Precise calibration of equilibrium oxygen isotope fractionations between dissolved phosphate and water from 3 to 37 °C

The stable oxygen isotope composition of orthophosphate (delta O-18(PO4)) is a widely used (paleo) temperature indicator and more recently, a useful tracer of phosphorus-cycling. In natural aqueous systems (e.g., oceans, rivers, soil/ground water) the largest reactive phosphorus pool is dissolved inorganic phosphate. Here, we present a new experimentally-determined equation for thermodynamic equilibrium O-isotope fractionations between dissolved phosphate and water, catalyzed by the enzyme inorganic pyrophosphatase (PPase) between 3 and 37 degrees C; 1000 ln alpha((PO4-H2O)) = 14.43(+/- 0.39) 1000/T (K) - 26.54(+/- 1.33) r(2) = 0.99 The new equation is slightly offset by +0.5 to +0.7 parts per thousand from recent empirically-determined fractionations based on biogenic apatite, with both based on modern cf-irms TC/EA analysis of Ag3PO4. Dissolved phosphate-water fractionations are offset by +0.9 to +2.3 parts per thousand from the earlier empirical relation for biogenic phosphate-water fractionation determined using fluorination of BiPO4. The equation presented here is thus, specific to equilibrium fractions between dissolved phosphate and water and appropriate for use in recent/future oxygen isotope studies of dissolved phosphate using similar cf-irms TC/EA analytical methods. (C) 2014 Elsevier Ltd. All rights reserved.