Golden perch (Macquaria ambigua) otolith microchemistry: modern validations and ancient applications

Abstract

Inland archaeological sites in the Australian arid zone contain few records of past environments. For those archives that do exist, such as sedimentary records, it can be difficult to associate the environmental conditions that they record directly with the time scales of human occupation. At the world heritage site of Lake Mungo, in north western New South Wales, lake shore dunes preserve a record of human occupation, and of alternating phases of wet and dry conditions in the adjacent lake. These two records provide a promising opportunity to generate commensurate behavioural and palaeoenvironmental information. As further surveying of the lunettes is completed and a more detailed and robust chronology using direct Optically Stimulated Luminescence (OSL) dating of the lunettes is constructed, a fuller more nuanced picture of changes in lake level, human occupation and climate will emerge. By finding new samples, new materials and new methods of analysis the chronology of human occupation and lake level changes at Lake Mungo and other Quaternary sites will become more detailed. This study investigates the potential of golden perch otoliths, which are found throughout the shoreline dunes of Lake Mungo, for providing additional detail about lake level fluctuations and general environmental conditions. Fish otoliths are bone-like structures that form in the inner ears of bony fish. They develop by the incremental deposition of calcium carbonate onto an organic matrix, forming annual growth rings. As otolith grow they take up and preserve a record of the trace element and isotopic composition of the ambient water. Some of these chemical markers are affected by changes in water level and temperature. This study analyses the δ18OCaCO3 values and trace element (Sr/Ca and Ba/Ca ratios) composition across the age increments of golden perch otoliths. The δ18OCaCO3 values of modern golden perch from tanks of known δ18OH2O values and temperature conditions were used to validate the assumption that golden perch otoliths form in isotopic equilibrium with the ambient water. Further analyses of modern otoliths from river populations of golden perch and from populations who died in an evaporating lake were examined to determine if known flooding and drying events were preserved in their microchemistry. The same analytical methods were applied to a collection of ancient otoliths excavated from the shorelines of Lake Mungo in the 1970s to investigate changes in water conditions (flooding and drying events) through time. These ancient otoliths were also radiocarbon dated to establish a more detailed chronology of the site. This study also investigates how mass balance models and ancient otolith δ18OCaCO3 values can be used to test scenarios of lake level change at Lake Mungo.