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Rationale: Compound‐specific stable isotope analysis of hydrogen (δD) and carbon (δ13C) in organic compounds is a valuable tool in biogeochemical research. A key limitation of this method is the relatively large amount of sample required to achieve desirable precision. Methods: We developed a large‐volume (20 μL) injection method that allows for high throughput analysis of less concentrated samples and tested it for δ13C and δD measurements of n‐alkanes. We also conducted a comparison of reference standards and assessed several methods to normalize and correct n‐alkane δD and δ13C measurements. Results: The mean precision of the δD method based on 233 environmental n‐alkane samples (two to three replications per sample) is 4.0‰ (1σ, estimated from the weighted mean of the pooled unbiased standard deviations) and 0.46‰ (1σ) for δ13C from 37 environmental samples (two to three replications per sample). The evaluation of reference standards shows that the use of n‐alkane standards with large offsets in δD values in adjacent n‐alkane chains can lead to biases in measurement correction. Conclusions: The large‐volume injection method shows good reproducibility of δ13C and δD measurements of n‐alkanes and reduces the required sample concentration by about 80%. We propose that for δD measurements, a reference standard set should be used in which each reference standard has a limited range of δD values and no adjacent n‐alkane chains, to minimize memory effects. [ABSTRACT FROM AUTHOR]

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]

Recent technological advancements in spectral imaging core-scanning techniques have proved to be a promising tool to study lake sediments at extremely high resolution. We used this novel analytical approach to scan core AU3 of the Pleistocene Auel maar, Western Germany. The resulting ultra-high-resolution RABD670 spectral index, a proxy for the lake's primary production, shows an almost complete succession of Greenland Interstadials of the NGRIP ice core chronology back to around 60,000 years. Using the ELSA-20 chronology and its anchor points to the NGRIP record as a stratigraphic basis, we were able to compare and fine-tune prominent climate signals occurring in both regions. This in-depth correlation yields strong evidence that the climates of Greenland and Central Europe were not only strongly coupled on timescales of stadials and interstadials but even on multidecadal scales, showing prominent climate cycles between 20 and 125 years. As climate changes in these regions were ultimately driven by variations in the North Atlantic meridional heat transport, their strong coupling becomes most apparent during cold and arid intervals. In contrast, longer-lasting warmer and more humid phases caused the activation of various regional feedback mechanisms (e.g., soil formation, forest growth), resulting in more complex patterns in the proxy records. [ABSTRACT FROM AUTHOR]

Millennial-scale climate variations during the last glacial period, such as Dansgaard–Oeschger (DO) cycles and Heinrich events, have been extensively studied using ice core and marine proxy records. However, there is a limited understanding of the magnitude of these temperature fluctuations in continental regions, and questions remain about the seasonal signal of these climate events. This study presents a 60 000-year-long temperature reconstruction based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) extracted from lake sediments from the Eifel Volcanic Field, Germany. brGDGTs are bacterial membrane-spanning lipids that are known to have a strong relationship with temperature, making them suitable for temperature reconstructions. We test several temperature calibration models on modern samples taken from soils and multiple maar lakes. We find a negative bias in brGDGT-based temperature estimates associated with water depth and anoxic conditions that can be corrected for by accounting for a brGDGT isomer that is only produced in anoxic conditions. The corrected temperature reconstruction correlates with proxy and climate model estimates of temperature spanning the same time period, validating the calibration approach we selected. However, millennial-scale variability is significantly dampened in the brGDGT record, and in contrast to other Northern Hemisphere climate records, during several Heinrich stadials, temperatures actually increase. We demonstrate that these apparent discrepancies can be explained by the unique seasonal response of the brGDGT paleothermometer to temperatures of months above freezing (TMAF). Our data support the view that warm-season temperatures in Europe varied minimally during the last glacial period and that abrupt millennial-scale events were defined by colder, longer winters. Our continuous high-resolution temperature reconstruction provides important information about the magnitude of seasonal climate variability during the last glacial period that can be used to test climate models and inform studies of paleoecological change. [ABSTRACT FROM AUTHOR]

Millennial-scale climate variations during the last glacial period, such as Dansgaard–Oeschger (D/O) cycles and Heinrich events, have been extensively studied using ice core and marine proxy records. However, there is a limited understanding of the magnitude of these temperature fluctuations in continental regions, and questions remain about the seasonal signal of these climate events. This study presents a 60,000-year long temperature reconstruction based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) extracted from lake sediments from the Eifel volcanic field, Germany. brGDGTs are bacterial membrane lipids that are known to have strong relationship with temperature, making them suitable for temperature reconstructions. We test several temperature calibration models on modern samples taken from soils and multiple maar lakes. We find a bias associated with water depth and anoxic conditions that can be corrected for by accounting for a brGDGT isomer that is only produced in anoxic conditions. The corrected temperature reconstruction correlates with proxy and model temperature record spanning the same time period, validating the calibration model. However, millennial-scale variability is significantly dampened in the brGDGT record, and in contrast to other northern hemisphere climate records, during several Heinrich stadials, temperatures actually increase. We demonstrate that these apparent discrepancies can be explained by the unique seasonal response of the brGDGT paleothermometer to temperatures of months above freezing (TMAF). Our data support the view that warm season temperatures in Europe varied minimally during the last glacial period, and that abrupt millennial-scale events were defined by colder, longer winters. Our continuous high-resolution temperature reconstruction provides important information about the magnitude of seasonal climate variability during the last glacial period that can be used to test climate models and inform studies of paleoecological change. [ABSTRACT FROM AUTHOR]

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 bacteriochlorophylls- 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 10.2–3.4 kyr BP, with dominance of PSB over GSB from 8.8–3.4 kyr BP indicating 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. [ABSTRACT FROM AUTHOR]

Mesomediterranean vegetation with abundant evergreen broadleaved trees and shrubs is dominant along the coast of northern Greece. Well-dated palynological records are available from the inland submediterranean zone, where evergreen broadleaved trees and shrubs are absent or rare, but less is known about the vegetation history of the coastal area. For instance, it is unclear when and why evergreen broadleaved vegetation expanded during the Holocene. Here we present a new record from a mesomediterranean site, Limni Volvi, near the coast of the Aegean Sea. To understand Holocene vegetation history, we combined palynology, microscopic charcoal analysis and biogeochemical proxies (Ti, Ca, RABD655–680max). At the start of the Holocene, open steppe vegetation and high fire activity prevailed around Limni Volvi. Afforestation by deciduous trees was likely delayed compared to sites further inland, due to the lack of moisture at the start of the Holocene, but vegetation composition was generally similar (i.e. submediterranean), with barely any evergreen broadleaved element. Around 8150 cal. BP (6200 cal. BC) trees and shrubs declined in the open woodlands or parklands, and the landscape returned to steppe conditions at 8000 cal. BP (6050 cal. BC), likely because of drier and colder conditions during the “8.2 ka event.” Around the same time, cultural indicators suggest the start of Neolithic agriculture in the region. The present-day mesomediterranean vegetation with evergreen oaks only began to develop after 6000–5000 cal. BP (4050–3050 cal. BC), likely in response to increasing winter temperatures and reduced frost occurrence, further advantaged by human impact during the past 3600 years. [ABSTRACT FROM AUTHOR]