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In this study, the ‘dead carbon proportion’ (DCP) calculated from combined U-Th and radiocarbon analyses was used to explore the carbon isotope systematics in Corchia Cave (Italy) speleothems, using the example of stalagmite CC26 which grew during the last ∼12 ka. The DCP values in CC26 are among the highest ever recorded in a stalagmite, spanning the range 44.8–68.8%. A combination of almost closed-system conditions and sulphuric acid dissolution (SAD) are proposed as major drivers in producing such a high DCP with minor contribution from old organic matter from the deep vadose zone. The long-term decrease in both DCP and δ 13 C most likely reflects post-glacial soil recovery above the cave, with a progressive increase of soil CO 2 contribution to the total dissolved inorganic carbon (DIC). Pronounced millennial-scale shifts in DCP and relatively small coeval but antipathetic changes in δ 13 C are modulated by the effects of hydrological variability on open and closed-system dissolution, SAD and prior calcite precipitation. Hence, the DCP in Corchia Cave speleothems represents an additional proxy for rainfall amount. [ABSTRACT FROM AUTHOR]

The quality of climate proxy data from speleothem archives depends to varying degrees on crystallization processes, which result in diverse fabrics. Here, we document shifts in calcite growth mechanisms, from ion-by-ion to nanoparticle/nanocrystal attachment, in stalagmites from the tropical island of Atiu (South Pacific). Changes in solution stoichiometry and organic matter content result in the development of two columnar fabrics that are common elsewhere in settings characterized by seasonal contrast. A porous columnar fabric, characterized by intracrystalline micro and nanoporosity grows via a non-classical pathway through amorphous calcium carbonate (ACC) nanoparticles (∼2–4 nm in diameter) and calcite nanocrystal attachment. Despite subsequent transformation of nanoparticles/crystals into a large calcite crystal, the porous columnar fabric appears to preserve a δ18O signal that faithfully reflects that of the parent fluid via quasi-equilibrium fractionation. Furthermore, the porous fabric shows random and fuzzy lateral distributions of Sr, another hydrological proxy, yet this element's incorporation follows equilibrium partitioning. The chemical properties of compact columnar fabrics, which appear to grow by classical ion-by-ion attachment, may not directly reflect those of the original depositional environment because of degassing, the presence of growth inhibitors (such as Na) and very early diagenetic modifications. Columnar porous calcite fabrics that formed through non-classical pathways in other settings may faithfully record the original properties of the parent drip water, whereas compact fabrics that formed through classical pathways elsewhere may not. It is concluded that the study of fabrics at the nano-scale is a necessary complement to speleothem research to identify the influence of crystallization pathways on the accuracy of proxy data. • Stalagmite calcite forms through complex crystallization pathways. • Both classical and non-classical processes are involved. • Particle and nanocrystal attachment operate to yield a porous fabric. • Columnar porous calcite may faithfully record the original properties of the parent drip water. • Oxygen isotopes and trace element concentration correlate with fabrics through crystallization. [ABSTRACT FROM AUTHOR]

