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Human activities have contributed to significant disruptions of the phosphorus (P) cycle on Earth’s surface. Yet, there is little information about when and how humans started to influence the global P cycle in the past. In this study, we reconstruct lake-wide P burial rates during the Holocene based on sediment-P data of 108 lakes across the globe. The results indicate the first distinct increases in lake P burial rates after the mid-late Holocene (at around 4000 years before present BP) at global scales and in Europe. Yet, different land-use histories have caused different timings of the first increases in lake P records in other regions, with ~2000 BP in China and ~550 BP in North America. We further show that global lake P-sequestration rate from ~4000 BP to 1850 Common Era (CE) has doubled compared with that in the period before 4000 BP. Since 1850 CE, the value increased ~six-fold compared with the period before 4000 BP. These findings indicate that anthropogenic activities have been affecting the global P cycle over a pre-industrial background for millennia.

Pigments produced by anoxygenic phototrophic bacteria are valuable proxies of past anoxia in lacustrine and marine environments. Pigment measurement typically requires time-consuming and costly chemical extractions and chromatographic analyses, which limits the temporal resolution of paleoenvironmental reconstructions based on sedimentary pigments. Here, we evaluate the potential of in situ hyperspectral imaging (HSI) core scanning as a rapid, non-destructive method to document high-resolution changes in oxygenic and anoxygenic phototrophic communities at meromictic Lake Cadagno, Switzerland. Three distinct groups of pigments can be detected with the HSI method in the sediments of Lake Cadagno; each pigment group represents a different phototrophic community. Oxygenic phototrophs are indicated by total chloropigments (TChl; chlorophyll a, b, and derivatives). Two types of anoxygenic phototrophs were distinguished - purple sulfur bacteria (PSB), represented by bacteriochlorophyll a, and green sulfur bacteria (GSB), represented by bacteriochlorophyll c, d, and e. HSI pigment indices were validated by pigment measurements performed on extracted samples using spectrophotometer and high-performance liquid chromatography (HPLC). Bacteriochlorophylls were present throughout the past 10 kyr, confirming geochemical evidence of nearly continuous stratification and sulfidic conditions at Lake Cadagno. Major shifts in the anoxygenic phototropic communities are recorded at decadal to millennial scales. GSB and PSB communities coexisted from 10.2-8.8 kyr BP. Dominance of PSB over GSB from 8.8-3.4 kyr BP indicates strongly stratified conditions in the lake and strong light radiation at the chemocline. From 3.4-1.3 kyr BP, PSB were mostly absent, and GSB became dominant, implying lower light intensity at the chemocline due to a combination of factors including deforestation in the lake surroundings, increased flood frequency, cooler climatic conditions, and changes in groundwater solute concentrations. The high-resolution HSI data show that frequent flood events and mass movements disturbed the chemocline and the anoxygenic bacterial communities and that the PSB were particularly sensitive and slow to recover following these disturbance events. This study demonstrates for the first time that HSI can detect GSB-related pigments, making the method uniquely valuable as a rapid tool to study samples containing pigments of both oxygenic and anoxygenic phototrophs., Biogeosciences, 20 (12), ISSN:1726-4170

Varved lake sediments are exceptional archives of paleoclimatic information due to their precise chronological control and annual resolution. However, quantitative paleoclimate reconstructions based on the biogeochemical composition of biochemical varves are extremely rare, mainly because the climate–proxy relationships are complex and obtaining biogeochemical proxy data at very high (annual) resolution is difficult. Recent developments in high-resolution hyperspectral imaging (HSI) of sedimentary pigment biomarkers combined with micro X-ray fluorescence (µXRF) elemental mapping make it possible to measure the structure and composition of varves at unprecedented resolution. This provides opportunities to explore seasonal climate signals preserved in biochemical varves and, thus, assess the potential for annual-resolution climate reconstruction from biochemical varves. Here, we present a geochemical dataset including HSI-inferred sedimentary pigments and µXRF-inferred elements at very high spatial resolution (60 µm, i.e. > 100 data points per varve year) in varved sediments of Lake Żabińskie, Poland, over the period 1966–2019 CE. We compare these data with local meteorological observations to explore and quantify how changing seasonal meteorological conditions influenced sediment composition and varve formation processes. Based on the dissimilarity of within-varve multivariate geochemical time series, we classified varves into four types. Multivariate analysis of variance shows that these four varve types were formed in years with significantly different seasonal meteorological conditions. Generalized additive models (GAMs) were used to infer seasonal climate conditions based on sedimentary variables. Spring and summer (MAMJJA) temperatures were predicted using Ti and total C (Radj2=0.55; cross-validated root mean square error (CV-RMSE) = 0.7 ∘C, 14.4 %). Windy days from March to December (mean daily wind speed > 7 m s−1) were predicted using mass accumulation rate (MAR) and Si (Radj2=0.48; CV-RMSE = 19.0 %). This study demonstrates that high-resolution scanning techniques are promising tools to improve our understanding of varve formation processes and climate–proxy relationships in biochemical varves. This knowledge is the basis for quantitative high-resolution paleoclimate reconstructions, and here we provide examples of calibration and validation of annual-resolution seasonal weather inference from varve biogeochemical data.

The East African (hydro-)climate response to perturbations during the deglacial transition (e.g. Older and Younger Dryas) is complex and expressed heterogeneously in different paleoclimatic records. Lake Victoria (LV), Africa’s largest lake, desiccated entirely during the dry last glacial (>16.3 kyr BP) and subsequently refilled as climate conditions got more humid, reaching a highstand during the Early Holocene. However, existing sediment records from LV do not have sufficient resolution to fully resolve short-term hydroclimate changes during the deglacial transition (especially between 14 and 11 kyr BP). There is little direct evidence of late-glacial lake level fluctuations in LV so far because intermediate water depth coring sites suitable to record intermittent lowstands are missing.By analysing sediment cores along a near-shore/shallow water (current water depth 22 m) to offshore/deep water (current water depth 63 m) coring transect covering the past 16,000 years, we aim at a more accurate spatial and temporal reconstruction of LV’s deglacial lake level history in response to regional hydroclimate changes.Core stratigraphy and geochemical evidence, combined with a robust radiocarbon chronology, demonstrate a stepwise infilling of the Lake Victoria basin after its last complete desiccation (< 16.3 kyr BP). Following the dry late glacial Heinrich 1 event, an intermediate water level prevailed between 16.3 and 14.4 kyr BP, with uninterrupted deposition of fine-grained, organic matter-rich pelagic muds at our deep-water site and coarser, sandy-clay deposits at the near shore site. A second dry episode during the Older Dryas (~14 kyr BP) is marked by an abrupt decline in lake level with deposition of coarse mollusc shell bearing sediments at the near shore site indicating a littoral depositional environment. This shift in hydroclimate in the Lake Victoria basin is congruent with a brief period of cooling and drying during the Bölling/Alleröd (Dansgard Oeschger Event 1), which is also recorded in other East African Lakes. Subsequently, Lake Victoria reached maximum water levels with the onset of the African Humid Period in the early Holocene at around 11 kyr BP, which is expressed by elevated input of chemically weathered material (e.g. Rb/K) and deposition of fine-grained muds at both the near shore and offshore sites.

