Steele Island Midden Damage
Climate change is having catastrophic consequences for the survival of important elements of Tasmania’s Aboriginal heritage. Already, sea level rise is leading to increased wave action. This, in turn, is threatening our coastal rock art and undermining thousand year old shell middens and washing them into the sea.
All these changes are rapidly altering an environment that has been relatively stable for the past 6000 years. Ancient sites that have been established upon that stable land surface are now being destroyed. Tasmania’s Aboriginal heritage is unique, non-renewable and of world significance. Much of it is going to be lost to erosion in the next 30-50 years.
Some reports suggest that sea level is predicted to rise 10-26 cm by 2050 and by up to 88 cm by the end of the century (Solomon et al. 2007).
But a simple rise in sea level is not the sole problem affecting Aboriginal heritage. Seas coming ashore break and run up the beach. Breaking waves, especially at High Water Mark, erode the toe of the foreshore, undermine it and cause successive slumping. It is in this area, where foreshore meets waves, that many of Tasmania’s important Aboriginal coastal sites are situated.
The geometry of the beach and the effects of breaking waves create a magnifying effect encapsulated in the ‘Bruun Rule’. The Bruun Rule suggests that for every one centimetre rise in sea level the foreshore will erode 50-100 cms inland. In other words, it can be expected that sandy foreshores, could retreat a
further 5-26 metres by 2050 and 80 metres by the end of the century.
The effects of the Bruun Rule on our coasts are further exacerbated by other phenomenon associated with climate change, including:
Midden blow-out, West Coast
· increased frequencies of periods of lower barometric pressure (which cause the sea to rise locally by 1 cm for every hectopascal fall in pressure),
· increased wave heights,
· increased wind speeds, and
· greater frequency of storm surges.
All these, multiplied by an increased ‘return rate’ - the rate of occurrence of these extreme phenomena - critically magnify the impacts of the sea on our foreshore heritage.
Rock art is particularly vulnerable as it is often situated on the coast and has frequently been executed on very friable rock and often faces westerly winds. Preminghana for example, a significant Aboriginal rock art site on the West Coast, was created on calcarenite, which is almost as friable as sugar cake, and continues to be under threat. Rock markings have now been found that are commonly submerged by the sea and are protected only be seaweed.
Once stable coastlines, now under active erosion, are commonplace around the State.
On a beach in Southwest Tasmania, old established trees have recently been undercut by water action as a result of recent changes in the high water regime. Along this eroding coastline are five rock art sites under threat as well as shell middens which are actively disappearing.
Oyster middens at the Hazards Beach in Freycinet Peninsula have been eroding for years. Middens on the South West Coast are under threat.
Midden erosion, Hazards Beach
Aboriginal quarries and their related stone tool making workshops, often containing thousands if not millions of artefacts, are now found out of context, as eroded lag deposits littering beaches. These quarries and associated workshops are rare and important sites. As sources for the stone tools distributed across the land, they form a basis for understanding Aboriginal occupation, spatial patterning, dating, technology, economy, and transport – information that is quickly becoming lost to us.
Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.) 2007.
Climate Change 2007: The Physical Science Basis, Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007, Cambridge University Press: Cambridge, United Kingdom and New York, NY, USA.
For further reading see: