Trace elements in bivalve shells from the Rio Cruces, Chile

In May 1960, the largest recorded earthquake in the earth's history struck southern Chile, and lowered the course of the Rio Cruces by approximately 2 m. This created a wetland, which was colonised by the waterweed Egeria densa and, subsequently, by large populations of the black- necked swan, which fed on the weed. Reported catastrophic declines in the weed and swan popula- tions in 2004 coincided with the opening of a large cellulose plant upstream, leading to popular and scientific condemnation of the plant. In 2008, samples of the freshwater bivalve Diplodon chilensis were retrieved from the bed of the Rio Cruces at several locations downstream of the plant, and the growth patterns of the bivalves were studied in thin section. Trace element profiles of bivalves were determined by laser ablation inductively coupled plasma mass spectrometry (LA-ICP/MS). Some of the bivalves had ages of almost 50 yr, indicating that they colonised the drowned riverbed less than a decade after the earthquake. There were no growth recessions or abnormalities in the shells corre- sponding to 2004, the year the plant opened. Patterns of trace elements in the shells describe aspects of the evolution of the watershed, track annual cycles in rainfall and indicate patterns of soil erosion and/or sediment input. Sr, Ba and Mn have strong annual cyclicity; Ba and Mn are viewed as indica- tors of soil erosion. Peaks in Ba were large immediately after the earthquake, whereas Ba peaks in the last few decades were of much smaller amplitude. The changes in Ba patterns probably indicate changes in patterns of soil erosion: high erosion rates immediately after the earthquake, slowing as the basin fills. Cu appears as annual low-level peaks in abundance in about 1975, which continue to the present. This may be the result of spraying of fungicides in the watershed, which is heavily used by agriculture. Similarly, scattered peaks in As around 1980 may indicate the use of pesticides. There is no evidence in the bivalve shells of a chemical spill in 2004.