(Table 1) Boron concentration and d11B ratios of ODP Hole 130-803D

The pH of the surface ocean is a sensitive function of its alkalinity and total inorganic carbon concentration, properties which also control the partial pressure of atmospheric carbon dioxide (Broecker and Peng, 1982). Thus, an accurate proxy for past ocean pH could yield information about variations in atmospheric CO2. Recently, it has been suggested that the boron isotopic composition of foraminiferal tests depends on the pH of sea water as well as its isotopic composition (Vengosh et al., 1991, doi:10.1016/0016-7037(91)90139-V; Hemming and Hanson, 1992, doi:10.1016/0016-7037(92)90151-8). Here we present boron isotope and elemental data for sedimentary pore fluids and isotope data for bulk foraminiferal samples from a deep-sea sediment core. The composition of the pore waters implies that sea water boron concentrations and isotopic composition have been constant during the past 21 Myr, allowing us to reconstruct past ocean pH directly from the foraminiferal isotope data. We find that 21 Myr ago, surface ocean pH was only 7.4 ±0.2, but it then increased to 8.2 ±0.2 (roughly the present value) about 7.5 Myr ago. This is consistent with suggestions (Popp et al., 1989; Cerling, 1991; Arthur et al., 1991) that atmospheric CO2 concentrations may have been much higher 21 Myr ago than today.表层海水（surface ocean）的pH值是其碱度（alkalinity）与总无机碳浓度（total inorganic carbon concentration）的敏感函数，而这两项性质同时调控大气二氧化碳分压（partial pressure of atmospheric carbon dioxide）（Broecker与Peng，1982）。因此，精准的过去海水pH代用指标（proxy）可为研究大气CO₂的变化提供有效信息。近期有研究提出，有孔虫壳体（foraminiferal tests）的硼同位素组成（boron isotopic composition）不仅受海水同位素组成影响，同时与海水pH值相关（Vengosh等，1991，doi:10.1016/0016-7037(91)90139-V；Hemming与Hanson，1992，doi:10.1016/0016-7037(92)90151-8）。本文针对某深海沉积岩心（deep-sea sediment core）中的沉积孔隙流体（sedimentary pore fluids）给出硼同位素与元素数据，并提供该岩心内混合有孔虫样品（bulk foraminiferal samples）的同位素数据。孔隙水的组成特征表明，过去21 Myr以来，海水硼浓度与同位素组成始终保持恒定，这使得我们可以直接通过有孔虫同位素数据重建过去的海水pH值。研究结果显示，21 Myr前表层海水pH仅为7.4±0.2，而在约7.5 Myr前，其pH值上升至8.2±0.2（大致与现代海水pH值相当）。这一结果与此前多项研究提出的观点一致（Popp等，1989；Cerling，1991；Arthur等，1991），即21 Myr前的大气CO₂浓度远高于现代水平。