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The Cryogenian period (720–635 million years ago) in the Neoproterozoic era featured two phases of global or near-global ice cover, termed ‘Snowball Earth’. Climate models of all kinds indicate that the inception of these phases must have occurred in the course of a self-amplifying ice–albedo feedback that forced the climate from a partially ice-covered to a Snowball state within a few years or decades. The maximum concentration of atmospheric carbon dioxide (CO2) allowing such a drastic shift is difficult to determine because it depends on the choice of model and the boundary conditions prescribed in the model. Many previous studies report values or ranges for this CO2 threshold but typically test only very few different boundary conditions. Furthermore, most studies include some kind of variability internal to the climate system but exclude variability due to volcanism. Here we present a comprehensive sensitivity study considering different scenarios for the Cryogenian continental configuration, orbital geometry, and short-term volcanic cooling effects in a consistent model framework, using the climate model of intermediate complexity CLIMBER-3α. The continental configurations comprise palaeogeography reconstructions for both Snowball-Earth periods from two different sources, as well as two idealised configurations with either uniformly dispersed continents or a single polar supercontinent. Orbital geometries are sampled as multiple different combinations of the parameters obliquity, eccentricity, and argument of perihelion. For volcanic eruptions, we differentiate between single and globally homogeneous perturbations, single and zonally resolved perturbations, and random sequences of globally homogeneous perturbations with realistic statistics. The CO2 threshold lies between 10 and 250 ppm for all simulations. While the idealized continental configurations span a difference of around 200 ppm for the threshold, the continental reconstructions differ by only 20–40 ppm. Changes in orbital geometry account for variations in the CO2 threshold by up to 32 ppm. The effects of volcanic perturbations largely depend on the orbital geometry. A very large peak reduction of net solar radiation by around –20 W/m2 can shift the CO2 threshold by the same order of magnitude as the orbital geometry. Even larger eruptions of up to –40 W/m2 may shift the threshold by up to 50 ppm. However, the smaller, more frequent eruptions mostly have much lower impacts than the changes in continental configuration and orbital geometry. Eruptions near the equator tend to, but do not always, cause larger shifts than eruptions at high latitudes. Realistic sequences of eruptions lower the long-term temperature and have a bigger effect on the CO2 threshold than single large eruptions of comparable magnitude.

Coarsely crystalline cryogenic cave carbonates (CCC for short) are unique speleothems that precipitated in small water pools in cave ice via slow freezing. Their formation requires cave air temperatures very close to the 0°C isotherm. CCC have become increasingly important over the last decade and are one of the best terrestrial archives providing highly precise chronological constraints on paleo-permafrost degradation. Although a growing number of studies use CCCs as a time-marker of permafrost thawing, the details of their formation mechanism remain unanswered.CCC found at over a dozen sites in the ice-free parts of Eisriesenwelt (Tennengebirge, Eastern Alps) cover a wide spectrum of morphologies from skeletal crystals to more complex beak-like and hemispheric structures. At some sites one or two morphologies are dominant, while at other sites several different types occur together. The mechanisms of formation of many of the morphologies remain enigmatic.Here we present first results of high-resolution trace element (Mg, Sr, Ba, and U) and stable isotope (C and O) analyses of a white pea-like spherulite (~8 mm) and two beak-like aggregates (~12 and 15 mm in diameter) from Eisriesenwelt. The latter type shows an amber-coloured inner core, overgrown by more porous light brown to almost whitish calcite.Oxygen isotope ratios of one of the beak-like aggregates reduce progressively by 4.8‰ from the core towards the rim, while carbon isotope values increase by 2.1‰, reflecting the progressive freezing of the water. The dark brown calcite is characterized by low Mg, Sr, and Ba but high U values compared to the light brown calcite. Trace element analyses also indicate an increase in Mg and Sr towards the rim, whereas U shows the opposite trend. In the white hemispheric spherulite sample, oxygen isotope ratios show a larger (up to 2‰) intracrystalline variability, and a 5.8‰ decrease over the last 2 mm of the sample. In contrast, carbon isotope values vary only by 0.5‰. Mg values show an increasing trend through time, from core to rim. Sr, U, and Ba concentrations do not exhibit a clear pattern, except for a sharp change before the cessation of CCC growth. This is likely related to the presence of a thin (hundreds of 100µm), translucent calcite rim, expressed as an increase (decrease) in Mg, Sr, U (and Ba) concentrations. In both morphologies Ba follows a similar pattern as Sr, however Ba values decrease progressively before the cessation of CCC growth.Formation of CCC of the coarse crystalline variety has not been observed in-situ, limiting their significance as robust indicators of thawing conditions in ice caves. Studying individual CCC samples using a combination of high-resolution stable isotope and trace-element analyses is an important, but currently underdeveloped, approach to improving our understanding of CCC formation.

The reconstruction and analysis of palaeoseasonality from speleothem records remains a notoriously challenging task. Although the seasonal cycle is obscured by noise, dating uncertainties and irregular sampling, its extraction can identify regime transitions and enhance the understanding of long-term climate variability. Shifts in seasonal predictability of hydroclimatic conditions have immediate and serious repercussions for agricultural societies.We present a highly resolved speleothem record (ca. 0.22 years temporal resolution with episodes twice as high) of palaeoseasonality from Yok Balum cave in Belize covering the Common Era (400-2006 CE) and demonstrate how seasonal-scale hydrological variability can be extracted from δ13C and δ18O isotope records. We employ a Monte-Carlo based framework in which dating uncertainties are transferred into magnitude uncertainty and propagated. Regional historical proxy data enable us to relate climate variability to agricultural disasters throughout the Little Ice Age and population size variability during the Terminal Classic Maya collapse.Spectral analysis reveals the seasonal cycle as well as nonstationary ENSO- and multi-decadal-scale variability. Variations in both the subannual distribution of rainfall and mean average hydroclimate pose limitations on how reliably farmers can predict crop yield. A characterization of year-to-year predictability as well as the complexity of seasonal patterns unconver shifts in the seasonal-scale variability. These are discussed in the context of their implications for rainfall dependent agricultural societies.

