Abstract Previous paleolimnological studies demonstrated that the sediments of Garba Guracha, situated at 3950 m asl in the afro-alpine zone of the Bale Mountains of Ethiopia, provide a complete Late Glacial and Holocene paleoclimate and environmental archive. We revisited Garba Guracha in order to retrieve new sediment cores and to apply new environmental proxies, e.g. charcoal, diatoms, biomarkers, and stable isotopes. Our chronology is established using 210 Pb dating and radiocarbon dating of bulk sedimentary organic matter, bulk n -alkanes, and charcoal. Although bedrock was not reached during coring, basal ages confirm that sedimentation started at the earliest ~ 16 cal kyr BP. The absence of a systematic age offset for the n -alkanes suggests that “pre-aging” is not a prominent issue in this lake, which is characterised by a very small afro-alpine catchment. X-ray fluorescence scans and total organic carbon contents show a prominent transition from minerogenic to organic-rich sediments around 11 cal kyr BP coinciding with the Holocene onset. While an unambiguous terrestrial versus aquatic source identification seems challenging, the n -alkane-based P aq proxy, TOC/N ratios, δ 13 C values, and the sugar biomarker patterns suggest a predominantly autochthonous organic matter source. Supraregional climate events, such as the African Humid Period, the Younger Dryas (YD), a 6.5 cal kyr BP short drying event, and the 4.2 cal kyr BP transition to overall drier climate are recorded in our archive. The Garba Guracha record suggests that northern hemisphere forcings played a role in the Eastern African highland paleoclimate.
Abstract. Lanzhou, which is located in a steep alpine valley in western China, is one of the most polluted cities in China during the wintertime. In this study, an Aerodyne high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS), a seven-wavelength aethalometer, and a scanning mobility particle sizer (SMPS) were deployed during 10 January to 4 February 2014 to study the mass concentrations, chemical processes, and sources of submicrometer particulate matter (PM1). The average PM1 concentration during this study was 57.3 µg m−3 (ranging from 2.1 to 229.7 µg m−3 for hourly averages), with organic aerosol (OA) accounting for 51.2 %, followed by nitrate (16.5 %), sulfate (12.5 %), ammonium (10.3 %), black carbon (BC, 6.4 %), and chloride (3.0 %). The mass concentration of PM1 during winter was more than twice the average value observed at the same site in summer 2012 (24.5 µg m−3), but the mass fraction of OA was similar in the two seasons. Nitrate contributed a significantly higher fraction to the PM1 mass in winter than summer (16.5 % vs. 10 %), largely due to more favored partitioning to the particle phase at low air temperature. The mass fractions of both OA and nitrate increased by ∼ 5 % (47 to 52 for OA and 13 to 18 % for nitrate) with the increase of the total PM1 mass loading, while the average sulfate fraction decreased by 6 % (17 to 11 %), indicating the importance of OA and nitrate for the heavy air pollution events in Lanzhou. The size distributions of OA, nitrate, sulfate, ammonium, and chloride all peaked at ∼ 500 nm, with OA being slightly broader, suggesting that aerosol particles were internally mixed during winter, likely due to frequently calm and stagnant air conditions during wintertime in Lanzhou (average wind speed: 0.82 m s−1).The average mass spectrum of OA showed a medium oxidation degree (average O ∕ C ratio of 0.28), which was lower than that during summer 2012 (O ∕ C = 0.33). This is consistent with weaker photochemical processing during winter. Positive matrix factorization (PMF) with the multi-linear engine (ME-2) solver identified six OA sources, i.e., a hydrocarbon-like OA (HOA), a biomass burning OA (BBOA), a cooking-emitted OA (COA), a coal combustion OA (CCOA), and two oxygenated OA (OOA) factors. One of the OOAs was less oxidized (LO-OOA), and the other one more oxidized (MO-OOA). LO-OOA was the most abundant OA component (22.3 % of OA mass), followed by CCOA (22.0 %), COA (20.2 %), MO-OOA (14.9 %), BBOA (10.8 %), and HOA (9.8 %). The mass fraction of primary OA ( = HOA + BBOA + COA + CCOA) increased during high PM pollution periods, indicating that local primary emissions were a main reason for the formation of air pollution events in Lanzhou during winter. Radiocarbon (14C) measurement was conducted on four PM2.5 filter samples from this study, which allowed for a quantitative source apportionment of organic carbon (OC). The non-fossil sources on average accounted for 55 ± 3 % of OC, which could be mainly from biomass burning and cooking activities, suggesting the importance of non-fossil sources for the PM pollution in Lanzhou. Together with the PMF results, we also found that a large fraction (66 ± 10 %) of the secondary OC was from non-fossil OC.