Pigments produced by anoxygenic phototrophic bacteria are amongst the most reliable indicators of water column anoxia preserved in sediments [1], yet conventional pigment measurement techniques are labor intensive and prohibit high-resolution analyses of these biomarkers. In this study, we assess the potential of hyperspectral imaging (HSI) core-scanning to reconstruct high-resolution variations in anoxygenic phototrophic communities using sediment cores from meromictic Lake Cadagno, Switzerland. Three different pigment groups were detected, and each pigment group is diagnostic of different phototrophic communities. Mixolimnetic aerobic primary production is recorded by chlorophyll a (and derivatives), whereas anoxygenic phototrophs are split into two groups – purple sulfur bacteria (PSB) represented by bacteriochlorophyll a, and green sulfur bacteria (GSB) represented by bacteriochlorophylls c, d, e. Spectrophotometer and high-performance liquid chromatography (HPLC) pigment analyses were used to validate the HSI-inferred pigment data. Near-continuous presence of bacteriochlorophylls confirms previously published geochemical evidence for persistent anoxic/sulfidic conditions at Lake Cadagno throughout the past 9.8 ka [2]. Furthermore, major shifts in the anoxygenic phototrophic communities appear to be related to environmental factors that affect lake stratification and light penetration at the chemocline. PSB and GSB became established after ca. 9.8 ka BP, and high abundances of both groups are inferred from 9.8-8.8 ka BP. PSB became dominant during the mid-Holocene from 8.8-3.4 ka BP, likely indicating a stable and shallow chemocline with high light penetration to the sulfidic chemocline. From 3.4-1.4 ka BP, PSB were substantially reduced and GSB became more dominant, likely due to a combination of cooler temperatures and land cover changes in the catchment that led to weakened lake stratification and more frequent turbid underflows. The high-resolution data show GSB established prior to PSB during the initial development of sulfidic conditions at 9.8 ka BP, and show that GSB tend to re-establish more quickly following mass movements and floods. This study demonstrates for the first time the capability of hyperspectral imaging to detect GSB related pigments, and shows the strong potential for high-resolution reconstruction of anoxygenic phototrophic bacteria communities using this technique.1. Sinninghe Damsté, J. S. and Schouten, S.: Biological markers for anoxia in the photic zone of the water column, in: Handbook of Environmental Chemistry, Volume 2: Reactions and Processes, Springer-Verlag, Berlin, Heidelberg, 127–163, https://doi.org/10.1007/698_2_005, 2006.2. Wirth, S. B., Gilli, A., Niemann, H., Dahl, T. W., Ravasi, D., Sax, N., Hamann, Y., Peduzzi, R., Peduzzi, S., Tonolla, M., Lehmann, M. F., and Anselmetti, F. S.: Combining sedimentological, trace metal (Mn, Mo) and molecular evidence for reconstructing past water-column redox conditions: The example of meromictic Lake Cadagno (Swiss Alps), Geochim. Cosmochim. Acta, 120, 220–238, https://doi.org/10.1016/j.gca.2013.06.017, 2013.

Lake Victoria is the largest tropical lake on the planet. Located in East Africa at an altitude of 1135 m asl, it lies across the limits between two major climatic zones with a temperature and moisture gradient and associated tropical biomes, the rain forest, and the savanna. At higher altitudes > 1200–2500 m a.s.l. temperatures are significantly lower and vegetation forms an Afromontane belt. Primarily triggered by climate shifts, these three biomes and fire regimes have been dynamically interspersing over the last 17,000 years.Here, we present a robust 14C chronology mainly based on macroscopic charcoal using the MICADAS system of LARA at the University of Bern, new palynological data used as biostratigraphic control, and the first continuous charcoal record in Lake Victoria to establish the fire history.Our pollen and macro–charcoal records, support the assumption that throughout time regional fire dynamics are controlled by biome’s changes, and that climate was the main driver of these vegetation shifts at least until the Iron Age. Our results indicate that during the Last Glacial Maxima and Heinrich Stadial 1, under dry and colder climates the savanna was dominating, with low fire regimes before 15,000 cal yr BP and increased fire occurrence between 15,000 and 14,000 cal yr BP. After this period, the Afromontane forest started to expand, and warmer and humid climates promoted the growth of rain forests and reduced fire events, which is particularly observed in the African Humid Period (between ca. 11,500 and 5000 cal yr BP). Subsequently, our records indicate a global maximum of fire occurrence at 5000 cal yr BP, followed by unexpectedly low fire regimes during the Iron Age and the subsequent periods.This work is part of a SINERGIA project funded by the Swiss National Foundation which seeks to unravel the long-term causes and consequences of Lake Victoria’s ecosystem dynamics with a special focus on the evolution of fish species and other biotas from the late Pleistocene to the present.

Variation in molybdenum (Mo) concentration and isotope composition is an established tracer for redox changes in marine environments. Here we apply Mo as a proxy of past water column oxygenation in ancient ferruginous and hyposulfidic Lake Towuti. Lake Towuti is >1.2 Myrs old, up to 200m deep, weakly stratified and anoxic below ~100 m water depth, and surrounded by an ultramafic bedrock-dominated catchment in South Sulawesi, Indonesia. Despite the current permanent stratification, deeper water mixing and oxygenation occurred periodically in the past due to Towuti’s sensitivity to climate change. To better reconstruct the redox changes through time, we present Mo concentration and isotope data from laterite profiles overlying ultramafic bedrock, lake surface sediments, and a ~30 kyr sediment piston core from Lake Towuti, Indonesia.Although Mo concentrations in laterite profiles are overall higher compared to the underlying bedrock, the absence of a significant Mo isotopic variability with values close to the unweathered ultramafic protolith, varying from –0.16‰ to –0.04‰ δ98MoNIST3134, suggests low aqueous mobility of Mo during weathering due to the formation of laterite Fe-oxides. In contrast to the laterite samples, Mo isotopic variability in lake surface sediments show a larger variability varying from –1.15‰ to –0.13‰ δ98MoNIST3134 with a significantly lighter Mo isotopic composition in sediments deposited under oxic bottom water conditions. This light Mo isotopic composition is likely caused by early diagenetic redox cycling of Mo and Fe at the sediment-water interface. In the deeper, anoxic parts of the basin, Mo isotopic compositions show values close to the laterite input with elevated sedimentary Mo concentrations which are likely the result of an authigenic Mo enrichment from the water column. Mo isotope variability in sediments taken from a piston core in the deep part of the lake is in the range of Mo isotope compositions from modern surface sediments, varying from –0.14‰ to –0.66‰ δ98MoNIST3134. Interestingly, this variability is well correlated with local and global indicators of climate change from previous studies. Sediments deposited between ~30 kyr and ~10 kyr exhibit Mo isotope signatures similar to present-day oxygenated shallow water sites, thereby suggesting enhanced lake mixing and bottom water oxygenation under drier and colder climate conditions of the last glacial period. Sediments deposited since ~10 kyr under wetter and warmer climate conditions exhibit Mo isotope signatures similar to present-day anoxic deeper water settings. These suggest that Mo isotope compositions of lake sediments are potential quantitative indicators of past climate-driven water column oxygenation.

