Boron isotopic ratios and concentrations of pore fluids and carbonates from ODP Site 138-851 (Table 1)

We have determined the boron content and isotopic composition of bulk carbonates and associated pore waters from Ocean Drilling Program Site 851. The delta11B of the carbonates range from 23 per mil to as low as -5.5 per mil, greatly extending the range of reported carbonate delta11B values. In contrast, pore fluid isotopic compositions are relatively constant. Based on the correlation of delta11B and 1/B (r = 0.99) we conclude that the bulk carbonate at this site is a mixture of a biogenic component (~7 ppm with delta11B of 23 per mil) and a recrystallized component (~2.5 ppm with delta11B of -5.5 per mil). The empirical "low pH" water-carbonate fractionation factor implied by the data is 0.959 +/- 0.003. A boron concentration minimum occurs in pore waters at a depth of approximately 200 m below the seafloor and this is interpreted as reflecting essentially quantitative uptake of B at this depth and the horizontal advection of pore fluids with water residence times below the minimum on the order of 3,600 years.