti/Al, K/Al and grain size distribution. The chronology of the sediment sequence is well constrained, based on Pb, Cs and AMS C dates. Between 3400 and 2100 BCE, isostatic conditions favored enhanced deep water exchange between Gåsfjärden and the open Baltic Sea. At that time, Gåsfjärden was characterized by relatively high productivity and salinity, as well as frequently occurring hypoxic-anoxic bottom water, despite the relatively large connection with the Baltic Sea. The most severe interval of oxygen depletion is recorded between 2400 and 2100 BCE, and appears to coincide with a similar hypoxic event in the Gotland Basin in the open Baltic Sea. As regional climate became wetter and colder between 2100 BCE and 700 BCE, salinity declined and bottom water oxygen conditions improved. Throughout the record, grain size, Ti/Al and K/Al data indicate an evolution towards a more enclosed coastal system, as suggested by reconstructions of the post-glacial shoreline regression. Gåsfjärden shifted to close to modern conditions after 700 BCE, and was characterized by less hypoxia and lower salinity compared with 3400-700 BCE. The timing of the shift corresponds with the Sub-Boreal/Sub-Atlantic transition in Europe. Human-induced erosion in the catchment is observed as early as 600 CE, and is particularly prominent since regional copper mining activity increased around 1700 CE. A sharp increase in sediment C concentration is recorded since the 1950s, indicating significant anthropogenic impact on biogeochemical cycles in the coastal zone, as observed elsewhere in the Baltic Sea. [ABSTRACT FROM AUTHOR]

In the Asian monsoon margin of northwest China, millennial-scale precipitation and effective moisture changes during the Holocene differ from those observed in the primary monsoon area. Whether these differences were caused by a change in precipitation or other hydroclimate factors remains controversial. We selected Huahai Lake, located in the western portion of the Hexi Corridor, at the northwest margin of the Asian monsoon area, to address this question. Using paleoclimate proxies (mineralogical assemblages and immobile elements) and results from a previous study, we inferred hydroclimate changes in this area during the early and middle Holocene (10.5-5.5 cal ka BP). Heavy precipitation and abundant runoff occurred during the early Holocene (10.5-8.8 cal ka BP). Proxies (carbonate content, total organic carbon and C/N) for precipitation minus evaporation (P − E) in the same section, however, revealed low P − E during the early Holocene and highest P − E in the middle Holocene (8.8-5.5 cal ka BP). Therefore, on a millennial timescale, precipitation amount and effective moisture changes were asynchronous during the early and middle Holocene. The precipitation and effective moisture pattern in the study area during that time span was different from patterns in both the Asian monsoon and westerly wind-influenced areas, suggesting an interplay between the two climate features. High precipitation during the early Holocene corresponded to a strengthened Asian monsoon. Highest P − E in the study area occurred during the middle Holocene and may have been caused by low evaporation, rather than high precipitation. [ABSTRACT FROM AUTHOR]

Phytoplankton species are the primary producers in lakes and play important roles in food-web structures. Any shift in their diversity and productivity has an impact on other aquatic life forms. We use a range of environmental variables to explore the possible drivers influencing phytoplankton composition over the last 14,500 years in a temperate lake Lielais Svētiņu, eastern Latvia. Using pollen, non-pollen palynomorphs, temperature reconstructions and lithological information as proxies of environmental factors, we statistically test their associations with the fossil phytoplankton community composition. Our results reveal that during the Late Glacial, the climate warming, the decrease in landscape openness, and increase in organic matter were significant environmental variables affecting dynamics of phytoplankton communities , especially in the prevalence of Botryococcus, Tetraedron, Scenedesmus and Pediastrum. According to the Redundancy Analysis and Generalized Least Squares models, Pediastrum, Scenedesmus and Tetraedron were positively associated with waterlogging tolerance that indicates moist soils in surroundings of the lake, during the Early Holocene. The 8.2 ka cold event with a 2-3 °C cooling led to a strong environmental disturbance for nearly 700 years, indicated by an increased chlorophyta accumulation rates and a decrease in the organic matter. Our results indicate that Coelastrum reticulatum and C. polychordum are characteristic for the 8.2 ka cold event. Positive association between cyanobacteria and mean air summer temperature suggests that a warming favoured cyanobacteria over other phytoplankton taxa between 8000 and 4000 cal yr BP. High nutrient loads and water turbidity were more important for the dynamics of cyanobacteria from 4000 to 2000 cal yr BP. Human-driven trophic level change was recorded in the last 2000 years by abundances of fungi Sporormiella and Sordaria, and by the peaks of Gloeotrichia pisum, C. reticulatum and C. polychordum indicating eutrophication. [ABSTRACT FROM AUTHOR]

