MULTIPLE GEOCHRONOMETER STUDY OF A HIGH-ELEVATION PLEISTOCENE MARINE TERRACE IN NORTHERN CHILE – IMPLICATIONS FOR COASTAL UPLIFT RATES

Dating of Chilean marine terrace sequences and determination of coastal uplift rates have been hampered by a nearly complete absence of coral in Quaternary terrace deposits. The presence of solitary coral (Sphenotrochus sp.) in shell-bearing deposits on a ~125 meter elevation early or middle Pleistocene terrace at Morro de Copiapo (27°10’S) provides an opportunity to use multiple geochronometers to constrain terrace age and to provide a long-term uplift rate. Dating of the terrace also allows an assessment of paleontologically based dating of terrace deposits, previously used to infer terrace age and uplift rate.

The Morro de Copiapo terrace contains a warm-water fauna (Marquardt et al., 2004), interpreted to indicate an MIS 11 (~425-375 ka) age. However, a combination of coral uranium-series ages, coral and mollusc Sr-isotope values, and mollusc amino acid ratios suggests an older age. Most of the six U-series analyses of corals that were conducted yielded apparent ²³⁰Th/²³⁴U ages older than MIS 11, but all but two analyses gave back-calculated initial ²³⁴U/²³⁸U values lower than modern seawater. This is an uncommon situation, as the reverse is more typical, with higher initial ²³⁴U/²³⁸U values. Because it is uncertain how to interpret the lower ²³⁴U/²³⁸U values, our age estimates rely primarily on Sr-isotope ratios and amino acid ratios. The mean ⁸⁷Sr/⁸⁶Sr value for five of the same six and two additional corals is 0.709121±0.000006, indicating an age of 1136-1377 ka (2σ) using the calibration curve of Howarth and McArthur (2001). Six Protothaca sp. shells from the same site yielded D-alloisoleucine to L-isoleucine ratios that cluster at 1.09±0.05. A parabolic kinetic model calibrated with a likely last-interglacial ratio of ~0.45 (Leonard and Wehmiller, 1992) indicates an age of ~1000 ka for the shells. Two of the shells were also analyzed for ⁸⁷Sr/⁸⁶Sr, yielding values similar to the corals.

Taken together, the three approaches indicate that the warm-fauna-bearing terrace is older than MIS 11, most likely ≥1000 ka. These results indicate that uplift rates at Morro de Copiapo are lower than previously thought, likely by a factor of at least two. More importantly, they indicate that the assumption that a warm fauna indicates an MIS 11 age, the basis of much recent work in Chile and elsewhere, is not universally applicable.