Marine radiocarbon reservoir age variation in Donax obesulus shells from northern Peru: Late Holocene evidence for extended El Nino

For at least 6 m.y., El Nino events have posed the greatest environmental risk on the Peruvian coast. A better understanding of El Nino is essential for predicting future risk and growth in this tropical desert. To achieve this we analyzed archaeological and modern pre-bomb shells from the surf clam Donax for the radiocarbon reservoir effect (Delta R) to characterize late Holocene coastal upwelling conditions in northern Peru (8 degrees 14'S). Mean Delta R values from these shells suggest that modern upwelling conditions in this region were likely established between A.D. 539 and A.D. 1578. Our radiocarbon data suggest that upwelling conditions ca. A.D. 539 were less intense than those in modern times. The observed coastal water enrichment in C-14 may be consequence of frequent strong El Nino events or extended El Nino-like conditions. These Delta R-inferred marine conditions are in agreement with proposed extended El Nino activity in proxy and archaeological records of ca. A.D. 475-530. Extended El Nino conditions have been linked to political destabilization, societal transformation, and collapse of the Moche civilization in northern Peru. A return to such conditions would have significant impacts on the dense population of this region today and in the near future.