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Paleoclimate data provide important information about the character of natural climate variability. However, records with sufficient length and resolution to resolve high‐frequency (decadal‐scale) variability across the Holocene are scarce. We present a 10,800‐year reconstruction of spring and summer temperature at three‐year resolution based on biochemical varves from Lake Żabińskie, Poland. The reconstruction is based on Ca/Ti ratio, which are significantly correlated with instrumental spring and summer temperature spanning 240 years. Major climate events of the Holocene period are represented in the reconstruction, including the Holocene Thermal Maximum, 8.2 ka Event, Medieval Climate Anomaly, and Little Ice Age. A low‐frequency 8,000‐year decreasing trend in warm‐season temperatures is driven by declining summer insolation. Temperature variability is highest during the early Holocene, likely related to warmer and drier conditions. The rate of warming during the past 90 years is extremely unusual, if not unprecedented for the Holocene, based on our reconstruction. Plain Language Summary: Studying past climate change helps us understand how the climate system works. One aspect of past climate that is not well understood is how much temperatures changed from year to year and over decades during warm periods of Earth's history. In this study, we measured chemical properties of lake sediments from Poland and found that chemical changes were related to spring and summer temperature changes. We used this data to estimate spring and summer temperatures during the past 10,800 years. Relatively warm summers occurred around 9,000–7,500 years ago, a period known from other studies to be relatively warm in the northern hemisphere. We also found greater variability in temperatures during this warm period. Our long record of spring and summer temperatures shows that the warming rate of the past 90 years is most likely faster than any time during the past 10,800 years, showing that human‐caused global warming is more extreme than natural climate variations. Key Points: 10,800‐year‐long, 3‐year‐resolution, warm‐season temperature reconstruction from biochemical varves from Lake Żabińskie, PolandPeak summer temperatures occurred during the early Holocene with declining summer insolation driving cooling during the past 8,000 yearsThe rate of modern warming is highly unusual, at least 4 standard deviations above the mean temperature trend of the past 10,800 years [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Global spread of hypoxia and altered mixing regimes in freshwater systems is a growing major 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 multi-proxy approach combining high-resolution Hyperspectral Imaging (HSI) pigment data, with specific HPLC chlorophylls and carotenoids to examine Holocene trophic state changes and anoxia evolution in 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 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. [ABSTRACT FROM AUTHOR]

The El Niño–Southern Oscillation (ENSO) is the earth's dominant mode of inter‐annual climate variability. It alternates between warm (El Niño) and cold (La Niña) 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 approach 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 Niña‐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 volcanic eruptions of the last 1000 years, there is no reconstruction coherency with regard to the state of ENSO. This study provides new ENSO reconstructions based on a large, updated collection of proxy records. We identify the periods from 1580 to the end of the 17th century and from 1825 to present as most robust to quantify ENSO within the last millennium, and reveal disagreement among various ENSO reconstructions during five periods within 1700–1786, possibly related to teleconnection changes in the west Pacific domain. We find a significant response of ENSO towards more La Niña‐like conditions 3–5 years after major volcanic events. [ABSTRACT FROM AUTHOR]

High-resolution climate reconstructions from a range of natural archives across the world are fundamental to place current climate change into perspective. Paleoclimate records for the Southern Hemisphere are scarce and only a few quantitative high-resolution reconstructions exist for the past millennium. We present a record of annually laminated sediments of Lago Plomo (46°59′S, 72°52′W,203 m a.s.l.) located east of the Northern Patagonian Ice Field (NPI). Radiometric dating (210Pb, 137Cs, 14C AMS) is consistent with counts of millimetre-scale laminae, confirming the annual nature of the laminae couplets with a light summer and a dark winter layer. The varves were analyzed for thickness, mass accumulation rate (MAR), scanning x-ray fluorescence (XRF) and scanning reflectance spectroscopy in the visible range (VIS-RS). MAR data were calibrated against austral winter (JJA) precipitation data (CRU TS 3.0) for the period ad 1930–2002 (r = 0.67, p(aut) < 0.05). Using a linear inverse regression model we reconstructed winter precipitation for Lago Plomo back to ad 1530. The root mean squared error of prediction (RMSEP) is small (13.3 mm/month; 12% of the average precipitation) compared with the pronounced decadal and multidecadal variability, suggesting that most of the reconstructed variability is significant. Wetter phases (reference ad 1930–2002) were observed around ad 1600, ad 1630–1690 and ad 1780–1850, and a prolonged drier period ad 1690–1780 with a multidecadal minimum centered on ad 1770. The spatial correlation for South America suggests that the JJA precipitation record from Lago Plomo is representative for large areas in the southwest between c. 41°S and 51°S. [ABSTRACT FROM AUTHOR]

