Trachycardium procerum valves from coastal Peru were measured through ontogeny for radiocarbon via accelerator mass spectrometry (AMS). A ~4.5 year old specimen collected in 1984 near Casma, Peru (~9° 30'S) grew before and during the 1982-83 El Niño/southern oscillation (ENSO) warm event and was sampled at 6-8 week temporal resolution. The shell morphology recorded El Niño warming as a shallow growth break with subsequent alteration of aragonite crystal microstructure. Therefore, the region of the shell that grew during El Niño is recognized as a change in valve curvature. The presence of this growth pattern ensured that shell material was precipitated during the warm event and that each AMS sample could be independently identified to represent a defined period in the ENSO cycle.

Samples taken from portions of the shell precipitated prior to the El Niño warm event (before the diagnostic growth break) had a mean value of 1.0‰, with a maximum seasonal range of 1.8‰. During the warm event (after the growth break) there was an abrupt increase to 11.1‰. Aragonite precipitated near the margin of the valve had radiocarbon values approaching those present before the growth break.

We attribute this radiocarbon distribution to variations in vertical mixing of bicarbonate from modern surface waters and more ancient upwelled water. As an El Niño event begins and the thermocline deepens, less vertical mixing occurs. Thus, radiocarbon values in shell precipitated during El Niño appear younger (more positive) relative to non-El Niño periods, which represent periods of more intense upwelling. The results from the modern specimen validate the use of molluscan radiocarbon as a proxy of upwelling conditions related to ENSO and suggest its utility in both physical oceanography and paleoclimatology.

A T. procerum valve dating to 5830±90 uncalibrated yrs. BP. (based on stratigraphically bracketed charcoal samples), excavated from the archaeological site of Ostra, was sampled using the same methods as the modern valve to illustrate a potential application of the proxy. This sample's combined radiocarbon and oxygen isotope profiles permit evaluation of predicted variation in mid-Holocene changes in ENSO-related upwelling intensity. These preliminary data suggest that upwelling off northern Peru may have intensified after 5000 yrs. BP.