Dating the paleobeaches of Pampa Mejillones, northern Chile, by cosmogenic chlorine-36

Abstract

Cosmogenic 36Cl accumulated in shells, gravel, and boulders was used to date Pleistocene beach ridges deposited on the coastal plain of Pampa Mejillones in northern Chile. First, a new method for processing rock samples and making AgCl targets for analysis of 36CI was developed. Small (1-5 g) carbonate and silicate samples were digested in the presence of CI carrier (spike) in a pressure bomb and AMS targets were prepared. The results from three replicates of each rock sample were compared to AMS results from unspiked samples. The three carbonate and three silicate samples gave 36Cl/ CI results that, with one exception, are within one standard deviation from the un spiked values. These results demonstrated the feasibility of the new sample preparation technique. Preparation of shell material for analysis of 36Cl required assessment and removal of any seawater Cl taken up by the shell after its deposition. A leaching experiment was performed to determine the behavior of Cl in shell carbonates. The results show that 36Cl is removed during leaching, and suggest that the maximum age is closest to the true exposure age of the sample. Chlorine-36 ages on shells from five distinct paleoshorelines are 50±2 ky, 126±5 ky, 201±12 ky, 226±10 ky and 258±9 kyo Whereas these ages are much younger than the ages previously assigned to these paleobeach sequences, each of the 36Cl ages corresponds to a sea-level high stand in SPECMAP and other records of global sea levels. The results indicate either that 36Cl ages underestimate the true paleobeach ages, or that the previous chronologies need revisions.