The Reservoir Age Effect Varies With the Mobilization of Pre-Aged Organic Carbon in a High-Altitude Central Asian Catchment

Lake sediments provide excellent archives to study past environmental and hydrological changes at high temporal resolution. However, their utility is often restricted by chronological uncertainties due to the “ reservoir age effect ” (RAE), a phenomenon that results in anomalously old radiocarbon ages of total organic carbon (TOC) samples that is mainly attributed to the contribution of pre-aged carbon from aquatic organisms. Although the RAE is a well-known problem especially in high altitude lakes, detailed studies analyzing the temporal variations in the contribution of terrestrial and aquatic organic carbon (OC) on the RAE are scarce. This is partially due to the complexity of isolating individual compounds for subsequent compound-speci fi c radiocarbon analysis (CSRA). We developed a rapid method for isolating individual short-chain (C 16 and C 18 ) and long-chain ( > C 24 ) saturated fatty acid methyl esters (FAMEs) by using high-pressure liquid chromatography (HPLC). Our method introduces only minor contaminations (0.50 ± 0.22 µg dead carbon on average) and requires only few injections ( ≤ 10), therefore offering clear advantages over traditional preparative gas chromatography (prep-GC). Here we show that radiocarbon values ( Δ 14C) of long-chain FAs, which originate from terrestrial higher plant waxes, re fl ect carbon from a substantially pre-aged OC reservoir, whereas the Δ 14C of short-chain FAs that originate from aquatic sources were generally less pre-aged. 14 C ages obtained from the long-chain FAs are in closer agreement with 14 C ages of the corresponding bulk TOC fraction, indicating a high control of pre-aged terrestrial OC input from the catchment on TOC-derived 14 C ages. Variations in the age offset between terrestrial and aquatic biomarkers are related to changes in bulk sediment log(Ti/K) that re fl ect variations in detrital input from the catchment. Our results indicate that the chronological offset between terrestrial and aquatic OC in this high-altitude catchment is mainly driven by temporal variations in the mobilization of pre-aged OC from the catchment. In conclusion, to obtain accurate and process-speci fi c lake sediment chronologies, attention must be given to the temporal dynamics of the RAE. Variations in the apparent ages of aquatic and terrestrial contributions to the sediment and their mass balance can substantially alter the reservoir age effect

Seasonal deposition processes and chronology of a varved Holocene lake sediment record from Lake Chatyr Kol (Kyrgyz Republic)

A finely laminated lake sediment record with a basal age of 11,619 ± 603 years BP was retrieved from Lake Chatyr Kol (Kyrgyz Republic). Microfacies analysis reveals the presence of seasonal laminae (varves) from the sediment basis to ~ 360 ± 40 years BP. The Chatvd19 floating varve chronology covers the time span from 360 ± 40 years BP to the base and relies on replicate varve counts on overlapping petrographic thin sections with an uncertainty of ± 5 %. The uppermost non-varved interval was chronologically constrained by 210Pb and 137Cs γ-spectrometry and interpolation based on varve thickness measurements of adjacent varved intervals with an assumed uncertainty of 10 %. Six varve types were distinguished, are described in detail and show a changing predominance of clastic-organic, clastic-calcitic or -aragonitic, calcitic-clastic, organic-clastic and clastic-diatom varves throughout the Holocene. Variations in varve thickness and the number and composition of seasonal sublayers are attributed to 1) changes in the amount of summer or winter/spring precipitation affecting local runoff and erosion and/or to 2) evaporative conditions during summer. Radiocarbon dating of bulk organic matter, daphnia remains, aquatic plant remains and Ruppia maritima seeds reveal reservoir ages with a clear decreasing trend up core from ~ 6,150 years in the early Holocene, to ~ 3,000 years in the mid-Holocene, to ~ 1,000 years and less in the late Holocene and modern times. In contrast, two radiocarbon dates from terrestrial plant remains are in good agreement with the varve-based chronology

Ecosystem shifts at two Mid-Holocene tipping points in the alpine Lake Son Kol (Kyrgyzstan, Central Asia)

Tipping points can be defined as critical ecosystem thresholds that start self-enforced dynamics pushing systems into new stable states. Many lake ecosystems of arid Central Asia are sensitive to hydrological changes as they are located at the intersection of the influence of the dry Siberian Anticyclone and the relatively humid mid-latitude Westerlies, and their sediment records can be used to study past tipping points. We studied subfossil chironomid remains preserved in a ca. 6000-year-long sediment record from the Central Asian lake Son Kol (Central Kyrgyzstan) to reconstruct past ecosystem dynamics. Our results show abrupt transitions from a chironomid fauna dominated by macrophyte-associated, salinity-indicating taxa, to a vegetation-independent fauna, and subsequently to a macrophyte-associated, freshwater-indicating fauna. A comparison of the chironomid-based environmental reconstruction to other proxy indicators from the same record suggests a phase of increased Westerly strength starting about 4900 cal. yr BP. This increase led to enhanced precipitation and sediment fluxes into the lake, which in turn led to high turbidity levels and consequently to a macrophyte collapse causing abrupt changes in the chironomid fauna. At 4300 cal. yr BP, a weakening of the Westerlies in combination with higher lake levels led to lower turbidity and ultimately to the recovery of the macrophyte population and associated changes in the chironomid assemblage. These two sequences of events show how the occurrence of a gradual change in an external trigger (Westerlies) can trigger a cascade of within-lake processes (turbidity, macrophyte density) and may ultimately lead to an abrupt reorganisation of the ecosystem (chironomid fauna), providing models for tipping points

TEM analysis of boride-based ultra-high temperature ceramics

Hafnium and zirconium borides are leading candidate materials for use in ultra-high-temperature applications thanks to their excellent combination of physical, mechanical and oxidation resistance properties. It has been shown that the addition of MoSi2 allows the densification without the application of pressure, improves the oxidation resistance and the mechanical properties at high temperatures. Despite the use of this sintering additive for several ultra high temperature ceramics, the densification mechanisms are still unclear and matter of debate. Transmission electron microscopy (TEM) is a powerful tool to explore microstructure at small length scale. A careful literature analysis reveals that neither detailed TEM work nor reports on densification mechanisms are available for this class of materials. In the present work, the microstructure of pressureless sintered ZrB2-MoSi2 and HfB2-MoSi2 composites was analyzed by scanning and transmission electron microscopy in order to disclose the mechanisms leading to densification and to understand the role of MoSi2 during sintering. The formation of solid solutions was observed in ZrB2-MoSi2 system, whilst the solubility of Mo into HfB2 lattice seems to be more limited. For both composites the presence of (TM,Mo)5SiB2, where TM=Hf or Zr, was detected. The formation of secondary phases is analysed and discussed in accordance with thermodynamical calculations and phase diagram

Microstructure characterization of boride-based ultra-high-temperature ceramics

Hafnium and zirconium borides are leading candidate materials for use in ultra-high-temperature applications thanks to their excellent combination of physical, mechanical and oxidation resistance properties. It has been shown that the addition of MoSi2 allows the densification without the application of pressure, improves the oxidation resistance and the mechanical properties at high temperatures. Despite the use of this sintering additive for several ultra high temperature ceramics, the densification mechanisms are still unclear and matter of debate. Transmission electron microscopy (TEM) is a powerful tool to explore microstructure at small length scale. A careful literature analysis reveals that neither detailed TEM work nor any reports on densification mechanism are available for this class of materials. In the present work, the microstructure of HfB2-MoSi2 and ZrB2-MoSi2 composites was analyzed in detail in order to gain an insight into the densification mechanism during pressureless sintering. The formation of solid solutions was observed in ZrB2-MoSi2 system, whilst the solubility of Mo into HfB2 lattice seems to be more limited. For both composites the presence of (TM,Mo)5SiB2, where TM=Hf or Zr, was detected. The formation of secondary phases is analysed and discussed in accordance with thermodynamical calculations and the phase diagram

