Anomalous eNd change of the Antarctic Intermediate Water in the Pacific sector of the Southern Ocean during the Middle Miocene Climate Transition

The Middle Miocene Climatic Optima (MMCO; 17–14.5 Ma) is warmest over the last 23 Myrs, with higher pCO₂ (400–500 ppmv; Foster et al., 2012, Super et al., 2018), ~6 °C warming in a mid-latitude compared to the present (Flower and Kennett, 1994), and a large reduction (30–36 m) in Antarctic ice volume (Gasson et al., 2016). The eruption of CO₂ from the Columbia River flood basalt has been suggested as a primary cause of the MMCO. However, the mechanisms of the progressive global cooling after the MMCO remain highly controversial. Here, we provide novel paleoceanographic information on the Antarctic Intermediate Water (AAIW) in the Pacific sector of the Southern Ocean to improve our understanding of climate–ocean conditions during the MMCO. In this study, we analyzed the middle Miocene biopelagic sediments (> 90% CaCO₃ contents) from ODP Site 1120 (50°3.8′S, 173°22.3′E), located on the central Campbell Plateau off the South Island of New Zealand (Ando et al., 2011). Because the middle Miocene paleo-water depth at Site 1120 is estimated to be similar to the present water depth (~600 m), analyses of oxygen and carbon isotopic compositions of benthic foraminifer and neodymium (Nd) isotopes of fossil fish teeth/debris allow us to characterize the AAIW during the deposition. We present newly measured 17.5–8.5 Myr records of Nd isotopes of fossil fish teeth/debris, planktonic foraminiferal δ¹⁸O, δ¹³C, Mg/Ca, and Ba/Ca from Site 1120. The ε_Nd values ranged from -7.5 to -3.2 at 17.5–8.5 Ma. In contrast, during the Middle Miocene Climate Transition (MMCT, 14.5–13.5 Ma), ε_Nd values shifted rapidly toward more radiogenic values (~-3.2) and then gradually returned to less radiogenic values (-7 to -6). Such a large long-term variation of ε_Nd values has not been reported in previous datasets, which is the first records to represent the characteristics of the AAIW in the Pacific sector of the Southern Ocean during the MMCT. We argue for the two possible causes of these ε_Nd changes in the AAIW: 1) there might have been an anomalous supply of radiogenic Nd due to the intense physical weathering in West Antarctica caused by the onset of glaciation and 2) the equatorial surface water, characterized by high ε_Nd values, might have expanded toward the high latitudes and a part of the water mass was incorporated into the intermediate layer in the Southern Hemisphere.