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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 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]

Trace amounts of sulphur in speleothems suggest that stalagmites may act as archives of sulphur deposition, thereby recording aspects of atmospheric variability in sulphur content. Accurate interpretation of this novel sulphur archive depends upon understanding how biogeochemical cycling in the soil and epikarst above the cave may modify the precursor atmospheric values of sulphur concentration and isotopic composition prior to incorporation into the speleothem record. Dual isotope analysis of δS-SO and δO-SO is used to trace biogeochemical transformations of atmospheric sulphur through the cave system at Grotta di Ernesto in the Italian Alps and builds towards a framework for interpretation of speleothem sulphur archives which depends on overlying ecosystem dynamics and karst hydrological properties. A three component model of atmospheric sulphate signal modification is proposed to be driven by (1). vegetation and soil cycling, (2). the degree of groundwater mixing in the karst aquifer; and (3). redox status. The relative influence of each process is specific to individual drip flow sites and associated stalagmites, rendering each sulphur archive a unique signal of environmental conditions. Under conditions found in the soil and epikarst above Grotta di Ernesto, the dual isotope signatures of sulphate sulphur and oxygen incorporated into speleothem carbonate, closely reflect past conditions of industrial sulphur loading to the atmosphere and the extent of signal modification through biogeochemical cycling and aquifer mixing. [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: Fabric and stable isotopic composition of a Holocene stalagmite (CR1) from a cave in northern Sicily record changes in paleorainfall in the early Holocene. High δ13C stable isotope values in the calcite deposited from ca. 8500 to ca. 7500 yr ago are interpreted as reflecting periods of high rainfall. The wet phase was interrupted by two periods of multi-century duration characterized by relatively cool and dry winters centered at ca. 8200 and ca. 7500 yr ago, highlighted by low δ13C and δ18O values. A high variability of δ13C values is recorded from ca. 7500 to ca. 6500 yr ago and indicates that the transition from a pluvial early Holocene to the present-day climate conditions was punctuated by decadal-scale periods of relatively dry winters. In northern Sicily, the traditional elements of the Neolithic appear at ca. 7700 yr ago. It is possible that changes in rainfall influenced the passage from hunter-gathering to farming and sheep-herding economies. [Copyright &y& Elsevier]

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]