Reconstructing past precipitation from lake levels and inverse modelling for Andean Lake La Cocha

Knowledge of paleoclimates and past climate change is important to put recent and future climate change in perspective. In the absence of well-developed methodology to reconstruct paleoprecipitation the majority of climate reconstructions focus on temperature, whereas precipitation is an equally important climate parameter. This paper explores the possibility of inferring paleoprecipitation from lake-level records by inverse hydrological modelling. Pollen spectra of a lacustrine sediment core were used to infer changes in past temperatures and lake levels during the past 14,000 years. A hydrological model that calculates lake levels using meteorological parameters and a digital terrain model were developed for the catchment area of Lake La Cocha. After calibration the model accurately simulated modern lake levels. A sensitivity analysis shows that the model results are most sensitive to temperature and precipitation. This hydrological model was subsequently used to estimate mean annual precipitation needed to reproduce the pollen-based reconstructed lake levels (inverse modelling). The lake currently discharges through the permanent Guamués River, with a modelled mean annual discharge of 3.6 m³ s^?1. However, past lake levels and hydrological modelling results suggest that Lake La Cocha has been free of discharge during most of the Holocene, and after an intermittent phase only recently started discharging permanently. The uncertainty in the inferred precipitation during the discharge-free period is estimated at ~22 mm. Quasi stable lake levels seem to justify using equilibrium conditions when reconstructing precipitation. Early Holocene lake levels were ~10 m lower than modern values, implying that precipitation must have been 30–40 % less than today.     

A high-resolution sedimentary archive from landslide-dammed Lake Mengda,north-eastern Tibetan Plateau

Lacustrine sediments have been widely used to investigate past climatic and environmental changes on millennial to seasonal time scales. Sedimentary archives of lakes in mountainous regions may also record non-climatic events such as earthquakes. We argue herein that a set of 64 annual laminae couplets reconciles a stratigraphically inconsistent accelerator mass spectrometry (AMS) ¹⁴C chronology in a ~4-m-long sediment core from Lake Mengda, in the north-eastern Tibetan Plateau. The laminations suggest the lake was formed by a large landslide, triggered by the 1927 Gulang earthquake (M = 8.0). The lake sediment sequence can be separated into three units based on lithologic, sedimentary, and isotopic characteristics. Starting from the bottom of the sequence, these are: (1) unweathered, coarse, sandy valley-floor deposits or landslide debris that pre-date the lake, (2) landslide-induced, fine-grained soil or reworked landslide debris with a high organic content, and (3) lacustrine sediments with low organic content and laminations. These annual laminations provide a high-resolution record of anthropogenic and environmental changes during the twentieth century, recording enhanced sediment input associated with two phases of construction activities. The high mean sedimentation rates of up to 4.8 mm year^?1 underscore the potential for reconstructing such distinct sediment pulses in remote, forested, and seemingly undisturbed mountain catchments.     

Late Holocene change in climate and atmospheric circulation inferred from geochemical records at Kepler Lake, south-central Alaska

Climate records during the last millennium are essential in placing recent anthropogenic-induced climate change into the context of natural climatic variability. However, detailed records are still sparse in Alaska, and these records would help elucidate climate patterns and possible forcing mechanisms. Here we present a multiple-proxy sedimentary record from Kepler Lake in south-central Alaska to reconstruct climatic and environmental changes over the last 800?years. Two short cores (85 and 101?cm long) from this groundwater-fed marl lake provide a detailed stable isotope and sediment lithological record with chronology based on four AMS ¹⁴C dates on terrestrial macrofossils and ²¹⁰Pb analysis. The ??¹⁸O values of inorganic calcite (CaCO₃) range from ?17.0 to ?15.7???, with the highest values during the period of 1450?C1850 AD, coeval with the well-documented Little Ice Age (LIA) cold interval in Alaska. The high ??¹⁸O values during the cold LIA are interpreted as reflecting shifts in atmospheric circulation. A weakening of the wintertime Aleutian low pressure system residing over the Gulf of Alaska during the LIA would have resulted in ¹⁸O-enriched winter precipitation as well as a colder and possibly drier winter climate in south-central Alaska. Also, elevated calcite contents of >80?% during the LIA reflect a lowering of lake level and/or enhanced seasonality (warmer summer and colder winter), as calcite precipitation in freshwater lakes is primarily a function of peak summer temperature and water depth. This interpretation is also supported by high ??¹³C values, likely reflecting high aquatic productivity or increased residence times of the lake water during lower lake levels. The lower lake levels and warmer summers would have increased evaporative enrichment in ¹⁸O, also contributing to the high ??¹⁸O values during the LIA. Our results indicate that changes in atmospheric circulation were an important component of climate change during the last millennium, exerting strong influence on regional climate in Alaska and the Arctic.     

