Deglacial foraminiferal neodymium isotopes form the low latitude Atlantic

Radiogenic neodymium isotopes have been used as a water mass mixing proxy to investigate past changes in ocean circulation. Here we present a new depth transect of deglacial neodymium isotope records measured on uncleaned planktic foraminifera from five cores spanning from 3300 to 4900 m on the Mauritanian margin, in the tropical eastern Atlantic as well as an additional record from 4000 m on the Ceara Rise in the equatorial western Atlantic. Despite being located under the Saharan dust plume, the eastern Atlantic records differ from the composition of detrital inputs through time and exhibit similar values to the western Atlantic foraminiferal Nd across the deglaciation. Therefore we interpret the foraminiferal values as recording deep water Nd isotope changes. All six cores shift to less radiogenic values across the deglaciation, indicating that they were bathed by a lower proportion of North Atlantic Deep Water during the Last Glacial Maximum (LGM) relative to the Holocene. The eastern Atlantic records also show that a neodymium isotope gradient was present during the LGM and during the deglaciation, with more radiogenic values observed at the deepest sites. A homogeneous water mass observed below 3750 m in the deepest eastern Atlantic during the LGM is attributed to the mixing of deep water by rough topography as it passes from the western Atlantic through the fracture zones in the Mid-Atlantic Ridge. This implies that during the LGM the low latitude deep eastern Atlantic was ventilated from the western Atlantic via advection through fracture zones in the same manner as occurs in the modern ocean. Comparison with carbon isotopes indicates there was more respired carbon in the deep eastern than deep western Atlantic during the LGM, as is also seen in the modern Atlantic Ocean.

放射成因钕同位素（radiogenic neodymium isotopes）常被用作水团混合示踪剂，用以研究古海洋环流的演化变化。本文报道了一套全新的冰消期钕同位素深度断面数据：该数据取自热带东大西洋毛里塔尼亚陆缘5个水深3300~4900 m的岩芯中未清洗的浮游有孔虫（planktic foraminifera），同时还包含赤道西大西洋塞阿拉海岭（Ceara Rise）4000 m水深处的额外一条记录。尽管这些东大西洋样品处于撒哈拉粉尘羽流影响区域之下，但其记录的同位素组成随时间的变化与陆源碎屑输入的组成并不一致，且在整个冰消期内，其数值与西大西洋浮游有孔虫的钕同位素值保持一致。因此我们认为，浮游有孔虫的钕同位素数值记录了深水钕同位素的演化变化。全部6个岩芯在冰消期均呈现出放射成因特征减弱的数值变化，这表明相较于全新世（Holocene），末次冰盛期（Last Glacial Maximum, LGM）期间，这些站位所暴露的北大西洋深层水占比更低。东大西洋的记录还显示，在末次冰盛期及冰消期均存在钕同位素梯度，水深越深的站位其同位素值的放射成因特征越强。末次冰盛期时，东大西洋最深水域（水深>3750 m）观测到的均一水团，可归因于深层水团从西大西洋穿过大西洋中脊（Mid-Atlantic Ridge）的断裂带时，受崎岖地形影响发生混合。这表明，在末次冰盛期，低纬度东大西洋深水层通过与现代海洋相同的机制，经由断裂带的平流作用从西大西洋获得通风。与碳同位素数据对比后发现，末次冰盛期东大西洋深水层的呼吸碳含量高于西大西洋深水层，这一现象同样出现在现代大西洋中。