Isotopic fractionation of boron in growing corals and itspalaeoenvironmental implication

摘要

Isotopic compositions of boron delta super(11) B sub(c)) value and elemental concentrations of B, Ca, Mg, Fe and Al in growing corals from Sanya Bay, China and Kavaratti Lagoon, India along with delta super(11) B sub(sw) values and pH values of the coexisting sea water have been measured in this study using TIMS. The delta super(11) B sub(c) values of growing corals behave inconsistently and are not related to the concentrations of Fe and Al in corals. The higher delta super(11) B sub(c) values of corals imply additional amount of trigonal species of boron to be incorporated into corals. Based on the measured delta super(11) B values of growing corals and coexisting sea water as well as the pH of co-existing sea water, a new isotopic fractionation factor alpha sub(43) between two boron species, B(OH) sub(4) super(-) and B(OH) sub(3), has been measured to be 0.980, consistent with the well accepted alpha sub(43) of 0.981 established by earlier workers. The significance of alpha sub(43) value established by recalculating the sea water pH considering the published delta super(11) B values of marine carbonates and using the reported alpha sub(43) values of 0.968, 0.981, 0.984 and one alpha sub(43) of 0.980 observed in this study, gave the average pH values of sea water to be 8.85, 8.17, 7.76 and 8.22 respectively. This indicates the closeness of alpha sub(43) value of 0.980 with that of the well-accepted reported alpha sub(43) of 0.981 as the recalculated pH values of sea water obtained using these are nearly the same. The alpha sub(43) value of 0.980 observed in this study can therefore be considered an acceptable value for reconstructing the past sea water pH values.