The reconstruction of robust past climate records from speleothems requires a prior understanding of the environmental and hydrological conditions that lead to speleothem formation and the chemical signals encoded within them. On regional-scales, there has been little quantification of the dependency of cave dripwater geochemistry on meteorology (net infiltration, temperature), environmental and geographical factors (elevation, latitude, soil activity, vegetation cover, atmospheric aerosol composition) and geological properties of the aquifer (lithology, porosity and thickness). In the present study, we analysed over 200 karst waters collected in 11 caves of the Trentino region (NE Italy). The caves span sub-humid Mediterranean to cold-humid temperate climates and infiltration elevations ( Z inf ) ranging from 355 to 2400 m a.s.l., corresponding to infiltration mean annual temperatures (MAT inf ) between 12 and 0 °C. Since all the caves developed in pure carbonate rocks, soil pCO 2 is found to be the main factor controlling the carbonate dissolution. For this reason, the parameters controlling the carbonate–carbonic acid system and calcite saturation state (SI CC ) are directly correlated with the MAT inf , which influences the vegetation zones and eventually the production of CO 2 in the soil. SI CC linearly depends on MAT inf (SI CC = 0.09 MAT inf − 0.4) and SI CC = 0 is reached at Z inf = 1.66 km a.s.l., corresponding to a MAT inf = 4.4 °C. This point identifies the “ speleothem limit ” defined here as the elevation (or corresponding MAT inf ) above which no sparitic speleothem precipitation usually occurs. We demonstrate that due to temperature-forced changes in the soil and vegetation and subsequently SI CC , the speleothem limit shifts to higher altitudes during maximum interglacial conditions. Speleothems from high altitude caves (1.5–2.5 km a.s.l.) thus can identify optimum interglacial periods. By contrast, speleothems formed at lower altitudes are better suited as archives of hydrological proxies. At altitudes below 1.2 km a.s.l., prior calcite precipitation (PCP) modifies percolating waters, particularly during periods of reduced infiltration. We introduce the use of the SiO 2 /Ca and SO 4 /Ca ratios in cave waters to complement Mg/Ca and Sr/Ca ratios as markers of PCP. SO 4 and SiO 2 are derived from atmospheric deposition and siliciclastic minerals in the soil zone, rather than carbonate host rocks (as in the case of Mg and Sr). By combing shifts to higher Mg/Ca, SiO 2 /Ca and SO 4 /Ca ratios along their characteristics PCP lines, we improve the robustness of the interpretation that this resulted from increasing PCP, rather than incongruent calcite dissolution (ICD). Our method permits the quantification of PCP between 0% and 40% for low elevation cave waters. This novel approach has important implications for speleothem-based paleoclimate studies where the distinction between PCP and ICD can be ambiguous and, in combination with Mg/Ca and Sr/Ca ratios, permits the quantification of net infiltration and/or rainfall amount from speleothem records. [ABSTRACT FROM AUTHOR]

Sulphate concentrations in speleothems identify major volcanic eruptions, provide useful information on soil and aquifer dynamics and, in similar fashion to the 14 C bomb peak, its Anthropocene peak can be used to date recent cave formations. However, the transmission of S from the atmosphere to cave dripwater and its incorporation in speleothems is subjected to biogeochemical cycling and accurate studies of each cave site are needed in order to assess how the S atmospheric signal is modified and eventually encoded in speleothems. This study investigates the role of biogeochemical cycling and aquifer hydrology by utilising published and new dripwater and speleothem data from Grotta di Ernesto (ER) in northern Italy. Here we provide the first long-term record of sulphate concentration in cave dripwater based on over 20 years of measurements. Fast drip site st-ER1 is characterised by a continuous decrease in SO 4 concentration from a high of 7.5 ± 0.8 mg/l in 1993–1994 to a low of 2.2 ± 0.2 mg/l in 2013–2014, and replicates with a delay of ∼15 years the decline in the atmospheric SO 2 emissions. The S-series of slow flow ER78 site is further delayed by ∼4.5 years in relation to the S retention in the aquifer matrix. The dripwater data are used to extend the previously published S record (1810–1998 AD) of stalagmite ER78 and reconstruct the anthropogenic S-peak: this displays a delay of ∼20 years with respect to the atmospheric S emission peak due to biogeochemical cycling and aquifer storage. However, sulphur recycling above the cave did not operate with the same degree of efficiency through time, which resulted in a variable time delay between S deposition and incorporation into the stalagmite. In the pre-Anthropocene era, and in particular during the cold Little Ice Age, biogeochemical cycling was far less efficient than today, and the fast transmission of the atmospheric signal allowed capture of S released during major volcanic eruptions by stalagmites. [ABSTRACT FROM AUTHOR]