Stalagmites are an extraordinarily powerful resource for the reconstruction of climatological palaeoseasonality. Here, we provide a review of different types of seasonality preserved by stalagmites and methods for extracting this information. A new drip classification scheme is introduced, which facilitates the identification of stalagmites fed by seasonally responsive drips and which highlights the wide variability in drip types feeding stalagmites. This hydrological variability, combined with seasonality in Earth atmospheric processes, meteoric precipitation, biological processes within the soil, and cave atmosphere composition means that every stalagmite retains a different and distinct (but correct) record of environmental conditions. Replication of a record is extremely useful but should not be expected unless comparing stalagmites affected by the same processes in the same proportion. A short overview of common microanalytical techniques is presented, and suggested best practice discussed. In addition to geochemical methods, a new modelling technique for extracting meteoric precipitation and temperature palaeoseasonality from stalagmite δ18O data is discussed and tested with both synthetic and real-world datasets. Finally, world maps of temperature, meteoric precipitation amount, and meteoric precipitation oxygen isotope ratio seasonality are presented and discussed, with an aim of helping to identify regions most sensitive to shifts in seasonality. © 2021 Elsevier Ltd

Identification of volcanic signals preserved in paleoenvironmental records can provide key insights into the timing and consequences of explosive volcanism. Yet the eruption record is incomplete and this confounds our ability to link volcanic eruptions to their impacts on climate and environments. Studies have suggested that stalagmite records can help to address these gaps, through the identification of transient geochemical variability associated with incorporation of elements derived from erupted material. However, the utility of stalagmites for tracing volcanism is poorly constrained globally. Here, we present a high-resolution geochemical dataset for stalagmite NIED08–05 from Niedźwiedzia Cave (Poland). We do this with two primary aims: (1) to test the suitability of NIED08–05 as a record of volcanism since 3 ka BP, and (2) to ascertain whether stalagmites grown in temperate regions preserve volcanic signals with success comparable to those grown in tropical regions. We find transient enrichments of 16 trace elements and the rare-earth-elements Y, La, Nd, which coincide with the timing of some known eruption events. Using Principal Component Analysis (PCA) we find that elements atypically incorporated into calcite (e.g., Fe) co-vary. Similarly, filtering PC1 (17% of the dataset variability) for high magnitude deviations from a baseline signal yields tentative agreement between PC scores and some known large eruptions with tephra found in Poland. We use our analysis to discuss the complexities involved in associating volcanic signals with stalagmite chemistry in temperate regions far from the source of large eruptions. The transport pathway from volcanic source to stalagmite growth surfaces includes the complex Niedźwiedzia Cave hydrological system and is influenced by dense forest above the cave site. Together these factors increase the potential for attenuation of volcanic-derived chemical signals prior to reaching the stalagmite, and so make it difficult to unambiguously link trace element enrichments in NIED08–05 to volcanic eruptions. Our results provide strong evidence that in a temperate depositional environment far from active volcanoes, climate and hydrology conspire to mute the strength of volcanic geochemical signals. Therefore, this work provides important incentives for future research in this area by highlighting that stalagmites grown in a comparatively simple hydrological regime and grown in caves overlain by thin vegetation and soil cover , may preserve volcanic signatures with greater success than those grown in temperate environments.

For millions of people living in the humid neotropics seasonally predictable rainfall is crucial for agricultural success and food security. Understanding long-term stability and volatility of seasonal rainfall distributions should be of concern to researchers and policy makers. However, reconstructions of paleorainfall seasonality in the neotropics have been constrained by a lack of precisely dated and sub-annually resolved records. We present a 1,600-year rainfall paleoseasonality reconstruction from speleothem sample Yok G, from Yok Balum Cave located in southern Belize, Central America. Yok G grew continuously from 400 C.E. to 2,006 C.E. and its age is constrained by 52 U-series dates with a mean error of ~7 years. The isotope record consists of 7,151 δ18O and δ13C measurements at ~0.22-year resolution allowing us to detect the presence and amplitude of annual wet-dry cycles. In Belize rainfall distribution and seasonality controls are currently dominated by the annual migration of the intertropical convergence zone (ITCZ) with marked meridional contrast. The Yok G record suggest distinct changes in seasonality at multi-centennial intervals. The earliest portion of the record (400-~850 C.E.) shows little intra-annual seasonal variation, the period from ~850-1400 C.E. has highly variable annual oscillations and periods of low seasonality, while the period from 1,400-2,006 C.E. shows well developed seasonal signals. Element ratios (Mg/Ca, Sr/Ca, and U/Ca) are used to assess Prior Carbonate Precipitation in the epikarst system. We review these changes and the isotopic record from Yok G and discuss tools for interpreting the stability and volatility in seasonal rainfall distributions and possible implications for past and modern agricultural societies.

Neotropics data evince expansion and contraction of the inter-tropical convergence zone due to cooling and warming, respectively., Large changes in hydroclimate in the Neotropics implied by proxy evidence, such as during the Little Ice Age, have been attributed to meridional shifts of the intertropical convergence zone (ITCZ), although alternative modes of ITCZ variability have also been suggested. Here, we use seasonally resolved stalagmite rainfall proxy data from the modern northern limit of the ITCZ in southern Belize, combined with records from across the Neotropics and subtropics, to fingerprint ITCZ variability during the Common Era. Our data are consistent with models that suggest ITCZ expansion and weakening during globally cold climate intervals and contraction and intensification during global warmth. As a result, regions currently in the margins of the ITCZ in both hemispheres are likely transitioning to more arid and highly variable conditions, aggravating current trends of increased social unrest and mass migration.

The Younger Dryas is considered the archetypal millennial-scale climate change event, and identifying its cause is fundamental for thoroughly understanding climate systematics during deglaciations. However, the mechanisms responsible for its initiation remain elusive, and both of the most researched triggers (a meltwater pulse or a bolide impact) are controversial. Here, we consider the problem from a different perspective and explore a hypothesis that Younger Dryas climate shifts were catalysed by the unusually sulfur-rich 12.880 ± 0.040 ka BP eruption of the Laacher See volcano (Germany). We use the most recent chronology for the GISP2 ice core ion dataset from the Greenland ice sheet to identify a large volcanic sulfur spike coincident with both the Laacher See eruption and the onset of Younger Dryas-related cooling in Greenland (i.e. the most recent abrupt Greenland millennial-scale cooling event, the Greenland Stadial 1, GS-1). Previously published lake sediment and stalagmite records confirm that the eruption's timing was indistinguishable from the onset of cooling across the North Atlantic but that it preceded westerly wind repositioning over central Europe by ∼ 200 years. We suggest that the initial short-lived volcanic sulfate aerosol cooling was amplified by ocean circulation shifts and/or sea ice expansion, gradually cooling the North Atlantic region and incrementally shifting the midlatitude westerlies to the south. The aerosol-related cooling probably only lasted 1–3 years, and the majority of Younger Dryas-related cooling may have been due to the sea-ice–ocean circulation positive feedback, which was particularly effective during the intermediate ice volume conditions characteristic of ∼ 13 ka BP. We conclude that the large and sulfur-rich Laacher See eruption should be considered a viable trigger for the Younger Dryas. However, future studies should prioritise climate modelling of high-latitude volcanism during deglacial boundary conditions in order to test the hypothesis proposed here.

