OCEANIC PH CONTROL ON THE BORON ISOTOPIC COMPOSITION OF FORAMINIFERA - EVIDENCE FROM CULTURE EXPERIMENTS

Culture experiments were carried out with the species Orbulina univers a at four different pH values (7.7+/-0.05, 8.15+/-0.05, 8.60+/-0.05, a nd 9.0+/-0.10) in order to establish the pH-dependence of boron isotop e fractionation between seawater and foraminifera. A clear relationshi p between the boron isotopic composition of the foraminifera and the p H of the seawater culture solutions was found, showing heavier boron i sotopic composition at higher pH. This finding supports the viability of boron isotopes as a paleo-pH tool. It is important to note that Orb ulina universa cultured in natural seawater, as well as those obtained from coretop samples, have a significantly lighter boron isotopic rat io than Globigerinoides sacculifer from coretop samples, suggesting th at at least for this species, a vital effect is active.