Human Impact Since Medieval Times and Recent Ecological Restorationin a Mediterranean Lake: The Laguna Zoñar, Southern Spain

The multidisciplinary study of sediment cores from Laguna Zoñar (37°29′00′′ N, 4°41′22′′ W, 300 m a.s.l., Andalucía, Spain) provides a detailed record of environmental, climatic and anthropogenic changes in a Mediterranean watershed since Medieval times, and an opportunity to evaluate the lake restoration policies during the last decades. The paleohydrological reconstructions show fluctuating lake levels since the end of the Medieval Warm Period (ca. AD 1300) till the late 19th century and a more acute dry period during the late 19th century – early 20th century, after the end of the Little Ice Age. Human activities have played a significant role in Laguna Zoñar hydrological changes since the late 19th century, when the outlet was drained, and particularly in the mid-20th century (till 1982) when the spring waters feeding the lake were diverted for human use. Two main periods of increased human activities in the watershed are recorded in the sediments. The first started with the Christian conquest and colonization of the Guadalquivir River Valley (13th century) particularly after the fall of the Granada Kingdom (15th century). The second one corresponds to the late 19th century when more land was dedicated to olive cultivation. Intensification of soil erosion occurred in the mid-20th century, after the introduction of farm machinery. The lake was declared a protected area in the early 1980s, when some agricultural practices were restricted, and conservation measures implemented. As a consequence, the lake level increased, and some littoral zones were submerged. Pollen indicators reflect this limnological change during the last few decades. Geochemical indicators show a relative decrease in soil erosion, but not changes in the amount of chemical fertilizers reaching the lake. This study provides an opportunity to evaluate the relative significance of human vs. climatic factors in lake hydrology and watershed changes during historical times. Paleolimnological reconstructions should be taken into account by natural resources agencies to better define lake management policies, and to assess the results of restoration policies.     

Historical and paleolimnological evidence for expansion of Lake Athabasca (Canada) during the Little Ice Age

A multi-proxy paleolimnological record obtained from a small, lowland closed-drainage basin located in the Peace-Athabasca Delta (Alberta, Canada), 10 km northwest of the present-day shore of Lake Athabasca, captures evidence of pronounced hydroecological changes over the past ~400 years. Consistent with historical maps produced by early European explorers of western Canada, paleolimnological data support the existence of a Lake Athabasca highstand during the Little Ice Age (LIA), c. 1600–1900 Common Era (CE). This contrasts with interpretations from previous analyses on sediment cores from an upland closed-drainage basin located centrally within the Peace sector of the delta that indicate low water levels during this interval. The different paleohydrological records at these two basins reflect the relative influence of different controls on the lake water balances. During the LIA, the lowland site was influenced by high levels in Lake Athabasca, whereas the elevated basin was outside the range of water-level rise in the lake and its distributaries, and was instead controlled by dry atmospheric conditions that led to evaporative drawdown. Integration of paleolimnological records and historical sources demonstrates that the ecosystem has undergone marked climate-driven hydroecological change over the past century, which is important information for effective management.     