The hydroclimate of the South Pacific is highly variable on a range of timescales, however, the brevity of instrumental data in the region complicates the detection and attribution of this variability. Annually laminated speleothems may fill this knowledge gap by providing paleo-hydroclimate proxy data extending beyond the instrumental period. In this study, replicated oxygen isotope (δ18O) time series from two stalagmites from Pouatea (Pu17) and Nurau (Nu16) caves in the southern Cook Islands were used to reconstruct the variability of effective infiltration over the past 350 years (1672–2019 CE) based on observational rainfall data and cave monitoring and calcite farming experiments. The reconstructed infiltration was then compared to available reconstructions of known climate drivers of the South Pacific. Significant dry and wet periods over the last 350 years were identified based on the δ18O signal of compact/porous calcite fabric and their δ13C signals. The hydroclimate significance of these proxies was validated through comparison with the instrumental records. We could not reach an annual resolution of reconstructed infiltration, due to a combination of stable isotope sampling limitations and mixing effects within the aquifer. Nevertheless, the combined δ18O record of Pu17 and Nu16 identifies that the main driver of infiltration and, hence rainfall variability, in the southern Cook Islands is the location of the South Pacific Convergence Zone (SPCZ), which in turn is modulated by the El Niño Southern Oscillation (ENSO). This study demonstrates the hydroclimate significance of speleothem fabrics when combined with O and C stable isotopes, and adds paleoclimate information to a region that has a paucity of data. • ENSO and movements of the SPCZ are the main drivers of rainfall in the southern Cook Islands. • Joint use of speleothem fabrics, δ18O and δ13C serves as a proxy of effective infiltration. • Reconstruction of effective infiltration variability over the past 350 years (1672–2019 CE). • Insights into large-scale controls on rain and speleothem δ18O variability. [ABSTRACT FROM AUTHOR]

Past climate archives show the Last Interglacial (LIG) period as similar to slightly warmer than current temperatures. However, there is a lack of LIG proxy evidence regarding variations of the climate across large topographic features and how this manifests at different altitudes. Here, we analysed two flowstones from Bigonda Cave, northeast Italy, to reconstruct LIG climate conditions in a region where precipitation is strongly influenced by the presence of the Alps. Stable isotope ratios, trace element concentrations and speleothem petrography from 133 ka to 105 ka allowed the detection of various hydroclimate and environmental conditions. Composite speleothem δ18O records from northeast Italy were found to closely follow the Northern Hemisphere summer insolation curve, attesting to the influence of solar isolation on Earth's hydroclimate. Our reconstructions indicate a northward shift of the Intertropical Convergence Zone during the LIG maximum, permitting Atlantic-derived moisture to travel further east across North Africa than at present, before being directed north towards the Alps. The latter part of the LIG exhibited frequent, extreme precipitation events in the SE Prealps due to orographic lifting of moisture from both the Atlantic and Mediterranean seas. Socio-economic planning must prepare for extreme autumnal flooding events and serious summer droughts, particularly important in vulnerable mountainous regions. • Last Interglacial speleothem multi-proxy records from the NE Italian Prealps. • Three infiltration sources identified permits robust hydroclimate reconstruction. • Solar insolation was the main forcing mechanism of the oxygen isotope record. • The LIG was punctuated by hot dry summers and extreme cave flooding events. [ABSTRACT FROM AUTHOR]

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]

The so-called "Altamura Man" is a Neanderthal skeleton found in 1993 in the Lamalunga karstic system (southern Italy). The skeleton, dated between 172 and 130 ka, still lies deep within a spectacular cave setting, partly embedded within speleothems and extensively covered by coralloid concretions. These peculiar location and conditions would entail major difficulties in excavating and preserving ex-situ this skeleton. Moreover, a decision-making impasse among the institutions involved complicates the problem. Yet, this specimen is one of the best preserved human fossils known. From 2016 to 2020 we conducted extensive in situ investigations of the specimen. Digital techniques and equipment specifically designed for or adapted to the difficult underground context were used in carrying out the most detailed assessment possible of the skeleton. The data thus collected provide an insight into human evolution in Europe, and a first glance at the amount of information this amazing specimen could provide. In this contribution, we want to update the information on the problematic context of Lamalunga, and address the innovative solutions put in place in 2016–2020 to study and preserve the Neanderthal skeleton in this fragile environment. • The so-called "Altamura Man" is one of the most complete Neanderthal fossils. • The site and condition of preservation have made the study problematic for decades. • New studies in situ allowed a better characterization of the fossil and its context. • We review what is currently known of the specimen and its site in southern Italy. • We also present some of the innovative methods used for the study. [ABSTRACT FROM AUTHOR]