Anthropogenic eutrophication and spreading anoxia in freshwater systems is a global concern. Little is known about anoxia in earlier historic times under weaker human impact, or under prehistoric natural conditions with different trophic, land cover and climatic regimes. We use a novel approach that combines high‐resolution hyperspectral imaging with μ‐XRF and HPLC‐pigment data, which allows us to assess chloropigments (productivity) and bacteriopigments (anoxia) at seasonal subvarve‐scale resolution. Our ~9700 cal a BP varved sediment record from NE Poland suggests that productivity increased stepwise from oligotrophic Early Holocene conditions (until ~9200 cal a BP) to mesotrophic conditions in the Mid‐ and Late Holocene. Natural eutrophication was mainly a function of progressing landscape evolution with intense weathering under dense forest and warm‐moist climatic conditions. Generally, anoxia increased with increasing productivity. Seasonal anoxia and some multi‐decadal periods of meromixis were the common mixing patterns throughout the Holocene except for a period of persisting meromixis between ~5200 and 2000 cal a BP. Anthropogenic deforestation around 400 cal a BP resulted in substantially better lake oxygenation despite high productivity. In this small lake, aquatic productivity and lakeshore forest cover (wind shield) were more important factors controlling oxic/anoxic conditions than Holocene temperature variability.

A multiproxy workflow was used to assess >60 plant protection products (PPPs) in sediment samples from a Swiss lake under heavy agricultural pressure. The results show the appearance of PPPs for the first time in the early 1960s with an overall detection of 34 PPPs and with herbicides and fungicides found in equal proportions. Paleolimnological data [e.g., chronology, hyperspectral imaging of sedimentary green pigments, and semiquantitative elemental composition (μXRF scans)] suggest that PPP concentrations and fluxes to the sediment over time are not related to land surface processes such as soil erosion or lake biogeochemistry but are attributed mainly to PPP application (inferred from sales) or regulatory measures (bans). Additional compounds with similar sources of contamination as the target PPPs captured by nontarget trend analysis (≥2000 unknown profiles) reveal significant inputs of contaminants to the lake starting in the 1970s, followed by a decrease of contamination at the beginning of the 1990s and a constant increase by ∼28% of the unknown compounds since the year 2000. An ecological risk assessment conducted on detected PPPs indicates that since the 1980s, the sediment quality is insufficient with risk quotient values displaying maximum levels in the most recent sediments (∼2010) despite bans of specific PPPs and environmental regulations.

Detailed knowledge about the history of vegetation, fire and land use is scarce in Northern Greece. We analysed lake sediments from Limni Zazari (Northern Greece) to reconstruct the past local vegetation and fire history with a special focus on land use and its impacts on erosion and lake eutrophication. Our data suggest a rather dense steppic vegetation after ca 20,000 cal bp (18050 cal bc). Forest expansion with Pinus sylvestris and admixed Quercus pubescens started around 14,500 cal bp (12550 cal bc). After the onset of the Holocene, mixed deciduous sub-mediterranean oak forests expanded, accompanied by rapidly decreasing soil erosion rates and increasing aquatic biological productivity. Pollen of cereals and Plantago lanceolata suggests continuous farming activities in the region after 8,200 cal bp (6250 cal bc), in agreement with archaeological evidence. Fairly closed mixed pine-oak forests dominated the landscape until ca 3,500 cal bp (1550 cal bc) that were only temporarily reduced during the Neolithic around 7,100 and 6,500 cal bp (5150 and 4550 cal bc). Land cover changes and aquatic biogeochemistry were closely linked during this period. Forest phases corresponded to lake eutrophication and hypolimnetic anoxia (meromixis), whereas during periods of deforestation (e.g. around 8,200 cal bp/6250 cal bc) soil erosion rates and lake mixing increased, while aquatic productivity decreased. After 3,500 cal bp (1550 cal bc) humans disrupted forests and open land vegetation expanded (e.g. Artemisia, Rumex-type, Cichorioideae, Chenopodiaceae). With the onset of the Iron Age (ca. 3,050 cal bp/1100 cal bc) grassland communities expanded massively and pine-oak forests gradually declined. Anthropogenic pressure on forests increased even more during the past 500 years. Finally, forest recovery during the recent decades led to decreased erosion and increased lake productivity. We conclude that over the millennia, intense pastoral and arable activities shaped both aquatic and terrestrial environments, ultimately creating a humanized vegetation mosaic in which the original natural mixed deciduous oak forests only form relict stands. Future climate warming and decreasing anthropogenic pressure may release a rapid spread of mixed deciduous oak forests around Limni Zazari.

The El Nino–Southern Oscillation (ENSO) is the earth’s dominant mode of inter- annual climate variability. It alternates between warm (El Nino) and cold (La Nina) states, with global impacts on climate and society. This study provides new ENSO reconstructions based on a large, updated collection of proxy records. We use a novel reconstruction approa ch that employs running principal components, which allows us to take covariance changes between proxy records into account and thereby identify periods of likely teleconnection changes. Using different implementations of the principal component analysis enables us to identify periods within the last millennium when quantifications of ENSO are most robust. These periods range from 1580 to the end of the 17th century and from 1825 to present. We incorporate an assessment of consistency among our new and existing ENSO reconstructions leading to five short phases of low agreement among the reconstructions between 1700 and 1786. We find a consistent spatial pattern of proxy covariance during these four phases, differing from the structure seen over the instrumental period. This pattern points towards changes in teleconnections in the west Pacific/Australasian region, compared to the present state. Using our new reconstructions, we find a significant response of ENSO towards more La Nina-like conditions 3–5 years after major volcanic events. We further show that our new reconstructions and existing reconstructions largely agree on the state of ENSO during volcanic eruptions in the years 1695 and 1784, which helps put into perspective the climatic response to these events. During all other large volcan ic eruptions of the last 1000 years, there is no reconstruction coherency with regard to the state of ENSO.

Varved lake sediments are recognized as valuable archives of paleoclimatic information due to their precise chronological control. However, paleoclimate reconstructions based on the composition of biochemical varves are relatively rare (Zolitschka et al., 2015). We applied novel high-resolution scanning techniques to the varved sediments of Lake Żabińskie, Poland to obtain spatially resolved geochemical data at a resolution of 60 μm covering the period 1966-2019. Relative abundances of elements were measured in resin-embedded sediment slabs using a Bruker M4 Tornado micro-XRF scanner. Chloropigments-a and bacteriopheopigments-a were measured on a wet sediment core using a Specim Hyperspectral core scanner (Butz et al., 2015). The high resolution of the scanning data, and the relatively thick well-preserved varves (average thickness = 6.4 mm), enables a close examination of seasonal scale sediment composition and varve formation processes. Time series of geochemical variables within each varve year were classified into 4 varve type groups based on the dissimilarity measure ψ for multivariate time series (Benito and Birks, 2020; Gordon and Birks, 1974). Based on a Multivariate Analysis Of Variance test, these groups of years experienced significant (p ReferencesBenito, B. M. and Birks, H. J. B.: distantia: an open‐source toolset to quantify dissimilarity between multivariate ecological time‐series, Ecography (Cop.)., 43(5), 660–667, doi:10.1111/ecog.04895, 2020.Butz, C., Grosjean, M., Fischer, D., Wunderle, S., Tylmann, W. and Rein, B.: Hyperspectral imaging spectroscopy: a promising method for the biogeochemical analysis of lake sediments, J. Appl. Remote Sens., 9(1), 096031, doi:10.1117/1.jrs.9.096031, 2015.Gordon, A. D. and Birks, H. J. B.: Numerical methods in Quaternary palaeoecology: II. Comparison of pollen diagrams, New Phytol., 73(1), 221–249, doi:10.1111/j.1469-8137.1974.tb04621.x, 1974.Zolitschka, B., Francus, P., Ojala, A. E. K. and Schimmelmann, A.: Varves in lake sediments - a review, Quat. Sci. Rev., 117, 1–41, doi:10.1016/j.quascirev.2015.03.019, 2015.

Lake Victoria (LV), Africa’s largest lake is situated in the African Great Rift Valley. Due to its shallowness (max.68 m; mean 40 m) and limited river inflow, LV is very sensitive to variations in climate and lake level fluctuations. As a result, LV has undergone repeated low stand periods, or even complete desiccation during the Late Pleistocene with profound effects on the aquatic ecosystem. One example is the emergence of a unique biodiversity of endemic cichlid species following the lake’s last desiccation event during the last glacial and subsequent refilling commencing ~15,000 years ago.In an interdisciplinary project we aim at reconstructing linkages between paleoenvironmental variability, disturbances and adaptive species radiation by combining approaches from paleogenomics, paleoecology and paleolimnology. For this purpose, four sediment cores along a depth-transect (near-shore to offshore), covering ca. the past 14,000 years, are analyzed.We present first paleolimnological results of long-term changes of using (isotope-)geochemical indicators including: Sedimentary pigments and biogenic silica to infer aquatic productivity supported by micro X-ray Fluorescence (XRF) derived element geochemistry, 13C and 15N, and sedimentary phosphorus fraction analyses providing information on changes in sediment composition.The results suggest that the infilling of the LV basin was a long-term step-wise process. This is shown by elevated and variable indicators for lithogenic input (e.g Ti, Zr and K) and interpreted as mobilization of substrate from the shorelines by a dynamic lake level prior to its stabilization in the Early and Mid-Holocene. This process is mainly reflected in the core taken at the greatest water depth (65 m). Simultaneously, the aquatic productivity (BSi and chloropigments) increased rapidly after the refilling of the lake basin in the Late-Glacial. A gradual drying of the climate and a following shift to a more oxygenated water column is observed in the Mid-to Late Holocene indicated by a decline in chemically weathered material (e.g Rb/K & K/Al ratios) and abundance of Mn.

Global spread of anoxia in aquatic ecosystems has become a major issue that may potentially worsen due to global warming. The reconstruction of long-term hypolimnetic anoxia records can be challenging due to lack of valid and easily measurable proxies. The sedimentary Mn/Fe ratio measured by X-ray fluorescence (XRF) is often used as a proxy for past lake redox conditions. Yet the interpretation of this ratio can be problematic when Fe and Mn accumulation is not solely redox driven. We used the varved sediments of Lake Moossee (Switzerland) to examine the partitioning of Fe and Mn in seven fractions by sequential extraction under various oxygen conditions over the last 15,000 years. We combined this data with XRF scans and an independent diagnostic proxy for anoxia given by a hyperspectral imaging (HSI)-inferred record of bacteriopheophytin, to validate the use of the XRF-Mn/Fe ratio as redox proxy. In the 15,000-year long record, Fe was bound to humins and amorphous, crystalline, sulfide and residual forms. Mn was mainly present in carbonate and amorphous forms. Higher erosion, prolonged anoxia, diagenesis and humic matter input affected Fe and Mn accumulation. Under holomixis the XRF-Mn/Fe ratio successfully reflected lake redox conditions. Periods with higher detrital Fe input obscured the applicability of the ratio. During phases of permanent anoxia, intensified early diagenetic processes trapped Mn in the sediments in carbonate, crystalline oxide and humic forms. Our study shows that the single use of the XRF-Mn/Fe ratio is often not conclusive for inferring past lake redox conditions. The application of the XRF-Mn/Fe as a proxy for anoxia requires taking into account the individual lake characteristics and changes in lake environmental conditions, which affect the accumulation of Fe and Mn in the sediments., Science of The Total Environment, 755 (Part 2), ISSN:0048-9697, ISSN:1879-1026