Annual proxy data from Lago Grande di Monticchio (southern Italy) between 76 and 112 ka: new chronological constraints and insights on abrupt climatic oscillations

We present new annual sedimentological proxies and sub-annual element scanner data from the Lago Grande di Monticchio (MON) sediment record for the sequence 76–112 thousand years before present (ka). They are combined with the previously published decadal to centennial resolved pollen assemblage in order to provide a comprehensive reconstruction of six major abrupt stadial spells (MON 1–6) in the central Mediterranean during the early phase of the last glaciation. These climatic oscillations are defined by intervals of thicker varves and high Ti-counts and coincide with episodes of forest depletion interpreted as Mediterranean stadial conditions (cold winter/dry summer). Our chronology, labelled as MON-2014, has been updated for the study interval by tephrochronology and repeated and more precise varve counts and is independent from ice-core and speleothem chronologies. The high-resolution Monticchio data then have been compared in detail with the Greenland ice-core δ¹⁸O record (NorthGRIP) and the northern Alps speleothem δ¹⁸O_calcite data (NALPS). Based on visual inspection of major changes in the proxy data, MON 2–6 are suggested to correlate with Greenland stadials (GS) 25–20. MON 1 (Woillard event), the first and shortest cooling spell in the Mediterranean after a long phase of stable interglacial conditions, has no counterpart in the Greenland ice core, but coincides with the lowest isotope values at the end of the gradual decrease in δ¹⁸O_ice in NorthGRIP during the second half of the Greenland interstadial (GI) 25. MON 3 is the least pronounced cold spell and shows gradual transitions, whereas its NorthGRIP counterpart GS 24 is characterized by sharp changes in the isotope records. MON 2 and MON 4 are the longest and most pronounced oscillations in the MON sediments in good agreement with their counterparts identified in the ice and spelethem records. The length of MON 4 (correlating with GS 22) supports the duration of stadial proposed by the NALPS timescales and suggests ca. 500 year longer duration than calculated by the ice-core chronologies GICC05_modelext and AICC2012. Absolute dating of the cold spells provided by the MON-2014 chronology shows good agreement among the MON-2014, the GICC05_modelext and the NALPS timescales for the period between 112 and 100 ka. In contrast, the MON-2014 varve chronology dates the oscillations MON 4 to MON 6 (92–76 ka) as ca. 3500 years older than the most likely corresponding stadials GS 22 to GS 20 by the other chronologies

Biomolecular Evidence of Early Human Occupation of a High-Altitude Site in Western Central Asia During the Holocene

Reconstructions of early human occupation of high-altitude sites in Central Asia and possible migration routes during the Holocene are limited due to restricted archeological sample material. Consequently, there is a growing interest in alternative approaches to investigate past anthropogenic activity in this area. In this study, fecal biomarkers preserved in lake sediments from Lake Chatyr Kol (Tian Shan, Kyrgyzstan) were analyzed to reconstruct the local presence of humans and pastoral animals in this low-human-impact area in the past. Spanning the last ∼11,700 years, this high-altitude site (∼3,500 m above sea level) provides a continuous record of human occupancy in Western Central Asia. An early increase of human presence in the area during the mid-Holocene is marked by a sharp peak of the human fecal sterol coprostanol and its epimer epicoprostanol in the sediments. An associated increase in 5β-stigmastanol, a fecal biomarker deriving from herbivores indicates a human occupancy that most probably largely depended upon livestock. However, sterol profiles show that grazing animals had already occupied the catchment area of Lake Chatyr Kol before and also after a significant presence of humans. The biomarker evidence in this study demonstrates an early presence of humans in a high-altitude site in Central Asia at ∼5,900–4,000 a BP. Dry environmental conditions during this period likely made high altitude regions more accessible. Moreover, our results help to understand human migration in Western Central Asia during the early and mid-Holocene as part of a prehistoric Silk Road territory

