Abstract. The middle Miocene climate transition (MMCT, ~14.5–13.0 Ma) was associated with a significant expansion of Antarctic ice, but the mechanisms triggering the event remain enigmatic. We present a new clumped isotope (∆₄₇) bottom water temperature (BWT) record from 16.0 Ma to 12.2 Ma from Ocean Drilling Program (ODP) Site 747 in the Southern Ocean, and compare it to existing BWT records. We show that BWTs in the Southern Ocean were ~8–10 °C during the middle Miocene greenhouse, and thus considerably warmer than today. Nonetheless, bottom water δ¹⁸O (calculated from foraminiferal δ¹⁸O and ∆₄₇) suggests substantial amounts of land ice throughout the interval of the study. Our dataset demonstrates that BWTs at Site 747 decreased by ~3–5 °C across the MMCT. This cooling preceded the stepped main increase in global ice volume, and appears to have been followed by a transient bottom water warming starting during or slightly after the main ice volume increase. We speculate that a regional freshening of the upper water column at this time may have increased stratification and reduced bottom water heat loss to the atmosphere, counteracting global cooling in the bottom waters of the Southern Ocean and possibly even at larger scales. Additional processes and feedbacks required for substantial ice growth may have contributed to the observed decoupling of Southern Ocean BWT and global ice volume.

中文翻译：

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中新世中期气候过渡期间南大洋底水冷却和冰盖扩张

摘要。中新世中期气候变迁（MMCT，〜14.5-13.0 Ma）与南极冰的显着扩张有关，但触发该事件的机制仍是未知的。我们提出了南大洋钻探计划（ODP）站点747从16.0 Ma到12.2 Ma的新的丛集同位素（∆ ₄₇）底水温度（BWT）记录，并将其与现有的BWT记录进行了比较。我们显示，在中新世中期温室中，南大洋的BWTs约为8-10°C，因此比今天的温度要高得多。然而，底部水δ ¹⁸ O（从有孔虫δ计算¹⁸ O和Δ ₄₇）建议在整个研究过程中大量的陆冰。我们的数据集表明，在MMCT上，站点747的BWT降低了约3-5°C。这种冷却先于总体冰量的逐步主要增加，然后似乎是在主要冰量增加期间或之后开始的短暂的底部水加热。我们推测，此时高位水柱的区域性清新可能会增加分层并减少底部水向大气的热损失，从而抵消南大洋底部水域甚至更大规模的全球冷却。大量冰块生长所需的其他过程和反馈可能导致观测到的南大洋BWT和全球冰块之间的脱钩。