Recognition and mapping of lacustrine relict coastal features using high resolution aerial photographs and LiDAR data

Shallow lakes in semiarid regions experience frequent water level fluctuations. Each long-lasting episode of water-level lowering leaves behind abandoned littoral forms and deposits whose identification and mapping is hampered by their smooth relief. Given the difficulty of recognising these possible relict forms using traditional geomorphological techniques, two sources of information were employed in the present work: high resolution (1:15,000) aerial photographs and a digital terrain model (DTM) generated from LiDAR data. The improved definition of surface elevation enhanced the quality of geomorphological mapping as well as the accurate delineation of subtle geoforms. The method was applied to Gallocanta Lake, a highly fluctuating shallow lake 14 km² in area and less than 3 m deep located in a mountainous semiarid area of NE Spain. As a result, a sequence of relict coastal features (RCF) with high lateral continuity has been identified around the lakebed. These include well-preserved spits with recurved hooks, counter-spits, bays closed by barrier islands, beach ridges, deltas and cliffs. The highly precise LiDAR-derived topographic maps suggest a much greater extension of the lacustrine environment during the Late Pleistocene, reaching at least 51 km² of water surface and about 13 m of depth above the present lake bottom. The method presented in this paper generates very detailed palaeogeographical maps that are particularly useful for reconstructing lake changes in semiarid environments as a function of climate change.     

Effects of long term nutrient and climate variability on subfossil Cladocera in a deep,subalpine lake (Lake Garda,northern Italy)

Albeit subfossil Cladocera remains are considered as a reliable proxy for tracking historical lake development, they have been scarcely studied in large subalpine lakes south of the Alps. In this study, subfossil Cladocera remains from Lake Garda in northern Italy were analyzed to track the lake’s environmental changes since the Middle Ages. One core was retrieved from the largest sub-basin of Lake Garda (Brenzone, 350 m deep) and two cores were retrieved from the profundal and littoral zone of the smaller lake sub-basin (Bardolino, 80 and 40 m deep, respectively). The species distribution of Cladocera remains in recent sediment layers was similar to that observed in contemporary water samples. The deepest sections of the three cores were characterized by species sensitive to water temperature, which suggest a key role of major climatic events in driving the lake response during the late Holocene. The most evident change in Cladocera assemblages in the studied cores was observed during the 1960s, when planktonic taxa such as Daphnia spp. and Bosmina spp. became dominant at the expense of littoral taxa. Despite the highly coherent general pattern of subfossil Cladocera, the cores showed a set of minor differences, which were attributed to different morphological and hydrological features of the two basins forming Lake Garda. Multivariate analysis revealed a clear relation of Cladocera to climate variability during periods of low lake nutrient levels (i.e. before the 1960s). This provides additional information on past ecological responses of Lake Garda, as contrast previous data from subfossil diatom and pigment analyses did not fully capture effects of climate change during the same period. Since the 1960s shifts in assemblage composition of Cladocera and diatoms were highly coherent, in response to nutrient increase and the following, less pronounced, decrease in nutrient concentrations. During the last five decades, the response of the Cladocera to climate variability appeared to be masked by nutrient change. This work points up the potential of the multi-proxy approach for disentangling the multifaceted biological responses to multiple environmental stressors in large and deep lakes.     

Solar and anthropogenic imprints on Lake Masoko (southern Tanzania) during the last 500 years

The Masoko crater-lake in southern Tanzania provides a continuous record of environmental changes covering the last 500 years. Multi-proxy studies were performed on a 52 cm sediment core retrieved from the deepest part of the lake. Magnetic, organic carbon, geochemical proxies and pollen assemblages indicate a dry climate during the ‘Little Ice Age’ (AD 1550–1850), confirming that the LIA in eastern Africa resulted in marked and synchronous hydrological changes. However, the direction of response varies between different African lakes (low versus high lake-levels), indicating strong regional contrasts that prevent the clear identification of climate trends over eastern Africa at this time. Inferred changes in Masoko lake-levels closely resemble the record of solar activity cycles, indicating a possible control of solar activity on the climate in this area. This observation supports previous results from East African lakes, and extends this relationship southward. Finally, anthropogenic impact is observed in the Masoko sediments during the last 60 years, suggesting that human disturbance significantly affected this remote basin during colonial and post-colonial times.     

