Search for: All records

Stalagmites are excellent archives of past climate change. They can grow continuously for tens of thousands of years, be precisely dated, and record a wide range of climate variables through their chemistry. In the last quarter century, numerous studies have focused on (sub)tropical hydroclimate variability, particularly from South Asia, the Maritime Continent, and northern Australia. This talk will discuss how stalagmite paleoclimate research is conducted and what important findings have resulted from this work. 

With relatively few exceptions, dendrochronological records in Australia have been developed from conifer species. Over the last 20 years, some effort has successfully extracted records from some Eucalyptus species and Toona ciliata. However, very little attempt has been made to examine a broader range of species. There has been some success with extracting records from the African Boab species. We collected 10mm core samples from around 30 A. gregorii trees in the Kimberly and will be assessing the potential of the species by examining variability in vessel characteristics, isotopic and elemental tracers. We hope to use elemental tracers in the wood to corroborate a fire record from speleothems in the region. 

With relatively few exceptions, dendrochronological records in Australia have been developed from conifer species. Over the last 20 years, some effort has successfully extracted records from some Eucalyptus species and Toona ciliata. However, very little attempt has been made to examine a broader range of species. There has been some success with extracting records from the African Boab species. We collected 10mm core samples from around 30 A. gregorii trees in the Kimberly and will be assessing the potential of the species by examining variability in vessel characteristics, isotopic an elemental traces. We hope to use elemental traces in the wood to corroborate a fire record from speleothems in the region. 

Understanding the long-term interactions among vegetation, fire, and climate is critical for interpreting ecosystem responses to climatic perturbations. Project Prometheus investigates Holocene paleofire dynamics, vegetation shifts, and climate variability in the Mediterranean, using speleothem records from caves in Italy (Alps, Apennines, Sardinia) and the Balkans. By integrating multiple proxies, including polycyclic aromatic hydrocarbons (PAHs) as fire markers and n-alkanes as a proxy for vegetation composition and terrigenous input, this project aims to provide insights into the environmental drivers of fire activity from millennial to sub-centennial timescales, thus creating a high resolution fire history for the Mediterranean region. Speleothems offer a novel paleoenvironmental archive, and we apply an advanced hydrocarbon extraction protocol adapted from a study on Australian stalagmites1. This method, which includes slow acid dissolution in a clean-room setting to minimize contamination and maximize compound yields, has significantly improved the detection limits and expanded the range of PAHs identified2. Uranium-thorium (U-Th) dating ensures a precise chronological framework, enabling robust correlation between fire, vegetation, and climate proxies. Here we present results from the initial phase of the project, analyizing a dozen archives from Italy, Greece, and Northern Macedonia, at low resolution (millennial- and sub-millennial-scale). Preliminary results, will provide a first indication of technique effectiveness, archive quality, and regional historical variations (if any) in paleofire regimes. Comparative studies with paleofire data from lake sediments in Italy, where shifts in fire regimes have been previously documented, as well as with modern fire data derived from registries and satellite observations, will help contextualizing our findings within broader regional fire histories. This research advances our understanding of vegetation-wildfire-climate interactions in the Mediterranean by contributing high-resolution, multi-proxy reconstructions from an understudied archive. By linking past fire and vegetation responses to climatic variability, it provides critical context for assessing future ecosystem resilience and informing land management policies under changing climate conditions. 

