Hydrological characterization of cave drip waters in a porous limestone: Golgotha Cave, Western Australia

Cave drip water response to surface meteorological conditions is complex due to the heterogeneity of water movement in the karst unsaturated zone. Previous studies have focused on the monitoring of fractured rock limestones that have little or no primary porosity. In this study, we aim to further understand infiltration water hydrology in the Tamala Limestone of SW Australia, which is Quaternary aeolianite with primary porosity. We build on our previous studies of the Golgotha Cave system and utilize the existing spatial survey of 29 automated cave drip loggers and a lidar-based flow classification scheme, conducted in the two main chambers of this cave. We find that a daily sampling frequency at our cave site optimizes the capture of drip variability with the least possible sampling artifacts. With the optimum sampling frequency, most of the drip sites show persistent autocorrelation for at least a month, typically much longer, indicating ample storage of water feeding all stalactites investigated. Drip discharge histograms are highly variable, showing sometimes multimodal distributions. Histogram skewness is shown to relate to the wetter-than-average 2013 hydrological year and modality is affected by seasonality. The hydrological classification scheme with respect to mean discharge and the flow variation can distinguish between groundwater flow types in limestones with primary porosity, and the technique could be used to characterize different karst flow paths when high-frequency automated drip logger data are available. We observe little difference in the coefficient of variation (COV) between flow classification types, probably reflecting the ample storage due to the dominance of primary porosity at this cave site. Moreover, we do not find any relationship between drip variability and discharge within similar flow type. Finally, a combination of multidimensional scaling (MDS) and clustering by k means is used to classify similar drip types based on time series analysis. This clustering reveals four unique drip regimes which agree with previous flow type classification for this site. It highlights a spatial homogeneity in drip types in one cave chamber, and spatial heterogeneity in the other, which is in agreement with our understanding of cave chamber morphology and lithology. © Author(s) 201

A post-wildfire response in cave dripwater chemistry

Surface disturbances above a cave have the potential to impact cave dripwater discharge, isotopic composition and solute concentrations, which may subsequently be recorded in the stalagmites forming from these dripwaters. One such disturbance is wildfire; however, the effects of wildfire on cave chemistry and hydrology remains poorly understood. Using dripwater data monitored at two sites in a shallow cave, beneath a forest, in southwest Australia, we provide one of the first cave monitoring studies conducted in a post-fire regime, which seeks to identify the effects of wildfire and post-fire vegetation dynamics on dripwater δ18O composition and solute concentrations. We compare our post-wildfire δ18O data with predicted dripwater δ18O using a forward model based on measured hydro-climatic influences alone. This helps to delineate hydro-climatic and fire-related influences on δ18O. Further we also compare our data with both data from Golgotha Cave – which is in a similar environment but was not influenced by this particular fire – as well as regional groundwater chemistry, in an attempt to determine the extent to which wildfire affects dripwater chemistry. We find in our forested shallow cave that δ18O is higher after the fire relative to modelled δ18O. We attribute this to increased evaporation due to reduced albedo and canopy cover. The solute response post-fire varied between the two drip sites: at Site 1a, which had a large tree above it that was lost in the fire, we see a response reflecting both a reduction in tree water use and a removal of nutrients (Cl, Mg, Sr, and Ca) from the surface and subsurface. Solutes such as SO4 and K maintain high concentrations, due to the abundance of above-ground ash. At Site 2a, which was covered by lower–middle storey vegetation, we see a solute response reflecting evaporative concentration of all studied ions (Cl, Ca, Mg, Sr, SO4, and K) similar to the trend in δ18O for this drip site. We open a new avenue for speleothem science in fire-prone regions, focusing on the geochemical records of speleothems as potential palaeo-fire archives. © Author(s) 2016

Chemical characterisation and source identification of atmospheric aerosols in the Snowy Mountains, south-eastern Australia

Characterisation of atmospheric aerosols is of major importance for: climate, the hydrological cycle, human health and policymaking, biogeochemical and palaeo-climatological studies. In this study, the chemical composition and source apportionment of PM2.5 (particulate matter with aerodynamic diameters less than 2.5 μm) at Yarrangobilly, in the Snowy Mountains, SE Australia are examined and quantified. A new aerosol monitoring network was deployed in June 2013 and aerosol samples collected during the period July 2013 to July 2017 were analysed for 22 trace elements and black carbon by ion beam analysis techniques. Positive matrix factorisation and back trajectory analysis and trajectory clustering methods were employed for source apportionment and to isolate source areas and air mass travel pathways, respectively. This study identified the mean atmospheric PM2.5 mass concentration for the study period was (3.3 ± 2.5) μg m−3. It is shown that automobile (44.9 ± 0.8)%, secondary sulfate (21.4 ± 0.9)%, smoke (12.3 ± 0.6)%, soil (11.3 ± 0.5)% and aged sea salt (10.1 ± 0.4)% were the five PM2.5 source types, each with its own distinctive trends. The automobile and smoke sources were ascribed to a significant local influence from the road network and bushfire and hazard reduction burns, respectively. Long-range transport are the dominant sources for secondary sulfate from coal-fired power stations, windblown soil from the inland saline regions of the Lake Eyre and Murray-Darling Basins, and aged sea salt from the Southern Ocean to the remote alpine study site. The impact of recent climate change was recognised, as elevated smoke and windblown soil events correlated with drought and El Niño periods. Finally, the overall implications including potential aerosol derived proxies for interpreting palaeo-archives are discussed. To our knowledge, this is the first long-term detailed temporal and spatial characterisation of PM2.5 aerosols for the region and provides a crucial dataset for a range of multidisciplinary research. Crown Copyright © 2018 Published by Elsevier B.V

