Date of Award

Summer 1987

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Program/Concentration

Oceanography

Committee Director

Gregory A. Cutter

Committee Member

David J. Burdige

Committee Member

Thomas M. Church

Committee Member

William Dunstan

Committee Member

George T. F. Wong

Abstract

Investigation of the various chemical forms of selenium and sulfur in sediments and pore waters can provide information about various oxidation/reduction processes. Five cores were obtained from the Great Marsh, from April 1985 to June 1986. Sampling times coincided with the seasonal redox cycle known to occur within the marsh system. Sediments were analyzed for various selenium and sulfur phases.

Iron monosulfides and elemental sulfur both display large seasonal changes in concentration and distribution with depth, indicating a coupling with redox conditions. In contrast, the depth distribution of greigite did not show appreciable changes with season. Pyrite underwent large concentration changes in the upper 15 cm of sediment during spring, but remained relatively constant with respect to concentration and distribution below this zone. Using a mass balance approach for the upper marsh sediment (0-15 cm), sulfur needed for the observed rapid pyritization is found to be derived from elemental sulfur, iron monosulfide, and sulfate reduction. In the deeper sediments (15-30 cm), diagenetic modeling confirms that greigite is an intermediate in pyrite formation.

The depth distribution of total sedimentary selenium shows minor variations with season. Concentrations are generally higher in the surface layers and then decrease with depth. Elemental selenium exhibits a trend with depth similar to total selenium. Chromium reducible selenium was generally undetectable in most cores and shows little seasonality. In contrast, sedimentary (selenite + selenate) shows marked seasonality. In spring, sedimentary is less than 10% of the total sedimentary selenium throughout the profile. However in summer, a broad maximum (30% of the total selenium) occurs just above the redoxcline. Below the redoxcline sedimentary accounts for less than 10% of the total sedimentary selenium. Pore water selenium exhibits a seasonal trend, concurrent with the cyclic changes in sedimentary. Diagenetic modeling shows that the loss of total sedimentary selenium is controlled by the decrease in elemental selenium. Mass balance modeling indicates that the major export of selenium from the marsh sediment is either gaseous emissions of selenium or the flux of Spartina alterniflora litter from the marsh system. Gaseous selenium is a potentially important source of selenium to the atmosphere.

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DOI

10.25777/z5cp-zc86

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