Hydrothermal variation in several deep caves of the Nullarbor Karst Plain, Australia

Abstract

The Nullarbor karst plain in Australia contains many large caves along its southern coastal margin, 14 of which penetrate the regional groundwater basin near sea level to depths >10m. Since the mid-Miocene the Nullarbor has largely had an arid to semi-arid climate. Heat exchange between the atmosphere, the caves and their initial lake surfaces varies between caves.

In the west, divers have recorded water temperatures within the submerged passage systems of a close group of four caves (Cocklebiddy, Murra-El-Elevyn, Pannikin Plains and Tommy Grahams Caves). These water temperatures vary both within and between caves (range 18-24C). Similar effects have been noted in Warbla Cave ~350kms to the east and, in the same vicinity, in Weebubbie Cave.

Specifically, in stable conditions (no recent floods) Cocklebiddy Cave water temperature rises northwards from 18C in the entry lake to 24C through 6.5km of flooded passage. In Tommy Graham’s Cave temperature remains a consistent 23.5C thoughout. In Murra El Elevyn temperature rises from 19.1C in the entry lake to 20.5C in a narrow passage 300m inside the system where warmer water appears to enter the cave through the floor. Divers can ‘feel’ this water and a thermocline is clearly visible. In Weebubbie Cave the majority of the system is a consistent 18.6C with a distinctly bounded warm zone of 19.4C identified.

Though point sources have not been located in all caves, warm water may be entering these caves through their floors in zones which may be geomorphically identifiable via current investigations coupled with water chemistry analysis. This may have significant implications for a possible geothermal inception at depth of large deep Nullarbor caves, in two distinct nodes in the western and eastern ends of the karstfield. Research continues into these phenomena.