Greenland stadials and interstadials (GS/GI) were millennial climate oscillations during the last glacial period that were originally identified in Greenland ice cores but that have been correlated with environmental change around much of the globe, including in monsoon regimes, with enhanced monsoon rainfall coincident with North Atlantic warming. Hydroclimate variability associated with GS/GI have been investigated in detail using terrestrial (primarily oxygen isotopes in stalagmites) and marine records, particularly for the Southeast Asian monsoon. However, a considerably smaller number of terrestrial records preserve these events in the Indian summer monsoon (ISM), the primary water source for ~2 billion people across South Asia. Here we present the first glacial-age speleothem stable isotope time series from Nepal, located in the ISM regime. UK-1 is a 187 mm tall aragonite stalagmite from the Pokhara Valley of central Nepal, ~150 km west of Kathmandu. The chronology of UK-1, which was established by 8 U/Th dates, all of which fall in stratigraphic order (within the errors), reveals continuous growth from 34,350-31,500 yr BP (Marine Isotope Stage 3); age uncertainties average ±84 yr. Stable isotope samples were measured every 1 mm, corresponding to a temporal resolution of 18 yr. Oxygen isotope ratios range from -5.6‰ to -7.6‰, and share the same timing and structure as Greenland (inter)stadials GS/GI 6 and 5.2 in the NGRIP record. We interpret this as reflecting an amount effect response to a strengthened ISM driven by more (less) poleward migration of the intertropical convergence zone during periods of northern hemisphere warming (cooling). This clear millennial signal in UK-1 is a somewhat unexpected result given that amount effects in oxygen isotopes in precipitation are weak (R^2=0.1) in this area today. UK-1 carbon isotope ratios range from -3‰ to -6‰ (excluding a small number of negative spikes) and exhibit variability coarsely similar to the NGRIP record, with lower (higher) values generally corresponding to GI (GS), possibly due to prior calcite precipitation in voids above the cave concomitant with changes in precipitation. Some periods of antiphasing between carbon and oxygen are also apparent and may reflect flushing of soil carbon dioxide during particularly wet phases. 

Because traditional paleofire archives (e.g., burn scars on trees, charcoal in lake sediments) are not available in all settings, new ways of reconstructing past fire activity are needed. We focus here on polycyclic aromatic hydrocarbons (PAHs) in stalagmites. PAHs are organic molecules composed of two or more fused aromatic rings formed through incomplete combustion of organic matter, and vary in molecular weight depending on combustion conditions. Because the use of PAHs in stalagmites as a paleofire indicator is still in its infancy and because the production, deposition, and transport of PAHs into a cave is a complex and multi-faceted system, we tested the reproducibility of PAHs in two coeval and precisely-dated aragonite stalagmites – KNI-51-F and KNI-51-G - from KNI-51 (15.3°S, 128.6°E), a shallow cave located in the Kimberley region of tropical Western Australia. KNI-51-F and KNI-51-G span 1110-1620 CE and 1310-1630 CE, respectively. Each was hand-milled for analysis in continuous sections spanning approx. 2 mm-tall intervals at Ca’ Foscari University. Owing to differences in growth rate, temporal resolutions for KNI-51-F and KNI-51-G were 3±2 and 1±0.4 yr/sample, respectively. Chemical preparations and analysis methods follow those of Argiriadis et al. (2019) Analytical Chemistry, volume 91. In order to assess replication between the two stalagmites, we compared total abundances of low molecular weight (LMW: Napthalene, Acenaphthylene, Acenaphthene, Fluorene), medium molecular weight (MMW: Phenanthrene, Anthracene, Fluoranthene, Pyrene, Benzo(a)Anthracene, Chrysene, Retene), and high molecular weight (HHM: Benzo(b)Fluoranthene, Benzo(k)Fluoranthene, Benzo(e)Pyrene, Benzo(a)Pyrene, Perylene, Benzo(ghi)Perylene, Indeno(1,2,3-c,d)Pyrene, Dibenzo(A,H)Anthracene) PAHs. Total abundances of LMW, MMW, and HMW PAHs are similar (<10 ng/g) except for HMW PAHs in KNI-51-G, which are generally <1 ng/g. Total LMW and MMW abundance time series replicate well, with multiple synchronous multidecadal periods characterized by consistently low PAH abundances, suggestive of reduced bushfire activity, punctuated by intervals of high PAH abundances, likely reflecting frequent bushfire. Less coherence exists between HMW PAHs. 

