This dissertation work determined the changing scope of lead (Pb) contamination in the North Pacific Ocean since the phase-out of leaded gasoline in most of the world. Chapters 1 and 2 consisted of validating our method for determining Pb concentrations and isotopic compositions in seawater. Chapter 3 established a baseline of Pb isotopic compositions (PbICs) in the western and central North Pacific in 2002. This was an ideal time to establish such a baseline because China had recently (mid-2000) ceased their use of leaded gasoline and simultaneously began consuming increasingly large amounts of coal, known to have relatively high Pb concentrations. We found subsurface waters were contaminated with Asian industrial Pb, predominantly Chinese coal emissions. In contrast, the abyssal waters were a mix of Asian industrial Pb and background (i.e., natural) Pb.

Chapter 4 revisited the western and central North Pacific in 2009 - 2011 to determine what, if any, changes had occurred in this short time period. We found that Pb in subsurface and abyssal waters of the western North Pacific were similar to Chinese aerosols. Such a large change in the PbICs of abyssal water in 9 years was unanticipated and attributed to the relatively large flux of particle-bound Pb from the euphotic zone to the deep ocean, which was in isotopic equilibrium with the reservoir of dissolved Pb. In contrast, the central North Pacific abyssal water PbICs were similar to values previously reported because of the relatively lower particulate export. Based on comparisons to baseline PbIC data, we determined that abyssal waters in the western and central North Pacific would be isotopically indistinguishable from surface waters in the next three decades.

Sources of Pb to coastal California waters were reevaluated in Chapter 5. Prior studies had found that surface waters of the California Current System (CCS) were isotopically consistent with both Asian industrial Pb and US leaded gasoline, still in use at that point in time. In 2010 and 2011, we found that surface and subsurface waters of the CCS were isotopically similar to Asian industrial emissions. However, remobilized US gasoline Pb from sediments in the San Francisco Bay, California, were accumulating in the "mud belt" on the continental shelf and changing the isotopic composition of overlying waters. During periods of intense upwelling, this historic Pb was brought to the surface of the water. However, the much larger quantity of Pb from Asian industrial emissions made the isotopic composition of Pb from historic US gasoline unidentifiable in off-shore waters.

A secondary research focus of this dissertation was to improve my own teaching abilities. Chapter 6 explored the intersection of system thinking and aquatic toxicology in undergraduate education. Among a wealth of information, I found that group concept mapping was no more useful to student learning than the same activity done individually. This was due to poor implementation of team learning strategies by me and inadequate time for students to adjust to non-traditional instruction methodologies.

The effects of wildfires and deforestation on the remobilization of persistent inorganic contaminants are not completely understood. In forests, trace metal contaminants (e.g., lead) are sequestered in terrestrial sediment, soil organic matter, and vegetation, where they are relatively immobile. However, the combustion of soil organic matter and vegetation releases these contaminants in more labile forms. The associated deforestation and accelerated weathering increase the mobilization of trace metals from forests by simply increasing rates of erosion. These remobilized contaminants are transported to water bodies, increasing the loads of contaminants in these aquatic systems.

This project used analyses of trace metal inventories and lead isotopic compositions to study the remobilization and transport of toxic metals with respect to forest fires and deforestation associated with land-use. Chapters 2 and 3 present the results of investigation of the pyrogenic remobilization of trace metals at two contrasting sites in California: the relatively pristine Santa Barbara and the more populated and industrialized Los Angeles Metropolitan Area. The results indicate that wildfires are increasing the remobilization and transport of persistent contaminants (e.g., lead) in both study areas. Chapter 4 presents the results of a similar study conducted in Chile, in which geochronology and lead isotopic compositions were used to investigate the impact of historic (about a century ago) slash and burn practices on the deposition of trace metals in Lake Thompson, Patagonia. The results show a positive relationship between fire size and the flux of metals to the lake. These findings are significant because the frequency and intensity of wildfires are expected to increase with global climate change. Chapter 5 presents the results of a study in which contaminant remobilization was linked to land-use practices by characterizing temporal changes in the flux of trace metals to Lake Tanganyika, Africa. The results indicate that deforestation is the predominant factor accelerating trace metals transport to the lake. This land-use associated remobilization of contaminants is projected to increase with population growth and industrialization in Africa.