A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format. Fil: Kaufman, Darrell. Northern Arizona University.; Estados Unidos Fil: McKay, Nicholas. Northern Arizona University.; Estados Unidos Fil: Routson, Cody. Northern Arizona University.; Estados Unidos Fil: Erb, Michael. Northern Arizona University.; Estados Unidos Fil: Davis, Basil. University Of Lausanne; Suiza Fil: Heiri, Oliver. University Of Basel; Suiza Fil: Jaccard, Samuel. University Of Bern; Suiza Fil: Tierney, Jessica. University of Arizona; Estados Unidos Fil: Dätwyler, Christoph. University Of Bern; Suiza Fil: Axford, Yarrow. Northwestern University; Estados Unidos Fil: Brussel, Thomas. University of Utah; Estados Unidos Fil: Cartapanis, Olivier. University Of Bern; Suiza Fil: Chase, Brian. Universite de Montpellier; Francia Fil: Dawson, Andria. Mount Royal University; Canadá Fil: de Vernal, Anne. Université du Québec a Montreal; Canadá Fil: Engels, Stefan. University of London; Reino Unido Fil: Jonkers, Lukas. University Of Bremen; Alemania Fil: Marsicek, Jeremiah. University of Wisconsin-Madison; Estados Unidos Fil: Moffa Sánchez, Paola. University of Durham; Reino Unido Fil: Morrill, Carrie. University of Colorado; Estados Unidos Fil: Orsi, Anais. Université Paris-Saclay; Francia Fil: Rehfeld, Kira. Heidelberg University; Alemania Fil: Saunders, Krystyna. Australian Nuclear Science And Technology Organisation; Australia Fil: Sommer, Philipp. University Of Lausanne; Suiza Fil: Thomas, Elizabeth. University At Buffalo; Estados Unidos Fil: Tonello, Marcela Sandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Tóth, Mónika. Balaton Limnological Institute; Hungría Fil: Vachula, Richard. Brown University; Estados Unidos Fil: Andreev, Andrei. Alfred Wegener Institut Helmholtz Centre for Polar and Marine Research; Alemania Fil: Bertrand, Sebastien. Ghent University; Bélgica Fil: Massaferro, Julieta. Administración de Parques Nacionales. Parque Nacional "Nahuel Huapi"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Global spread of hypoxia and less frequent mixing in lakes is a major growing environmental concern. Climate change and human impact are expected to increasingly deteriorate aquatic ecosystems. The study of processes and drivers of such changes in the past provides a great asset for prevention and remediation in the future. We used a multiproxy approach combining high-resolution bulk pigment data measured by hyperspectral imaging (HSI) with lower-resolution specific chlorophyll types and carotenoids measured by HPLC to examine Holocene trophic state changes and anoxia evolution in the meromictic Lake Jaczno, NE Poland. A redundancy analysis (RDA) including pollen-inferred vegetation cover, temperature and human impacts provides insight into specific conditions and drivers of changing trophic and redox states in the lake. Anoxic and sulfidic conditions were established in Lake Jaczno after initial basin infilling 9500 years ago. Until 6700 cal BP, lake trophy was relatively low, water turbidity was high and green sulfur bacteria (GSB) were abundant within the phototrophic community, suggesting a deep oxic–anoxic boundary and weak stratification. The period between 6700–500 cal BP is characterized by constantly increasing lake production and a gradual shift from GSB to purple sulfur bacteria (PSB), suggesting a shallower oxic–anoxic boundary and pronounced stratification. Yet, the presence of spheroidene and speroidenone in the sediments indicates intermittent anoxia. After 500 cal BP, increasing human impact, deforestation and intensive agriculture promoted lake eutrophication, with a shift to PSB dominance and establishment of permanent anoxia and meromixis. Our study unambiguously documents the legacy of human impact on processes determining eutrophication and anoxia.

Knowledge about the vegetation history of Sardinia, the second largest island of the Mediterranean, is scanty. Here, we present a new sedimentary record covering the past ~ 8,000 years from Lago di Baratz, north-west Sardinia. Vegetation and fire history are reconstructed by pollen, spores, macrofossils and charcoal analyses and environmental dynamics by high-resolution element geochemistry together with pigment analyses. During the period 8,100–7,500 cal bp, when seasonality was high and fire and erosion were frequent, Erica arborea and E. scoparia woodlands dominated the coastal landscape. Subsequently, between 7,500 and 5,500 cal bp, seasonality gradually declined and thermo-mediterranean woodlands with Pistacia and Quercus ilex partially replaced Erica communities under diminished incidence of fire. After 5,500 cal bp, evergreen oak forests expanded markedly, erosion declined and lake levels increased, likely in response to increasing (summer) moisture availability. Increased anthropogenic fire disturbance triggered shrubland expansions (e.g. Tamarix and Pistacia) around 5,000–4,500 cal bp. Subsequently around 4,000–3,500 cal bp evergreen oak-olive forests expanded massively when fire activity declined and lake productivity and anoxia reached Holocene maxima. Land-use activities during the past 4,000 years (since the Bronze Age) gradually disrupted coastal forests, but relict stands persisted under rather stable environmental conditions until ca. 200 cal bp, when agricultural activities intensified and Pinus and Eucalyptus were planted to stabilize the sand dunes. Pervasive prehistoric land-use activities since at least the Bronze Age Nuraghi period included the cultivation of Prunus, Olea europaea and Juglans regia after 3,500–3,300 cal bp, and Quercus suber after 2,500 cal bp. We conclude that restoring less flammable native Q. ilex and O. europaea forest communities would markedly reduce fire risk and erodibility compared to recent forest plantations with flammable non-native trees (e.g. Pinus, Eucalyptus) and xerophytic shrubland (e.g. Cistus, Erica).

A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format., Measurement(s)climateTechnology Type(s)digital curationFactor Type(s)temporal interval • geographic location • proxy typeSample Characteristic - Environmentclimate systemSample Characteristic - LocationEarth (planet) Machine-accessible metadata file describing the reported data: https://www.ncdc.noaa.gov/paleo/study/27330

Eutrophication and anoxia are increasing in lakes worldwide. However, our understanding of variations of primary productivity and anoxia in lakes over thousands of years is limited. Long-term records are needed to understand the natural variability of lake ecosystems and to improve our understanding of drivers of productivity and anoxia. In this study, we used the varved sediment record of Lake Żabinskie, Poland to answer the following research questions: 1) How have primary production and water column oxygen concentrations varied during the past 10,800 years?; 2) what role did natural and anthropogenic forces have in driving changes in primary production or lake mixing regime? Recently developed hyperspectral imaging (HSI) techniques were used to quantify sedimentary chloropigments-a and bacteriopheopigments-a (Bphe-a) at sub-annual resolution. These data, combined with elemental data from micro X-ray fluorescence (μ-XRF) and pigment assemblage data from high-performance liquid chromatography (HPLC) measurements, were used to reconstruct paleolimnological conditions. Bphe-a was used as an indicator of anoxia, and its presence suggests that an extensive anoxic zone was present nearly continuously from 10.8 to 2.8 ka BP. Anoxic conditions, driven by thermal stratification, were promoted by closed forest cover during that time, which limited wind-driven mixing of the water column. After 2.8 ka BP, water column oxygenation occurred more frequently, particularly during periods of increased human agricultural activity and forest opening. Pronounced anoxia was again present continuously from ~610 to 1470 CE, concurrent with a period of reforestation. After ~1610 CE, deforestation caused increases in erosion rates, algal production, and water column oxygenation. Pigment assemblages indicate that the algal community during the past 150 years was different from any other time during the Holocene. This study demonstrates a clear link between lake biogeochemical processes and forest cover and shows the potential of HSI to produce extremely high-resolution records of past productivity and redox conditions from varved lake sediments.

Knowledge utilization depends on how well the scientific community communicates knowledge to its target audiences’ needs. We argue that policy-relevant science communication can increase the real-l...