Abstract: The relationship between summer-autumn floods in Central Europe and climate warming is poorly constrained by available instrumental, historical, proxy and model data. To investigate this relationship, a complete record of paleofloods, regional glacier length changes (and associated climate phases) and regional glacier advances and retreats (and associated climate transitions) are derived from the varved sediments of Lake Silvaplana (ca. 1450 BC–AD 420; Upper Engadine, Switzerland). In combination, these records provide insight into the behavior of floods (i.e. frequency) under a wide range of climate conditions. Eighty-five paleofloods are identified from turbidites in the sediments of Lake Silvaplana. Regional glacier length changes (and associated cool and/or wet and warm and/or dry climate phases) are inferred from centennial anomalies in the square root of low-pass (LP) filtered Mass Accumulation Rates (MARLP 1/2). Regional glacier advances and retreats (and associated cooling and/or wetting and warming and/or drying climate transitions) are inferred from centennial trends in MARLP. This is the first continuous record of glacier length changes in the Lake Silvaplana catchment for this time period. These data agree with regional records of land-use, glacier activity and lake levels. More frequent turbidites are found during cool and/or wet phases of ca. 1450 BC to AD 420. However, no relationship to climate transitions is discerned. Consistently, June–July–August (JJA) temperatures dating ca. 570 BC–AD 120 are inversely correlated to the frequency of turbidites. The rate that turbidite frequency increases with cooler JJA temperatures is not linear. Finally, 130 analogues for a 21st century climate in the Alps between ca. 570 BC–AD 120 (i.e. 50year windows with a warming trend and average JJA temperature exceeding AD 1950–AD 2000 values from nearby meteo station Sils Maria) are considered. These reveal that turbidites are less frequent than between ca. 1450 BC–AD 420. [Copyright &y& Elsevier]

Abstract: The present interglacial, the Holocene, spans the period of the last 11,700 years. It has sustained the growth and development of modern society. The millennial-scale decreasing solar insolation in the Northern Hemisphere summer lead to Northern Hemisphere cooling, a southern shift of the Intertropical Convergence Zone (ITCZ) and a weakening of the Northern Hemisphere summer monsoon systems. On the multidecadal to multicentury-scale, periods of more stable and warmer climate were interrupted by several cold relapses, at least in the Northern Hemisphere extra-tropical area. Based on carefully selected 10,000-year-long time series of temperature and humidity/precipitation, as well as reconstructions of glacier advances, the spatiotemporal pattern of six cold relapses during the last 10,000 years was analysed and presented in form of a Holocene Climate Atlas (HOCLAT; see http://www.oeschger.unibe.ch/research/projects/holocene_atlas/). A clear cyclicity was not found, and the spatiotemporal variability of temperature and humidity/precipitation during the six specific cold events (8200, 6300, 4700, 2700, 1550 and 550 years BP) was very high. Different dynamical processes such as meltwater flux into the North Atlantic, low solar activity, explosive volcanic eruptions, and fluctuations of the thermohaline circulation likely played a major role. In addition, internal dynamics in the North Atlantic and Pacific area (including their complex interaction) were likely involved. [Copyright &y& Elsevier]

