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The aeolian transport of aerosols (mineral dust from desert areas, smoke and ash from biomass burning, and from anthropogenic emissions) is an important process for introducing bioactive trace elements to the surface ocean and can have a large impact on marine primary production. All material that enters the ocean from the atmosphere must pass through the air-sea interface, or sea surface microlayer. The microlayer is the physical link between the sea surface and lower atmosphere and is therefore tied to the global biogeochemical cycling of trace elements. The microlayer (50 – 200 µm thickness) is a unique environment with different physical, chemical, and biological properties compared to the underlying water column. The microlayer is dynamic in nature due to numerous non-equilibrium processes such as temperature fluctuations, salinity gradients, irradiance, and wind and wave actions that influence its biogeochemical properties. However, the microlayer is mechanically more stable than the underlying water column due to the higher concentration of surface-active organic compounds; creating a more rigid film-like layer over the surface of the ocean. It is an important, yet often ignored component in the biogeochemical cycling of trace elements in the marine environment due to the lack of trace element clean sampling and analysis methods. A novel technique, a hollow cylinder of ultra-pure SiO₂ (quartz glass) with a plastic handle, was developed to sample the microlayer for trace elements. This research also developed and optimized clean trace element techniques to accurately measure nine trace metals (Al, Mn, Fe, Co, Ni, Cu, Zn, Cd, and Pb) in the dissolved and particulate fractions of the microlayer and underlying water column. Initially, our research focused on the behavior of dissolved and particulate Al, Mn, Fe, Co, Cu, Zn, Cd, and Pb in the microlayer in a controlled tank experiment using a Saharan dust source. The residence times of the dissolved trace elements ranged from 1.8 hours for Fe to 15 hours for Cd. The residence times for the particulate trace elements ranged from 1.0 minutes for Al and Fe to 1.4 minutes for Mn. There was an initial release of dissolved trace elements to the microlayer from the Saharan dust. However, the reactive fraction of the suspended particles increased over time, indicative of scavenging. Based on the artificial dust deposition experiment, aerosols should be retained in the sea surface microlayer long enough to undergo chemical and physical alteration that affects the bioavailability of trace elements. Opportunistic bacteria (example: Vibrio spp.) have been shown to experience rapid growth during dust deposition events. Aerosols and microlayer samples were collected in the Florida Keys over the course of two years for analysis of dissolved and particulate Al, Mn, Fe, Co, Ni, Cu, Zn, and Pb. Trace element concentrations increased by factors of 2 to 5 in the microlayer during significant Saharan dust events. Residence times of dissolved trace elements ranged from 0.12 hours for Mn to 2.4 hours for Cu. Residence times of particulate trace elements ranged from 1.1 minutes for Co to 2.4 minutes for Mn. The particulate residence times were comparable between the artificial deposition experiment and the natural deposition event observed in the Florida Keys. The relatively short residence times for dissolved trace elements compared to the artificial deposition event suggest external forces, such as wind and wave actions, mixed the dissolved metals faster than by simple molecular diffusion. Despite the short residence times, Vibrio spp. in the microlayer increased by factors of 2 to 10 after the passage of a Saharan dust event, which suggests that there was an initial pulse of bioavailable trace elements and other nutrients to the system. These findings demonstrate the dynamic nature of the sea surface microlayer and the large role atmospheric deposition can play when introducing trace elements to the surface oceans. It also sheds light on the need for more interdisciplinary research to deconvolute and quantify the processes occurring in the microlayer. / A Dissertation submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the Doctor of Philosophy. / Fall Semester 2016. / December 9, 2016. / atmospheric deposition, sea surface microlayer, trace elements / Includes bibliographical references. / William M. Landing, Professor Directing Dissertation; Albert E. Stiegman, University Representative; Angela N. Knapp, Committee Member; Sven A. Kranz, Committee Member; Vincent J. M. Salters, Committee Member.

Adsorption to sinking particulate matter, known as scavenging, is an important yet elusive term in the budgets of many bioactive and contaminant trace metals in the ocean. Scavenging is also involved in the cycling of other trace metals used in ocean sediments to learn about ocean processes, such as circulation rates or biological productivity, of the geologic past. This dissertation is an oceanographic exploration of the naturally-occurring radionuclides 230Th, 232Th and 231Pa aimed to better understand how their scavenging behavior can be utilized to trace chemical flux in the modern and the past ocean. While 230Th and 231Pa are produced by the radioactive decay of uranium dissolved in seawater, 232Th is added to the ocean via the input of continental material. Removal of Th by scavenging should be equivalent for both Th isotopes based on chemical principles. I use measured 232Th inventories from the North Pacific water column and apply a removal rate based on 234U:230Th disequilibria to estimate the flux of dissolved trace elements, such as the micro-nutrient Fe, to the ocean through the deposition of mineral dust (Chapter 2). Results from this study also raise the possibility of differing chemical behavior of the two Th isotopes in shallow water (<500 m depth) related to size-partitioning between truly dissolved and colloidal phases. Finally, results from the U.S. GEOTRACES North Atlantic Transect are used to investigate the relationship between 231Pa and 230Th in the water column and several of their proposed paleoceanographic purposes. The accuracy of 230Th as a proxy for constant sediment flux can be directly quantified to 40±10% with observations of boundary scavenging near the coast of Northwest Africa (Chapter 4). The 231Pa/230Th ratio in the water column is not clearly controlled by either ocean ventilation (Chapter 4) or particle composition (Chapter 5) alone. Near-bottom enhanced scavenging of both isotopes is also occurring over large regions, in benthic layers of resuspended sediments and in a hydrothermal plume emanating from the mid-Atlantic ridge. These novel observations must be reconciled in a new conceptual model, likely requiring future numerical modeling work, in order to clarify the use of the 231Pa/230Th ratio as a proxy for the Atlantic meridional overturning circulation.

