Hazardous, volcanic sediment-laden flows called lahars are among the largest sediment flows on Earth, yet their impacts beyond the coastline remain essentially unknown. Although lahars frequently reach the ocean, a lack of offshore observations limits understanding of their contribution to marine sediment budgets, biogeochemical cycling, ecosystem impacts, and hazards to coastal communities and critical infrastructure. Based on the first offshore observations of ocean-entering lahars, we show that the prevailing conceptual model of direct, short-lived sediment delivery is fundamentally-incomplete. Instead, lahar-derived sediment is transferred offshore through complex, delayed and long-lived process cascades, conveying substantially greater volumes of material to the deep sea than previously-recognized, producing widespread damage. A single lahar in a small, normally dry river catchment delivered >1,600 tonnes of sediment to the ocean in ~20 minutes, while others damaged subsea telecommunications cables and economically-important fisheries. Lahars from a single volcanic island contribute 7% of the annual sediment flux from all ocean islands worldwide, revealing lahars are a dominant and previously-overlooked agent of volcanic sediment delivery to the ocean. As climate change intensifies rainfall, lahar-driven ocean sediment fluxes and associated hazards are likely to increase, with far-reaching implications for marine systems and global infrastructure at hundreds of coastal volcanoes worldwide.

JCp-1-NP is a commercially distributed pressed nano-powder pellet calcium carbonate reference material. It is widely used as a matrix-matched reference material in Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) analyses, yet its composition is not well-documented. In this study, we report the uncertainty of the pellet composition by analysing 20 element-to-Ca ratios measured in three different laboratories. The interlaboratory comparison dataset was outlier-corrected and provides a robust assessment of the composition and long-term stability of this nano-powder pressed pellet. In addition, a review of all publicly available compositional data using this reference material published so far is compiled. The dataset is available online at the Figshare repository.

Coral tissue depth reflects organismal health and is influenced by environmental stressors. Reconstructing its past variability on inter- and intra-annual timescales, however, is not yet possible. Here we reconstructed seasonal tissue depth by measuring spatial offsets between growth cycles in corallite porosity and theca geochemistry (Lithium/Magnesium and Barium/Calcium ratios) of a single Siderastrea siderea core collected in Barbados. We show spatial offsets and thus tissue depth vary systematically over multiyear timescales, with decreasing values associated with thermal stress that impact extension rate and calcification in subsequent growth cycles. Large environmental disturbances such as the 2021 volcanic eruption of La Soufrière (St. Vincent) also impact tissue depth, in this case likely due to the release of bioactive metals upon ash deposition. This study investigates the robustness of the offset signal within a single colony and with further validation across multiple colonies could help reconstruct regional to global environmental and ecological stressors. Coral tissue depth is influenced by environmental stressors, directly impacting skeletal extension and calcification; it may therefore be used as a proxy for past coral health, according to analysis of a coral core from Barbados.

X-radiography of massive scleractinian coral skeletons reveal light and dark couplets termed “growth bands”, which are commonly related to seasonal fluctuations in environmental parameters including insolation and sea surface temperature (SST). Massive corals grow by extension of skeletal structures followed by thickening within the surface tissue layer. Therefore, an understanding of the depth in which skeletal thickening occurs is important to aid the interpretation of seasonal banding patterns. In this study, two colonies of Caribbean coral Siderastrea siderea were sampled from the north-west coast of Barbados at water depths of 5 and 15 m. The three-dimensional skeletal structure of each sample was reconstructed at high spatial resolutions using micro-computed tomography (µCT) scanning. A pixel segmentation algorithm was developed to classify different microstructures within the skeleton and to quantify spatial variations in corallite and theca porosity at the micrometer scale. The porosity reconstructions of the deeper sample reveal clearer growth banding, with a more dominant signal originating from within the corallite. Skeletal thickening occurs within the top two-thirds of the total depth of soft tissues and the rate of thickening varies between microstructures. Seasonality in the shallower sample is less clear, although porosity variability with depth is more similar across microstructures. The difference in signal origin and clarity between the two samples is attributed to the varying stability of water depth-dependent variables (i.e., insolation and wave energy). This study provides a new, powerful method of reconstructing and understanding growth strategies in massive scleractinian corals.

