Unsaturated zone hydrology and implications for paleo-climate speleothem reconstructions

Abstract

Speleothem growth relies on the supply of water which percolates from the surface, through the unsaturated zone and discharges into cavernous voids. The flow path of water feeding individual speleothems varies considerably depending on the karst architecture e.g. micro-fractures, solution pipes, structural voids in the karst, storage reservoirs, etc., all of which may alter the composition of drip waters over the flow route. By monitoring drip waters, we can determine: 1) unsaturated zone flow regimes; 2) connectivity between the surface and cave discharge zone; and 3) thresholds for groundwater recharge. This information can be used to identify suitable speleothems in caves for reconstruction of past climatic and hydrologic variability, at least over the last few thousand years of similar mean climate state. High-frequency, spatially-dense monitoring was conducted in Harrie Wood Cave, Yarrangobilly, Snowy Mountains over a 15 month period to characterise the flow regimes at 14 sites along a depth profile within the cave. Sites were monitored using acoustic drip loggers (stalagmates®). Discharge rates and response to significant rainfall events were highly variable between sites. A moderate relationship was found between decreasing discharge rates and increasing depth (r2 = 0.40). We suggest unsaturated zone storage and mixing, unrelated to depth, also have a significant impact on flow regimes. Using a statistical approach, five different drip types, which often had no spatial commonality, were identified. This information was used to inform the choice of speleothems for paleo-climate reconstruction, using stalagmites with differing hydrological regimes feeding growth, of which the preliminary data 18 will be presented here. The study highlights the need to understand unsaturated zone hydrology at the individual drip discharge level, prior to any speleothem study for paleo-climate, to truly appreciate the drip water signal it is recording. Copyright (C) The Authors