A comprehensive understanding of Holocene hydroclimate variability in the European Alps remains challenging because of the great spatial and temporal disparities between the northern and southern Alps, mainly caused by changes in atmospheric circulation patterns and different climate settings. Most of the hydroclimate studies are based on lake level and high-resolution flood reconstructions that can be potentially biased by catchment-specific effects and anthropogenic impacts. Moreover, floods are only single events and just one important aspect of paleohydrology. Phases of enhanced evaporation, transpiration and droughts are equally important ecologically and can occur between flooding events. Stable isotopes (δ18O) in speleothems and lake carbonates were applied to track past changes in atmospheric circulation and hydrology, but in the northern Alps, such studies mainly focus on the Late Glacial and Early Holocene. We present the first compound-specific δ2H record based on terrestrial (n-C31) and aquatic (n-C25) n-alkanes from a sediment core collected from Schliersee, a pre-alpine lake located in Bavaria (Germany), and covering the Late Holocene (past ~4.3 ka). Based on previous calibration studies and new data, we use the δ2H record of n-C31 as a proxy for the isotopic composition of precipitation. We find that δ2Hn-C31 from Schliersee shows depleted values between ~1200 and ~500 cal. yr BP and enriched values before (2500 – 1200 cal. yr BP) and thereafter (500 cal. yr BP until today). This pattern is in good agreement with speleothem δ18O from Spannagel cave, Austria, and compound-specific δ2H from Lake Ghirla, southern Alps and was previously interpreted to reflect changes in moisture source. Therefore, our results support the concept that northern hemispheric cooling and changes in the North Atlantic Oscillation cause changes in moisture source related to shifts in the position of the Westerlies. Based on our results we conclude that this mechanism seem to have affected the isotopic composition of precipitation in both northern and southern Alps. Moreover, aquatic δ2Hn-C25 is enriched by several tens of permille compared to terrestrial δ2Hn-C31, because of evaporative enrichment of lake water (Grafenstein & Labuhn, 2021 in: Ramstein et al., Springer Cham). Thus, we use their isotopic difference, expressed by Δaq–terr, as a proxy for evaporative enrichment. Our Δaq–terr shows a striking coincidence with tree-ring based drought reconstructions for Europe since the Medieval. This highlights that a “warm and dry” hydroclimate occurred during the Medieval (~1000 cal. yr BP), whereas “cool and wet” conditions prevailed during the Little Ice Age (~600 cal. yr BP). Furthermore, minima in Δaq–terr during the Little Ice Age seem to correspond to minima in solar forcing. High evaporative enrichment coincides with the observed anthropogenic warming during the last 250 years. Our δ2H-record from Schliersee is consistent with other regional reconstructions and provides additional insights into the paleohydrology of the northern Alps. This highlights the potential of compound-specific δ2H analyses as a powerful tool for paleohydrological reconstructions and helps to better understand the hydroclimate dynamics across the Alps.
Abstract It is generally accepted that a weakening of the North Atlantic thermohaline circulation caused the Younger Dryas cooling. Although the role of seasonality was emphasized previously, this aspect is rarely considered yet, and it remains elusive how this impacted hydroclimate during winters and summers across Central Europe. Here, we coupled biomarker-based δ 18 O and δ 2 H from Bergsee in southern Germany to reconstruct deuterium excess as a proxy for evaporation history from the Bølling-Allerød to the Preboreal. We compared this dataset with other biomarker isotope records in Central Europe. They are all lacking a strong isotopic depletion during the Younger Dryas, which is best explained by the summer sensitivity of the biomarker proxies: As Younger Dryas summers were relatively warm, there is an absence of the strong winter cooling signals recorded in annual water isotope records like Greenland or Lake Steißlingen. Lake evaporation at Bergsee together with other paleohydrological reconstructions draw a coherent picture of the Late Glacial hydroclimate, with strong evidence for warm and dry Younger Dryas summers. Rather than a southward shift of the Westerlies during winter, we suggest that a recently proposed feedback mechanism between North Atlantic sea ice extend, strong winter cooling and summer atmospheric blocking serves as a suitable explanation for summer dryness. Additional confidence to the robustness of these biomarker records is provided by the overall agreement of paleohydrological fluctuations during the Preboreal.