The recent development of the MIni CArbon DAting System (MICADAS) allows researchers to obtain radiocarbon (14C) ages from a variety of samples with miniature amounts of carbon ( µg C) by using a gas ion source input that bypasses the graphitization step used for conventional 14C dating with accelerator mass spectrometry (AMS). The ability to measure smaller samples, at reduced cost compared with graphitized samples, allows for greater dating density of sediments with low macrofossil concentrations. In this study, we use a section of varved sediments from Lake Żabińskie, NE Poland, as a case study to assess the usefulness of miniature samples from terrestrial plant macrofossils for dating lake sediments. Radiocarbon samples analyzed using gas-source techniques were measured from the same depths as larger graphitized samples to compare the reliability and precision of the two techniques directly. We find that the analytical precision of gas-source measurements decreases as sample mass decreases but is comparable with graphitized samples of a similar size (approximately 150 µg C). For samples larger than 40 µg C and younger than 6000 BP, the uncalibrated 1σ age uncertainty is consistently less than 150 years (±0.010 F14C). The reliability of 14C ages from both techniques is assessed via comparison with a best-age estimate for the sediment sequence, which is the result of an OxCal V sequence that integrates varve counts with 14C ages. No bias is evident in the ages produced by either gas-source input or graphitization. None of the 14C ages in our dataset are clear outliers; the 95 % confidence intervals of all 48 calibrated 14C ages overlap with the median best-age estimate. The effects of sample mass (which defines the expected analytical age uncertainty) and dating density on age–depth models are evaluated via simulated sets of 14C ages that are used as inputs for OxCal P-sequence age–depth models. Nine different sampling scenarios were simulated in which the mass of 14C samples and the number of samples were manipulated. The simulated age–depth models suggest that the lower analytical precision associated with miniature samples can be compensated for by increased dating density. The data presented in this paper can improve sampling strategies and can inform expectations of age uncertainty from miniature radiocarbon samples as well as age–depth model outcomes for lacustrine sediments.

The recent development of the MIni CArbon DAting System (MICADAS) allows researchers to obtain radiocarbon (14C) ages from a variety of samples with miniature amounts of carbon (

Eutrophication, prompted by anthropogenic activities and climate change has led to multiple adverse effects in freshwater systems across the world. As instrumental measurements are typically short, lake sediment proxies of aquatic primary productivity (PP) are often used to extend the observational record of eutrophication back in time. Sedimentary pigments provide specific information on PP and major algal communities, but the records are often limited in the temporal resolution. Hyperspectral imaging (HSI) data, in contrast, provide very high seasonal (sub-varve-scale) resolution, but the pigment speciation is limited. Here, we explore a combined approach on varved sediments from the Ponte Tresa basin, southern Switzerland, taking the advantages of both methods (HSI and high performance liquid chromatography, HPLC) with the goal to reconstruct the recent eutrophication history at seasonal to interannual resolution. We propose a modified scheme for the calibration of HSI data (here: Relative Absorption Band Depth between 590 and 730 nm RABD590–730) and HPLC-inferred pigment concentrations (here: ‘green pigments’ {chlorophyll a and pheophytin a}) and present a calibration model (R2 = 0.82; RMSEP ~ 12%). The calibration range covers > 98% of the spectral index values of all individual pixels (68 µm × 68 µm) in the sediment core. This allows us to identify and quantify extreme pigment concentrations related to individual major algal blooms, to identify multiple algal blooms within one season, and to assess interannual variability of PP. Prior to the 1930s, ‘green pigment’ concentrations and fluxes (~ 50 µg g−1; ~ 2 µg cm−2a−1, chlorophyll a and pheophytin a) and interannual variability was very low. From the 1930s to 1964, chlorophyll a and pheophytin a increased by a factor of ~ 4, and ββ-carotene appeared in substantial amounts (~ 0.4 µg cm−2a−1). Interannual variability increased markedly and a first strong algal bloom with ‘green pigment’ concentrations as high as 700 µg g−1 is observed in 1958. Peak eutrophication (~ 12 µg cm−2a−1 chlorophyll a and pheophytin a) and very high interannual variability with extreme algal blooms (‘green pigment’ concentrations up to 1400 µg g−1) is observed until ca. 1990, when eutrophication decreases slightly. Maximum PP values after 2009 are likely the result of internal nutrient cycling related to repeated deep mixing of the lake.

Increased cultural eutrophication since the 20th century, caused by phosphorus (P) enrichment, has become a major problem worldwide. In deep, stratified lakes, eutrophication-induced hypolimnetic anoxia often stimulates the release of labile P from the sediment into the water column. This positive feedback, termed internal P loading, maintains or even accelerates eutrophication. However, most studies on internal P loading have focused on recent times. Little is known about whether such positive feedbacks caused by labile P release from sediments also played a role under natural conditions with little or no human impact. We investigated a high-resolution 15,000-year sediment record of paleoproduction, anoxia, and five sedimentary P fractions from a small, deep lake, Soppensee, on the Swiss Central Plateau. We estimated long-term qualitative internal P loading by comparing the Holocene record of diatom-inferred epilimnetic total P (DI-TP) concentrations with labile P fraction (Fe P) concentrations in sediments under changing trophic state, redox, and lake mixing regimes. Intensified P cycling from sediments into the water column (enhanced internal P loading) apparently occurred as a positive feedback to natural eutrophication with persistent bottom-water anoxia during the early to mid-Holocene (~9000–6000 cal BP). However, this positive feedback was not inferred for other eutrophic phases. Fe-rich layers formed during seasonal mixing of the lake in the late Holocene (~2000–200 cal BP) and magnetite-type minerals produced by magnetotactic bacteria (MTB) internal P loading during anoxic phases in the mid- to late Holocene (~6000–2000 cal BP) appeared to prevent internal P loading. MTB presence resulted in high concentrations of potentially labile Fe P in sediments. Our study demonstrates the potential contribution of internal P loading during long-term natural eutrophication of deep stratified lakes and has wide implications for lake management and restoration. Our results highlight the importance of the coupled geochemical cycles of P and Fe in the long-term trophic state evolution of stratified, ferruginous, low-sulfate-water lakes, conditions that have been reported to serve as analogs for the Archaean Ocean.

We investigated diatom assemblages in surface sediments of 46 lakes in northern Poland and developed a diatom-based transfer function to infer epilimnetic total phosphorus (TP) concentrations. Multivariate ordination techniques (DCA, CCA) were used to identify major environmental gradients and to evaluate the effect of environmental parameters on the distribution of diatoms in the modern diatom dataset. The transfer function was developed using PLS, WA and WA-PLS models, and applied to a varved sediment core from Lake Żabińskie, AD 1888-2010. Annually-resolved quantitative reconstruction of TP concentrations shows that multidecadal changes in the TP level reflect the local settlement history, land-use changes and development of agriculture and tourism. The period of high trophic levels with maximum values of TP was documented until the late 1920s. In the 1930s-1970s period, TP generally decreased and eutraphentic flora was partly replaced by oligotraphentic and oligo-mesotraphentic diatom taxa. The reconstructed TP concentrations have started to increase from the 1970s. After the 1950s, strong short-term fluctuations of TP values were noted and explained by interactions between meteorological conditions, water column mixing and nutrient cycling in the lake.

