Reconstructing past ocean temperatures is crucial to determining the sensitivity of the tropics to global climate change. One tool scientists use to help them in this endeavor lies in the skeletons of massive coral reefs: the ratio of the elements strontium (Sr) and calcium (Ca). During coral growth, these elements are incorporated into the reef skeleton in varying amounts. Calcium absorption remains relatively constant, but the water temperature affects the strontium uptake, thereby giving scientists a means to reconstruct ocean temperatures of years gone by. Any other influences on strontium uptake, however, could interfere with this coral thermometer. Now a new study suggests that algal symbiotic organisms living within the coral may do just that. According to a report published online today by the journal Science, up to 65 percent of the Sr/Ca ratio in corals containing algae does not reflect water temperature.

Anne Louis Cohen and colleagues at the Woods Hole Oceanographic Institution examined two groups of coral growing near the New England coast, one with symbiotic algae and one without. For the first year, the elemental ratios of strontium and calcium were similar for the two corals. Data from the following three years, however, indicate that the Sr/Ca ratio in corals with symbionts "oscillates with much greater amplitude, and water temperature is clearly not the primary control." Using this ratio to estimate sea surface temperatures yielded an apparent warming of the ocean near Woods Hole of six degrees Celsius. The actual average sea surface temperatures, however, declined by 0.5 degree C. Ratios measured in corals without symbionts, in contrast, provided temperatures that agreed with the direct measurements.

The researchers report that photosynthesis by the algae decreases the relative uptake of strontium by the coral. To reliably estimate past sea surface temperatures, they conclude, ratios of strontium to calcium should be measured in reefs without symbiotic species. Alternatively, sections of symbiotic coral reef skeletons accreted at night (when photosynthesis is minimal) could be used.