Aerosol samples were collected in Zurich, Switzerland, at an urban background site and were analyzed with size exclusion chromatography (SEC) and laser/desorption ionization mass spectrometry (LDI‐MS) for water‐soluble organic compounds with high molecular weight. Daily samples were collected during two campaigns in winter and summer, for 1 month each. The concentration of high‐molecular‐weight compounds (humic‐like substances (HULIS)) was between 0.4 and 4 μg/m 3 in winter and summer. The most intense signals in the LDI‐MS mass spectra were measured between m/z150 and 500, comparing well with the mode of the two main high mass peaks determined with SEC corresponding to masses between 200 and 600 Da. For the maximum molecular weight, however, different results were obtained by the two techniques: whereas a maximum molecular weight between 1300 and 3300 Da was found with SEC, hardly any peaks above m/z700 were measured with LDI‐MS. During summer the maximum molecular weight of HULIS (determined with SEC) correlates positively with several parameters such as ozone and increased temperature indicative of enhanced atmospheric photo‐oxidation. The HULIS concentration also correlates positively with the oxalic acid concentration in the particles. This suggests that HULIS are generated by secondary processes in summer. The lack of such correlations during winter suggests that other sources and processes might be important during colder seasons.
Alpine ecosystems are particularly affected by climate change due to their high sensitivity to temperature variations. Understanding the response of vegetation to rapid temperature increase and human impact is necessary to produce accurate and reliable predictions of future mountain vegetation and mitigate climate change impacts. Our multiproxy study reconstructs climate, fire, and vegetation dynamics over the last 14,200 cal. BP at Lai da Vons (1991 m a.s.l.), a subalpine lake situated at the treeline ecotone in Eastern Switzerland. Our results based on geochemical analysis (XRF), pollen and charcoal analysis, and high-resolution macrofossil analysis reveal that early afforestation started already at the onset of the Holocene, dominated by Pinus cembra and Larix decidua. Larch/stone pine forests formed closed forests at the site at 10,300 cal. BP. From 9500 cal. BP onwards, when summer temperatures were warmer than today, Picea abies expanded simultaneously with Abies alba. In the past 6500 years, increasing human impact, culminating during two main phases of landscape opening and intensification of fire regime, led to a progressive increase of species richness. Considering future climate change, an upward shift of treeline and timberline is expected in the absence of domesticated herbivores. We advocate that low-intensity pastoralism in such a cultural landscape is essential to preserve the plant diversity of species-rich mountain meadows.
Carbonaceous aerosols are a major fraction of airborne particulate matter. They contribute to climate and health effects of the total aerosol burden of the atmosphere by counteracting the current trend of global warming and inducing respiratory and cardiovascular diseases, respectively. In spite of this general importance, only little is known about composition and sources of carbonaceous aerosols. Analysis of the long-lived radioactive isotope 14C (radiocarbon) is a unique source apportionment tool: it unambiguously separates fossil from non-fossil sources, as 14C has completely decayed in fossil fuels, whereas modern materials have the contemporary radiocarbon level. A novel separation method was developed for the direct determination of 14C in organic carbon (OC) and elemental carbon (EC), two sub-fractions of total carbon (TC). The implementation of a gas ion source for measurement with accelerator mass spectrometry (AMS) made 14C analysis more simple and robust. Based on this technique, all major contributions to the carbonaceous aerosol can be identified, which originate from fossil-fuel, biomass-burning and biogenic emissions. A survey of results from different field campaigns is shown.
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