Fish otolith microchemistry: Snapshots of lake conditions during early human occupation of Lake Mungo, Australia

The δ¹⁸O, Strontium/Calcium and Barium/Calcium values recorded in golden perch otoliths collected from two evaporative lakes, modern Lake Hope and ancient Lake Mungo, have been used to reconstruct changes in water composition and environmental conditions during the life of the fish. Lake Hope was filled by floodwaters in 1989 and 1990, then a period of lake drying was followed by a natural fish death event in 1994. Otoliths from these fish have δ¹⁸O profiles reflecting the earlier floods, and the progressive evaporation of the lake. Sr/Ca ratios start to follow the δ¹⁸O trend only after evaporation is well advanced, probably after the fish became stressed. Otoliths from a period of early human occupation at Lake Mungo, ¹⁴C age range ca. 37–42 cal kBP, record a different history. Most otoliths show a relatively stable δ¹⁸O profile throughout the life of each fish, with no evidence of significant lake flooding or drying. Sr/Ca ratios are similarly stable, indicating that over a period of ca. 5 ka evaporation and inflow remained in relative balance. All the otoliths have high Ba/Ca ratios during the early years of the fish, likely a juvenile biological effect in common. The Mungo otoliths differ, in also showing a rise in Ba/Ca ratios in the outermost layers, as yet unexplained. One Mungo otolith, ¹⁴C dated at ca. 19.3 cal kBP, does show evaporation and stress trends in δ¹⁸O and Sr/Ca ratios respectively, consistent with other evidence that Lake Mungo was subject to frequent drying at that time.