CORALS, PALEOTEMPERATURE RECORDS, AND THE ARAGONITE-CALCITE TRANSFORMATION

Oxygen isotopic records from corals play an increasing role in paleocl imatic research and climatic modeling, Proxy records of sea-surface te mperature are produced by isotopically analyzing samples drilled from aragonitic (CaCO3) coral skeletons, However, drilling aragonite causes a polymorphic inversion to calcite accompanied by O-18 enrichment. Th e isotopic enrichment may occur through atmospheric exchange, CO2 dega ssing, or fractionation during digestion. Regardless of the mechanism, dry drilling produced isotopic shifts as large as 0.8 parts per thous and, corresponding to an erroneous temperature decrease of 3 degrees C . This error is larger than the global temperature rise since the Indu strial Revolution, and a substantial part of the global temperature va riation over the past 10 ka. Because the isotopic shift is inconsisten t and can occur without the production of measurable calcite, detectin g and correcting the isotopic shift may be difficult.