Abstract Macrocrysts (large crystals) in magmas offer a premier record of pre‐eruptive magma storage conditions encoded in their chemistry and texture. Careful study of macrocryst zoning can deconvolve the conditions of crystal growth and the relative time a magma spends in a given physical‐chemical state prior to eruption. Importantly, identifying discrete macrocryst zones requires consideration of both chemistry and texture simultaneously. Here, we employ a novel image segmentation approach to characterize zoning from 2D chemical maps of plagioclase macrocrysts. We apply the method to 15 volcanic eruptions, across three stratigraphic sections, to track statistical differences in crystal zoning through relative time at an arc volcano (Mount Liamuiga, Saint Kitts). Plagioclase from the 15 eruptions are described by 7 unique textural‐chemical zoning populations, which we term “zoning groups,” each of which has a unique An# fingerprint. Two of the studied stratigraphic sections overwhelmingly record low‐An# zoning groups (ZGs), whereas the other section records mostly high‐An# ZGs, suggestive of two distinct storage conditions. Using observations from equilibrium experiments relevant to Saint Kitts bulk magma compositions, we show that differences in melt H2O are the primary drivers of the An# variability. Negative whole rock K2O versus predicted H2O trends are suggestive of ubiquitous H2O‐saturated conditions throughout the middle and upper crust, with a correlation between H2O‐saturated storage pressure (Psat) and eruptive dynamics; magmas stored in the upper‐crust (0.48 ± 0.28–1.08 ± 0.45 kbar) produce larger‐volume, pumice‐rich eruptions compared to magmas stored in the middle crust (3.29 ± 0.87–3.88 ± 1.05 kbar) which generally produce smaller‐volume, centimeter to decimeter‐thick fall deposits.

The interaction between volcanic eruptions and coral reefs are commonly reported to be detrimental to corals. However, recent laboratory experiments on cultured corals exposed to moderate concentrations of volcanic ash, reveal quite the opposite: ash-exposed corals exhibit surprising health improvements compared to the control counterparts.

Massive scleractinian corals provide high-resolution, continuous archives of ocean chemistry.Their wide distribution around the (sub)tropic zones makes corals a powerful resource for researchers.Most studies have focused on reconstructing physicochemical processes with temporal resolutions limited to annual seasonality.Shorter-term environmental variations or disturbances, however, are more challenging to identify due to the difficulty in constraining timescales (daily-weekly resolutions).Intra-annual variations in extension rates make spatially pinpointing events difficult, whilst vital effects and signal averaging during geochemical analyses mask and distort the original signal.To better constrain timescales, we conduct targeted LA-ICP-MS trace element analyses of skeletal features (columella, theca, septa and dissepiments) on coral core samples subjected to volcanic ashfall to determine which feature provides the highest temporal resolution.Massive colonies of Siderastrea siderea were cored from the northwest fringing reefs of Barbados, Lesser Antilles, 15 months following the April 2021 eruption of La Soufrière, St. Vincent.Ash is reported to have resided in the coastal environment for several weeks.Our results will provide important information concerning which skeletal feature to analyse to better identify and reconstruct short-term environmental disturbances in the massive Caribbean coral Siderastrea siderea.

The past decades have seen tremendous advances in analytical capabilities regarding the sensitivity, spatial selectivity, and instrumental precision of U-Th-Pb zircon geochronology. Along with improved zircon pretreatment to mitigate the effects of Pb-loss, these advancements have resulted in the emergence of U-Th-Pb dating as the most widely used geochronometer. In parallel, it became increasingly obvious that modern analytical techniques can resolve zircon age dispersal beyond instrumental uncertainties and that this dispersion cannot be attributed to Pb-loss or inheritance. Hence, there is a pressing need to refine statistical procedures for displaying and interpreting dispersed age data from volcanic and plutonic rocks, where zircon ages were traditionally assigned to the quasi-instantaneous events of eruption and magma emplacement, respectively. The ability to resolve zircon age spectra, which often range over timescales of 103–106 years, also offers new opportunities to monitor magmatic processes, because zircon crystallization directly relates to the temperature and composition of its host melt. This relation is, at least for typical subalkaline melt compositions, well calibrated by multiple zircon saturation experiments, although absolute saturation temperatures derived from them can vary by tens of degrees. Moreover, zircon saturation thermometry is supported by the trace element and isotopic inventory of zircon, which records the thermochemical and compositional evolution of melts at high fidelity. Here, we first review the properties of true zircon age spectra that are defined by a statistically robust overdispersion relative to analytical uncertainties. Secondly, we evaluate existing models and present new models that aim to quantitatively translate the properties of zircon age spectra into parameters controlling the longevity and thermal evolution of crustal magma bodies such as magma recharge flux and duration. These developing approaches, which aspire to capture all processes that affect the formation and dispersal of zircon in dynamic crustal magma systems, have the potential to foster an improved understanding of magmatism with implications for volcanic hazard assessment, geothermal energy uses, and the origins of ore deposits.

