Mg/Ca ratios of Globigerina bulloides and stable isotope data of ODP Hole 177-1092B in the Southern Ocean

The middle Miocene climate approximately 14 Ma ago was characterized by the glaciation of Antarctica, deep-ocean cooling and variations in the global carbon cycle. Although the Southern Ocean underwent significant oceanographic changes, there is limited information on their spatial extent and timing. However, such knowledge is crucial for understanding the role of the Southern Ocean and the Antarctic Circumpolar Current (ACC) for Antarctic glaciation and the coupling between the ocean and continental climate. We have reconstructed surface temperatures and seawater oxygen isotopes at Ocean Drilling Program (ODP) Site 1092 in the Polar Frontal Zone of the Atlantic sector of the Southern Ocean from foraminiferal oxygen isotopes (d18O) and magnesium to calcium ratios (Mg/Ca). Sea surface cooling by ~ 4 °C and freshening indicated by the ~ 1per mill reduction of seawater d18O (d18Osw) at 14.2 Ma precede the major step in Antarctic ice sheet growth at 13.8–13.9 Ma. This pattern qualitatively mirrors previous findings from the Pacific sector, and we interpret the surface hydrographic changes to reflect the circum-Antarctic northward shift of the Southern Ocean fronts and specifically at Site 1092 the passage of the Subantarctic Front. The magnitude of change in reconstructed d18Osw requires a d18Osw: salinity gradient significantly higher than the modern value (~ 0.52 per mill) and it possibly exceeded 1.1 per mill. This implies the Polar Frontal Zone was influenced by freshwater derived from Antarctica, which in turn confirms higher than modern continental precipitation. The latter has previously been suggested to have contributed to Antarctic glaciation.

约14 Ma前的中新世中期气候，以南极冰盖发育、深海降温及全球碳循环波动为典型特征。尽管南大洋在此期间发生了显著的海洋学变化，但相关变化的空间范围与发生时序的记录仍较为匮乏。然而，此类信息对于理解南大洋与南极绕极流（Antarctic Circumpolar Current, ACC）在南极冰盖发育过程中的作用，以及海洋与大陆气候之间的耦合机制至关重要。本研究基于有孔虫氧同位素（foraminiferal oxygen isotopes, δ¹⁸O）与镁钙比（magnesium to calcium ratios, Mg/Ca），重建了南大西洋扇区极锋带内大洋钻探计划（Ocean Drilling Program, ODP）1092站位的海表温度与海水氧同位素记录。14.2 Ma时，海表温度下降约4℃，同时海水氧同位素（δ¹⁸Osw）降低约1‰，指示海水盐度降低；这一变化早于13.8~13.9 Ma南极冰盖规模扩张的关键阶段。该变化模式与太平洋扇区的既往研究结果定性一致；我们认为，此次海表水文学变化反映了南大洋锋面向环南极区域北移，在1092站位处具体表现为亚极锋（Subantarctic Front）的过境事件。重建得到的δ¹⁸Osw变化幅度表明，δ¹⁸Osw与盐度的梯度显著高于现代水平（约0.52‰），甚至可能超过1.1‰。这意味着极锋带受到源自南极的淡水影响，进而证实当时南极大陆的降水量高于现代水平；此前已有研究指出，该现象曾对南极冰盖发育起到促进作用。