Computer Science
Scientific paper
Dec 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995e%26psl.136..665g&link_type=abstract
Earth and Planetary Science Letters, vol. 136, Issue 3-4, pp.665-676
Computer Science
18
Scientific paper
The first analyses of modern and fossil corals (calcite-free) using a positive ion (Cs 2 BO 2 + ) TIMS technique and a chemical procedure for B extraction are reported. The 11 B for modern corals fluctuates between 23.3 and 25.5 . The range of variation of fossil corals from the last interglacial period is greater, from 23.5 to 27.3 , and the lowest 11 B corresponds to the corals close to Last Glacial Maximum. Boron concentrations show a variation from 49 to 58 ppm in the recent corals, and from 39 to 52 ppm in ancient corals. For the ancient corals, linear correlations are observed in both 11 B vs. B concentration and 11 B vs. 1/ B plots, but these correlations are not observed for modern corals. Two interpretations are presented to account for these observations and to test the potential of 11 B for reconstructing paleo-ocean pH. The first interpretation assumes a thermodynamic partitioning of B between seawater and carbonates and shows that both boron isotopic compositions and concentrations can be used to calculate the pH of seawater in which corals grow. For our data, such a model requires that both the pH and the CO 2 of the deglacial ocean was lower than present-day values. The second interpretation is based on the correlation observed for ancient corals between boron isotopic compositions and 1/ B . We suggest that the measured boron isotopic compositions of ancient corals are caused by the mixing of a primary aragonite (the coral skeleton) and secondary aragonite precipitated from 10 B-enriched reef pore fluids. The characteristics ( 11 B and B concentrations) of the diagenetic aragonite and the fluids from which it precipitates can be constrained. If this proves correct, only the non-recrystallised parts of ancient corals can be used for paleo-pH reconstructions. These recrystallisation processes did not affect the 234 U/ 238 U ratios of the corals.
Gaillardet Jérôme
Jean Allègre Claude
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