Large‐scale mass movements recorded in the sediments of Lake Hallstatt (Austria)–evidence for recurrent natural hazards at a UNESCO World Heritage site

The Bronze to Iron Age underground salt mining complex of Hallstatt (Austria) is widely recognised for its cultural importance and wealth of archaeological artefacts. However, while the daily life in the salt mines is archaeologically well documented and environmental effects of the mining activity have been investigated recently, the impact of natural hazards on the prehistoric mining community is still poorly understood. For instance, while it is well established that the prehistoric underground mines have repeatedly been destroyed by large-scale mass movements, only little is known about the characteristics and extent of these events as well as about mass-movement recurrence during more recent times. To shed light on past mass-movement activity in the vicinity of the Hallstatt salt mines, we investigated sediment cores from adjacent Lake Hallstatt. Within the regular lake sediments we identified three large-scale event deposits, which are interpreted to originate from spontaneous or seismically induced mass movements in the mid-19th and late 9th century ce and the mid-4th century bce. While the age of the latter event is in good agreement with the abandonment of the famous Iron Age cemetery at Hallstatt, the younger events indicate that large-scale mass movements also occurred repeatedly during the Common Era

Enzymatic and spectroscopic properties of a thermostable [NiFe]‑hydrogenase performing H2-driven NAD+-reduction in the presence of O2

Biocatalysts that mediate the H2-dependent reduction of NAD+ to NADH are attractive from both a fundamental and applied perspective. Here we present the first biochemical and spectroscopic characterization of an NAD+-reducing [NiFe]‑hydrogenase that sustains catalytic activity at high temperatures and in the presence of O2, which usually acts as an inhibitor. We isolated and sequenced the four structural genes, hoxFUYH, encoding the soluble NAD+-reducing [NiFe]‑hydrogenase (SH) from the thermophilic betaproteobacterium, Hydrogenophilus thermoluteolus TH-1T (Ht). The HtSH was recombinantly overproduced in a hydrogenase-free mutant of the well-studied, H2-oxidizing betaproteobacterium Ralstonia eutropha H16 (Re). The enzyme was purified and characterized with various biochemical and spectroscopic techniques. Highest H2-mediated NAD+ reduction activity was observed at 80 °C and pH 6.5, and catalytic activity was found to be sustained at low O2 concentrations. Infrared spectroscopic analyses revealed a spectral pattern for as-isolated HtSH that is remarkably different from those of the closely related ReSH and other [NiFe]‑hydrogenases. This indicates an unusual configuration of the oxidized catalytic center in HtSH. Complementary electron paramagnetic resonance spectroscopic analyses revealed spectral signatures similar to related NAD+-reducing [NiFe]‑hydrogenases. This study lays the groundwork for structural and functional analyses of the HtSH as well as application of this enzyme for H2-driven cofactor recycling under oxic conditions at elevated temperatures

Impact of Specifically Adsorbing Anions on the Electroless Growth of Gold Nanotubes

Electroless metal deposition on nanochannel-containing templates is a versatile route towards metal nanotubes and nanowires if the plating reaction can be sufficiently controlled. In this study, disulfitoaurate-formaldehyde-based gold plating baths were modified by the addition of halides, pseudohalides, and EDTA. The introduction of specifically adsorbing anions strongly affected the heterogeneously autocatalyzed plating reaction and allowed the regulation of the reaction rate and the product morphology. The new plating baths showed enhanced stability and allowed the synthesis of homogeneous nanotubes of high aspect ratios (>150) in 30 μm thick ion track-etched polymer templates. Depending on the reaction conditions, solid and porous structures consisting of gold nanoparticles of differing size and shape were accessible. The presented strategy offers adapted gold thin films, nanotubes, and nanowires for applications in catalysis or sensing