THE EFFECT OF COLONY TOPOGRAPHY ON CLIMATE SIGNALS IN CORAL SKELETON

The stable oxygen isotope composition of the aragonitic skeleton of he rmatypic corals is a potential archive of paleotemperature and rainfal l data. Biological processes also influence coral delta(18)O although it has been difficult to determine which processes are involved and wh ether or not they dominate the stable isotope signal. We show here tha t colony topography, or surface bumpiness, is associated with signific ant differences in delta(18)O, delta(13)C, and the timing of high-dens ity band formation between same-age corallites in the central and fast est-growing region of a coral colony. These differences reflect change s experienced by individual corallites as they grow from the summit of a bump toward the bottom of a valley. Corallites on the bump record i sotopic temperatures more than 1 degrees C higher and accrete high-den sity skeleton about 2 months earlier than their valley counterparts ju st 20 mm away. We propose that these changes are not caused by coralli te ''aging'' but rather by changes in the overall rate and timing of l ight-enhanced calcification, which is lower and occurs later in shaded valleys than it does on exposed bumps. Although we conclude that sea temperature is the dominant influence on delta(18)O values in our cora l, our results show that significant isotopic variations may be expect ed over a small surface area in a single colony. The production of acc urate and reproducible coral-based climate records thus requires an un derstanding of the complexities of coral growth processes and incorpor ation of this knowledge into sampling strategies and interpretation of data. Copyright (C) 1997 Elsevier Science Ltd.