Hypolimnetic anoxia in eutrophic lakes can delay lake recovery to lower trophic states via the release of sediment phosphorus (P) to surface waters on short timescales in shallow lakes. However, the long-term effects of hypolimnetic redox conditions and trophic state on sedimentary P fraction retention in deep lakes are not clear yet. Hypolimnetic withdrawal of P-rich water is predicted to diminish sedimentary P and seasonal P recycling from the lake hypolimnion. Nevertheless, there is a lack of evidence from well-dated sediment cores, in particular from deep lakes, about the long-term impact of hypolimnetic withdrawal on sedimentary P retention. In this study, long-term sedimentary P fraction data since the early 1900s from Lake Burgäschi provide information on benthic P retention under the influence of increasing lake primary productivity (sedimentary green-pigment proxy), variable hypolimnion oxygenation regimes (Fe∕Mn ratio proxy), and hypolimnetic withdrawal since 1977. Results show that before hypolimnetic withdrawal (during the early 1900s to 1977), the redox-sensitive Fe∕Mn-P fraction comprised ∼50 % of total P (TP) in the sediment profile. Meanwhile, long-term retention of total P and labile P fractions in sediments was predominantly affected by past hypolimnetic redox conditions, and P retention increased in sedimentary Fe- and Mn-enriched layers when the sediment-overlaying water was seasonally oxic. However, from 1977 to 2017, due to eutrophication-induced persistent anoxic conditions in the hypolimnion and to hypolimnetic water withdrawal increasing the P export out of the lake, net burial rates of total and labile P fractions decreased considerably in surface sediments. By contrast, refractory Ca–P fraction retention was primarily related to lake primary production. Due to lake restoration since 1977, the Ca–P fraction became the primary P fraction in sediments (representing ∼39 % of total P), indicating a lower P bioavailability of surface sediments. Our study implies that in seasonally stratified eutrophic deep lakes (like Lake Burgäschi), hypolimnetic withdrawal can effectively reduce P retention in sediments and potential for sediment P release (internal P loads). However, after more than 40 years of hypolimnetic syphoning, the lake trophic state has not improved nor has lake productivity decreased. Furthermore, this restoration has not enhanced water column mixing and oxygenation in hypolimnetic waters. The findings of this study are relevant regarding the management of deep eutrophic lakes with mixing regimes typical for temperate zones., Biogeosciences, 17 (10), ISSN:1726-4170

The climate of the Southern Hemisphere (SH) is strongly influenced by variations in the El Niño-Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Due to the temporally very limited instrumental records in most parts of the SH, very little is known about the relationship between these two key modes of variability and its stability over time. Here, we use proxy-based reconstructions and climate model simulations to quantify changes in tropical-extratropical SH teleconnections as represented by the correlation between the ENSO and SAM indices. Reconstructions indicate mostly negative correlations back to around 1400 CE confirming the pattern seen in the instrumental record over the last few decades. An ensemble of last millennium simulations of the model CESM1 confirms this pattern with very stable ensemble mean correlations around −0.3. Individual forced simulations, the pre-industrial control run and the proxy-based reconstructions indicate intermittent periods of positive correlations and particularly strong negative correlations. The fluctuations of the ENSO-SAM correlations are not significantly related to solar nor volcanic forcing in both proxy and model data, indicating that they are driven by internal variability in the climate system. Pseudoproxy experiments indicate that the currently available proxy records are able to reproduce the tropical-extratropical teleconnection patterns back to around 1600 CE. We analyse the spatial temperature and sea level pressure patterns during periods of positive and particularly strong negative teleconnections in the CESM model. Results indicate no consistent pattern during periods where the ENSO-SAM teleconnection changes its sign. However, periods of very strong negative SH teleconnections are associated with negative temperature anomalies across large fractions of the extra-tropical Pacific and a strengthening of the Aleutian Low.

Lake Lugano is one of several deep lakes in Switzerland that have not yet recovered from eutrophication after large reductions of external phosphorus (P) loadings. Persistent eutrophication has been attributed mainly to internal P loadings from sediments. To achieve the restoration goals, it is critically important to evaluate the sediment P availability and release risk in this lake. In this study, we combined sequential P extraction (four fractions) with enzyme hydrolysis to assess distribution characteristics of P forms and potential bioavailability of organic P in an anoxic sediment profile from the Ponte Tresa basin of Lake Lugano, southern Switzerland. Labile P forms, i.e. mostly redox-sensitive iron bound P and metal oxides bound P (Al/Fe-P), comprised ~70% of total P in the sediment profile (1959–2017 CE), suggesting a high potential for P release from the anoxic sediment. Potentially bioavailable organic P forms (determined by addition of substrate specific enzymes) were considerably higher in the surface sediments (top 5 cm), which is very likely to release P in the near future with early diagenesis. The net burial rates (NBR) of redox sensitive Fe-P fraction and total P in sediments both showed significant decreasing trends from 1959 to 2017 CE, when trophic levels of the lake increased from mesotrophic to hypertrophic status. We suggest that, in the Ponte Tresa basin, higher eutrophication conditions led to enhanced sediment P release (mainly from redox sensitive Fe-P fraction), thus reducing P-NBR in sediments. This study highlights the concern that in deep monomictic lakes, eutrophication restoration might be hindered by extensive internal P cycling and reduced capacity of sediment P-trapping.

We present a reconstruction of environmental changes from sediments of high-altitude Laguna Chepical in the subtropical Andes of Central Chile (32°16′S; 70°30′W, 3050 m a.s.l.) for the past 3100 years. Based on subfossil pollen, microscopic charcoal and diatoms, we inferred changes in moisture (related to precipitation) and ice-cover/ice-free season (related to summer temperature) at decadal to millennial scales. Sustained wetter and colder summer temperatures than today prevailed between 1100 BC and ca. AD 1. Afterward, decreasing pollen accumulation rates and increased fire activity suggest drier conditions and possibly enhanced seasonality and/or inter-annual climate variability. Frequent changes between cold and warm summers were observed, particularly for the last 1000 years. About AD 1250 (during the Medieval Climate Anomaly), wet years and early break up of ice-cover occurred in central Chile, which is today typical for El Nino-like mean conditions. Conversely, and with the exception of a few wet pulses, a generally dry period with extended ice-cover (cool summers) was observed between AD 1400 and AD 1850 (Little Ice Age). This can be interpreted as a trend toward more La Nina-like mean conditions. Recent climate change and human disturbances during the last 100 years have prompted changes in diatom and plant communities that are unprecedented in the late Holocene. First, planktonic diatoms increased as a result of hydraulic interventions in the lake during the late 19th century, and secondly, the Andean vegetation shifted upward as result of recent warming, and the frequency of arboreal taxa was significantly reduced. At the same time peaks of fire activity were observed.

Increased erosion triggered by land-use changes is a major process that influences lake sedimentation. We explored the record of erosion intensity in annually laminated sediments of Lake Żabinskie, northeast Poland. A 1000-year-long, annually resolved suite of sedimentological (varve thickness, sediment accumulation rate) and geochemical data (scanning XRF, loss on ignition, biogenic silica) was analyzed with multivariate statistics. PCA indicated erosion was a major process responsible for changes in the chemical composition of the sediments. Analysis of sedimentary facies enabled identification of major phases of erosion that influenced lake sedimentation. These phases are consistent with the history of land use, inferred from pollen analysis. From AD 1000 to 1610, conditions around and in Lake Żabinskie were relatively stable, with low erosion intensity in the catchment and a dominance of carbonate sedimentation. Between AD 1610 and 1740, higher lake productivity and increased delivery of minerogenic material were caused by development of settlements in the region and widespread deforestation. The most prominent changes were observed between AD 1740 and 1880, when further land clearance and increased agricultural activity caused intensified soil erosion and higher lake productivity. Landscape clearance also created better conditions for water column mixing, which led to changes in redox conditions in the hypolimnion. The most recent period (AD 1880–2010) was characterized by partial reforestation and a gradual decrease in the intensity of erosional processes.

