Graduation Year

2025

Document Type

Dissertation

Degree

Ph.D.

Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

Marine Science

Major Professor

Tim M. Conway, Ph.D.

Committee Member

Robert H. Byrne, Ph.D.

Committee Member

P. Dreux Chappell, Ph.D.

Committee Member

Maeve C. Lohan, Ph.D.

Committee Member

Joseph J. Tamborski, Ph.D.

Keywords

Biogeochemistry, Coastal Margins, Rivers, Sediments, Submarine Groundwater, Trace Metals

Abstract

Dissolved iron (dFe) is an essential micronutrient for sustaining life in the surface ocean. Nearly four decades of research dedicated to understanding sources, cycling and biogeochemical modifications of Fe sources in the global oceans has generated a significant body of research supporting the idea that the concentration of Fe in the surface ocean exerts significant controls on primary production and carbon uptake in the surface ocean. Creation of the GEOTRACES program greatly increased the spatial and temporal resolution of high-quality trace element and trace element isotope data revealing new insights into the global Fe cycle, especially through the use of dissolved iron isotopic compositions (δ56Fe). It has been clearly demonstrated that dissolved Fe isotopic compositions are a useful tracer of point source enrichments and biogeochemical processes in the interior ocean. However, key questions remain regarding the utility of δ56Fe as a tracer of margin sources over long distances, and the effects of fractionation during dFe loss processes. Motivated by these key knowledge gaps, this dissertation investigates continental margin sources of dissolved iron, including rivers, sediments, and submarine groundwater, to the global oceans using dissolved iron isotopic compositions (δ56Fe). Further, it examines the impact of circulation on the distribution of dFe and δ56Fe in distinct water masses, and the utility of δ56Fe as a point source tracer over long distances in subsurface waters.

Chapter Three examines the influence of the Congo River, sedimentary margins, and water mass mixing on the distribution of dFe and δ56Fe to the South Atlantic Gyre using samples collected from the German GEOTRACES GA08 Namibian-Congo margin campaign. This dataset provides a unique opportunity to compare three distinct oceanographic regimes, a river dominated margin, an open ocean oligotrophic gyre, and a highly productive upwelling margin. Organic matter binding dFe and rapid offshore advection contributes to the preservation of the Congo River Plume dFe enrichment over 1000 km from the river outlet. At depth, the δ56Fe distributions are largely described by water mass mixing with some additional light Fe addition from the sedimentary margin at intermediate depths. Under the Benguela Upwelling System, seasonal anoxia in bottom waters over the shelf contributes to the extensive dFe enrichment by reductive dissolution in sediments. The dFe enrichment over the shelf carries the same isotopic composition as anoxic porewaters from the same margin, suggesting the dFe can diffuse into the overlying waters with little to no fractionation and δ56Fe can be used to effectively trace the existence of enrichments from sediments through the water column.

Chapter Four focuses on the Japanese GEOTRACES section GP02 and the oceanographic time series Line P in the subarctic Pacific Ocean. The East Asian Sedimentary Plume (EASP) is a well-studied feature with respect to dFe concentrations, however, the longevity of the δ56Fe signature and modification with transport has never been characterized. The EASP influences dFe and δ56Fe distributions in both the upper and lower fractions of North Pacific Intermediate Water (NPIW). The EASP is transported in L-NPIW along the 27.72 ? neutral density surface with negligible change in δ56Fe as far as the Gulf of Alaska, suggesting particle scavenging occurs without fractionation in this basin. Water mass mixing between lower North Pacific Intermediate water and Pacific Deep Water largely explains the change in δ56Fe in L-NPIW in this basin along 47˚N. Distal hydrothermal plume samples collected from the Juan de Fuca ridge system show no change in isotopic composition with a dramatic decrease in dFe away from the vent, further suggesting δ56Fe can be an effective tracer of marginal sources of dFe over long distances.

Chapter Five investigates seasonality of dFe and δ56Fe from five rivers and nine groundwater wells important to the West Florida Shelf. The West Florida Shelf is an excellent natural laboratory for the determination of seasonality of marginal sources of macro and micronutrients. The uniquely wide continental margin with a high degree of submarine groundwater discharge and riverine inputs, influences the delicate chemistry of the region. The offshore groundwater wells exhibit a lower degree of dFe and δ56Fe seasonality in the West Florida Shelf compared to rivers, yet carry a higher range between wells, suggesting higher spatial variability. The riverine dFe and δ56Fe endmembers significantly vary, but are all influenced by salinity, river flow rate and groundwater source contributions. Further characterization of terrestrial groundwaters is required to fully assess the impact of each aquifer system on the distribution of dFe and δ56Fe in each river system.

Overall, these findings provide new insights for using δ56Fe to investigate the importance of sedimentary margins as sources of dFe to the global oceans. Further, the documentation of remarkable preservation of Fe isotopic compositions away from the margins during water mass circulation informs and strengthens the use of δ56Fe as a source tracer at the basin scale, advancing our use of δ56Fe distributions to understand dFe transport away from the margins.

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