Planktic and benthic boron and carbon isotopes from ODP sites 121-758, 165-999, 154-926 and 122-761

The boron isotope composition (d11B) of planktic foraminiferal calcite, which reflects seawater pH, is a well-established proxy for reconstructing palaeo-atmospheric CO2 and seawater carbonate chemistry. However, to translate d11B measurements determined in calcareous fossils into pH we need to know the boron isotope composition of the parent seawater (d11Bsw). While a number of d11Bsw reconstructions exist, the discrepancies between them reveals uncertainties and deficiencies that need to be addressed. Here we present a new d11Bsw record based on the d11B difference between planktic and benthic foraminifera and an estimate of the pH gradient between surface and deep water. We then calculate d11Bsw two different ways. One variant of our method assumes that the pH gradient between surface and deep has remained the same as today over the past 23 Ma; the other uses the d13C gradient between surface and deep to represent change in the pH gradient through time. The results of these two methods of calculating d11Bsw are broadly consistency with each other, however, based on extensive carbon cycle modelling using CYCLOPS and GENIE we favour the d13C gradient method. In our favoured d11Bsw reconstruction, d11Bsw is around 2 per mil lower than today at ~37.5 per mil during the early and middle Miocene and increases to the modern value (39.61 per mil) by ~5 Ma. A similar pattern of change is evident in the seawater composition of three other stable isotope systems, Mg, Li and Ca. Concurrent shifts in the seawater isotopic composition of all four of these elements during the late Miocene, suggest a common forcing mechanism. We hypothesise the most likely cause of these shifts is a change in the isotopic composition of the riverine input, potentially driven by an increase in secondary mineral formation since ~15 Ma.

浮游有孔虫方解石（planktic foraminiferal calcite）的硼同位素组成(d11B)可反映海水pH值，是重建古大气CO₂与海水碳酸盐化学的成熟代用指标。然而，若要将钙质化石中的d11B测量值转化为pH值，我们需要知晓母源海水的硼同位素组成(d11Bsw)。尽管目前已有多项d11Bsw重建研究，但彼此间的差异暴露了仍需解决的不确定性与不足。本研究基于浮游有孔虫与底栖有孔虫（benthic foraminifera）间的d11B差值，以及表层与深层海水的pH梯度估算值，构建了一套全新的d11Bsw记录。我们随后采用两种不同方法计算d11Bsw：其一假设过去23 Ma以来表层与深层海水的pH梯度与现今保持一致；其二则利用表层与深层海水的碳同位素组成(d13C)差值来表征随时间变化的pH梯度。这两种d11Bsw计算方法得到的结果整体上彼此吻合；不过，基于使用CYCLOPS与GENIE开展的大规模碳循环模拟，我们更倾向于采用d13C梯度法。在我们所优选的d11Bsw重建结果中，中新世（Miocene）早中期的d11Bsw较现今低约2‰，数值约为37.5‰，至~5 Ma时升高至现代值(39.61‰)。镁（Mg）、锂（Li）与钙（Ca）这另外三种稳定同位素体系的海水组成也呈现出类似的变化模式。中新世晚期这四种元素的海水同位素组成同步发生偏移，表明存在共同的驱动机制。我们推测，导致这些偏移最有可能的原因是河流输入的同位素组成发生改变，这一变化可能由约15 Ma以来次生矿物形成作用增强所驱动。