Karst areas are widespread landforms present on all continents, formed by the dissolution of carbonate or evaporite host rock. Little is known about the composition and nature of dissolved organic matter (DOM) as it moves through karst systems, although karst DOM has been recognized as important for a range of natural processes. Microbial communities living in karst systems are some of the most diverse and intriguing on the planet, and their metabolism and life cycle can give clues related to the development of a host of different life forms. Karst areas are also of interest due to their mostly subterranean hydrology, and the repercussions of these processes on local carbon cycles. We illustrate some of the processes acting on DOM in karst waters through the analysis of soil, drip and cave pool waters at the tropical site of Yok Balum Cave, in southern Belize. Water samples were analyzed using ultra-high resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS), a technique that enables the resolution of single molecular formulae within a DOM spectrum. We perform multivariate statistics to detect trends in the data and identify provenance of detected molecular components. In addition to karst waters, four aliquots of a powdered stalagmite sample from the same cave system are analyzed. Our results show a clear gradient between the soil and the cave system. We hypothesize that both sorption on mineral surfaces and microbial reworking are responsible for the observed trend in DOM composition. The stalagmite extracts show an anomalous DOM pattern, which may be due to a variety of factors, including microbial activity on the stalagmite surface and different affinities of compounds to incorporation in the carbonate. The goal of this study was to follow the molecular transformations of DOM on its journey from the surface to the cave, and to provide a molecular basis for the establishment of stalagmite DOM proxies in karst systems.

Understanding the influence of changes in solar activity on Earth’s climate and distinguishing it from other forcings, such as volcanic activity, remains a major challenge for palaeoclimatology. This problem is best approached by investigating how these variables influenced past climate conditions as recorded in high precision paleoclimate archives. In particular, determining if the climate system response to these forcings changes through time is critical. Here we use the Wiener-Granger causality approach along with well-established cross-correlation analysis to investigate the causal relationship between solar activity, volcanic forcing, and climate as reflected in well-established Intertropical Convergence Zone (ITCZ) rainfall proxy records from Yok Balum Cave, southern Belize. Our analysis reveals a consistent influence of volcanic activity on regional Central American climate over the last two millennia. However, the coupling between solar variability and local climate varied with time, with a regime shift around 1000–1300 CE after which the solar-climate coupling weakened considerably.millennia. However, the coupling between solar variability and local climate varied with time, with a regime shift around 1000–1300 CE after which the solar-climate coupling weakened considerably.

Precisely-dated stalagmites are increasingly important archives for the reconstruction of terrestrial paleoclimate at very high temporal resolution. In-depth understanding of local conditions at the cave site and of the processes driving stalagmite deposition is of paramount importance for interpreting proxy signals incorporated in stalagmite carbonate. Here we present a sub-decadally resolved dead carbon fraction (DCF) record for a stalagmite from Yok Balum Cave (southern Belize). The record is coupled to parallel stable carbon isotope (delta C-13) and U/Ca measurements, as well as radiocarbon (C-14) measurements from soils overlying the cave system. Using a karst carbon cycle model we disentangle the importance of soil and karst processes on stalagmite DCF incorporation, revealing a dominant host rock dissolution control on total DCF. Covariation between DCF, delta C-13, and U/Ca indicates that karst processes are a common driver of all three parameters, suggesting possible use of delta C-13 and trace element ratios to independently quantify DCF variability. A statistically significant multi-decadal lag of variable length exists between DCF and reconstructed solar activity, suggesting that solar activity influenced regional precipitation in Mesoamerica over the past 1500 years, but that the relationship was non-static. Although the precise nature of the observed lag is unclear, solar-induced changes in North Atlantic oceanic and atmospheric dynamics may play a role. (C) 2016 Elsevier Ltd. All rights reserved.

Speleothem organic matter can be a powerful tracer for past environmental conditions and karst processes. Carbon isotope measurements (δ13C and 14C) in particular can provide crucial information on the provenance and age of speleothem organic matter, but are challenging due to low concentrations of organic matter in stalagmites. Here, we present a method development study on extraction and isotopic characterization of speleothem organic matter using a rapid procedure with low laboratory contamination risk. An extensive blank assessment allowed us to quantify possible sources of contamination through the entire method. Although blank contamination is consistently low (1.7 ± 0.34 – 4.3 ± 0.86 μg C for the entire procedure), incomplete sample decarbonation poses a still unresolved problem of the method, but can be detected when considering both δ13C and 14C values. We test the method on five stalagmites, showing reproducible results on samples as small as 7 μg C for δ13C and 20 μg C for 14C. Furthermore, we find consistently lower non-purgeable organic carbon (NPOC) 14C values compared to the carbonate 14C over the bomb spike interval in two stalagmites from Yok Balum Cave, Belize, suggesting overprint of a pre-aged or even fossil source of carbon on the organic fraction incorporated by these stalagmites.

Several stalagmite records have yielded important but discontinuous insights into northern Iberian climate since the Last Glacial. Here we present the first continuous Iberian stalagmite-based recon- struction of climate since the Bølling-Allerød interstadial, from a single stalagmite sample (GAR-01 from La Garma Cave, Cantabria). The ~13.5 ka GAR-01 record provides the opportunity for replication, continuation, and aggregation of previously published records from northern Spain. The GAR-01 record reveals shifts in oxygen isotope ratios that are inexplicable by appealing to a single control (i.e., exclu- sively temperature, rainfall amount, etc.). Herein we explore the potential role of rainfall and tempera- ture seasonality shifts on the new d18O record using a simple Monte Carlo approach to estimate the seasonal distribution of rainfall and the annual temperature range at 100-year timeslices across the record. This model is corroborated by intervals of monthly-resolved laser ablation trace element data, providing glimpses into past Iberian seasonality shifts. The most salient features of the modelled results include extremely dry Younger Dryas winters (~12.9-11.6 ka BP) and several intervals during the mid- Holocene with almost no summer rainfall (e.g., at 4.2 and 9.0 ka BP). By 1.6 ka BP, a near-modern rainfall seasonality was established. According to the modelling results, seasonal rainfall and temperature distribution variability can account for 95% of the record. The model presented here provides a new tool for extracting critical missing seasonality information from stalagmite d18O records. Intervals where the model does not converge may represent transient climate anomalies with unusual origins that warrant further investigation This research was funded by FMcD’s Enterprise Ireland Basic Research Grants Scheme and conducted in parallel with IJF’s Natural Environment Research Council (NERC) RAPID climate change programme funded ASCRIBE Project (Grant NER/T/S/2002/00448). FMcD acknowledges support from Science Foundation Ireland through Research Frontiers Grants 07/RFP/GEOF265 and 08/FRP/ GEO1184. The Archaeological work at La Garma has been funded by the Consejería de Educaci on, Cultura y Deporte del Gobierno de Cantabria. We thank Jens Fohlmeister and two anonymous reviewers for their helpful comments which improved the manuscript. We also thank Ana Moreno for her editorial inputs and useful comments that similarly improved the manuscript.

