By Kelsie

Hi all! I am part of the small but unique Archaeogeochemistry group here at the Research School of Earth Sciences (RSES).

We are focused on bridging the gap between analytical sciences and archaeological research. For my part I completed my undergraduate degree in archaeology and then for my honours project I moved into the realms of palaeoenvironmental reconstruction at the archaeologically and geologically significant site of Lake Mungo. I am continuing this work for my PhD. In the months to come I want to rope in some of the other archaeogeochemists to tell you about the cool and interesting stuff they are doing but for this post I will introduce you to Lake Mungo and the wonderful realm of fish otolith geochemistry.

Lake Mungo is one of seven now dry lakes that together make up the Willandra lakes world heritage area situated in south western, New South Wales. These lakes once formed the major overflow outlet for the Lachlan River and are enormous, in total covering some 240,000 hectares! I visited Lake Mungo for a couple of days in 2012 and again in 2014 and can attest to the areas almost ethereal beauty. As you walk along the ancient lakes’ shorelines, giant lunette outcrops tower over you. Exposed over time by wind and water, these structures provide not only welcome shade on the rather barren dunes, but also evidence for cyclic lake level fluctuations stretching back over the last 100,000 years.

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A view across Lake Mungo from the lunettes, two humans for scale (extra points if you can spot them).

In the 1970s human remains discovered eroding out of Mungo’s shoreline lunettes pushed dates for human occupation of Australia to beyond 20,000 years BP, radically changing our understanding of Australian pre-history at the time. Since then the development of new dating technologies and the discovery of further human remains have extended the timing of human occupation to beyond 40,000 years BP (not very old in a geological sense but for Australian archaeologists this was pretty epic).  Other evidence for human occupation found in the lunettes includes stone tools and fish bone fireplaces.

The lakeside lunettes of Lake Mungo are the focus of ongoing excavations and geological mapping by Nicola Stern and colleagues at the La Trobe University in Melbourne. Evidence for human occupation has turned up during some periods but not others. Why? I want to build up more information about what the lakes were like when humans were visiting them thousands of years ago and explore why certain lake conditions were more attractive than others. To this end I am assessing how the geochemistry of fish otoliths can be used as recorders of ambient water conditions.

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Excavations at one of the fish bone fireplaces at Lake Mungo (or maybe an area with stone tools, this was taken in 2012 and I can’t quite remember)

Otoliths are structures that form within the inner ear of bony fish by the incremental deposition of calcium carbonate forming annual rings similar to those of trees. As calcium carbonate is laid down elements from the surrounding water, some with distinctive isotopic compositions, become incorporated into the otolith matrix. By cutting transversely through fish otoliths their age rings can be exposed. Then we can analyse their unique elemental and isotopic compositions  using high resolution in situ techniques, such as the Sensitive High resolution Ion Microprobe (SHRIMP) here at the RSES.

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Clockwise from top left: an otolith and some burnt fish bones at Lake Mungo (the scale bar has 1 cm increments); two otoliths from Mungo encased in resin; a modern otolith in resin sectioned to expose annual rings; a modern otolith thin section showing those beautiful annual rings.

At the moment I am determining how the geochemistry of modern fish otoliths record water composition change, such as during evaporation or flooding, and whether or not we can pick up a temperature signal. Once I have established this I will then apply the same techniques to  ancient otoliths from Lake Mungo. Preliminary results from the modern fish studies show that large changes in the ambient water chemistry during the fish’s life, such as during evaporation or flooding, can be easily identified  but temperature effects are not as easily discernible.

I will leave you with the following comic because it made me laugh and now I wish we had interdepartmental pun-offs!

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Archaeologists vs Geologists in a pun-off