In order to better constrain the limnological impacts from recent climate change relative to those of the Holocene, we developed a high-resolution multi-proxy paleoenvironmental record from a small lake on eastern Baffin Island, Arctic Canada. Carbon and nitrogen elemental and isotopic compositions from sediment organic matter, algal pigments, and diatom assemblages are integrated to provide robust indices of paleoclimatic variability. In particular, the ratio between individual carotenoid pigments (lutein:diatoxanthin) reveals a shift in dominant primary production from 'green' taxa (chlorophytes, higher plants, and bryophytes) during the Holocene Thermal Maximum (HTM) to 'brown' taxa (diatoms and chrysophytes) over the mid- to late Holocene. Green pigment abundance appears most sensitive to mean summer temperatures, and their increased relative abundance in the past serves as an indicator of warm times. Regionally, the HTM occurred shortly after local deglaciation (10 ka), persisting until ~7 ka. This timing agrees with that revealed by chironomid assemblages and ice-core records elsewhere in the Canadian Arctic, but is significantly earlier than suggestions from palynology on Baffin Island. This study provides additional evidence that this discrepancy represents the ecesis for higher plant dispersal and colonization on distal, freshly deglaciated landscapes. Pigment and diatom data indicate that mid Holocene cooling began between 7 and 6 ka, intensifying after 3 ka. All proxies show pronounced change after 1.5 ka, with the greatest divergence from average Holocene values occurring during the Little Ice Age (LIA), supporting the growing consensus that the LIA was the coldest multi-centennial interval of the Holocene. In the twentieth century, most proxies, including sedimentary carotenoid ratios, abruptly returned to a similar state as the Holocene Thermal Maximum, while diatom species assemblages present a more muted response. This underscores that anthropogenic alteration of the Earth system has created conditions with no exact analog in the past 10,000 years. Collectively, these results add new information on the dimensions of Arctic lake responses to Holocene climate change, which in turn can be used to reconcile paleoclimate reconstructions from diverse proxies. [ABSTRACT FROM AUTHOR]

The Bezedna site represents a unique lake-mire complex in the Lublin chalkland, eastern Poland. This karst region contains Upper Cretaceous carbonate rocks under the influence of continental climate. Using sedimentologic, palaeobotanical, and fossil ostracode analyses as well as radiocarbon dating on cores, we were able: (1) to reconstruct the morphogenetic mechanisms related to lake formation, (2) to describe the main stages of lake evolution, and (3) to identify the environmental conditions during this evolution. The first stage was the formation of an accumulation basin in a periglacial setting related to permafrost thawing in the last phase of the Weichselian and at the beginning of the Holocene. During the second stage, in the successive phases of the Holocene, lake evolution was determined by climate changes and local conditions, and in the youngest Subatlantic period-also by human activity in several settlement phases. Natural water bodies in a karstic region create unique conditions for water circulation, especially for groundwater. During the change from the periglacial to temperate climate of the Holocene in the Lublin chalkland, the response of substratum (deeply frozen during the Weichselian) was influential in the development of surface water conditions in the study area. The strong fracturing of the Upper Cretaceous bedrock and general change of groundwater circulation associated with the activation of an ascending groundwater supply favored the intensive dissolution of carbonate and the progressive ground subsidence. Based on the reconstruction and correlation with paleoenvironmental investigations of the adjacent Volhynia chalkland, we were able to estimate and compare the influences of local (morphologic and geologic) conditions versus global climate changes on the paleoenvironmental record. Climate-driven lake-level fluctuations coincided over great areas of the Northern Hemisphere and are well reflected in changes of the Ostracoda fauna. [ABSTRACT FROM AUTHOR]

This study presents a detailed analysis of geochemical and biotic proxies in a lake sediment profile to assess the effects of local and regional environmental drivers on the Holocene development of Lake Loitsana, situated in the northern boreal forest of NE Finland. Multi-proxy studies, in particular those that include a detailed plant macrofossil record, from the part of the northern boreal zone of Fennoscandia which has not been affected by treeline fluctuations, are scarce and few of these records date back to the earliest part of the Holocene. A 9-m sediment sequence of gyttja overlying silts representing the last c. 10,700 cal year, allowed for a high-resolution study with emphasis on the early to mid-Holocene lake history. The lacustrine sediments were studied using lithology, loss-on-ignition and C/N ratios, micro- and macro-fossils of aquatic and wetland taxa, diatoms, chironomids and accelerator mass spectrometry C dating on terrestrial plant macrofossils. Our study shows that the local development at Loitsana was complex and included a distinct glacial lake phase and subsequent drainage, a history of fluvial input affected by nearby wetland expansion, and lake infilling in an eventual esker-fed shallow lake. Enhanced trophic conditions, due to morphometric eutrophication, are recorded as Glacial Lake Sokli drained and open water conditions became restricted to a relatively small Lake Loitsana depression. pH appears to have been stable throughout the Holocene with a well-buffered lake due to the local carbonatite bedrock (Sokli Carbonatite Massif). The fossil assemblage changes are best explained by a complex mixture of drivers, including water-body conditions (i.e. depth, turbidity and turbulence), rate of sediment input, and the general infilling of the lake, highlighting the need to carefully evaluate the possible influence of such local factors as palaeoenvironmental conditions are reconstructed based on aquatic proxies. [ABSTRACT FROM AUTHOR]