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

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.