Carbonate-associated sulphate (CAS) is a useful carrier of palaeoenvironmental information throughout the geologic record, particularly through its stable isotope composition. However, a paucity of experimental data restricts quantitative understanding of sulphate incorporation into carbonates, and consequently CAS concentrations and their diagenetic modifications are rarely interpreted. However, in the case of calcite speleothems, the remarkably high-resolution CAS records which are obtainable via modern microanalytical techniques represent a potentially invaluable source of palaeoenvironmental information. Here, we describe the results of controlled experiments of sulphate co-precipitation with calcite in freshwater solutions where pH, saturation state, and sulphate concentration were varied independently of each other. Solution pH is confirmed as the principal control on sulphate incorporation into calcite. The relative efficiency of incorporation was calculated as a partition coefficient D SO 4  = (mSO 4 /mCO 3 ) solid /(mSO 4 /mCO 3 ) solution . High crystal growth rates (driven by either pH or saturation state) encouraged higher values of D SO 4 because of an increasing concentration of defect sites on crystal surfaces. At low growth rates, D SO 4 was reduced due to an inferred competition between sulphate and bicarbonate at the calcite surface. These experimental results are applied to understand the incorporation of sulphate into speleothem calcite. The experimentally determined pH-dependence suggests that strong seasonal variations in cave air PCO 2 could account for annual cycles in sulphate concentration observed in stalagmites. Our new experimentally determined values of D SO 4 were compared with D SO 4 values calculated from speleothem-drip water monitoring from two caves within the Austrian and Italian Alps. At Obir cave, Austria, D SO 4 (×10 5 ) varies between 11.1 (winter) and 9.0 (summer) and the corresponding figures for Ernesto cave, Italy, are 15.4 (winter) and 14.9 (summer). These values approximate predicted D SO 4 values based on our chamber experiments containing both low (2 ppm) and high (20 ppm) sulphate concentrations. Our experimental values of D SO 4 obtained at crystal growth rates typical of stalagmites, closely match those observed in other cave sites from around the world. This validates the universality of the controls behind D SO 4 and will enhance the use of speleothem CAS as a palaeoenvironmental proxy. [ABSTRACT FROM AUTHOR]

Speleothem records of past environmental change provide an important opportunity to explore fire frequency and intensity in the past, and the antecedent climatic conditions leading to fire events. Here, fire sensitive geochemical signals in a stalagmite from Yonderup Cave, a shallow cave in Western Australia, are compared to well-documented wildfire events that occurred in recent decades. The results are extended to identify wildfires during the growth interval of the speleothem (1760 CE–2005 CE). Principal component analysis of the stalagmite time-series revealed distinct peaks in a combination of phosphorus and metal (aluminium, zinc, copper and lead) concentrations in response to known fire events, which are interpreted to have come from ash. Varying responses in the geochemical signal in the pre- and post-European colonisation period are likely linked to changes in land management, fire frequency and fire intensities. Of note is what we infer to be a particularly intense fire event concluded to have occurred in 1897 ± 5 CE, which climate sensitive proxies (δ18O and bedrock-derived elements) indicate was preceded by a multi-decadal dry period that began in the late 1860s. The intensity of the deduced fire event is supported by a peak in P that is 6.3 times higher than elsewhere in the speleothem, accompanied by a peak in Zn. It was also inferred that changes to the surface-cave hydrology occurred as a result of this event, indicated by increased input of colloidal organic material onto the stalagmite that was interpreted to be caused by shallow karst bedrock fracturing from the heat-induced deformation that is observed after intense fires. These findings show the potential for speleothems to provide dated records of fire intensity and recurrence intervals. Further development could lead to a better understanding of the climate-fire relationship and the effects of land-management practices on wildfire frequency and intensity. [ABSTRACT FROM AUTHOR]

The spatiotemporal variability of the Asian Monsoon (AM) over the last two millennia has been attributed to a combination of external solar and volcanic forcing and/or internal coupled atmosphere-ocean dynamics, but the relative importance of these mechanisms remains unresolved. The present knowledge of multidecadal to centennial-scale AM variability over Mainland Southeast Asia is not well-constrained, despite substantial progress in understanding seasonal to decadal variability from tree ring records. Here we present the first high-resolution stable isotope (δ 13 C and δ 18 O) speleothem record from northern Laos spanning the Common Era (∼50 BCE to 1880 CE). The δ 13 C record reveals substantial centennial-scale fluctuations primarily driven by local water balance. Notably, the driest period at our site occurred from ∼1280 to 1430 CE, during the time of the Angkor droughts, supporting previous findings that this megadrought likely impacted much of Mainland Southeast Asia. In contrast, variations in stalagmite δ 18 O reflect changes in rainfall upstream from our study site. Interestingly, the δ 18 O record exhibits a positive correlation with solar activity that persists after 1200 CE, contrary to the findings in previous studies. Solar-forced climate model simulations reveal that these δ 18 O variations may be driven by solar-forced changes in upstream rainout over the tropical Indian Ocean, which modify the δ 18 O of moisture transported to our study site without necessarily affecting local rainfall amount. We conclude that future rainfall changes in Mainland Southeast Asia are likely to be superimposed on multidecadal to centennial-scale variations in background climate driven primarily by internal climate variability, whereas solar forcing may impact upstream rainout over the Indian Ocean. • New high-resolution speleothem record from northern Laos spanning ∼50 BCE to 1880 CE. • δ 13 C reflects local water balance and records centennial-length dry periods during the Little Ice Age. • δ 18 O reflects upstream rainout over the Indian Ocean and may be influenced by solar forcing. [ABSTRACT FROM AUTHOR]

Speleothems are usually considered as one of the most amenable palaeoclimate archives for U-series dating. A number of studies in recent years, however, report cases of diagenetic alteration which compromises the use of U-series systematics in speleothems, resulting in inaccurate U-Th ages. Here we present the results of a high-resolution U-Th dating study of a stalagmite (CC26) from Corchia Cave in Italy where we document a number of departures from an otherwise well-defined age-depth model, and explore potential causes for these outliers. Unlike examples illustrated in previous studies, CC26 contains no visible evidence of neomorphism, and appears, at least superficially, ideally suited to dating. Good reproducibility obtained between multi-aliquot U-Th analyses removes any possibility of analytical issues contributing to these outliers. Furthermore, replicate analyses of samples from the same stratigraphic layer yielded ages in stratigraphic sequence, implying very localized open-system behavior. Uranium loss is suggested as a causative mechanism on account of the fact that all the outliers are older than their assumed true age. A limited number of micro-voids were observed under micro-CT analyses, and it is proposed that these were pathways for U loss. Uranium-loss modelling allows us to constrain the possible timing of diagenetic alteration and indicates that the precursor for the outlier with the largest age discrepancy (309%) must have been aragonite. This study indicates that visibly unaltered speleothems may still contain small domains that have experienced post-depositional alteration. Such “cryptic” diagenesis, as recorded in this stalagmite, has implications for the constancy of accuracy of the U-series dating technique, and suggests a need for careful examination of speleothems prior to dating, particularly in low-resolution U-Th studies. [ABSTRACT FROM AUTHOR]