Gli speleotemi sono ampiamente utilizzati come archivio paleoclimatico in ragione di alcune caratteristiche come la crescita continua e regolare, la resistenza all’alterazione, la datazione precisa e la possibilità di ottenere record ad alta risoluzione. La ricostruzione di paleofuochi dagli speleotemi è un innovativo ambito di ricerca che sta velocemente prendendo piede con lo sviluppo di tecniche analitiche dedicate e l’utilizzo di nuovi proxy geochimici. Di recente, abbiamo proposto un metodo per la determinazione di idrocarburi policiclici aromatici in tracce e n-alcani nelle stalagmiti, dimostrando la validità del metodo e dei proxy attraverso una serie di test effettuati su stalagmiti provenienti dalla grotta KNI-51, situata nella regione del Kimberley, in Australia nord-occidentale. Il sito è particolarmente adatto per questo tipo di studio, trattandosi di una zona soggetta a frequenti incendi e di una grotta poco profonda le cui stalagmiti hanno una crescita media di 1-2 mm/anno. Attualmente è in fase di completamento il record ad alta risoluzione ottenuto da tre stalagmiti che coprono l’ultimo millennio, includendo quindi la colonizzazione europea e il conseguente cambiamento del regime di fuoco dovuto alla soppressione del controllo tradizionalmente effettuato dalla popolazione aborigena e all’introduzione di animali da pascolo. Alle nostre latitudini, il fuoco sta acquisendo un’importanza crescente a livello ambientale, climatico e sociale. Comprendere la complessità coinvolta nelle interazioni tra fuoco, clima ed ecosistemi beneficia della conoscenza delle dinamiche del passato. Pertanto, nell’ottica di testare questo approccio in un contesto come quello Mediterraneo, è stato finanziato il progetto PRIN-PNRR PROMETHEUS, che prevede di testare lo stesso metodo su campioni provenienti da aree situate in Italia e in Europa meridionale e caratterizzate da diversi microclimi. 

Pyrogenic compounds, such as polycyclic aromatic hydrocarbons (PAHs), can track the type and intensity of fires and are preserved in many environmental matrices including speleothems. We recently reported on a stalagmite record of PAH abundance distributions from cave KNI-51, located among the eucalypt savanna in the Ningbing range of tropical Western Australia. In order to better understand the manner by which PAHs from local bushfires are deposited on the land surface and transported into caves, we performed a controlled burn and irrigation experiment at cave KNI-140, located near to and in the same bedrock as cave KNI-51. Samples of soil, vegetation, ash, and air were collected prior to and immediately succeeding the prescribed burn. The fire, which burned predominantly grasses, was ignited by matches (no accelerants were used) and covered approximately 30,000 square meters upwind from the cave. The land surface above the cave was irrigated prior to and immediately succeeding the burn with resulting dripwater collected for analysis. Next, ash samples were deposited directly above the cave and then similarly irrigated, with the drip water also collected. The PAHs present in these samples were measured via gas chromatography-mass spectrometry at Ca’ Foscari University, Venice. Our results reveal that low molecular weight PAHs were the most abundant species of PAH in the drip water and heavier PAHs were substantially less abundant. This result is likely due to the low combustion temperature of the burn, with abundances increasing through each of the three stages of sample collection, demonstrating that deposition from smoke and cinders produces identifiable signals in dripwater (and thus stalagmite) PAHs, supporting the contention that KNI-51 stalagmites record fire activity occurring not just above the cave but within km of the cave. On-going analyses of soil, vegetation, and ash samples will further clarify the role of fire on production and transmission of PAHs at this site, and thus how these organic compounds preserved in speleothems can help delineate the fire history in the region. 

