Nd and Sm isotopic record from fossil fish teeth recovered from ODP Site 177-1090

Neodymium (Nd) isotopes were measured on 181 samples of fossil fish teeth recovered from Oligocene to Miocene sections at Ocean Drilling Program Site 1090 (3700 m water depth) on Agulhas Ridge in the Atlantic sector of the Southern Ocean. A long-term decreasing trend toward less radiogenic Nd isotope compositions dominates the record. This trend is interrupted by shifts toward more radiogenic compositions near the early/late Oligocene boundary and the Oligocene/Miocene boundary. Overall, epsilon-Nd values at Agulhas Ridge are more radiogenic than at other Atlantic locations, and are similar to those at Indian Ocean locations. The pattern of variability is remarkably similar to Nd isotope results from Walvis Ridge (South Atlantic) and Ninetyeast Ridge (Indian Ocean). In contrast, Agulhas Ridge and Maud Rise Nd isotope records do not show similar patterns over this interval. Results from this study indicate that deep water in the Atlantic flowed predominantly from north to south during the Oligocene and Miocene, and that export of Northern Component Water (NCW) to the Southern Ocean increased in the late Oligocene. There is also evidence for efficient exchange of deep waters between the Atlantic sector of the Southern Ocean and the Indian Ocean, although the direction of deep water flow is not entirely clear from these data. The shifts to more radiogenic Nd isotopic compositions most likely represent increases in the flux of Pacific waters through Drake Passage, and the timing of these events reflect development of a mature Antarctic Circumpolar Current (ACC). The relative timing of increased NCW export and ACC maturation support hypotheses that link deep water formation in the North Atlantic to the opening of Drake Passage.

研究针对南大洋大西洋海域阿古拉斯海岭的大洋钻探计划（Ocean Drilling Program）1090站位（水深3700米）所获渐新统至中新统地层的181件鱼牙化石样品，开展了钕（Nd）同位素测试分析。该同位素记录整体以向低放射成因钕同位素组成方向长期递减的趋势为主导，这一趋势被渐新世早/晚边界附近以及渐新世/中新世边界附近的高放射成因组成偏移事件所打断。总体而言，阿古拉斯海岭的ε钕（epsilon-Nd）值相较于大西洋其他站位更为放射成因，且与印度洋海域站位的测试结果相近。其同位素变化模式与南大西洋沃尔维斯海岭、印度洋九十东海岭的钕同位素研究结果极为相似；与之形成鲜明对比的是，该时段内阿古拉斯海岭与毛德隆起的钕同位素记录并未呈现类似的变化规律。

本研究结果显示，渐新世至中新世期间大西洋深层水整体呈由北向南的流动态势，且晚渐新世时期北水团组分（NCW）向南大洋的输出量有所提升。同时存在证据表明，南大洋大西洋海域与印度洋之间存在高效的深层水交换过程，尽管仅凭当前数据尚无法完全明确深层水的具体流动方向。此次观测到的高放射成因钕同位素组成偏移事件，大概率对应太平洋水体经德雷克海峡（Drake Passage）的通量增加，而这些事件的发生时间则反映了成熟南极绕极流（ACC）的形成进程。北水团组分输出增强与南极绕极流成熟的相对时间节点，为“北大西洋深水形成与德雷克海峡开启存在关联”的相关假说提供了有力支撑。