Organic phosphorus incorporated in calcite during laboratory precipitation experiments and in natural cave deposits was investigated by solid-state NMR spectroscopy. For calcite precipitated in the presence of organic phosphoesters of varying size and functionality, solid-state 31 P{ 1 H} CP/MAS NMR shows that the phosphoesters were incorporated intact into the solid. Systematic changes in the 31 P NMR chemical shift of the phosphate group were observed between the solid phosphoester and that incorporated in the solid precipitate, yielding 31 P NMR chemical shifts of the coprecipitates in the range of +1.8 to −2.2 ppm. These chemical shifts are distinct from that of similarly prepared calcite coprecipitated with inorganic phosphate, 3.5 ppm. Only minor changes were noted in the phosphoester 31 P chemical shift anisotropy (CSA) which suggests no significant change in the local structure of the phosphate group, which is dominated by C–O–P bonding. Close spatial proximity of the organic phosphate group to calcite structural components was revealed by 31 P/ 13 C rotational echo double resonance (REDOR) experiments for coprecipitates prepared with 13 C-labeled carbonate. All coprecipitates showed significant 31 P dephasing effects upon 13 C-irradiation, signaling atomic-scale proximity to carbonate carbon. The dephasing rate for smaller organophosphate molecules is similar to that observed for inorganic phosphate, whereas much slower dephasing was observed for larger molecules having long and/or bulky side-chains. This result suggests that small organic molecules can be tightly enclosed within the calcite structure, whereas significant structural disruption required to accommodate the larger organic molecules leads to longer phosphate–carbonate distances. Comparison of 31 P NMR spectroscopic data from the synthetic coprecipitates with those from calcite moonmilk speleothems indicates that phosphorus occurs mainly as inorganic orthophosphate in the natural deposits, although small signals occur with characteristics consistent with phosphate monoesters. The results of this study indicate that trace- to minor concentrations of dissolved organic molecules can be effectively taken up during calcite precipitation and incorporated in the structure, leaving a resilient record of materials present during crystallization. [ABSTRACT FROM AUTHOR]

Abstract: Diverse interpretations have been made of carbon isotope time series in speleothems, reflecting multiple potential controls. Here we study the dynamics of 13C and 12C cycling in a particularly well-constrained site to improve our understanding of processes affecting speleothem δ13C values. The small, tubular Grotta di Ernesto cave (NE Italy) hosts annually-laminated speleothem archives of climatic and environmental changes. Temperature, air pressure, pCO2, dissolved inorganic carbon (DIC) and their C isotopic compositions were monitored for up to five years in soil water and gas, cave dripwater and cave air. Mass-balance models were constructed for CO2 concentrations and tested against the carbon isotope data. Air advection forces winter pCO2 to drop in the cave air to ca. 500ppm from a summer peak of ca. 1500ppm, with a rate of air exchange between cave and free atmosphere of approximately 0.4days. The process of cave ventilation forces degassing of CO2 from the dripwater, prior to any calcite precipitation onto the stalagmites. This phase of degassing causes kinetic isotope fractionation, i.e. 13C-enrichment of dripwater whose δ13CDIC values are already higher (by about 1‰) than those of soil water due to dissolution of the carbonate rock. A subsequent systematic shift to even higher δ13C values, from −11.5‰ in the cave drips to about −8‰ calculated for the solution film on top of stalagmites, is related to degassing on the stalagmite top and equilibration with the cave air. Mass-balance modelling of C fluxes reveals that a very small percentage of isotopically depleted cave air CO2 evolves from the first phase of dripwater degassing, and shifts the winter cave air composition toward slightly more depleted values than those calculated for equilibrium. The systematic 13C-enrichment from the soil to the stalagmites at Grotta di Ernesto is independent of drip rate, and forced by the difference in pCO2 between cave water and cave air. This implies that speleothem δ13C values may not be simply interpreted either in terms of hydrology or soil processes. [Copyright &y& Elsevier]