Meteorological data collected during the post‐9/11 flight grounding in the USA suggested that the removal of contrails increased diurnal temperature range (DTR), but subsequent research has contested this result. The 2010 Eyjafjallajökull eruption resulted in a 97% flight cancellation rate across the UK, offering another rare opportunity to compare DTR under contrail‐free skies against those with contrails. Temperature data from 199 UK meteorological stations indicate that a +3.4 °C DTR anomaly occurred during the grounding interval across the region previously affected by the highest flight densities, substantially larger than the +1.1 °C anomaly previously observed but smaller than other DTR anomalies (up to ~+6 °C) that were independent of the grounding. Although the observed DTR anomalies are largely attributable to weather system migration, a contribution of up to +1°C from contrail absence appears reconcilable with both the observed time evolution in DTR during the Eyjafjallajökull grounding period and previous results.

Aragonitic speleothems are increasingly utilised as palaeoclimate archives due to their amenability to high precision U–Th dating. Proxy records from fast-growing aragonitic stalagmites, precisely dated to annual timescales, can allow investigation of climatic events occurring on annual or even sub-annual timescales with minimal chronological uncertainty. However, the behaviour of many trace elements, such as uranium, in aragonitic speleothems has not thus far been as well constrained as in calcitic speleothems. Here, we use uranium concentration shifts measured across primary calcite-to-aragonite mineralogical transitions in speleothems to calculate the distribution coefficient of uranium in aragonitic speleothems (derived DU = 3.74 ± 1.13). Because our calculated DU is considerably above 1 increased prior aragonite precipitation due to increased karst water residence time should strongly control stalagmite aragonite U/Ca values. Consequently, uranium concentrations in aragonitic speleothems should act as excellent proxies for effective rainfall.\ud \ud We test this using a high-resolution ICP-MS derived trace element dataset from a Belizean stalagmite. YOK-G is an aragonitic stalagmite from Yok Balum cave in Belize with an extremely robust monthly-resolved chronology built using annual δ13C cycles. We interpret seasonal U/Ca variations in YOK-G as reflecting changes in the amount and seasonality of prior aragonite precipitation driven by variable rainfall amounts. The U/Ca record strongly suggests that modern drying has occurred in Belize, and that this drying was primarily caused by a reduction in wet season rainfall. This is consistent with published stable isotope data from YOK-G also very strongly suggesting modern rainfall reductions, previously interpreted as the result of southward ITCZ displacement. Our results strongly suggest that U/Ca values in aragonitic speleothems are excellent proxies for rainfall variability. This new tool, combined with the exceptional chronological control characteristic of aragonitic stalagmites and the high spatial resolution afforded by modern microanalytical techniques, should facilitate the construction of new exquisitely resolved rainfall records, providing rare insights into seasonality changes as well as long-term changes in local recharge conditions.

Robust chronologies are crucial for the correct interpretation of climate proxy records and for detailed reconstructions of palaeoclimate. Stalagmites have garnered strong interest as recorders of past climate in part due to their amenability to U-series dating. However, many stalagmites are not dateable using this technique due to low 238U and/or high detrital Th concentrations (e.g., many tropical cave systems (Adkins et al., 2013)), and occasionally these issues affect stalagmites across wide geographical regions (e.g., large parts of Australia (Green et al. 2013)) complicating the use of stalagmites in these areas. Radiocarbon (14C) offers an alternative method of dating stalagmites, but issues associated with the ‘dead carbon fraction’ (DCF) have historically hindered this approach. Here, a novel 14C-based method for dating stalagmites is presented and discussed. The technique calculates a best-fit growth rate between a time-series of stalagmite 14C data and known atmospheric 14C variability. The new method produces excellent results for stalagmites that satisfy four requirements: i) the absence of long-term secular variability in DCF (i.e., stalagmite DCF varies around a mean value with no long-term trend), ii) stalagmite growth rate does not vary significantly (the technique identifies stalagmites with substantial growth rate variability), iii) the stalagmite record is long enough that measurable 14C decay has occurred, and iv) one ‘anchor’ point exists where the calendar age is known. The model produces good results for a previously U–Th dated stalagmite from Heshang Cave, China, and is then applied to an undated stalagmite from southern Poland. The new method will not replace high-precision U–Th measurements, because the precision of the technique is difficult to quantify. However, it provides a means for dating certain stalagmites undateable by conventional U–Th methods and for refining coarse U–Th chronologies.

The forcing mechanisms responsible for the mid-Holocene onset of the Mediterranean-type climate in south-western Europe are currently unclear, but understanding these is critical for accurate climate projections under future greenhouse gas warming. Additionally, regional studies that present conflicting patterns for the onset and advancement of Mediterranean climatic conditions complicate definitively ascribing causality. Here, we use a new high resolution stalagmite density record obtained non-destructively using Computed Tomography (CT scanning) to reconstruct southern Iberian climate between 9.3 and 2.9 ka BP. We suggest that stalagmite density can be used as a water-excess proxy, with lower densities associated with more variable drip rates, possibly reflecting increased seasonality consistent with expectations from previous studies of speleothem textures and crystal fabrics. Our results reveal an early Holocene humid interval and mid-Holocene year-round aridity that preceded the onset of Mediterranean climate at 5.3 ka BP in southern Iberia. Using this new dataset combined with previously published results, we link the gradual advancement of the Mediterranean climate to the southward migration of the North Atlantic Subtropical High induced by an orbitally driven decrease in Northern Hemisphere insolation. Future anthropogenic warming could result in a reversal of this trend, a northward migration of the North Atlantic Subtropical High, and a return to year-round aridity in south-western Europe.

Large multivariate trace element datasets produced by LA-ICP-MS speleothem analysis can pose difficulties for analysis and interpretation. Processes acting on various timescales and magnitudes affect trace element concentrations, and deconvolving the most important controls is often complex. Here Principal Component Analysis (PCA) is applied to identify the modes and timings of variation which best explain the overall variability in an exceptionally high-resolution (10 μm vertical resolution) multivariate trace element record produced by LA-ICP-MS from a modern (1979–2001) Belizean stalagmite with excellent age control. Principal Component 1 (PC1) in this dataset is defined by a weak correlation between multiple elements, and may reflect non-carbonate material incorporated within the speleothem. Elevated PC1 scores in ATM-7 occur following regional volcanic eruptions with ash clouds extending over the cave site, as demonstrated using NASA remote sensing data from the Total Ozone Mapping Spectrometer and HYSPLIT trajectory modelling. Spikes in PC1 occur at the beginning of the wet season, and this may reflect a seasonal flushing event that transports volcanogenic material through the karst and incorporates it within the speleothem. Our results suggest that PCA can simplify exploration of large laser ablation datasets, and that PCA is a valuable tool for identifying the dominant controls on stalagmite trace element chemistry. Future studies should evaluate how transferable this technique is to other sites with different environmental conditions where volcanic ashfall has occurred. This research potentially adds tephrochronology to the stalagmite dating toolkit or, conversely, opens the door to using stalagmites to identify previously unknown or uncertainly dated eruptions.

