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Cave carbonate mineral deposits (speleothems) contain trace elements that are intensively investigated for their significance as palaeoclimate and environmental proxies. However, chlorine, which is abundant in marine and meteoric waters, has been overlooked as a potential palaeo-proxy, while cosmogenic 36Cl could, in principle, provide a solar irradiance proxy. Here, total Cl concentrations analysed from various speleothems were low (3–14 mg/kg), with variations linked to crystal fabrics. High-resolution synchrotron radiation micro X-ray fluorescence (μ-XRF) trace element mapping showed Cl often associated with Na, Si, and Al. We propose that speleothems incorporate Cl in two fractions: (1) water soluble (e.g., fluid inclusions) and (2) water insoluble and strongly bound (e.g., associated with detrital particulates). However, disparities indicated that alternate unidentified mechanisms for Cl incorporation were present, raising important questions regarding incorporation of many trace elements into speleothems. Our first measurements of 36Cl/Cl ratios in speleothems required large samples due to low Cl concentrations, limiting the potential of 36Cl as a solar irradiance proxy. Critically, our findings highlight a knowledge gap into how Cl and other trace elements are incorporated into speleothems, how the incorporation mechanisms and final elemental concentrations are related to speleothem fabrics, and the significance this may have for how trace elements in speleothems are interpreted as palaeoclimate proxies. [ABSTRACT FROM AUTHOR]

Annually laminated stalagmites are an exceptional archive of high-resolution past hydroclimate changes and provide robust annually-resolved age models. Stalagmite chemical annual laminae are invisible through optical and fluorescence microscopy, but are revealed by X-ray fluorescence or mass-spectrometry techniques. Synchrotron radiation based micro-X-ray fluorescence (S-μXRF) has the unique advantage of creating 2-dimensional maps of element distribution, which allows visualizing lateral heterogeneity and porosity that pose problems when one dimensional line-scanning techniques, such as LA-ICP-MS, are used. Here, we show how S-μXRF fast element mapping allows obtaining robust age models and palaeoclimate data from stalagmites otherwise difficult to date. Analytical challenges and solution to problems are discussed for both thick polished slabs and thin sections. The elements detectable through the fast acquisition mode (dwell time 1 to 4 ms) are typically Ca, Sr, Br, while Fe, Zn, Ni, Cu, Mn, Y, and Rb are occasionally detected in few distinctive layers or, in association with grain inclusions and porosity. Only Sr, however, exhibits a consistent and clear annual cyclicity. At incident angles of 90° the attenuation depth for Sr is ca. 240 μm and, therefore thick samples are preferable as they produce stronger fluorescence signal and sharper images unaffected by non-linear drift. On the other hand, thick samples are more affected by geometric and edge effects that can be particularly severe in the presence of irregular lamination and when the laminae are dipping with variable angle within the sample. The Sr concentration in the tropical stalagmites analysed in this study varied from 15 to 600 ppm, which strongly influences the dwell time needed to obtain high-quality maps. For Sr concentration below 20 ppm the ideal dwell time exceeds 20 ms and this renders the acquisition of large maps (> 20 Megapixels) hardly feasible. For concentrations above 20 ppm the fast mapping (dwell time < 5 ms) through S-μXRF is by far the best methodology to visualise chemical annual cycles in stalagmites. [Display omitted] • Synchrotron μXRF is the optimal technique for mapping chemical annual laminae in stalagmites. • Synchrotron μXRF mapping provides calendar chronologies in tropical stalagmites. • Sr is the best element that marks annual cycles • Inelastic and elastic scattering maps highlight crystal structure, growth defects and porosity. [ABSTRACT FROM AUTHOR]

Abstract: Precise dating and correlation of past key volcanic eruptions over a wide geographic area in archives of past climate variability is necessary to support a direct causality between volcanism and climate changes. Research has mostly focused on ice cores and varved sediments, which capture a record of volcanic eruptions in geochemistry and the presence of tephra and criptotephra. Precisely dated cave carbonate deposits, collectively known as speleothems are other valuable palaeoclimate archives, and encode information on past volcanism in their sulphate concentration variability. Due to the physical characteristic of speleothems, detection of sulphate concentration variability requires techniques capable of high spatial resolution, very low limit of detection (ppm to ppb) and low background noise. Synchrotron radiation-based (SR) micro X-ray fluorescence (µXRF) and X-ray absorption near-edge spectrometry prove to be one of the most effective techniques to detect short-lived pulses of sulphate concentration increase, which may be interpreted as being related to atmospheric load due to volcanic eruptions. Here, we provide an overview of existing work as well as a novel interpretation of a SR µXRF-based sulphate series in an annually laminated stalagmite with robust chronology. Sulphate concentration peaks in the years 1815–1816, 1844 and 1947, possibly coinciding with Tambora, Krakatau, and Hekla eruptions. It is concluded that sulphate concentration in speleothems expand the potential to correlate volcanic eruption events at a global scale. [Copyright &y& Elsevier]

