BORON ISOTOPE GEOCHEMISTRY OF PALAEOZOIC BRACHIOPOD CALCITE: IMPLICATIONS FOR A SECULAR CHANGE IN THE BORON ISOTOPE GEOCHEMISTRY OF SEA WATER OVER THE PHANEROZOIC

The boron isotope composition of marine carbonate has been proposed as a palaeo-pH proxy and potential tool to reconstruct atmospheric pCO₂. The precise knowledge of the boron isotopic composition of ancient sea water represents the fundamental prerequisite for any palaeo-pH reconstruction. In order to estimate the boron isotope composition of Palaeozoic oceans, we measured boron isotope ratios of Silurian to Permian brachiopod calcite. Modern brachiopod shells show a similar dependence on d¹¹B of shell calcite and pH of ambient water suggesting that the boron isotope composition of fossil brachiopod calcite may be used to reconstruct d¹¹B of past oceans (Lécuyer et al. 2002). Brachiopod shells are composed of low-magnesium calcite that has high potential to preserve its primary geochemical compositions. The preservation state of the studied shells was verified by means of cathodoluminescence microscopy, trace element geochemistry as well as SEM. Only non-luminescent brachiopod shells revealing well-preserved microstructures, high strontium as well as low iron and manganese contents were accepted for further analysis. High boron contents (20 to 100 ppm B) are taken as additional argument for preservation of the primary geochemical signals since the high mobility of boron suggests a significant loss of boron if recrystallisation occurred.

The boron isotope ratios of  well-preserved Silurian, Devonian, Pennsylvanian and Permian  brachiopod calcite range from 6.8 to 11.0 ‰, 7.3 to 14.9 ‰, 12.4 to 15.8 ‰, and, 10.1 to 11.7 ‰, respectively. These boron isotope values are significantly depleted in ¹¹B in comparison to boron isotope values of modern brachiopod shells (16.8 to 19.7 ‰). Since these low ratios cannot be explained by a significantly lower oceanic pH,  Palaeozoic oceans are interpreted to have been depleted in ¹¹B by up to 10 ‰. Numeric modelling of the boron geochemical cycle suggests that enhanced riverine flux of boron from the continents to the oceanic reservoir can account for these low oceanic d¹¹B values. The outcome of this study indicates that the boron isotope ratio of sea water varied during the Phanerozoic as consequence of a variable boron exchange with the oceanic crust and preferentially due to changes in the boron delivery from the continents.

Lécuyer et al. (2002),  Chem. Geol., 186, 45-55