Ocean circulation plays an integral part in a multitude of Earth's processes including the transfer of heat and nutrients across the globe. Additionally, its role in initiating and/or responding to global climate change is thought to be significant though poorly constrained. One tool used to further understand the influence of changes in ocean circulation during climate transitions is paleocirculation records developed from deep sea cores. These records paint a picture of how ocean circulation changed throughout time and are composed of an array of elements and isotopes extracted from different sediment archives. Neodymium (Nd) isotopes have been applied to paleocirculation because of the geographic variability of these isotopes in seawater and their ability to be preserved in deep sea sediments. Nd isotope records have been extracted from Fe-Mn crusts, leachates of sediment coatings, fish debris and foraminifera dissolutions to investigate changes in circulation at both deep and shallow ocean depths. Several of these records have been developed to investigate changes in the amount of northern vs. southern sourced waters in the South Atlantic Ocean. The advancement of northern sourced waters into the South Atlantic and Southern Ocean is an important branch of the global ocean circulation system known as Atlantic Meridional Overturning Circulation (AMOC). In order to further investigate changes of AMOC in the South Atlantic, we have developed a Nd isotope record from Cape Basin core TN057-6 for the last ~400 kyr. In agreement with the literature, the developed Nd record shows a decrease in AMOC during the Last Glacial Maximum and for previous glacial stages. Interglacial or warm periods defined by increased AMOC of comparable magnitude to modern circulation. These findings are summarized in the first two chapters of this thesis. The potential for Nd isotopes (εNd) as a water mass tracer is dependent on a thorough understanding of Nd cycling within the water column. The use of Nd isotopes in the modern ocean is also a valuable tool for investigating biogeochemical cycles and environmental perturbations such as dust or freshwater inputs. The distribution of εNd within the oceans suggests quasi-conservative behavior, traces water masses and shows correlations with both salinity and silicate. However, one observation known as the "Nd-paradox" suggests there are some poorly constrained sources and sinks of Nd in the ocean. The "Nd-paradox" refers to an apparent decoupling of Nd isotopes and Nd concentration ([Nd]) within the water column. In order to explain such features and the Nd cycle as a whole, it is essential to expand the database of Nd seawater data. As part of the GEOTRACES initiative, there have been a growing number of studies to measure seawater Nd-composition. The last two chapters of this thesis focus on the Nd-composition of seawater samples collected along GEOTRACES cruise transect GA03 from Lisbon, Portugal to Cape Verde Islands to Woods Hole, USA. The major water masses sampled as part of this cruise are Mediterranean Outflow Water, Antarctic Intermediate Water, North Atlantic Deep Water and Antarctic Bottom Water. Additional features sampled are near shore and open ocean stations, the Saharan dust plume, an expansive oxygen minimum zone, nepheloid layers and a Mid-Atlantic Ocean Ridge hydrothermal site. For each sample we calculate a predicted Nd-composition based on water mass mixing. By comparing the predicted and measured Nd-composition, we are able to investigate how Nd deviates from conservative behavior. Results from this work show that εNd is predominately conservative at deep depths at open ocean stations and is sensitive to small changes in water mass end-member Nd-compositions. This finding has important implications for the way end-members are defined in paleoceanographic Nd studies. Hydrothermal inputs are shown to have no influence on the isotopic composition of nearby water mass εNd compositions. However, an expansive nepheloid layer in the deep western North Atlantic does influence seawater εNd. [Nd] does not show conservative behavior but rather follows the "Nd-Paradox" such that concentrations increase with depth across the basin and exceed predicted [Nd] values.

Graduation date: 1983

Thesis (Ph. D.)--York University, 1998. Graduate Programme in Chemistry. / Typescript. Includes bibliographical references (leaves 245-251). Also available on the Internet. MODE OF ACCESS via web browser by entering the following URL: http://wwwlib.umi.com/cr/yorku/fullcit?pNQ39305.

Thesis (Ph. D.)--University of Texas at Austin, 2000. / Vita. Includes bibliographical references (leaves 142-155). Available also in a digital version from Dissertation Abstracts.

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (p. 107-116).

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