Coral reefs, which are among the most productive ecosystems on earth, are in global decline due to rapid climate change. Volcanic activity also results in extreme environmental changes at local to global scales, and may have significant impacts on coral reefs compared to other natural disturbances. During explosive eruptions, large amounts of volcanic ash are generated, significantly disrupting ecosystems close to a volcano, and depositing ash over distal areas (10s - 1000s of km depending on i.a. eruption size and wind direction). Once volcanic ash interacts with seawater, the dissolution of metals leads to a rapid change in the geochemical properties of the seawater column. Here, we report the first known effects of volcanic ash on the physiology and elemental cycling of a symbiotic scleractinian coral under laboratory conditions. Nubbins of the branching coral Stylophora pistillata were reared in aquaria under controlled conditions (insolation, temperature, and pH), while environmental parameters, effective quantum yield, and skeletal growth rate were monitored. Half the aquaria were exposed to volcanic ash every other day for 6 weeks (250 mg L-1 week-1), which induced significant changes in the fluorescence-derived photochemical parameters (ΦPSII, Fv/Fm, NPQ, rETR), directly enhanced the efficiency of symbiont photosynthesis (Pg, Pn), and lead to increased biomineralization rates. Enhancement of symbiont photosynthesis is induced by the supply of essential metals (Fe and Mn), derived from volcanic ash leaching in ambient seawater or within the organism following ingestion. The beneficial role of volcanic ash as an important micronutrient source is supported by the fact that neither photophysiological stress nor signs of lipid peroxidation were detected. Subaerial volcanism affects micronutrient cycling in the coral ecosystem, but the implication for coral ecophysiology on a reef scale remains to be tested. Nevertheless, exposure to volcanic ash can improve coral health and thus influence resilience to external stressors.

Caldera-forming eruptions have the potential to induce drastic socioeconomic change. However, the criteria to identify volcanoes capable of producing large magnitude eruptions in the future are not well constrained. Here we compile and analyse data, revealing that volcanoes which have produced catastrophic caldera-forming eruptions in the past, show larger ranges of erupted magma geochemistry compared to those that have not. This suggests geochemical variability is related to the size of magmatic systems. Using heat transfer simulations, we show that differences in magma flux result in a dependency between chemical diversity and magma volume that is consistent with these observations. We conclude that compositional spread should be included in the catalogue of criteria to identify volcanoes with greater probability of producing future large eruptions. Importantly, this allows to identify stratovolcanoes with caldera-like geochemical signatures, which have not yet been recognized as systems with greater likelihood of producing large magnitude eruptions.

The physical and chemical properties of magma govern the eruptive style and behaviour of volcanoes. Many of these parameters are linked to the storage pressure and temperature of the erupted magma, and melt chemistry. However, reliable single-phase thermobarometers and chemometers which can recover this information, particularly using amphibole chemistry, remain elusive. We present a suite of single-phase amphibole and clinopyroxene thermobarometers and chemometers, calibrated using machine learning. This approach allows us to intimately track the range of pre-eruptive conditions over the course of a millennial eruptive cycle on an island arc volcano (Saint Kitts, Eastern Caribbean). We unpick the story of Mount Liamuiga, a stratovolcano that pops its upper-crustal (2 kbar), dacitic cork at the beginning of the Lower Mansion Series eruptive sequence. This permits a progressive increase in the thermal maturity of the magma arriving at the surface from the middle to upper crust (2 – 5.5 kbar) through time. The temperature increase correlates well with matrix plagioclase chemistry, which itself displays a remarkable progression to less evolved (more anorthitic) composition in time. We find that amphibole is a reliable themobarometer (SEE = 1.4 kbar; 40 ˚C), at odds with previous studies. We suggest it is the regression strategy, as opposed to the abject insensitivity to pressure, that has hindered previous calibrations of amphibole only thermobarometers. By recognising this, we have constructed a high-resolution, quantitative picture of the magma plumbing system beneath an arc volcano.