Information about decadal to millennial variability of El Niño Southern Oscillation (ENSO) is fundamental for the assessment of ENSO responses to natural and anthropogenic forcings. Despite a growing number of ENSO reconstructions, the overall picture of Holocene ENSO variability is inconsistent. Here, we revisit the iconic Holocene ENSO sediment record of Lake Pallcacocha, Ecuador (Rodbell et al., 1999). We asked: (i) How coherent are the records of clastic layers (flood layers) in the sediments of Lake Pallcacocha and adjacent Lake Fondococha? (ii) What are the synoptic-scale atmospheric conditions that lead to intense precipitation and, potentially, to alluvial activity promoting the deposition of clastic layers in these lakes? (iii) Is intense precipitation in this area associated with El Niño, or not? We analyzed clastic layers in Late-Holocene sediments from multiple cores in Lakes Pallcacocha and Fondococha from Cajas National Park, southern Ecuadorian Andes. Additionally, we investigated precipitation data from 13 nearby meteorological stations to test if intense precipitation (percentiles P 0.95 , P 0.99, P 0.995 ) is predominantly related to El Niño conditions or not (based on 15 different ENSO indices). Our results show that the absolute flood frequencies (clastic layers per 100 years) differ substantially from lake to lake. This indicates that the frequency of clastic layers reflects different sensitivities (thresholds of precipitation) of the catchments to alluvial activity. 210 Pb ages suggest that neither the 1982/83 nor the 1997/98 very strong El Niños produced clastic layers comparable to those found in the late Holocene. Daily precipitation records from meteorological stations close to Lake Pallcacocha including a high-altitude station from the western slope of the Andes did not show unusually high precipitation during the super El Niño 2015/16. We further find that intense precipitation in this area occurs at roughly equal probability under El Niño, La Niña and neutral conditions. Although the spectral properties of the late Holocene clastic layers in Lake Pallcacocha fall into the typical ENSO frequency band, we do not find evidence in the recent sediments and the meteorological data that would support a diagnostic link between alluvial activity in Lake Pallcacocha and strong El Niño events. Our data do not support the idea that the (late) Holocene flood record of Lake Pallcacocha is a conclusive paleo-El Niño record. [ABSTRACT FROM AUTHOR]

Human activity has fundamentally altered the global phosphorus (P) cycle. Yet our understanding of when and how humans influenced the P cycle has been limited by the scarcity of long-term P sequestration records, particularly outside Europe and North America. Lake sediments provide a unique archive of past P burial rates and allow the human-mediated disruption of the global P cycle to be examined. We compiled the first global-scale and continentally resolved reconstruction of lake-wide Holocene P burial rates using 108 lakes from around the world. In Europe, lake P burial rates started to increase noticeably after ∼4000 calendar years before 1950 CE (cal BP), whereas the increase occurred later in China (∼2000 cal BP) and in North America (∼550 cal BP), which is most likely related to different histories of population growth, land-use and associated soil erosion intensities. Anthropogenic soil erosion explains ∼86% of the observed changes in global lake P burial rates in pre-industrial times. We also provide the first long-term estimates of the global lake P sink over the Holocene (∼2686 Tg P). We estimate that the global mean lake sediment P sequestration since 1850 CE (100 cal BP) is ∼1.54 Tg P yr−1, representing approximately a six-fold increase above the mean pre-industrial value (∼0.24 Tg P yr−1; 11,500 to 100 cal BP) and around a ten-fold increase above the Early-Middle Holocene low-disturbance baseline of 0.16 Tg P yr−1. This study suggests that human activities have been affecting the global P cycle for millennia, with substantial alteration after industrial times (1850 CE). • We estimated Holocene phosphorus (P) burial rates in sediments of 108 global lakes. • Global lake P burial rates increased notably in the Late Holocene. • Human-induced soil erosion caused early increases in lake sediment P burial rates. • We observed different patterns of lake P burial rates among continents since the Late Holocene. • We provide the first estimates of long-term, global P sequestration in lake sediments throughout the Holocene. [ABSTRACT FROM AUTHOR]

