Hot rocks add heat to ice sheet models

An outcrop of Cambrian-aged granite at Sandefjiord Bay.
Cambrian-aged granites, such as this outcrop near Landing Bluff, Sandefjiord Bay, contain radioactive heat producing elements which contribute to regionally elevated sub-glacial heat flow in East Antarctica. (Photo: Edward Grew)
Detail of a Cambrian-aged granite in Antarctica. The Cambrian period lasted from about 542 to 488 million years ago.  Heat flow modelling diagram of a 275 km section of the Prydz Bay coastline. In figure ‘a’ the ellipses along the transect line show the location and extent of known high heat-producing Cambrian granites, with the average heat production for each outcrop in µW per cubic metre. Also shown is the possible sub-glacial extent of ‘hot’ granites inferred from geomagnetic surveys, the East Antarctic ice sheet, glaciers and floating ice shelves.Rocks in the Rauer Group of islands in northern Prydz Bay.

Incorporating the distribution of ‘hot rocks’ in East Antarctica into ice sheet models, will improve predictions of ice sheet behaviour and potential sea-level change, according to new Australian research.

Speaking at the ‘Strategic Science in Antarctica’ conference in June, Dr Chris Carson from Geoscience Australia, said naturally occurring ‘heat-producing elements’ – mainly uranium, thorium and potassium – present in certain rock types found in Antarctica, contribute to local and regional-scale variation in heat flow underneath the ice sheet. They do this by generating tiny amounts of heat by radioactive decay.

‘These regions of elevated heat flow potentially can contribute to ice surging and ice stream flow,’ Dr Carson said.

Sub-glacial heat flow under the West Antarctic ice sheet has been measured at a number of sites and found to be elevated due to active rifting and volcanism. However, crustal heat flow beneath the East Antarctic ice sheet is poorly understood, and instead, a broadly uniform heat flow across much of the region is often assumed in ice sheet models. Such assumptions ignore the natural variability of heat flow due to variations in the sub-glacial geology.

To illustrate the scale and importance of this variability, Dr Carson and colleagues from the Australian Antarctic Division, Antarctic Climate and Ecosystems Cooperative Research Centre, University of Melbourne and the University of Texas, recently published a paper in the Journal of the Geological Society, London, describing the distribution of hot rocks, and their impact on regional heat flow, in different parts of the Australian Antarctic Territory.

In a 275 km transect along the Prydz Bay coastline – running from the Vestfold Hills to the Amery Ice Shelf – heat production values for individual rock types (derived from geochemical analysis of the rocks) ranged from 0.02 µW per cubic metre to almost 66 µW per cubic metre (1 µW [micro Watt] is 0.000001 Watts).

‘Rocks of the northern Prydz Bay region – the Vestfold Hills and Rauer Group – have generally low heat production,’ Dr Carson said.

‘However, in southern Prydz Bay, there are numerous outcrops of Cambrian-aged granites which are characterised by elevated heat production. The presence of these hot rocks fundamentally affects the regional heat flow in the region.’

Available aeromagnetic data suggests that these hot granites may be more widespread underneath the ice sheet in southern Prydz Bay.

Dr Carson said a better knowledge of the sub-glacial location and distribution of such granites across East Antarctica is essential for understanding regional heat flow characteristics of the Antarctic crust. This information can then be factored into ice modelling studies.

‘The assumption that the crust of East Antarctica is thermally homogenous is inappropriate and it is critical that both local and regional geology are considered in ice modelling studies,’ he said.

‘As elevated and variable heat flow would have a fundamental effect on ice sheet behaviour, incorporating geological controls on heat flow into models could refine predictions of ice mass balance and sea-level change.’

Wendy Pyper
Australian Antarctic Division