Middle and late Holocene climate change and human impact inferred from diatoms,algae and aquatic macrophyte pollen in sediments from Lake Montcortès (NE Iberian Peninsula)

During the middle and late Holocene, the Iberian Peninsula underwent large climatic and hydrologic changes, but the temporal resolution and regional distribution of available palaeoenvironmental records is still insufficient for a comprehensive assessment of the regional variability. The high sedimentation rate in karstic, meromictic Montcortès Lake (Catalan pre-Pyrenees) allows for a detailed reconstruction of the regional palaeoecology over the last 5,340 years using diatom analysis, aquatic pollen, sedimentological data, and historic documentary records. Results show marked fluctuations in diatom species assemblage composition, mainly between dominant Cyclotella taxa and small Fragilariales. We suggest that the conspicuous alternation between Cyclotella comta and C. cyclopuncta reflects changes in trophic state, while the succession of centric and pennate species most likely reflects changes in the hydrology of the lake. The diatom assemblages were used to identify six main phases: (1) high productivity and likely lower lake levels before 2350 BC, (2) lower lake levels and a strong arid phase between 2350 and 1850 BC, (3) lake level increase between 1850 and 850 BC, (4) relatively high lake level with fluctuating conditions during the Iberian and Roman Epochs (650 BC–350 AD), (5) lower lake levels, unfavourable conditions for diatom preservation, eutrophication and erosion triggered by increased human activities in the watershed during the Medieval Climate Anomaly (900–1300 AD), and (6) relatively higher lake levels during the LIA (1380–1850 AD) and afterwards. The combined study of diatoms, algae and pollen provides a detailed reconstruction of past climate, which refines understanding of regional environmental variability and interactions between climate and socio-economic conditions in the Pyrenees.     

Natural and human-induced environmental changes in Eastern Europe during the Holocene: a multi-proxy palaeolimnological study of a small Latvian lake in a humid temperate zone

This study uses the Holocene lake sediment of Lake ?ū?i (Latvia, Vidzeme Heights) for environmental reconstruction with multi-proxy records including lithology, computerised axial tomography scan, grain-size analysis, geochemistry, diatoms and macrofossils, supported by AMS radiocarbon dating. Numerical analyses (PCA; CONISS) reveal three main phases in the development of the lake. Response to the Lateglacial–Holocene transition in Lake ?ū?i took place around 11,300 cal. BP. Organogenic sedimentation started with distinctive 5-cm-thick peat layer and was followed by lacustrine sedimentation of carbonaceous gyttja. Several findings of the peat layer with similar dated age and position at different absolute altitudes indicate that lake basin was formed by glaciokarstic processes. In the Early Holocene (until around 8,500 cal. BP), the lake was shallow and holomictic, surrounded by unstable catchment with erosion and inflow events. Predominance of diatom species of Cyclotella and Tabellaria, large numbers of respiratory horns of phantom midge pupae (Chaoboridae), high Fe/Mn ratio, as well as the presence of laminated sediments indicates the transition to a dimictic and oligo-mesotrophic lake conditions with high water level, anoxia in the near-bottom and stable catchment in the Middle Holocene (8,500–2,000 cal. BP). This contrasts with many hydrologically sensitive lakes in Northern and Eastern Europe in which the water level fell several meters during this period. During the Late Holocene (from 2,000 cal. BP to the present), the lithological and biotic variables reveal major changes, such as the increase in erosion (coarser grain-size fraction) and eutrophication [diatoms Aulacoseira ambigua (Grun.) Sim., Stephanodiscus spp., Cyclostephanos dubius (Fricke) Round]. Characteristics of lake-catchment system during the Late Holocene reflect anthropogenic signal superimposed on the natural forcing factors. To date, the Late Quaternary palaeolimnological reconstructions using lake sediment has been limited in the Baltic region. Therefore, findings from Lake ?ū?i provide important information about environmental and climatic changes that took place in this part of Eastern Europe. This study shows that the relative importance of climate and local factors has varied over the time and it is essential to consider the lake basin topography, catchment size and land cover as potential dominant forcing factors for changes in sedimentary signal.     