Abstract: Stalagmite ER78, from Grotta di Ernesto cave in NE Italy displays clear annual lamination consistent with its shallow depth below a forest ecosystem subject to autumnal peaking of water infiltration. Synchrotron radiation scanning micro-X-ray fluorescence analyses of heavy elements at European Synchrotron Radiation Facility beamline ID22, and light elements at ID21, with 1–3μm resolution has been combined with data from ion microprobe analyses to reveal chemical variability across the visible layers of these annual laminae. A series of elements display a symmetrical peak, centered around the thin, dark layer at the top of each lamina. The peak concentration is ordered Y>Zn, Cu and Pb>P and Br. This hierarchy is thought to reflect the selectivity of transport of these elements, possibly by organic colloids flushed from the soil zone during autumn infiltration. Ion microprobe analysis indicates Na and F also increase, as does H, the latter reflecting increased microporosity. Sr displays a trough around the dark and thin autumn layer implying that its incorporation may be limited by competition with other elements. Mg and S show a different pattern of annual variation and Fe displays none. The trace metals, Br and Y display peak abundance in the early 20th century, which appears to reflect a period of tree-felling rather than a climatic anomaly. The results demonstrate the power of the high spatial resolution and low detection limits of the synchrotron technique, and its ability to produce quantitative maps that allow distinction of layered structure from that of isolated particles, or irregular inhomogeneities. [Copyright &y& Elsevier]

Abstract: We report here the first speleothem time-series of the variability of sulphate, a species whose abundance in catchments is strongly influenced by atmospheric anthropogenic and volcanic sources. Annually-resolved archives of S, Mg, Si and P were generated by applying synchrotron radiation micro X-Ray Fluorescence (XRF) to two speleothems from different sites in northern Italy. X-ray absorption-edge spectrometry proves that the S is in the form of sulphate and XRF mapping demonstrates that S is within calcite and enriched zones are predominantly as layers. A post-1850 A.D. record from the Ernesto cave shows a substantial rise in sulphate, interpreted as reflecting the largely anthropogenically-forced variation of sulphate of the atmospheric boundary layer, moderated by some ecosystem storage. Analysis of the circa 5.2 to circa 5.0 ka interval of a speleothem from Savi cave, where ecosystem retention of S is likely to have been minimal, shows a spiky sulphate record, resembling that of ice cores. A series of sulphate peaks suggest that multiple volcanic sulphate aerosol emissions at that time. This probably enhanced summer temperature cooling thus favouring the preservation of the human mummy of Neolithic–Copper age, the “Iceman” on the watershed between Italy and Austria. Both examples illustrate the power of speleothems to record atmospheric sulphate variability. [Copyright &y& Elsevier]

Debate about sedimentary structures found in intertidal zones of highly evaporative environments such as sabkhas, has commonly been focused on the interpretation of their morphology as purely abiotic or microbial. This is relevant to the study of early signs of Life on Earth, of the resilience of Life in extreme environments and for the search of signs of Life on Mars and exoplanets. Critically, ancient rocks likely underwent multiple diagenetic phases that may have partially overprinted primary features, yet microbially-influenced structures may be stabilized by micrite and micrite itself can be stabilized by organic molecules.The aim of the present study is to investigate the best chemical tracer that highlights primary (or very early diagenetic) depositional features by utilising Synchrotron Radiation based micro-X Ray Fluorsecence (SR-µXRF) mapping. Here we present the first SR-µXRF investigation of living microbial mats collected in the Dohat Faishakh Sabkha (DF) of Qatar compared with a similar fossil counterpart, the loferite facies from the Late Triassic Dolomia Principale (DP) in the Brenta Dolomites (Italy).In the modern DF sample we found that As, Br, Fe and Mn were associated with the dark, microbial laminae (organic part), whilst Sr was associated with the "mineral" part, along with Ca and Rb. This pattern has been already observed by SR-µXRF investigation of continental microbial mats.In the Late Triassic DP laminated facies - commonly interpreted as fossilized cyanobacterial microbial mats that originally thrived in an evaporative environment similar to that of the Qatar sabkha - the laminae consist of dolomite, whose genesis is a matter of debate. However, novel nanostructural investigations of the laminites suggest that some primary structures are preserved and, critically, the SR-µXRF maps revealed that Sr is associated with micrite outlining "microbial laminae".It is well known that Sr is immobilized by calcification and totally related to the mineral parts of the mats. By contrast, other elements, such as Br are in the "organic part", and are preferentially carried away during diagenesis. Sr distribution in the DP is very similar to that in the DF samples, which, coupled with modern growth rates of the mat, hints to the idea that it marks primary carbonate deposition as related to annual cycles in environmental parameters. At the same time, it is interesting to note that all the elements associated with "organics" are not detectable in the Triassic samples.This finding provides a new tool for both assessing existence of a primary mineral component of a microbial mat in fossil samples and for understanding annual-scale processes that drove the formation of "lofer" facies. [ABSTRACT FROM AUTHOR]

There is a shortage of archives of sulfur that can be used to investigate industrial or volcanic pollution in terrestrial catchments, but the role of S as a nutrient, coupled with sparse published evidence, suggests that trees are promising targets. We focused on two conifer species (Picea abies (L) Karst and Abies alba Miller) from an Alpine site in NE Italy. Bulk analyses of Abies demonstrate that S concentrations were higher in the second half of the 20th century but with some high outliers possibly reflecting particulate impurities. X-ray synchrotron analyses confirmed the observed time trend, which is similar to that of a nearby stalagmite, and reflects an atmospheric pollution record mediated by storage in the soil and ecosystem. S and P were found to be localized in the inner cell wall (ca. 2μm wide), local thickenings of which probably account for some outlying high values of S in synchrotron studies. S occurs as a mixture of oxidation states (0 to +0.5, +2, +5, and +6) which are consistent in space and time. The results indicate that wood older than a few years contains archive-quality S but that robust conclusions require multiple replicate analyses. [ABSTRACT FROM AUTHOR]