Environmental pollution as a marine tracer: measuring and modelling anthropogenic lead in the ocean
File(s)
Author(s)
Olivelli, Arianna
Type
Thesis
Abstract
Lead (Pb) is a heavy metal that is naturally present in the ocean at trace levels. However, since the late 19th century, human activities have increased Pb concentrations and altered its natural biogeochemical cycle in the marine environment. The relative abundance of lead isotopes in seawater represents a powerful tool to reveal the sources, pathways, and sinks of Pb from land to the ocean and within it. This thesis focuses on assessing the present levels of Pb pollution and constraining the biogeochemical cycle of Pb in areas of the ocean that have been previously understudied. The approach followed is two-fold, and includes (i) the generation of new Pb concentration and isotope composition data and (ii) the application of machine learning to marine isotope geochemistry.
In Chapter 2, I determine natural and anthropogenic sources of Pb in the surface South Atlantic Ocean and show that average Pb concentrations decreased by 34% between the 1990s and 2011. This decrease in pollution is also supported by isotope data, which show that the Pb isotope composition of surface seawater has shifted towards a more natural signature.
In Chapter 3, I present a new dataset of Pb concentrations and isotope compositions for the South Atlantic Ocean. By comparing observed and modelled Pb distributions, I find evidence for vertical transport of anthropogenic Pb due to reversible scavenging. Therefore, I caution the use of Pb isotope ratios as tracers of ocean ventilation, especially in areas where suspended particulate matter loads are high.
Lastly, in Chapter 4, I develop three machine learning models to map the global distribution of Pb and its isotopes. I identify key patterns in the distribution of Pb and its cycling in the ocean. The model outputs offer benchmarks for monitoring future levels of pollution and refining process-based models of Pb cycling.
In Chapter 2, I determine natural and anthropogenic sources of Pb in the surface South Atlantic Ocean and show that average Pb concentrations decreased by 34% between the 1990s and 2011. This decrease in pollution is also supported by isotope data, which show that the Pb isotope composition of surface seawater has shifted towards a more natural signature.
In Chapter 3, I present a new dataset of Pb concentrations and isotope compositions for the South Atlantic Ocean. By comparing observed and modelled Pb distributions, I find evidence for vertical transport of anthropogenic Pb due to reversible scavenging. Therefore, I caution the use of Pb isotope ratios as tracers of ocean ventilation, especially in areas where suspended particulate matter loads are high.
Lastly, in Chapter 4, I develop three machine learning models to map the global distribution of Pb and its isotopes. I identify key patterns in the distribution of Pb and its cycling in the ocean. The model outputs offer benchmarks for monitoring future levels of pollution and refining process-based models of Pb cycling.
Date Issued
2025-01-20
Date Awarded
2025-04-01
Copyright Statement
Attribution-NonCommercial 4.0 International Licence (CC BY-NC)
License URL
Advisor
Rehkämper, Mark
van de Flierdt, Tina
Sponsor
Natural Environment Research Council (Great Britain)
Grant Number
NE/S007415/1
Publisher Department
Department of Earth Science & Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)