(Table S3) Benthic foraminiferal Mg/Ca, Li/Ca and stable isotope ratios of ODP Site 122-761

The sensitivities of benthic foraminiferal Mg/Ca and Li/Ca to bottom water temperature and carbonate saturation state have recently been assessed. Here we present a new approach that uses paired Mg/Ca and Li/Ca records to calculate simultaneous changes in temperature and saturation state. Using previously published records, we first use this approach to document a cooling of deep ocean waters associated with the establishment of the Antarctic ice sheet at the Eocene-Oligocene climate transition. We then apply this approach to new records of the Middle Miocene Climate Transition from ODP Site 761 to estimate variations in bottom water temperature and the oxygen isotopic composition of seawater. We estimate that the oxygen isotopic composition of seawater varied by ~1 per mil between the deglacial extreme of the Miocene Climatic Optimum and the glacial maximum following the Middle Miocene Climate Transition, indicating large amplitude variations in ice volume. However, the longer-term change between 15.3 and 12.5 Ma is marked by a ~1°C cooling of deep waters, and an increase in the oxygen isotopic composition of seawater of ~0.6 per mil. We find that bottom water saturation state increased in the lead up to the Middle Miocene Climate Transition and decreased shortly after. This supports decreasing pCO2 as a driver for global cooling and ice sheet expansion, in agreement with existing boron isotope and leaf stomatal index CO2 records but in contrast to the published alkenone CO2 records.

底栖有孔虫的Mg/Ca与Li/Ca比值对底层水温和碳酸盐饱和状态的敏感性近期已得到评估。本文提出一种新方法，通过配对Mg/Ca与Li/Ca记录计算温度与饱和状态的同步变化。利用已发表记录，我们首先应用该方法揭示了始新世-渐新世气候转型期南极冰盖形成相关的深海冷却事件；随后将其应用于ODP 761站位中新世中期气候转型的新记录，以估算底层水温和海水氧同位素组成的变化。结果显示，中新世气候适宜期冰消期极值与中新世中期气候转型后冰期最大值之间，海水氧同位素组成变化约1‰，表明冰量存在大幅度波动。而15.3至12.5 Ma期间的长期变化以深水冷却约1℃、海水氧同位素组成增加约0.6‰为特征。此外，底层水饱和状态在中新世中期气候转型前升高、转型后不久下降，这支持pCO₂降低是全球冷却与冰盖扩张的驱动因素，与现有硼同位素及叶片气孔指数CO₂记录一致，但与已发表的烯酮CO₂记录相悖。