Caves demonstrate decrease in rainfall recharge of southwest Australian groundwater is unprecedented for the last 800 years

Billions of people worldwide rely on groundwater. As rainfall in many regions in the future is projected to decrease, it is critical to understand the impacts of climate change on groundwater recharge. The groundwater recharge response to a sustained decrease in rainfall across southwest Australia that began in the late 1960s was examined in seven modern speleothems and drip waters from four caves. These show a pronounced increase or uptick in regional drip water and speleothem oxygen isotopic composition (δ¹⁸O) that is not driven by a change in rainfall δ¹⁸O values, but is in response to the shallow karst aquifers becoming disconnected from rainfall recharge due to regional drying. Our findings imply that rainfall recharge to groundwater may no longer be reliably occurring in this region, which is highly dependent on groundwater resources. Examination of the longer speleothem record shows that this situation is unprecedented over the last 800 years.Stacey C. Priestley, Pauline C. Treble, Alan D. Griffiths, Andy Baker, Nerilie J. Abram, Karina T. Meredit

Estimation of deep infiltration in unsaturated limestone environments using cave lidar and drip count data

Limestone aeolianites constitute karstic aquifers covering much of the western and southern Australian coastal fringe. They are a key groundwater resource for a range of industries such as winery and tourism, and provide important ecosystem services such as habitat for stygofauna. Moreover, recharge estimation is important for understanding the water cycle, for contaminant transport, for water management, and for stalagmite-based paleoclimate reconstructions. Caves offer a natural inception point to observe both the long-term groundwater recharge and the preferential movement of water through the unsaturated zone of such limestone. With the availability of automated drip rate logging systems and remote sensing techniques, it is now possible to deploy the combination of these methods for larger-scale studies of infiltration processes within a cave. In this study, we utilize a spatial survey of automated cave drip monitoring in two large chambers of Golgotha Cave, south-western Western Australia (SWWA), with the aim of better understanding infiltration water movement and the relationship between infiltration, stalactite morphology, and unsaturated zone recharge. By applying morphological analysis of ceiling features from Terrestrial LiDAR (T-LiDAR) data, coupled with drip time series and climate data from 2012 to 2014, we demonstrate the nature of the relationships between infiltration through fractures in the limestone and unsaturated zone recharge. Similarities between drip rate time series are interpreted in terms of flow patterns, cave chamber morphology, and lithology. Moreover, we develop a new technique to estimate recharge in large-scale caves, engaging flow classification to determine the cave ceiling area covered by each flow category and drip data for the entire observation period, to calculate the total volume of cave discharge. This new technique can be applied to other cave sites to identify highly focussed areas of recharge and can help to better estimate the total recharge volume

Ubiquitous karst hydrological control on speleothem oxygen isotope variability in a global study

Speleothem oxygen isotopic (δ¹⁸O) records are used to reconstruct past hydroclimate yet records from the same cave do not always replicate. We use a global database of speleothem δ18O to quantify the replicability of records to show that disagreement is common worldwide, occurs across timescales and is unrelated to climate, depth or lithology. Our global analysis demonstrates that within-cave differences in mean speleothem δ¹⁸O values are consistent with those of dripwater, supporting a ubiquitous influence of flowpaths. We present a case study of four new stalagmite records from Golgotha Cave, southwest Australia, where the isotopic differences between them are informed by cave monitoring. It is demonstrated that karst hydrology is a major driver of within-cave speleothem and dripwater δ¹⁸O variability, primarily due to the influence of fractures on flowpaths. Applying our understanding of water movement through fractures assists in quantitative reconstruction of past climate variability from speleothem δ¹⁸O records.Pauline C. Treble, Andy Baker, Nerilie J. Abram, John C. Hellstrom, Jagoda Crawford, Michael K. Gagan, Andrea Borsato, Alan D. Griffiths, Petra Bajo, Monika Markowska, Stacey C. Priestley, Stuart Hankin, David Paterso