Vegetation changes in the southern Pyrenean flank during the last millennium in relation to climate and human activities: the Montcortès lacustrine record

We report vegetation changes of the last millennium inferred from palynological analysis of a sediment core from Lake Montcortès, situated at ~1,000 m elevation in the southern pre-Pyrenean flank. The record begins in the Middle Ages (~AD 800) and ends around AD1920, with an average resolution of ~30 years. The reconstructed vegetation sequence is complex and shows the influence of both climate and humans in shaping the landscape. Pre-feudal times were characterized by the presence of well-developed conifer forests, which were intensely burned at the beginning of feudal times (AD 1000) and were replaced by cereal (rye) and hemp cultivation, as well as meadows and pastures. In the thirteenth century, a relatively short period of warming, likely corresponding to the Medieval Warm Period, was inferred from the presence of a low Mediterranean scrub community that is today restricted to <800 m elevation. This community disappeared during Little Ice Age cooling in the fifteenth century, coinciding with a decline in human activities around the lake. Forest recovery began around AD 1500, at the beginning of the Modern period, coinciding with wetter climate. Forests, however, declined again during the seventeenth century, coinciding with maximum olive and hemp cultivation. This situation was reversed in post-Modern times (nineteenth century), characterized by an intense agricultural crisis and a significant decline in population that favored forest re-expansion. Correlations with nearby Estanya Lake, situated about 350 m below, provide a regional picture of environmental change. Besides some climate forcing evident in both sequences, human activities seem to have been the main drivers of landscape and vegetation change in the southern Pyrenean flank, in agreement with conclusions from other studies in high-mountain environments.     

Environmental variability in Lake Naivasha,Kenya, over the last two centuries

Lake Naivasha, Kenya, is one of a number of freshwater lakes in the East African Rift System. Since the beginning of the twentieth century, it has experienced greater anthropogenic influence as a result of increasingly intensive farming of coffee, tea, flowers, and other horticultural crops within its catchment. The water-level history of Lake Naivasha over the past 200 years was derived from a combination of instrumental records and sediment data. In this study, we analysed diatoms in a lake sediment core to infer past lacustrine conductivity and total phosphorus concentrations. We also measured total nitrogen and carbon concentrations in the sediments. Core chronology was established by ²¹⁰Pb dating and covered a ~186-year history of natural (climatic) and human-induced environmental changes. Three stratigraphic zones in the core were identified using diatom assemblages. There was a change from littoral/epiphytic diatoms such as Gomphonema gracile and Cymbella muelleri, which occurred during a prolonged dry period from ca. 1820 to 1896 AD, through a transition period, to the present planktonic Aulacoseira sp. that favors nutrient-rich waters. This marked change in the diatom assemblage was caused by climate change, and later a strong anthropogenic overprint on the lake system. Increases in sediment accumulation rates since 1928, from 0.01 to 0.08 g cm⁻² year⁻¹ correlate with an increase in diatom-inferred total phosphorus concentrations since the beginning of the twentieth century. The increase in phosphorus accumulation suggests increasing eutrophication of freshwater Lake Naivasha. This study identified two major periods in the lake’s history: (1) the period from 1820 to 1950 AD, during which the lake was affected mainly by natural climate variations, and (2) the period since 1950, during which the effects of anthropogenic activity overprinted those of natural climate variation.     

A record of early to middle Holocene environmental change inferred from lake deposits beneath a sabkha sequence in the Central Sahara (Seggedim, NE Niger)

Seggedim is a remote site in NE Niger where previous studies demonstrated the presence of Holocene paleolake deposits beneath an existing sabkha. In 2005, we retrieved a 15-m core that contains a high-resolution record of early to middle Holocene environmental change. Mineralogical, chemical and petrographic data were used to infer paleoenvironmental conditions recorded by successive lake stages. An exceptionally stable lake regime existed from 10.6 to 7.3 cal ka BP. This regime, reflecting moist conditions, was characterized by deposition of continuously laminated diatomite with high organic matter content, formed under meromictic conditions, with very low detrital input. A lake regime shift occurred at 7.3 cal ka BP, interpreted as the beginning of a transition to arid conditions. This shift was initially marked by a change to predominantly clastic sedimentation, followed by increased sand input during later stages, culminating in the onset of the sabkha stage. Because of the presence of reworked organic material in the deposits, reliable radiocarbon dating of the succession of lake stages could not be achieved.     

Development of large shallow Lake Peipsi (North-Eastern Europe) over the Holocene based on the stratigraphy of phosphorus fractions

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.     

Effects of spring warming and mixing duration on diatom deposition in deep Tiefer See,NE Germany

Monitoring during three meteorologically different spring seasons in 2012, 2013, and 2014 revealed that temperature increase in spring, which influences spring lake mixing duration, markedly affected nutrient availability and diatom deposition in a sediment trap close to the bottom of deep Tiefer See, NE Germany. Deposition of Stephanodiscus taxa and small Cyclotella taxa was much higher after late ice out and a deep, short lake mixing period in spring 2013, compared to that after gradual warming and lengthy lake mixing periods in spring 2012 and 2014, when only brief or marginal ice cover occurred. Availability of dissolved Si and P was 33 and 20 % higher, respectively, in 2013 compared to 2014. The observed relation between high (low) diatom deposition and short (lengthy) mixing duration in spring was applied to varved sediments deposited between AD 1924 and 2008. Low detrital Si content in trapped material and a sediment core enabled use of µXRF-counts of Si as a proxy for diatom silica. The spring mixing duration for 1951–2008 was derived from FLake-model calculations. The spring warming duration related to lake mixing was approximated from air temperatures for 1924–2008 using the dates when daily mean air temperature exceeded 5 °C (start) and 10 °C (end). Diatom silica deposition showed a significant (p < 0.0001) inverse linear relationship with the modeled spring mixing duration (R² = 0.36) and the spring warming duration (R² = 0.28). In both cases, the relationship is strengthened when data from the period of low diatom production (1987–2005) is excluded (R² = 0.59 and R² = 0.35). Part of this low diatom production is related to external nutrient supply that favored growth of cyanobacteria at the expense of diatoms. This approach shows that diatom Si deposition was strongly influenced by the availability of light and nutrients, related to the duration of lake mixing and warming in spring, during most of the studied period. The remaining unexplained variability, however, indicates that additional factors influence Si deposition. Further tests in other deep, temperate lakes are necessary to verify if this relation is a common feature and consequently, if diatom Si can be used as a proxy for spring mixing duration in such lakes.     

Last deglaciation and Holocene environmental change at high altitude in the Pyrenees: the geochemical and paleomagnetic record from Marboré Lake (N Spain)

Sedimentological, geochemical and magnetic data in a ~ 7-m sequence from Marboré Lake (2612 m asl, central Pyrenees) provide information about environmental variability since the last glacier retreat (14.6 cal ka BP) in high-altitude Pyrenean environments. The sediment sequence is composed of millimeter- to centimeter-thick rhythmites made of finer greyish laminae and coarser-grain, carbonate-bearing laminae arranged in varied patterns throughout the sequence. Finer laminae are interpreted as deposition during periods of predominantly ice-covered conditions, whereas coarser, carbonate-bearing sediments reflect periods of higher runoff. The age model, based on 13 ¹⁴C dates and a reservoir effect assessed with ²¹⁰Pb and ¹³⁷Cs, is coherent with known synchronous vegetation changes across the Pyrenees. Warmer intervals such as GI-1 (14.6–12.8 cal ka BP, Bølling/Allerød period), 10.4–8.2 cal ka BP in the Early Holocene, 7.5–5.2 cal ka BP in the Mid Holocene and the Medieval Climate Anomaly (AD 900–1300), are characterized by peaks in productivity and higher carbonate preservation. Deposition during colder periods such as GS-1 (12.8–11.7 cal ka BP), the Neoglacial (ca. 5.2–3.5 cal ka BP) and the Little Ice Age (last 400 years) show an increase in finer laminae. The presence of magnetite throughout the whole section suggests that Marboré Lake maintained predominantly oxic conditions since its formation. Changes in magnetic properties and the increase in magnetite from 3.5 cal ka BP to present, however, indicate a more oxic environment at the lake bottom during the last few millennia. The occurrence of Pb concentration peaks in sediments of Roman and modern age demonstrates the global distribution of heavy metal deposition, even into high-mountain lakes.     

