Both Sr/Ca and U/Ca ratios in aragonite coral skeletons may be used as proxies for past sea surface temperature (SST). Numerous studies have applied this technique to the last glacial cycle, but older samples are plagued by diagenetic alteration. Laser ablation microanalysis is therefore useful because isolated intervals of alteration too small to identified by milling techniques, can be detected and avoided. Corals (Goniopora hilli) from the Neogene (~ 7 My) Gurabo Formation, Dominican Republic, were analyzed for Sr/Ca and U/Ca ratios using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS) at the Australian National University. Approximately 140 analyses were obtained per year of coral growth (as revealed by comparison of growth banding with sinusoids in Sr/Ca and U/Ca ratios). The raw data were smoothed by bundling them into 12 equal sections in order to achieve monthly temporal resolution. These data were then converted to monthly average SST using published relations for Sr/Ca (for a different species of the genus Goniopora) and U/Ca (for the genus Porites) vs water temperature. While the shapes of the Sr/Ca and U/Ca sinusoids are similar, Sr/ Ca-based seasonal SST variability is three times as high as seasonal SST variability calculated using U/Ca ratios. It would at first seem likely that the lack of a U/Ca vs SST calibration for the genus Goniopora would favor the Sr/Ca SST reconstruction, but Sr/Ca-based SST suggest an unrealistic 30 degree C range in SST. Correlation of Sr/Ca and U/Ca sinusoids with growth bands suggests that Sr/Ca is more sensitive to diagenetic alteration than U/Ca.
Michael Lommler, ’06 Springfield, IL
Majors: Biology, Environmental Studies
Ann F. Budd
University of Iowa
Sponsor: Rhawn Denniston