Earth Sciences of Lake Gnotuk & Lake Bullenmerri

The following information is taken from the 2001 'Lake Gnotuk & Bullenmerri Planning Stratgey Discussion Paper'

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The paper was prepared for presenation to the Corangamite Shire Council by Dr, Roger Jones, Senior Research Scientist, CSIRO Atmospheric Research, supported by significant representation from the scientific community.


Geological Features
Lake Gnotuk and Lake Bullenmerri are twin maars of the Newer Volcanic Province of southeastern Australia. They are amongst the latest eruptions of the province and are notable landforms within western Victoria. They have been classified by the Geological Society of Australia as being of international significance as geological features and on the basis of their historical geology. In another review based on their volcanic activity, state of preservation and geomorphology Rosengren (1994) classified Bullenmerri as internationally significant and Gnotuk as nationally significant.

Geochemistry
Geochemistry is the chemistry of geology and is used to understand the dynamics of geology and Earth systems over long time scales. The Lake Bullenmerri maar is notable because of inclusions of mantle material (xenoliths) with the pyroclastic material ejected by the maar eruption. This material, from 50-75+ km deep within the upper mantle can tell us a great deal about how mantle/crust processes shape the Earth. Ultramafic inclusions have been examined to diagnose how the mantle effects volcanism and to derive clues into the geological evolution of southeastern Australia (see Griffin et al.; Matsumoto et al.; O'Neill et al.)

Biogeochemistry
Biogeochemistry studies the chemical remains of past biological processes that have been preserved in geological material. Biochemical investigations rely on comparing current activities in uncontaminated environments with the record of past biochemistry to diagnose changes. Site contamination through altered chemical fluxes (e.g. fertilisers, sediments) can compromise such investigations. Strontium/calcium (Sr/Ca) ratios from ostracods (microcrustaceans) preserved in lake sediments have been used to diagnose past water salinities (e.g. Chivas et al.). Much of the carbonate formed around spring discharge near the lake shore also has a biological origin (Dodson, 1974).

Limnology and Palaeolimnology
Limnology is the study of freshwater, saline, and brackish environments of lakes. In some cases, the history of the water body itself is important, but typically information is used in a wider geographical and ecological context. Both lakes have been extensively studied (e.g. Timms; De Deckker) with data on aquatic species contributing to global databases used to map environmental change. Palaeomagnetic information from lake sediments has also been used to assess historical changes in the regional expression of the Earth's magnetic field (Barton et al.).

Palaeontology
Fossil and subfossil of both extinct and extant species are found around the lakes. Lake Bullenmerri is the type location for the extinct wombat, Vombatus pliocenus McCoy. Well-preserved tree-trunks and roots exposed around Lakes Gnotuk and Bullenmerri have been dated to about 2000 years old, but unfortunately currently are usually just burnt or left to rot after exposure. Older macrofossil remains of bones, leaves and bark identifiable to the species level, can be found in lake sediments. Fish, insect remains and microfossils can also be found in lake sediments (e.g. De Deckker, 1982).

Palynology
Palynology is the study of preserved pollen, which occurs in the sediments of both lakes. Palynological studies of both lakes have allowed an unbroken vegetation history of the region to 16,000 years before present to be constructed (e.g. Yezdani, 1970; Dodson, 1979). Together with historical studies, this allows a highly detailed history of land-use change in western Victoria to be constructed.

Climatology
The past climate of western Victoria has been reconstructed from palynology, biogeochemistry, sedimentology and lake water-balance modelling, providing a high-resolution record of past climate dating to 16,000 years before present (Jones et al., 1998). This has been matched with historical climate, to provide a very long-term baseline from which to measure the impact of climate change (Jones et al., 2001). This is one of the most detailed records gained from lakes anywhere in the world.

Hydrology
The climatology of the crater lakes is understood through its hydrology. One of the most difficult problems in Australian hydrology is separating out the effects of past land-use change and of climate on hydrological response. Historical clearing and subsequent use of the catchments for grazing appears not to have altered the lakes' hydrology. More intensive land-use must be avoided, as it may compromise what is, on the national scale, a very important attribute.

Education
The crater lakes are a very important site for education in the Earth sciences and in environmental education generally. A number of tertiary institutions visit the lakes annually, and there is a small but constant supply of international researchers and one-off field trips visiting the lakes. Given the amount of development around the craters to date, it is becoming more and more embarrassing to try and reconcile this development and the lakes' scientific importance to the international visitor.