Persistence of Artemisia steppe in the Tangra Yumco Basin,west-central Tibet,China: despite or in consequence of Holocene lake-level changes?

The closed Tangra Yumco Basin underwent the strongest Quaternary lake-level changes so far recorded on the Tibetan Plateau. It was hitherto unknown what effect this had on local Holocene vegetation development. A 3.6-m sediment core from a recessional lake terrace at 4,700 m a.s.l., 160 m above the present lake level of Tangra Yumco, was studied to reconstruct Holocene flooding phases (sedimentology and ostracod analyses), vegetation dynamics and human influence (palynology, charcoal and coprophilous fungi analyses). Peat at the base of the profile proves lake level was below 4,700 m a.s.l. during the Pleistocene/Holocene transition. A deep-lake phase started after 11 cal ka BP, but the ostracod record indicates the level was not higher than ~4,720 m a.s.l. (180 m above present) and decreased gradually after the early Holocene maximum. Additional sediment ages from the basin suggest recession of Tangra Yumco from the coring site after 2.6 cal ka BP, with a shallow local lake persisting at the site until ~1 cal ka BP. The final peat formation indicates drier conditions thereafter. Persistence of Artemisia steppe during the Holocene lake high-stand resembles palynological records from west Tibet that indicate early Holocene aridity, in spite of high lake levels that may have resulted from meltwater input. Yet pollen assemblages indicate humidity closer to that of present potential forest areas near Lhasa, with 500–600 mm annual precipitation. Thus, the early mid-Holocene humidity was sufficient to sustain at least juniper forest, but Artemisia dominance persisted as a consequence of a combination of environmental disturbances such as (1) strong early Holocene climate fluctuations, (2) inundation of habitats suitable for forest, (3) extensive water surfaces that served as barriers to terrestrial diaspore transport from refuge areas, (4) strong erosion that denuded the non-flooded upper slopes and (5) increasing human influence since the late glacial.     

A multidisciplinary study of Holocene sediment records from Hjort Sø on Store Koldewey, Northeast Greenland

Two sediment cores of 70 and 252 cm length were recovered from Hjort Sø, a small lake on Store Koldewey, Northeast Greenland, and studied with a multidisciplinary approach in order to reconstruct the local environmental history and to test the relevance of proxies for paleoenvironmental information. The basal sediments from the longer core are dominated by clastic matter, which was likely deposited during deglaciation of the lake basin. These clastic sediments are overlain by gyttja, which is also present throughout the shorter core. AMS radiocarbon dating was conducted on plant macrofossils of 11 samples from the gyttja in both cores. A reliable chronology was established for both cores, which dated the onset of organic accumulation at 9,500 cal. year BP. The Holocene temperature development, with an early to mid Holocene thermal maximum, is best reflected in the grain-size composition. Nutrient availability was apparently low during the early Holocene and led to low productivity in the lake and its vicinity. From ca. 7,000 cal. year BP, productivity in the lake increased significantly, probably induced by external nutrient input from goose excrements. From this time, micro- and macro-fossil remains reflect relatively well the climate history of East Greenland, with a cooling during the middle Holocene, the medieval warming, and the Little Ice Age. The amount of organic matter in the sequence seems to be more affected by lake ice cover or by nutrient supply from the catchment than by temperature changes. The record from Hjort Sø thus reveals the difficulties in interpreting sedimentary records from high arctic regions.     