Rapid ecosystem transitions and adverse effects on ecosystem services as responses to combined climate and human impacts are of major concern. Yet few long-term (i.e. >60 years) quantitative observational time series exist, particularly for ecosystems that have a long history of human intervention. Here, we combine three major environmental pressures (land use, nutrients and erosion) with quantitative summer and winter climate reconstructions and climate model simulations to explore the system dynamics, resilience and the role of disturbance regimes in varved eutrophic Lake Żabińskie (NE Poland) since AD 1000. The comparison between these independent sources of information allows us to establish the coherence and points of disagreements between such data sets. We find that climate reconstructions capture noticeably natural forced climate variability, while internal variability is the dominant source of variability during most parts of the last millennium at the regional scale, precisely at which climate models seem to underestimate forced variability. Using different multivariate analyses and change point detection techniques, we identify ecosystem changes through time and shifts between rather stable states and highly variable ones. Prior to AD 1600, the lake ecosystem was characterised by high stability and resilience against observed natural climate variability. During this period, the anthropogenic fingerprint was small; the lake ecosystem was buffered against the combined human and natural disturbance. In contrast, lake–ecosystem conditions started to fluctuate across a broad range of states after AD 1600. The period AD 1745–1886 represents the phase with the strongest human disturbance of the catchment–lake ecosystem. During that time, the range of natural climate variability did not increase. Analyses of the frequency of change points in the multi-proxy data set suggest that the last 400 years were highly variable and increased vulnerability of the ecosystem to the anthropogenic disturbances. This led to significant rapid ecosystem transformations.

In this study we present a quantitative, high resolution reconstruction of past austral winter length in the Chilean Andes at 38°S from AD 1920 to 2009. For Laguna Escondida, a nearly pristine lake situated on the flanks of the Andes at 1740 m above sea level, past variability in the duration of the winter season (Days T4 °C) was reconstructed. Because high elevation meteorological stations are absent in this region, the reconstruction provides novel insights into recent temperature trends in the central–southern Andes. As a cold-season temperature proxy, we used chrysophyte stomatocysts. This novel proxy for cold season temperature was so far applied successfully in the European Alps and Pyrenees but has not yet been tested in the Southern Hemisphere. The reconstruction in this study was based on a newly developed Transfer Function to estimate Days T4 °C (number of consecutive days with surface water temperatures at or below 4 °C) from sedimentary stomatocyst assemblages (R2boot = 0.8, RMSEPboot = 28.7 days (= half the standard deviation)). To develop a high quality TF model, sediment traps and thermistors were placed in thirty remote lakes along an altitude gradient (420–2040 m a.s.l.). Complete materials and data were collected in 24 lakes after one year. Detailed statistical analyses indicate that modern stomatocysts primarily respond to the length of the cold season. The TF model was then applied to the sedimentary stomatocysts from a 210Pb-dated short core of L. Escondida. Comparison to independent reanalysis data showed that reconstructed changes in Days T4°C provides detailed information on winter–spring temperature variability since AD 1920. The reconstruction shows that recent warming (onset in AD 1980) in the southern Chilean Andes was not exceptional in the context of the past century. This is in strong contrast to studies from the Northern Hemisphere. The finding is also in contrast to the cooling temperature trends which were detected using meteorological measurement data in low altitude sites along the Chilean coast. This finding confirms that coastal meteorological station data in this region do not reliably reflect recent temperature trends at high altitudes. Moreover, it implies a southward shift of the northern border of the Westerlies wind belt. This study clearly illustrates the importance of quantitative, high resolution studies from remote sites, in particular at high elevation mountain areas.

Lakes located downwind of active volcanoes serve as a natural repository for volcanic ash (tephra) produced during eruptive events. In this study, sediment cores from four lakes in Cajas National Park, southern Ecuador, situated approximately 200 km downwind of active volcanoes in the Northern Andes Volcanic Zone, were analysed to document the regional history of tephra fall extending back around 3,000 a cal BP. The ages of the lacustrine sedimentary sequences were constrained using a total of 20 AMS radiocarbon ages on plant remains. The tephra layers were correlated among the lakes based on their radiocarbon age, elemental composition, colour, and grain morphology. We found five unique tephra layers, each at least 0.2 cm thick, and further constrained their ages by combining the results from two age-depth modelling approaches (clam and rbacon). The tephra layers were deposited 3,034±621, 2,027±41, 1,557±177, 733±112, and 450±70 a cal BP. The ages of all but the youngest tephra layer overlap with those of known eruptions from Tungurahua. Some tephra layers are missing as macroscopic layers in several cores, with only two of the five tephra layers visible in the sediment of three lakes. Likewise, previous studies of lake sediment cores from the region are missing the four youngest tephra layers, further highlighting the need to sample multiple lakes to reconstruct a comprehensive history of fallout events. The newly documented stratigraphic marker layers will benefit future studies of lake sediments in Cajas National Park.

20th century eutrophication and global spread of anoxia is a threat for freshwater ecosystems. Little is known about Holocene anoxia and meromixis events when anthropogenic impacts were weaker and natural ecosystem variability played the dominant role. In this study, we examine the relationship between lake mixing and lake production, climate variability, vegetation cover, catchment erosion and (pre)historic anthropogenic impacts in Moossee (Switzerland), over the last 15,000 years. We use sub-annually resolved calibrated hyperspectral imaging data (total chlorophyll for paleoproduction, bacteriopheophytin for anoxia and meromixis) combined with X-ray fluorescence and pollen data. Production shows a first increase at 14,500 cal yr BP, a further increase after 7500 cal BP, relative maxima in the late Bronze, Iron and Middle Ages, and the unprecedented peak in the 20th Century. Until 7500 cal BP, the lake was well mixed with only scarce phases of seasonal to multiannual anoxia. Repeated meromixis events occurred between 7500 and 2500 cal BP when temperatures were high, forests closed, and lake production was already enhanced. After the forests were cleared (2500 cal BP) the lake remained mostly holomictic. Holocene meromixis events were systematically terminated by local deforestation related to Neolithic and Bronze Age lakeshore settlements: charcoal peaked, tree pollen dropped below a threshold of 80%, soil erosion and lake production increased and bacteriopheophytin disappeared. Meromixis re-established after the termination of lakeshore settlements and the onset of afforestation with tree pollen exceeding 80%. These repeated cycles unambiguously document how even early human societies affected the mixing regime and biogeochemical cycling in this lake.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.