Southwest Australian speleothem records – an update

Southwest Western Australia (SWWA) experienced a clear climatic change during the late 20thC and has been identified as highly vulnerable to future climate change (Hope et al., 2010). Paleoclimate records could assist in understanding SWWA climate but very few exist for this region. Early investigations into speleothems were very promising, demonstrating that speleothem-based proxies record the multidecadal reduction in rainfall since the 1970s (Treble et al., 2003, 2005; Fischer and Treble, 2008). Subsequent efforts to build a paleoclimate record revealed that the climate-speleothem signal was poorly understood in our studied cave e.g. disagreement between coeval records; non-linear response in the speleothem-climate signal. To address this, a cave monitoring program was launched in 2005 involving instrumentation of Golgotha Cave and monthly drip water collection. We present a summary of these findings during 2005-2013 from two contrasting high and low-flow drip sites, as well as our progress on building a high-resolution climate record spanning the last ~600 years

Calibrating climate-delta O-18 regression models for the interpretation of high-resolution speleothem delta O-18 time series.

Providing estimates of past climate changes on interannual - millenial timescales requires suitable regression models between climate and climate proxies. Many proxies appear to show relationships with climate that are timescale dependent. Any proxy-climate model should be able to replicate the major patterns that are observed at multiple timescales. Here we develop a new climate-isotope regression model for speleothems from a middle latitude site. In the low to middle latitudes, daily variation in precipitation isotopes (within individual months) is largely negatively correlated with daily rainfall amount. On interdecadal timescales, though, this relationship appears to be nonstationary. These two points provide a theoretical basis for a new climate-isotope regression model in which the slope and the intercept of a δ O-18(day) - P-day line for a given month are modulated by organized patterns of climate variability, such as the extratropical zonal waves (including the annular modes). In constructing this new regression model, we show how daily precipitation - δ O-18 relationships can be estimated using only monthly δ O-18 data and daily rainfall amounts. The new regression model provides a consistent picture of O-18 variability over a range of timescales, and this has not been the case with any previous climate-isotope regression model. © 2008, American Geophysical Unio

Trace elements: from sources to cave drip water, south-eastern Australia

Speleothem trace element time series are constructed from the infiltrating drip water geochemistry, and hence are examined under contrasting rainfall conditions associated with El Niño and La Niña phases of ENSO. The aim was to identify suitable inorganic element proxies for palaeoclimate interpretation in speleothem records from Harrie Wood Cave, Yarrangobilly. The drip water chemical composition at the stalactite tip reflects a contribution from different endmembers and processes; therefore it is necessary to study the different sources, pathways and processes that occur as water migrates through the atmosphere-soil-karst system. Here we present high resolution aerosol, rainfall and drip water 18O and inorganic drip-water datasets. Analysis of aerosol samples above the caves reveals the atmosphere supplies a suite of elements from automobile emissions, windblown soil, smoke, secondary sulfate and aged sea salt sources. The bedrock and aerosols were identified as contributory sources of solutes to the drip water. The clay-rich soil zone was recognised as a sink for inorganic elements, and a secondary source for Zn. In soil, a number of processes were demonstrated to modify the chemical composition of the resultant drip waters. The drip water chemistry is driven by the long-term gradient in the cumulative water balance. The flow paths feeding the drip sites were shown to be fracture flow, from a ventilated well-mixed pocket within the epikarst storage reservoir. Dilution and reduced prior calcite precipitation (PCP) controlled the drip-water chemistry during the La Niña/wet years whereas enhanced PCP was observed during the El Niño and dry periods. Mg and Sr show particular promise as paleoclimate proxies for drought and flood events, while with further research Na, K and Zn may also be reliably used. These findings will be applied in a modern speleothem record. © Author(s

Flow Classification and Cave Discharge Characteristics in Unsaturated Karst Formation