Fire regimes differ across tropical and subtropical biomes depending on multiple parameters whose interactions and levels of importance are poorly understood, particularly at multidecadal and longer time scales. In the catchment of Lake Victoria, savanna, rainforest, and Afromontane vegetation have interspersed over the last 17,000 years, which may have influenced the fire regime and vice versa. However, climate and humans are most often the primary drivers of fire regime changes, and analysing their respective roles is critical for understanding current and future fire regimes. Besides a handful of radiocarbon dates on grassy charcoal, the timescales of published studies of Lake Victoria sediment chronologies rely mostly on dates of bulk sediment, and chronological disagreements persist, mainly due to variation between estimations of the 14C reservoir effect. Here, we provide independent 14C chronologies for three Late Glacial and Holocene lacustrine sediment cores from various water depths and compare them with the biostratigraphy to establish a new chronological framework. We present the first continuous sedimentary charcoal records from Lake Victoria; these suggest that fire activity varied substantially during the past 17,000 years. Our new pollen records reveal the long-term vegetation dynamics. The available evidence suggests that before human impact increased during the Iron Age (ca. 2400 yr BP), biomass burning was linked to climate and vegetation reorganizations, such as warming, drying, and the expansion of rainforests and savannas. Our results imply that climate can trigger substantial fire regime changes and that vegetation responses to climate change can co-determine the fire regime. For instance, biomass burning decreased significantly when the rainforest expanded in response to increasing temperatures and moisture availability. Such insights into the long-term linkages between climate, vegetation, and the fire regime may help to refine ecosystem management and conservation strategies in a changing global climate. • Three age–depth models provide a highly reliable chronology of Lake Victoria. • A regional desiccation phase might have occurred from ca. 18,500 until 15,500 cal yr BP. • The savanna biome persisted until the onset of the Holocene. • During the AHP forest species expanded and fire activity declined steadily. • Long-term variations of 1–2 °C may drive biome reorganizations in the LV basin. [ABSTRACT FROM AUTHOR]

High-resolution records of calibrated proxy data for the past millennium are fundamental to place current changes into the context of pre-industrial natural forced and unforced variability. Although the need for regional spatially-explicit comprehensive reconstructions is widely recognized, the proxy data sources are still scarce, particularly for the Southern Hemisphere and especially for South America. We present a 600-year long warm season temperature record from varved sediments of Lago Plomo, a proglacial lake of the Northern Patagonian Ice field in Southern Chile (46°59′S, 72°52′W, 203 m a.s.l.). The thickness of the bright summer sediment layer relative to the dark winter layer (measured as total brightness; % reflectance 400–730 nm) is calibrated against warm season SONDJF temperature (1900–2009; r = 0.58, p (aut) = 0.056, RE = 0.52; CE = 0.15, RMSEP = 0.28 °C; five-year triangular filtered data). In Lago Plomo, warm summer temperatures lead to enhanced glacier melt and suspended sediment transport, which results in thicker light summer layers and to brighter sediments. Although Patagonia shows pronounced regional differences in decadal temperature trends and variability, the 600 years temperature reconstruction from Lago Plomo compares favourably with other regional/continental temperature records, but also emphasizes significant regional differences for which no data and information existed so far. These regional differences seem to be real as they are also reflected in modern climate data sets (1900–2010). The reconstruction shows pronounced subdecadal – multidecadal variability with cold phases during parts of the Little Ice Age (16th and 18th centuries) and in the beginning of the 20th century. The most prominent warm phase is the 19th century which is as warm as the second half of the 20th century. The exceptional summer warmth AD 1780–1810 is also found in other archives of Northern Patagonia and Central Chile. Our record shows the delayed 20th century warming in the Southern Hemisphere. The comparison between winter precipitation and summer temperature (inter-seasonal coupling) from Lago Plomo reveals alternating phases with parallel and contrasting decadal trends of winter precipitation and summer temperature (positive and negative running correlations R winter PP; summer TT ). This observation from the sediment proxy data is also confirmed by two sets of reanalysis data for the 20th century. Reanalysis data show that phases with negative correlations between winter precipitation and summer temperature (e.g., dry winters and warm summers) at Lago Plomo are characteristic for periods when circumpolar Westerly flow is displaced southward and enhanced around 60°S. [ABSTRACT FROM AUTHOR]

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 Żabińskie, 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. Unlabelled Image • Aquatic productivity and anoxia were inferred from a 10,800-year sediment core. • Sedimentary pigments were quantified using hyperspectral imaging (HSI). • Anoxia was persistent during the Holocene prior to human-mediated deforestation. • HSI on varves enables sub-annual reconstruction of past productivity and anoxia. [ABSTRACT FROM AUTHOR]

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. • First Late Glacial/Holocene high-resolution paleoproduction and meromixis record. • Sub-varve resolution hyperspectral data provide information about seasonal anoxia. • Early human impact influenced the lake mixing regime. • Closed forest (tree pollen>80%), eutrophication and warm summers prompted meromixis. [ABSTRACT FROM AUTHOR]