East Asian summer monsoon (EASM) rainfall impacts the world's most populous regions. Accurate EASM rainfall prediction necessitates robust paleoclimate reconstructions from proxy data and quantitative linkage to modern climatic conditions. Many precisely dated oxygen isotope records from Chinese stalagmites have been interpreted as directly reflecting past EASM rainfall amount variability, but recent research suggests that such records instead integrate multiple hydroclimatic processes. Using a Lagrangian precipitation moisture source diagnostic, we demonstrate that EASM rainfall is primarily derived from the Indian Ocean. Conversely, Pacific Ocean moisture export peaks during winter, and the moisture uptake area does not differ significantly between summer and winter and is thus a minor contributor to monsoonal precipitation. Our results are substantiated by an accurate reproduction of summer and winter spatial rainfall distributions across China. We also correlate modern EASM rainfall oxygen isotope ratios with instrumental rainfall amount and our moisture source data. This analysis reveals that the strength of the source effect is geographically variable, and differences in atmospheric moisture transport may significantly impact the isotopic signature of EASM rainfall at the Hulu, Dongge, and Wanxiang Cave sites. These results improve our ability to isolate the rainfall amount signal in paleomonsoon reconstructions and indicate that precipitation across central and eastern China will directly respond to variability in Indian Ocean moisture supply.

Precisely-dated, high-resolution stable isotope and trace element data from a stalagmite from La Garma Cave, northern Spain, reveal several stages of distinct climatic variability along the northern Iberian Atlantic margin, and provide new constraints on the latitude of North Atlantic westerlies during the Younger Dryas Event (YD). Westerly wind position (reconstructed using sub-annually resolved Mg data as a proxy for sea spray contributions associated with wind strength at this coastal cave site) during the early YD (12.85–12.15 kyr) oscillated meridionally, resembling the decadal-scale component of the modern North Atlantic Oscillation (NAO). Northward repositioning of westerly storm tracks over northern Iberia began at ∼12.15 kyr, consistent with other high-resolution wind proxy reconstructions from central and northern Europe, but occurred more gradually nearer the Atlantic margin. From ∼12.15 kyr to the YD termination, atmospheric circulation resembled a persistently positive NAO, with westerlies reaching their maximum northward extent at 11.8 kyr (reflected by a Mg concentration minimum at this time). Air temperature (reflected by our δ18O data) and Iberian wind strength were predominantly coupled throughout the YD suggesting that temperature modulated sea-ice extent, and consequently controlled westerly wind latitude. However our data suggest that abrupt warming at 12.1 kyr was followed by much more gradual northward shifts in westerly position, and that a lag existed between the warming and sea-ice retreat. This gradual return of the westerlies to the north beginning at 12.1 kyr is consistent with inferred changes in wind strength at other European sites. Additionally, atmospheric circulation inferred from our northern Iberian wind strength proxy record generally tracked low-frequency meridional shifts in Intertropical Convergence Zone position, corroborating past research that suggested closely coupled low- and high-latitude atmospheric teleconnections over this period.

Lead concentrations and isotope ratios from two speleothems from the Han-sur-Lesse cave in southern Belgium were measured in order to study the ability of speleothems to act as archives of atmospheric pollution. To address this aim we analyzed trace elemental Al and Pb compositions by LA-ICP-MS and ICP-MS as well as Pb isotopes by MC-ICP-MS. The results help to identify three intervals characterized by particularly high enrichment of Pb: from 1880 to 1905 AD, from 1945 to 1965 AD, and from 1975 to 1990 AD. The speleothem record shows similar changes as the known historical atmospheric pollution level in Belgium. Lead isotope ratios discriminate between Pb sources and confirm that coal and gasoline combustion, combined with regional metallurgical activities, were the predominant Pb pollution sources in the stalagmites during the last 250 years. This study opens possibilities to determine anthropogenic versus natural metal sources in well-dated speleothem archives.

The Younger Dryas is the most well-documented millennial-scale cooling event of the Quaternary, but the mechanisms responsible for its initiation remain elusive. Here we use a recently revised chronology for the GISP2 ice core ion dataset to identify a large volcanic sulphur spike coincident with both the sulphur-rich Laacher See volcanic eruption and the onset of Younger Dryas-related cooling (GS-1) in Greenland. Lake sediment and stalagmite records confirm that the eruption's timing was indistinguishable from the onset of cooling across the North Atlantic, but that it preceded westerly wind repositioning over central Europe by ~ 200 years. We suggest that the initial short-lived volcanic sulphate aerosol cooling was amplified by oceanic circulation shifts or sea ice expansion, gradually cooling the North Atlantic region and incrementally shifting the mid-latitude westerlies to the south. The aerosol-related cooling probably only lasted 2–4 years, and the majority of Younger Dryas-related cooling was instead due to this positive feedback, which was particularly effective during the intermediate ice volume conditions characteristic of ~ 13 ka BP. We conclude that the large and sulphur-rich Laacher See eruption should be considered a viable trigger for the Younger Dryas.

It is becoming increasingly clear that a substantial reservoir of carbon exists in the unsaturated zone of aquifers, though the total size of this reservoir on a global scale remains unquantified. Here we provide the first broad estimate of the amount of carbon dioxide gas found in this terrestrial reservoir. We calculate that between 2 and 53 PgC exists as gaseous CO2 in aquifers worldwide, generated by the slow microbial oxidation of organic particles transported into aquifers by percolating groundwater. Importantly, this carbon reservoir is in the form of CO2 gas, and is therefore transferable to the Earth's atmosphere without any phase change. On a coarse scale, water table depths are partially controlled by local sea level; sea level lowering therefore allows slow carbon sequestration into the reservoir and sea level increases force rapid CO2 outgassing from this reservoir. High-resolution cave air pCO2 data demonstrate that sea level variability does affect CO2 outgassing rates from the unsaturated zone, and that the CO2 outgassing due to sea level rise currently occurs on daily (tidal) timescales. We suggest that global mean water table depth must modulate the global unsaturated zone volume and the size of this carbon reservoir, potentially affecting atmospheric CO2 on geological timescales.

The presence of a low- to mid-latitude interhemispheric hydrologic seesaw is apparent over orbital and glacial-interglacial timescales, but its existence over the most recent past remains unclear. Here we investigate, based on climate proxy reconstructions from both hemispheres, the inter-hemispherical phasing of the Intertropical Convergence Zone (ITCZ) and the low- to mid-latitude teleconnections in the Northern Hemisphere over the past 2000 years. A clear feature is a persistent southward shift of the ITCZ during the Little Ice Age until the beginning of the 19th Century. Strong covariation between our new composite ITCZ-stack and North Atlantic Oscillation (NAO) records reveals a tight coupling between these two synoptic weather and climate phenomena over decadal-to-centennial timescales. This relationship becomes most apparent when comparing two precisely dated, high-resolution paleorainfall records from Belize and Scotland, indicating that the low- to mid-latitude teleconnection was also active over annual-decadal timescales. It is likely a combination of external forcing, i.e., solar and volcanic, and internal feedbacks, that drives the synchronous ITCZ and NAO shifts via energy flux perturbations in the tropics.