A multi-proxy approach to reconstruct hydrological changes and Holocene climate development of Nam Co, Central Tibet

Holocene lake level fluctuations were reconstructed from a 2.7-m sediment core from Nam Co, Central Tibet, China dating to >7.2 cal ka BP. Results were compared to existing lake records from the Tibetan Plateau to infer variations in the strength of the Asian Monsoon. Geomorphological features in the Nam Co catchment, such as beach ridges and lake terraces, indicate high lake stands during the late Glacial. A major low stand is suggested for the Last Glacial Maximum (LGM). Sands and sandy silts at the base of the core are transgressive facies, with material transported by melt water and deposited under rising lake level conditions that followed the LGM low stand. Variations in grain size, major elements, biomarker stable isotopes and minerals in the core suggest a climate evolution reflected in at least five depositional units and subunits. Sediments in Unit I (~7.2 to ~5.4 cal ka BP) were deposited at highest lake levels. Large amounts of allogenic minerals and allochthonous organic matter suggest high precipitation and melt water input, implying positive water balance. Increasing aquatic productivity points to favourable environmental conditions. Unit II (~5.4 to ~4.0 cal ka BP) marks a transition between favourable, stable hydrological conditions and lake level decrease. Lower lake levels were a consequence of drier climate with less monsoonal precipitation, higher evaporation rates, and increased moisture recycling in the catchment. Unit III (~4.0 to ~1.4 cal ka BP) reflects the driest periods recorded, at ~3.7 cal ka BP and 1.6 cal ka BP. Lake shrinkage and salinization was interrupted as suggested by the deposition of Unit IV (~1.4 to ~0.8 cal ka BP), when increased precipitation and runoff that might be related to the Medieval Warm Period, led to a stable, but still low lake level. Unit V (800 cal years BP—present) is characterized by progressive lake shrinkage due to intense evaporation. Large fluctuations in geochemical variables indicate humid and arid periods, respectively, at Nam Co between ~450 and ~200 cal years BP, with the latter assumed to correspond to the Little Ice Age. Modern hydrological data indicate the lake level is rising. Comparison of the Nam Co record with other lake records from the Tibetan Plateau suggests general agreement with the broader picture of Holocene environmental evolution. The timing of dry and wet climate conditions at lake sites across Tibet indicates a gradually decreasing influence of the southern monsoon during the Holocene, from NW to SE. Nevertheless, further research is needed to improve our understanding of Holocene spatio-temporal hydrological variations across the Asian continent.     

A 17,300-year record of mercury accumulation in a pristine lake in southern Chile

Anthropogenic mercury accumulation in lake sediments has been studied extensively, but natural processes that controlled mercury accumulation in the past are still poorly understood. We present a 17,300-year record of mercury accumulation in the sediments of Lake Hambre, southernmost Patagonia, Chile (53°S, 70°W), in combination with an investigation of environmental changes in the region. Mercury accumulation in the remote pristine lake varied appreciably, as much as 16×, exceeding the anthropogenic forcing of atmospheric mercury fluxes by a factor of 3–5. Principal Component Analysis revealed that short-term variations were not related to changes in atmospheric mercury deposition or to fluxes of mineral soil into the lake. Instead, there was significant covariation between short-term changes in mercury and catchment-derived trace elements, e.g. copper and yttrium, throughout the past 17,000 years. Covariation between mercury and carbon concentration in some parts of the record suggests that fluxes of particulate and dissolved organic matter from the catchment to the lake account for short-term variations in mercury accumulation. Nevertheless, over the long term, there is no common trend for mercury accumulation and organic matter flux. The median mercury accumulation rate was rather constant (29 μg m^?2 a^?1), whereas the flux of terrestrial organic matter into the lake increased through time. We hypothesize that this was a consequence of a progressive decrease in the input of terrestrial organic matter-bound mercury through time. Whereas production of terrestrial organic matter increased over the long term because of development of catchment vegetation and soils, following glacier retreat, amounts of mercury, copper and yttrium provided by atmospheric deposition and bedrock weathering remained relatively constant. As a consequence, despite increased fluxes of terrestrial organic matter to the lake, fluxes of mercury remained constant.