The neodymium isotopic composition of waters masses in the eastern Pacific sector of the Southern Ocean

The Antarctic Circumpolar Current is one of the key components of ocean circulation, and a knowledge of its isotopic composition is essential to the use of neodymium (Nd) isotopes to trace circulation now and in the past. Here we present 57 new analyses of the Nd isotopic composition of the water column in the eastern Pacific sector of the Southern Ocean, documenting both the variation in three dimensions as well as the controls on that variability. Nd isotopic data for the middle of the water column demonstrate the homogeneity of Circumpolar Deep Water (CDW) at an εNd value of −8.7 ± 0.1. This homogeneity reflects the large Nd inventory in the ACC flow, and the degree to which this large inventory buffers the Nd characteristics of the ACC against extra-oceanic inputs from the continents, either via dust from the atmosphere or through dissolved and particulate material from the adjacent continents. CDW upwells onto the Amundsen Sea shelf and, even here, its Nd isotopic properties are close to conserved in the middle of the water column (εNd = −8.0 ± 0.2 at 600 m). At the top and bottom of the shelf water column, however, the Nd isotopic and concentration characteristics are strongly modified (to εNd as high as −4.5). All the shelf water column data obtained here are consistent with net addition of Nd to bottom and surface waters with a contrasting isotopic composition that is matched by local sediment (εNd = −1 to −2), followed by conservative mixing of that water into intermediate levels. Mixing with this shelf composition also leads to significant modification of open ocean surface water (Nd isotopic shift around 1 epsilon unit) in the ACC as it flows eastwards. Modification of open ocean bottom waters by interaction with sediment is more subtle, but there is marked non-conservative removal of Nd accompanied by significant changes in isotopic composition in waters within 10 m of the seabed. The new data demonstrate the conservativity of Nd in the middle water column, especially for such large volume flows as the ACC. Though boundary exchange-type processes are clearly important in this region, and their imprint on both the shelf and open-ocean surface water is significant, there is no observable impact on the main core of CDW. This finding augurs well for the use of Nd isotopes as a conservative water-mass tracer now and in the past. For example, these data suggest that the only likely control on the temporal variability of southern-component deep water exported northwards into the Atlantic through the last glacial cycle is variations in the input of North Atlantic Deep Water in the Atlantic sector. On the other hand, the data also suggest caution in the use of sedimentary archives of bottom water Nd as records of deep water Nd isotopic characteristics, given the dramatic modification of bottom waters on the shelf and the more subtly non-conservative behaviour in the open ocean bottom water that are both suggestive of modification of the Nd characteristics of bottom waters by sediment.

Details