Graduation Year

2020

Document Type

Thesis

Degree

M.S.

Degree Name

Master of Science (M.S.)

Degree Granting Department

Marine Science

Major Professor

Tim Conway, Ph.D.

Committee Member

Robert Byrne, Ph.D.

Committee Member

William Homoky, Ph.D.

Keywords

biogeochemistry, GEOTRACES, Río de la Plata, sediments, Southern Ocean, trace metal

Abstract

Iron (Fe), used as a cofactor in nitrogen fixation and photosynthesis by oceanic microorganisms, has extremely low dissolved concentrations in the surface ocean, leading to widespread limitation of phytoplankton growth. Dissolved Fe isotope ratios (δ56Fe) have been shown to be useful in helping to quantify the sources and cycling of Fe in the oceans if Fe source signatures and fractionation processes are well understood. Here, this thesis presents data from GEOTRACES section GA10W, and investigate the isotopic signature of sediment-derived dissolved Fe from the South Atlantic margins. My results show that there are both shallow (δ56Fe of -0.2‰) and deep inputs (δ56Fe of -0.7‰) of dissolved Fe to the water column from sediments on the South American margin. Using a two-component mixing model, the data show that non-reductive sediment dissolution dominates surface inputs of Fe at shelf stations, while reductive release is more important at slope depths (~1250 m). This pattern appears to be driven by the sediment grain size and porosity rather than dissolved oxygen. Near the Uruguayan margin, the influence of a low- salinity plume from the Río de la Plata coincides with a large range in δ56Fe (-1.7 to +0.4‰), highlighting the complexities of Fe cycling in estuarine environments. Farther offshore, from 45°W to 25°W, average surface ocean δ56Fe signatures of +0.1‰ indicate that Fe derived from non-reductive sediment dissolution dominates Fe supply to the western South Atlantic. Farther east, from 20°W to 10°E, heavy δ56Fe in surface waters are linked to in situ surface processes occurring in the Fe-limited waters of the Southern Ocean. Sediment-derived Fe (δ56Fe of -0.5‰) is also observed near the South African margin, but it is not transported far from the shelf. Overall, my results demonstrate the importance of understanding both endmember δ56Fe signatures and in situ processes in order to use δ56Fe to quantify the sources and long-range transport of dissolved Fe.

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