Over the late Holocene, a variety of hydroclimate-sensitive proxies have identified substantial, multidecadal changes in Indian summer monsoon (ISM) precipitation, the most prominent of which is the “4.2 ka event”. This interval, dated to ~4.2-3.9 ka, is associated with severe droughts across South Asia that are linked to societal change. Given the absence of the 4.2 ka event in polar records, the 4.2 ka event is generally associated with low latitude forcings, but no clear consensus on its origins has been reached. We investigated the ISM response to the 4.2 ka event through analysis of aragonite stalagmites from Siddha cave, formed in the lower Paleozoic Dhading dolomite in the Pokhara Valley of central Nepal (28.0˚N, 84.1˚E; ~850 m.a.s.l.). The climate of this region is dominated by small monthly variations in air temperature (21±5˚C) but strong precipitation seasonality associated with the ISM: ~80% of the annual 3900 mm of rainfall occurs between June and September. High uranium and low detrital thorium abundances in these stalagmites yield precise U/Th ages that all fall within stratigraphic order. These dates reveal continuous growth from 4.30-2.26 ka, interrupted only by a hiatus from 3.27-3.10 ka. Overlap with coeval aragonite stalagmites reveals generally consistent trends in carbon and oxygen isotope ratios, suggesting that these stalagmites reflect environmental variability and not secondary (e.g., kinetic) effects. Many stalagmite-based paleomonsoon reconstructions rely on oxygen isotope ratios, which track amount effects in regional rainfall. However, our on-going rainwater collection and analysis program, as well as a previous study conducted in Kathmandu, 120 km the east of Siddha cave, reveals that amount effects in precipitation are weak in this region, particularly during the monsoon season, and thus we rely instead on carbon isotope ratios, which have been demonstrated to track site-specific effective precipitation. Siddha cave stalagmite carbon isotopes, in contrast to other South Asian proxy records, indicate that ISM rainfall increased at Siddha cave from 4.13-3.91 ka. As a further test of this result, we analyzed uranium abundances in the section spanning 4.3-3.4 ka. Uranium serves as an indicator of prior aragonite precipitation and thus of hydroclimate, and like carbon isotopes, suggests increased ISM rainfall coincident with the 4.2 ka event. This precipitation anomaly is nearly identical in timing and structure but anti-phased with stalagmites from Mawmluh cave, northeastern India. We investigated the climatic origins of this precipitation dipole using observational data from the Global Precipitation Climatology Centre (GPCC) and Hadley Center Sea Ice and Sea Surface Temperature (HadISST) products. Preliminary spatial composites suggest that large precipitation differences between Mawmluh and Siddha caves are associated with SST anomalies in the equatorial Pacific. Additionally, superposed Epoch Analysis shows relatively rapid eastern Indian Ocean cooling during the summer monsoon season coeval with large precipitation differences between these sites. Our findings lend support to a tropical Indo-Pacific origin of the 4.2 ka event. 

Human activity and climate change are altering natural rates and intensities of wildfire, but the scale and extent of burning prior to the modern era are poorly understood. Prehistoric fire activity can be reconstructed using a variety of records including charcoal deposited along with sediment at the bottom of lakes and burn scars on tree rings, but these are not available in all environmental settings. We are developing a new paleofire proxy: polycyclic aromatic hydrocarbons (PAHs) in stalagmites. PAHs are produced by the burning of vegetation, with molecular weights reflecting combustion conditions. After being formed in a fire, PAHs are transported downward by infiltrating rainwater and in cave areas can become incorporated into stalagmites as they crystallize from drip water in underlying caves (Perrette et al., 2008; Denniston et al., 2018). Thus, the potential exists for PAHs in stalagmites to preserve evidence of the presence and intensity of fire through time. Because this is a new method, several important tests need to be performed to evaluate its veracity. I assessed how well PAH abundances, ratios, and trends replicate between two coeval stalagmites from cave KNI-51 located in the tropics of Western Australia. Stalagmite KNI-51-F was previously analyzed and I analyzed the overlapping portion of stalagmite KNl-51-G: overlap in age from CE 1310-1630. This work was done at Ca' Foscari University, Venice, in the fall of 2022, under the direction of Dr. Elena Argiriadis. The results show similarities between the area of overlap in the G and F stalagmites. The commonalities of the concentrations of PAH in the stalagmites indicate confidence in the developed method in assessing pyrogenic compounds in coeval stalagmites.