Stable isotope ratios of various proxies are widely used for palaeoclimate reconstruction, and it is often assumed that isotope ratios reflect vegetation abundance or type. However, very little research exists on the isotopic equilibration of extant biomes under variable environmental conditions. In this study, carbon and oxygen isotope ratios from leaves of various Podocarpaceae genera, endemic to New Zealand, are linked to environmental parameters from the Land Environments New Zealand model. The dominant influence on stable isotope ratios within the majority of Podocarpaceae studied here is vapour pressure deficit (VPD). A simple latitudinal trend does not exist, and neither temperature nor rainfall (decoupled from VPD) controls the stable isotope ratios. The results suggest that modern spatial heterogeneity in VPD affects the stable isotope values of vegetation, and that historic VPD variability would change the stable isotope ratios of Podocarpaceae without necessitating a change in vegetation type, density, or productivity. This represents an alternative model for temporal isotope change within geochemical proxies and reinforces the need for increased stable isotopic research in modern plant ecosystems to better understand modern, and eventually palaeoclimatic processes affecting the terrestrial biosphere.

Accurately predicting future tropical cyclone risk requires understanding the fundamental controls on tropical cyclone dynamics. Here we present an annually-resolved 450-year reconstruction of western Caribbean tropical cyclone activity developed using a new coupled carbon and oxygen isotope ratio technique in an exceptionally well-dated stalagmite from Belize. Western Caribbean tropical cyclone activity peaked at 1650 A.D., coincident with maximum Little Ice Age cooling, and decreased gradually until the end of the record in 1983. Considered with other reconstructions, the new record suggests that the mean track of Cape Verde tropical cyclones shifted gradually north-eastward from the western Caribbean toward the North American east coast over the last 450 years. Since ~1870 A.D., these shifts were largely driven by anthropogenic greenhouse gas and sulphate aerosol emissions. Our results strongly suggest that future emission scenarios will result in more frequent tropical cyclone impacts on the financial and population centres of the northeastern United States.

The processes that govern the incorporation of (trace) elements into speleothems can often be linked to environmental changes. Although element incorporation into speleothem calcite is now reasonably well understood, current knowledge regarding trace element variability in speleothem aragonite is very limited. Of particular interest is whether trace element distribution coefficients are above or below one in order to assess the extent to which prior aragonite precipitation has affected speleothem aragonite trace element records. This study uses nine calcite-to-aragonite transitions in seven speleothems from diverse environmental settings to derive the first quantitative estimates of the distribution coefficients for several elements in speleothem aragonite: DMg(Ar) = 9.7E−5 ± 9.01E−5, DBa(Ar) = 0.91 ± 0.88, DSr(Ar) = 1.38 ± 0.53, and DU(Ar) = 6.26 ± 4.54 (1σ SD). For one speleothem from western Germany, the distribution coefficients are generally higher, which is potentially related to the very low growth rates (

A 13-month long, daily-scale drip water dataset collected by an automatic water sampling device placed in a small cave in Ireland provides evidence for seasonal shifts in hydrochemistry, including a pulse of colloidally-associated elements in the late summer. Ca, Sr, Mg, Na, Ba, P, Cu, Zn, Rb, Y, Cs, U, Th, and Pb concentrations of the collected water samples were determined, and flow rates were also calculated. Alkali and alkali earth metals decreased in concentration during a summer water deficit, whereas colloidally-associated element concentrations increased during the same interval and spiked dramatically in the late summer/early autumn. The observed increase in colloidally-associated element concentrations may have coincided with increased breakdown of soil organic material by microorganisms in the late summer/early autumn, which led to an increased flux of organic colloids in the drip water, or it may relate to increased rates of dry deposition from cave air. The decrease in alkali and alkali earth metal concentrations in the summer most likely resulted from the addition of dilute water linked to condensation of water vapor above the drip site. Drip water Sr and P concentrations are anticorrelated, and their variation over the study period resembles seasonal trends observed in stalagmite calcite at other sites. Because the Sr minima and P maxima are interpreted as reflecting different phenomena (increased proportion of condensation water relative to karst water, and an increased flux of organic colloids, respectively), the relative timing of these two mechanisms in the past may explain the shifting polarity of their correlation observed in some stalagmite trace element records.

The nature of cave ventilation is of interest to cavers, speleologists, and paleoclimatologists working with stalagmites. Because cave ventilation systematics may change over the growth span of a stalagmite, understanding what factors affect them is critical for determining events that may have affected climate proxies within the stalagmite. Similarly, understanding how the hydrology of the drips feeding a stalagmite evolves through time is key to building robust records of paleoclimate, particularly because stalagmite records have become critical archives of climate change information of the last 500,000 years. Here we present data from an extensive, on-going monitoring effort at Yok Balum Cave, Belize, initiated in 2011, that characterizes high-resolution ventilation dynamics at this site. Clear seasonal ventilation regimes exist, driven by thermally induced inside-outside air density differences. The winter regime is dominated by air inflow into the cave, decreased drawdown from the epikarst into the cave, and a limited diurnal signal. Conversely, summer ventilation is dominated by air outflow from the cave, greater CO2 drawdown and drip water degassing, and a strong diurnal signal. Active monitoring during a large (M7.4) earthquake in November 2012 provides a unique opportunity to assess the response of the cave atmosphere and hydrology to substantial seismic activity. Cave atmosphere dynamics and hydrology are found to be highly resilient to seismic activity, with no observable disturbance occurring around the earthquake, despite there being considerable evidence of physical disruption in the cave. Monitoring included different kinds of drips, and the earthquake affected none of the monitored drip types. This suggests that stalagmite-derived paleoclimate records are not affected by seismic activity, except in extreme cases where the stalagmite or conjugate stalactite is damaged or reoriented.

Hourly resolved cave air P CO 2 and cave drip water hydrochemical data illustrate that calcite deposition on stalagmites can be modulated by prior calcite precipitation (PCP) on extremely short timescales. A very clear second-order covariation between cave air P CO 2 and drip water Ca2+ concentrations during the winter months demonstrates the effects of degassing-induced PCP on drip water chemistry. Estimating the strength of the cave air P CO 2 control on PCP is possible because the PCP signal is so clear; at our drip site a one ppm shift in Ca2+ concentrations requires a P CO 2 shift of between 333 and 667 ppm. This value will undoubtedly vary from site to site, depending on drip water flow rate, residence time, drip water–cave air P CO 2 differential, and availability of low P CO 2 void spaces in the vadose zone above the cave. High-resolution cave environmental measurements were used to model calcite deposition on one stalagmite in Crag Cave, SW Ireland, and modelled growth over the study period (222 μm over 171 days) is extremely similar to the amount of actual calcite growth (240 μm) over the same time interval, strongly suggesting that equations used to estimate stalagmite growth rates are valid. Although cave air P CO 2 appears to control drip water hydrochemistry in the winter, drip water dilution caused by rain events may have played a larger role during the summer, as evidenced by a series of sudden drops in Ca2+ concentrations (dilution) followed by much more gradual increases in drip water Ca2+ concentrations (slow addition of diffuse water). This research demonstrates that PCP on stalactites, cave ceilings, and void spaces within the karst above the cave partially controls drip water chemistry, and that thorough characterisation of this process at individual caves is necessary to most accurately interpret climate records from those sites.

