Marine cycling of neodymium in the South Atlantic and the Southern Oceans

Abstract

Neodymium (Nd) isotopes have been used as a proxy to trace both modern and palaeo-ocean circulation of the deep ocean which is an important factor controlling the climate. Although there has been widespread utilisation of Nd isotopes as a proxy, the marine cycling of Nd is not fully understood. This thesis investigates the cycling of neodymium, together with other rare earth elements, by determining Nd isotopes in the South Atlantic Ocean and the Southern Oceans, which are critical locations in the global ocean circulation system. It consists of three projects focusing on area of the South Atlantic, the Bellingshausen Sea, and the Antarctic sub-ice shelf cavities, respectively.

Measurements on seawater neodymium isotopic composition (ƐNd) and carbon isotopic composition (δ13C) of 40°S transect in the South Atlantic are made to examine the cycling of Nd in this region. The results show that Nd is controlled horizontally by water mass mixing and vertically by reversible scavenging. The "Nd paradox" in the Atlantic and the Pacific Oceans can be explained by less sensitivity of Nd isotopic composition compared with Nd concentration changes during vertical cycling in the deep ocean. The result also reveals that Nd isotopes can be a robust ocean circulation proxy, compared with other tracers, that trace water mass mixing and distinguish the Pacific signal in the South Atlantic. North Atlantic Deep Water, Weddell Sea Deep Water, and Pacific Lower Circumpolar Deep Water are the three water masses occupying the deep South Atlantic, and this work shows that Nd isotopes can be used to quantify the different proportions of them at western and eastern side.

Porewaters have been extracted from marine sediment of three sites on the continental rise of the Bellingshausen Sea (Pacific sector of the Southern Ocean) to investigate the possible boundary processes. A suite of measurements made on seawater, porewater, and marine sediment show that boundary exchange is the prevailing process modifying porewater and bottom water by shifting their Nd isotopic ratios to more radiogenic values without significantly increasing the concentration of dissolved Nd in the waters. The boundary processes probably shift the Nd isotopic composition of AABW formed in the Ross Sea to values that are more radiogenic than in Circumpolar Deep Water.

Seawater and marine sediment samples underneath the Filchner, the Larsen C, and the George VI Ice Shelves of Antarctica were measured for Nd isotopes in this study. Seawater from beneath the Filchner Ice Shelf and the northern Larsen C Ice Shelf have ƐNd values close to the detritus and leachates, indicating modification from local input. The unexpected less radiogenic ƐNd values of seawater from underneath the southern Larsen C Ice Shelf and particularly the George VI Ice Shelf indicate an undefined source, which may be geologically deep, and associated with the unique set of parameters in this location (ice sheet, tectonic activity, etc.). Nevertheless, the local signals of the sub-ice shelf seawaters are not seen in the shelf waters, which means not likely to change the deep waters in the Bellingshausen Sea and the Weddell Sea.