Kinetic control of skeletal Sr/Ca in a symbiotic coral: Implications for the paleotemperature proxy

Modeling of past climates is critically dependent on estimates of past sea surface temperatures (SSTs), for which one of the principal techniques used is the measurement of Sr/Ca ratios in corals [Guilderson er al., 1994; McC ulloch et al., 1999; Hughen er al., 1999]. The link between coral Sr/Ca and SST is not well-understood and there have been a number of discrepant obse rvations [de Villiers et al., 1995; Alibert, 1998]. Corals with symbiotic z ooxanthellae are known to show large diurnal fluctuations in calcification rate associated with the photosynthetic activity of their symbionts. Using detailed measurements with the ion microprobe, we compared the Sr/Ca conten t of discrete daytime and nighttime skeletal structures in the massive herm atypic coral Porites lutea over the course of 1 year and a seasonal tempera ture range of 4 degreesC. The Sr/Ca content of daytime skeleton is always l ower than that of adjacent nighttime skeleton. While the slope of the night time Sr/Ca-SST correlation is close to that seen in inorganic aragonite pre cipitates, that of the daytime correlation is >4 times as steep. We attribu te these differences to the role of photosynthesis in calcification and con clude that bulk Sr/Ca is related principally to daytime calcification rate rather than directly to SST. More reliable estimates of past SST may be arr ived at through selective analysis of nighttime skeleton.