Cave calcite deposits, and stalagmites specifically, are proving to be critical recorders of paleoclimate. Stalagmites develop from the slow accumulation of calcite deposited after the degassing of carbon dioxide from percolation water in caves. Stalagmites are therefore theoretically capable of recording the hydrochemistry of every drip of water that impacts their surface, but complications involving hydrology, cave air P CO2 variability, intra-aquifer processing, and petrography preclude straightforward interpretation. Nevertheless, stalagmite climate proxy records using stable isotopes, particularly of oxygen, are reconstructing the timing, structure, and in some cases magnitude of climate shifts worldwide.Trace element to calcium ratios in stalagmite are a promising new proxy for both independent reconstruction of climate and for supporting oxygen isotope ratios. Extensive monitoring of individual cave sites is required to most accurately interpret stalagmite proxy records.

Caves are extreme and specialised habitats for terrestrial life that sometimes contain moonmilk, a fine-grained paste-like secondary mineral deposit that is found in subterranean systems worldwide. While previous studies have investigated the possible role of microorganisms in moonmilk precipitation, the microbial community ecology of moonmilk deposits is poorly understood. Bacterial and fungal community structure associated with four spatially isolated microcrystalline, acicular calcite moonmilk deposits at Ballynamintra Cave (S. Ireland) was investigated during this study. Statistical analyses revealed significant differences in microbial activity, number of bacterial species, bacterial richness and diversity, and fungal diversity (Shannon's diversity) among the moonmilk sites over an area of approximately 2.5 m(2). However, the number of fungal species and fungal community richness were unaffected by sampling location. SIMPER analysis revealed significant differences in bacterial and fungal community composition among the sampling sites. These data suggest that a rich assemblage of microorganisms exists associated with moonmilk, with some spatial diversity, which may reflect small-scale spatial differences in cave biogeochemistry.

This two-year study investigates the relative influence of meteorological variables (precipitation amount and temperature), atmospheric circulation, air mass history, and moisture source region on Irish precipitation oxygen isotopes (δ18Op) on event and monthly timescales. Single predictor correlations reveal that on the event scale, 20% of δ18Op variability is attributable to the amount effect and 7% to the temperature effect while on the monthly timescale the North Atlantic Oscillation accounts for up to 20% of δ18Op variability and the amount and temperature effects are not significant. In comparison, multivariate linear regression reveals that the interaction of temperature and precipitation amount explains up to 40% of δ18Op variance at event and monthly timescales. Five-day kinematic back trajectories suggest that the amount-weighted mean δ18Op value of southerly- and northerly-derived events are lower by 2‰ relative to events derived from the west. Because air mass history and atmospheric circulation appear to influence δ18Op in Ireland, Irish paleo-δ18Op proxy records are best interpreted as reflecting a combination of parameters, not just paleotemperature or paleorainfall.

Stable isotope ratios of whole rock carbonates and faunas from three low latitude Upper Ordovician sections demonstrate a coherent pattern of shifting subtropical and tropical water masses and associated climate belts. We suggest that tropical water beneath the Inter-tropical Convergence Zone (δ18O = − 10‰ to − 15‰) was isotopically light reflecting high sea-surface temperatures (SST) and reduced sea-surface salinity (SSS). Subtropical water in both the northern and southern hemispheres is characterised by δ18O values from − 4 to − 6‰. Our interpretation is consistent with the published results from physical–chemical modeling of Ordovician seawater composition and climate models. Deriving absolute temperature and salinity values from Ordovician δ18Ocarb data is problematic as they could reflect SST, SSS, ice volume and potentially a variable diagenetic overprint, and currently no independent constraints exist for any of these variables. The ITCZ was positioned at ~− 10°N of the Equator during the early Katian consistent with the likely position of the region of peak sea-surface temperature (SST). During the late Katian Boda Event the ITCZ moved across the Equator to ~− 10°S. By analogy with the Pliocene this resulted in a symmetrical distribution of sea-surface currents about the Equator. This led to a net increase in heat flow towards the poles and lower latitudinal temperature gradients, potentially explaining the migration of warm water faunas to higher southern latitudes during the Boda Event. During the Hirnantian the ITCZ shifted north of the Equator, resulting in a reduced poleward heat transport. ITCZ position has been proposed as a likely forcing mechanism of Neogene glaciation and the evidence from the Ordovician suggests a discrete palaeo-ITCZ whose position was controlled by solar insolation and latitudinal temperature gradients linked to ice volume changes and Hadley Cell dynamics. We hypothesize that the position of the ITCZ was sensitive to the growing Hirnantian ice sheet during the Late Ordovician. We propose that latitudinal movement of the ITCZ during the Late Ordovician resulted in a major atmosphere–ocean circulation re-organization. Higher pCO2 estimates for the Ordovician caused more extreme changes in this circulation that were the likely primary cause of biotic mass extinction. If the monsoon is considered an off-equator ITCZ then we provide direct evidence of an Ordovician monsoonal climate.

Cave air P CO 2 at two Irish sites varied dramatically on daily to seasonal timescales, potentially affecting the timing of calcite deposition and consequently climate proxy records derived from stalagmites collected at the same sites. Temperature-dependent biochemical processes in the soil control CO 2 production, resulting in high summer P CO 2 values and low winter values at both sites. Large Large-amplitude, high-frequency variations superimposed on this seasonal cycle reflect cave air circulation. Here we model stalagmite growth rates, which are controlled partly by CO 2 degassing rates from drip water, by considering both the seasonal and high-frequency cave air P CO 2 variations. Modeled hourly growth rates for stalagmite CC-Bil from Crag Cave in SW Ireland reach maxima in late December (0.063 μm h − 1 ) and minima in late June/early July (0.033 μm h − 1 ). For well-mixed ‘diffuse flow’ cave drips such as those that feed CC-Bil, high summer cave air P CO 2 depresses summer calcite deposition, while low winter P CO 2 promotes degassing and enhances deposition rates. In stalagmites fed by well-mixed drips lacking seasonal variations in δ 18 O, integrated annual stalagmite calcite δ 18 O is unaffected; however, seasonality in cave air P CO 2 may influence non-conservative geochemical climate proxies (e.g., δ 13 C, Sr/Ca). Stalagmites fed by ‘seasonal’ drips whose hydrochemical properties vary in response to seasonality may have higher growth rates in summer because soil air P CO 2 may increase relative to cave air P CO 2 due to higher soil temperatures. This in turn may bias stalagmite calcite δ 18 O records towards isotopically heavier summer drip water δ 18 O values, resulting in elevated calcite δ 18 O values compared to the ‘equilibrium’ values predicted by calcite–water isotope fractionation equations. Interpretations of stalagmite-based paleoclimate proxies should therefore consider the consequences of cave air P CO 2 variability and the resulting intra-annual variability in calcite deposition rates.

