Peruvian mollusk shells as multi-proxy archives: late Holocene upwelling variation and El Niño-induced biomineralization effects on trace elements

The purpose of this research is to characterize Peruvian upwelling during the late Holocene (last 2000 years) using molluscan proxies. Peruvian upwelling is a key component of El Niño/Southern Oscillation (ENSO) cycle, an important factor introducing interannual variability to Earth's weather. Thus by studying Peruvian paleoupwelling a better picture of past ENSO conditions can be inferred. High resolution sampling for radiocarbon and stable oxygen isotopes in modern pre-bomb Donax obesulus and Protothaca asperrima shells revealed sub-seasonal variations in Peruvian upwelling. Based on the shells' radiocarbon data a new reservoir effect correction (ΔR) was calculated for the Peruvian coast. ΔR, the radiocarbon age difference between global and local marine reservoirs, is also a qualitative proxy for deep water upwelling. A Trachycardium procerum shell that survived the 1982-1983 El Niño revealed that biomineralization changes induced by this event likely affected trace element incorporation into molluscan aragonite. Detected variations in mollusk biomineralization linked to El Niño suggest the need for coupled structural and chemical analyses in environmental proxy studies. Comparison between modern pre-bomb and archaeological ΔR obtained from D. obesulus shells revealed similar upwelling rates in northern Peru for the 20th and 16th centuries and lower rates for the 6th century. Low upwelling rates in northern Peru in the 6th century are in agreement with reported Mega- El Niño events that contributed to the political decline of Moche society.