ESR Analyses with Barnacles: A New Method for Dating Shell Middens and Sealevel Changes

Barnacles have never been dated by electron spin resonance (ESR) before. Barnacles occur in several Paleolithic archaeological sites, including those at the Pinnacle Point cave complex, South Africa. Live barnacles occur mainly in the intertidal zone close to sealevel, but die when exposed. Thus, dating barnacles in life position dates past sealevel changes.

Electron spin resonance (ESR) can date aragonite molluscs ranging from 5 ka to 500 ka. Using a protocol similar to that used for thin molluscs, surface sediment was removed from the barnacles sonicating in 0.1M acetic acid. After gently crushing the shells, they were etched in 0.1M acetic acid to remove the outer 20 μm. Shells were powdered to 100-200 mesh and resonicated in 0.1M acetic acid. Samples were irradiated with ⁶⁰Co γ to added doses of 1 to 100 krads, the annealed for 15 days. Samples were analyzed using an X band ESR spectrometer at room temperature.

Six barnacle samples from Norridgewock, Maine, and Khyex River, British Columbia, were tested for suitability for ESR dating. Due to Mn²⁺ interference, the Maine barnacles were not datable by ESR. Since the two barnacles from BC had no Mn²⁺ interference, they yielded ESR ages of 13.0 ± 1.0 ka and 14.0 ± 1.9 ka assuming linear U uptake. These ages correlate well with ¹⁴C dates on wood associated with the glaciomarine sediment (Blais-Stevens, 2006). SEM EDX performed on one giant barnacle, Balanus nubilus, from BC indicated that it contained insignificant Mn concentration. Although stability tests to calculate the mean dating signal lifetime and more ESR calibration tests against other barnacles of known age are needed to ensure the method's accuracy, ESR can indeed date Balanus, and thus, date sealevel changes or shell middens containing barnacles.