Abstract:
We have determined the boron content and isotopic composition of bulk carbonates and associated pore waters from Ocean Drilling Program Site 851. The δ11B of the carbonates range from 23%% to as low as -5.5%%, greatly extending the range of reported carbonate δ11B values. In contrast, pore fluid isotopic compositions are relatively constant. Based on the correlation of δ11B 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 δ11B of 23%%) and a recrystallized component (~2.5 ppm with δ11B of -5.5%%). 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.