In this study we utilize the spatial array of automated cave drip monitoring in two large chambers of the Golgotha Cave, SW Australia, developed in Quaternary aeolianite (dune limestone), with the aim of understanding infiltration water movement via the relationships between infiltration, stalactite morphology and groundwater recharge. Mahmud et al. (2015) used the Terrestrial LiDAR measurements to analyze stalactite morphology and to characterize possible flow locations in this cave. Here we identify the stalactites feeding the drip loggers and classify each as matrix (soda straw or icicle), fracture or combined-flow. These morphology-based classifications are compared with flow characteristics from the drip logger time series and the discharge from each stalactite is calculated. The total estimated discharge from each area is compared with infiltration estimates to better understand flow from the surface to the cave ceilings of the studied areas.The drip discharge data agrees with the morphology-based flow classification in terms of flow and geometrical characteristics of cave ceiling stalactites. No significant relationships were observed between the drip logger discharge, skewness and coefficient of variation with overburden thickness, due to the possibility of potential vadose-zone storage volume and increasing complexity of the karst architecture. However, these properties can be used to characterize different flow categories. A correlation matrix demonstrates that similar flow categories are positively correlated, implying significant influence of spatial distribution. The infiltration water comes from a larger surface area, suggesting that infiltration is being focused to the studied ceiling areas of each chamber. Most of the ceiling in the cave site is dry, suggesting the possibility of capillary effects with water moving around the cave rather than passing through it.Reference:Mahmud et al. (2015), Terrestrial Lidar Survey and Morphological Analysis to Identify Infiltration Properties in the Tamala Limestone, Western Australia, IEEE JSTARS, DOI: 10.1109/JSTARS.2015.2451088, in Press

Sulphur: a proxy for wildfires in stalagmites

Bushfires are a global hazard that can have catastrophic impacts on communities and ecosystems. However, there is limited baseline data on how fire frequency and intensity have responded to climate change in the past. A speleothem-based bushfire record will allow us to determine long-term natural fire regimes and better understand the relationship between bushfires and climate. Recent research has demonstrated the potential of using sulphur as a speleothem paleofire proxy1 but a full characterisation of S in the karst environments of SW Western Australia is lacking. Here, we quantify the relative contributions of different sources of sulphur in a modern cave environment through the characterisation of rainfall, soil, bedrock, vegetation and cave drip water sulphate concentration and isotopic values (δ18O- SO4 and δ34S- SO4) to assess the role of sulphur cycling through the biomass in response to burn events at this site. This information will be used to interpret the SO4 isotope record, supported by other proxies in a 2-12 ka speleothem from SW Western Australia. This is the first speleothem SO4 isotope study in the southern hemisphere and the first in the world to use speleothem S isotopes in a paleofire context. © Author(s

Drip hydrology monitoring in caves to inform stalagmite palaeoclimate records, Yarrangobilly, NSW.

Palaeoclimate research using speleothems has significantly increased over the last decade, owing to their potential to provide multi-proxy high resolution (sub-annual) terrestrial records of past climate variability. A crucial step in using these archives as high resolution proxies is understanding the connectivity between the surface climate and the signal transferred to the speleothem. This study investigates the modern karst hydrology at Yarrangobilly Caves, in the Snowy Mountains NSW. A high-frequency, spatially-dense drip water monitoring campaign in Harrie Wood Cave, was conducted over a 13 month period to characterise the hydrology of 14 sites within the same cave. By utilising the cave as a natural observatory we can determine 1) vadose-zone flow regimes, and 2) thresholds of recharge at the site. Using a statistical approach (PCA and AHC) 5 main drip hydrological regimes were established. Depth was found to have a moderate relationship (r2 = 0.4) with discharge, whereby increasing depth was associated with a dampening of flow and drip response. However, depth could not account for all the variability observed in the drip hydrology, suggesting complex controls unrelated to depth, such as unsaturated zone storage and mixing, appear to have a significant impact on vadose-zone flow regimes. As a speleothem is a function of the infiltrating drip water, we suggest that stalagmites fed by different drip types may thus contain different parts of the climate record i.e. smoothed mean annual vs. an extreme event record. These findings will be used to assess three suitable stalagmites for palaeoclimate reconstruction, fed by drip waters with different hydrological regimes and the preliminary results presented here. © Australasian Quaternary Association Inc

What determines the calcium concentration of speleothem-forming drip waters?

Cave drip water calcium ion concentration is a primary determinant of speleothem deposition and growth rate. The factors that determine drip water calcium ion concentrations are the soil and vadose zone CO2 concentrations, and the hydrogeochemical evolution of the water from soil to cave. Here, we use a systematic literature review of cave drip water calcium concentrations, combined with PHREEQC equilibrium modelling, to investigate the global relationship between calcium concentration and surface climate. Our results are discussed in the context of understanding the climatic and environmental controls on drip water calcium concentration, speleothem growth rates and proxies of past climate and environmental change. We use an empirical, global soil CO2 concentration–temperature relationship to derive PHREEQC modelled cave drip water calcium concentrations. The global mean modelled drip water calcium concentration is close to that observed, but it over-predicts at high and low temperatures, and significantly under-predicts at temperate conditions. We hypothesise that closed system hydrochemical evolution due to water saturation is an important control on carbonate dissolution at colder temperatures. Under warmer conditions, for example temperate climates with a dry and hot or warm summer, seasonally-limited water availability can lead to: < 100% soil cover; water-limitations on microbial and root respiration; wildfire; and prior calcite precipitation, all of which limit drip water calcium concentrations. In temperate climates with no dry season, higher CO2 concentrations than modelled from soil values are necessary to explain the observed drip water calcium values, which we propose is from an additional source of CO2 from microbial activity and root respiration in the vadose zone during open system hydrochemical evolution