The isotopic ecology of terrestrial snails from tropical island settings is not known despite the importance of such data sets for paleoenvironmental reconstructions. In this study, variations in carbon (13C/ 12C) and oxygen (18O/16O) isotope ratios obtained during whole-shell and sequential-rib analyses of the modern land snail, Cerion, reveal a record of diet and local environment on San Salvador, Bahamas. The mean δ13C value of adult Cerion shells collected from C4 vegetation is higher by 1.0‰ relative to snails collected from C3 plants, suggesting that carbon isotopes in shell carbonate reflect the dominant plant type in the diet, though the broad range of shell carbonate δ13C confirms a varied diet for this genus. The mean δ18O values of adult Cerion shells collected from the west coast of San Salvador are 0.8‰ higher than those collected from the east coast of the island. This difference may reflect the incorporation of water vapor derived from 18O-rich hypersaline lakes located in the isla...

© 2015 Macmillan Publishers Limited. The position of the intertropical convergence zone is an important control on the distribution of low latitude precipitation. Its position is largely controlled by hemisphere temperature contrasts. The release of aerosols by human activities may have resulted in a southward shift of the intertropical convergence zone since the early 1900s (refs) by muting the warming of the Northern Hemisphere relative to the Southern Hemisphere over this interval but this proposed shift remains equivocal. Here we reconstruct monthly rainfall over Belize for the past 456 years from variations in the carbon isotope composition of a well dated monthly resolved speleothem. We identify an unprecedented drying trend since ad 1850 that indicates a southward displacement of the intertropical convergence zone. This drying coincides with increasing aerosol emissions in the Northern Hemisphere and also marks a breakdown in the relationship between Northern Hemisphere temperatures and the position of the intertropical convergence zone observed earlier in the record. We also identify nine short lived drying events since ad 1550 each following a large volcanic eruption in the Northern Hemisphere. We conclude that anthropogenic aerosol emissions have led to a reduction of rainfall in the northern tropics during the twentieth century and suggest that geographic changes in aerosol emissions should be considered when assessing potential future rainfall shifts in the tropics.

The identification of vadose zone hydrological pathways that most accurately transmit climate signals through karst aquifers to stalagmites is critical for accurately interpreting climate proxies contained within individual stalagmites. A three-year cave drip hydrochemical study across a spectrum of drip types in Crag Cave, SW Ireland, reveals substantial variability in drip hydrochemical behaviour. Stalagmites fed by very slow drips ( 2 ml/min) sites, apparently unconnected with local meteorological events. Water from these drips was typically undersaturated with respect to calcite, and thus did not result in calcite deposition. Data presented here suggest that drips in this flow regime also experience flow re-routing and blocking, and that any stalagmites developed under such drips are unsuitable as mid- to high-resolution paleoclimate proxies. Most drip sites demonstrated seasonal [Ca2+] and [Mg2+] variability that was probably linked to water excess. Prior calcite precipitation along the flowpath affected the chemistry of slowly dripping sites, while dilution predominantly controlled the water chemistry of the more rapidly dripping sites. This research underscores the importance of understanding drip hydrology prior to selecting stalagmites for paleoclimate analysis and before interpreting any subsequent proxy data.

Increases in calcite deposition rates combined with decreases in δ13C and δ18O in three modern stalagmites from Brown's Folly Mine, Wiltshire, England, are correlative with a well-documented re-vegetation above the mine. Increased soil PCO2 resulted in greater amounts of dissolved CaCO3 in the drip waters, which consequently increased annual calcite deposition rates. The absence of deposition prior to 1916 (28 years after the mine was closed) indicates that vegetation had not yet sufficiently developed to allow higher PCO2 values to form in the soil. Lower δ13C values through time may reflect the increased input of isotopically light biogenic carbon to the total dissolved inorganic carbon (DIC). δ18O decreased synchronously with δ13C, reflecting the increased importance of isotopically light winter recharge due to greater biomass-induced summer evapotranspiration. This is the first empirical demonstration that vegetation density can control stalagmite growth rates, δ13C, and δ18O, contributing critical insights into the interpretation of these climate proxies in ancient stalagmites.

Abrupt first-order shifts in strontium and phosphorus concentrations in stalagmite calcite deposited in western Ireland during the 8200-year event (the major cooling episode 8200 years before the present) are interpreted as responses to a drier climate lasting about 37 years. Both shifts are centered on 8330 ± 80 years before the present, coinciding with a large oxygen isotope anomaly and a change in the calcite petrography. In this very high resolution (monthly) record, antipathetic second-order oscillations in phosphorus and strontium reveal decreased growth rates and increased rainfall seasonality. Growth rate variations within the event reveal a two-pronged structure consistent with recent model simulations.

Interpretations of high-resolution proxy datasets from stalagmites require support from long-term cave monitoring data and quantified changes in sample growth rate. One cave site for which the modern climate signal transfer systematics are relatively well characterised by cave monitoring is New St Michael's Cave, Gibraltar. This site provides a rare opportunity to reconstruct modern calcite growth, to link growth with the cave environment and local climate, and to test the sufficiency of existing growth rate theory on monthly to inter-annual timescales. Here, we use a numerical time-series growth rate model, driven by cave monitoring and local meteorological data, and the results of an experimental investigation into variation in dripwater film thickness as a function of stalagmite apex morphology to reconstruct the modern growth (AD 1951–2004) of ‘Gib04a’, a stalagmite retrieved from New St Michael's Cave. Our experimental measurements demonstrate that dripwater film thickness decreases linearly with increasing stalagmite curvature and that the presence of millimetre-scale surface microtopography reduces film thickness by an order of magnitude. We identified changes in growth laminae curvature from a Gib04a cut section to determine film thickness variability through time and combined this with estimated dripwater [Ca2+] and cave air pCO2 seasonality to drive the model. Our reconstruction exhibits strong seasonality and tracks variability in calcite [Sr2+], a trace metal whose incorporation into calcite is partially growth rate-controlled. Reconstructed growth also shows co-variation with seasonal changes in calcite fabric, with high growth corresponding to a greater density of calcite grain boundaries. We also link secular trends in karst recharge, film thickness and Gib04a growth, and assess the overall sensitivity of vertical growth rate to film thickness variability. This approach could be used to characterise the growth of other samples retrieved from well-monitored cave systems and may prove particularly useful in quantifying seasonal bias in geochemical proxy datasets, facilitating greater robustness of palaeoclimate reconstructions.