Minerals exhibit compositional zoning patterns that can be related to changes in the environment in which they grew. Using statistical methods that have been designed to segment optical images, we have developed a procedure to segment zoned crystals using elemental maps. For a single mineral phase, compositional zones in individual crystals are correlated between multiple crystals. This allows us to quantify the complexity and variability of chemical zoning between different geological samples. Specifically, we employ a simple linear iterative clustering algorithm, which splits the chemical maps into spatially constrained regions of similar chemistry. The result is a texturally segmented crystal, akin to what would be identified by the human eye. To aid the segmentation and correlation of zones, we also introduce a new method to classify multiple mineral phases within a single thin section. This is based on a finite mixture model approach, which proves very effective in removing mixed pixels that will only introduce noise into the segmentation. We provide an example using the mineral phase plagioclase. Using two contemporaneous samples from an eruptive unit on the island of St. Kitts (Eastern Caribbean) we show that a volcanic bomb (-10 cm) and a scoria clast (-2 cm) have similar rim compositions but distinctly different core compositions. Our methodology will enable a statistical characterization of 2D complexity of crystals in a variety of different geo-scientific disciplines. This will allow the genesis of different mineral phases to be directly compared.

The Toba volcanic system in Indonesia has produced two of the largest eruptions (>2,000 km3 dense-rock equivalent [DRE] each) on Earth since the Quaternary. U-Pb crystallization ages of zircon span a period of ∼600 ky before each eruptive event, and in the run-up to each eruption, the mean and variance of the zircons' U content decrease. To quantify the process of accumulation of eruptible magma underneath the Toba caldera, we integrated these observations with thermal and geochemical modeling. We show that caldera-forming eruptions at Toba are the result of progressive thermal maturation of the upper crustal magma reservoir, which grows and chemically homogenizes, by sustained magma influx at average volumetric rates between 0.008 and 0.01 km3/y over the past 2.2 My. Protracted thermal pulses related to magma-recharge events prime the system for eruption without necessarily requiring an increased magma-recharge rate before the two supereruptions. If the rate of magma input was maintained since the last supereruption of Toba at 75 ka, eruptible magma is currently accumulating at a minimum rate of ∼4.2 km3 per millennium, and the current estimate of the total volume of potentially eruptible magma available today is a minimum of ∼315 km3 Our approach to evaluate magma flux and the rate of eruptible magma accumulation is applicable to other volcanic systems capable of producing supereruptions and thereby could help in assessing the potential of active volcanic systems to feed supereruptions.

Establishing a quantitative link between magmatic processes occurring at depth and volcanic eruption dynamics is essential to forecast the future behaviour of volcanoes, and to correctly interpret monitoring signals at active centres. Chemical zoning in minerals, which captures successive events or states within a magmatic system, can be exploited for such a purpose. However, to develop a quantitative understanding of magmatic systems requires an unbiased, reproducible method for characterising zoned crystals. We use image segmentation on thin section scale chemical maps to segment textural zones in plagioclase phenocrysts. These zones are then correlated throughout a stratigraphic sequence from Saint Kitts (Lesser Antilles), composed of a basal pyroclastic flow deposit and a series of fall deposits. Both segmented phenocrysts and unsegmented matrix plagioclase are chemically decoupled from whole rock geochemical trends, with the latter showing a systematic temporal progression towards less chemically evolved magma (more anorthitic plagioclase). By working on a stratigraphic sequence, it is possible to track the chemical and textural complexity of segmented plagioclase in time, in this case on the order of millennia. In doing so, we find a relationship between the number of crystal populations, deposit thickness and time. Thicker deposits contain a larger number of crystal populations, alongside an overall reduction in this number towards the top of the deposit. Our approach provides quantitative textural parameters for volcanic and plutonic rocks, including the ability to measure the amount of crystal fracturing. In combination with mineral chemistry, these parameters can strengthen the link between petrology and volcanology, paving the way towards a deeper understanding of the magmatic processes controlling eruptive dynamics.