Tracing water masses and pollution in the Southern Ocean using neodymium and lead isotopes

• Main Author: Griffiths, Alexander Matthew
• Other Authors: Rehkämper, Mark, van de Flierdt, Tina, Natural Environment Research Council (NERC)
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: Earth Science & Engineering, Imperial College London 2020
• Subjects: Antarctic; Pacific; Southern Ocean; Wilkes Land; Antarc*
• Online Access: http://hdl.handle.net/10044/1/96454; https://doi.org/10.25560/96454

Trace elements and their isotopes play a vital role in the ocean as participants in, and tracers of, processes of interest to climate change and environmental pollution. This thesis focuses on the use of isotopic variations in neodymium (Nd) and lead (Pb) to understand the cycling of these elements in the Southern Ocean. Neodymium isotopes have been used as a palaeo-proxy to understand changes in Antarctic Bottom Water (AABW) circulation through time. The biogeochemical processes controlling Nd in seawater, however, remain under-constrained due to a paucity of modern observations in the Southern Ocean. In chapter 2, Nd isotope and rare earth element (REE) data are presented for the Wilkes Land continental margin. In this region, AABW exhibits a distinct εNd signature that is intermediate between Atlantic and Pacific sector AABW. The REE data confirm that the εNd signature is not caused by distinct local continental inputs but by mixing of advected AABW with (modified) Circumpolar Deep Water (CDW). Anthropogenic emissions from mining, smelting and fossil fuel combustion are important sources of oceanic Pb contamination. Seawater Pb isotope data, however, is currently severely limited by analytical challenges such as sample contamination, time-consuming extraction procedures and insufficient instrumental detection limits. In chapter 3, a novel method is presented for the determination of seawater Pb isotope compositions and concentrations. The method encompasses solid-phase extraction of Pb from seawater with Nobias chelate PA-1 resin followed by multi-collector inductively coupled mass spectrometry (MC-ICP-MS) analyses using a 207Pb-204Pb double-spike to correct for instrumental mass discrimination. When compared to an established double-spike procedure that employs thermal ionisation mass spectrometry (TIMS), the results are unbiased by systematic error and demonstrates improved precision for the and the minor 204Pb ratios (by about a factor of 2). In chapter 4, the new method is applied to seawater samples from the Australian sector of the Southern Ocean to assess the sources and processes governing the distribution of Pb in this region. Surface waters exhibit a high fraction of anthropogenic Pb (~30–50%). Reversible scavenging and equilibrium exchange are the dominant processes responsible for the vertical transport of this anthropogenic Pb to deeper waters. These processes may account for ~80% of the observed dissolved Pb isotope in the intermediate depth waters at the Polar Front. Overall, the thesis highlights the essential role that isotope analyses play in deconvolving the processes responsible for the biogeochemical cycling of Nd and Pb in the ocean. Open Access