Date of Award

Spring 2007

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Ocean & Earth Sciences

Committee Director

David J. Burdige

Committee Member

Desmond Cook

Committee Member

Robert F. Dias

Committee Member

Richard C. Zimmerman

Abstract

This dissertation presents the results of studies examining the role that seagrasses play in carbonate dissolution and early diagenesis of Bahamas Bank sediments. Three aspects of this problem are addressed: (1) stable carbon isotopes as an indicator of early diagenesis of carbonates, using results of a field study; (2) carbonate dissolution stoichiometry and carbonate reprecipitation, using the results from closed-system sediment incubation studies; (3) carbonate dissolution and reprecipitation across the broader Bahamas Bank. In Chapter II, I examined δ13C in the dissolved inorganic carbon (DIC) of sediments with various degrees of seagrass densities. In low seagrass density and bare oolitic sand sediments, isotope mass balance could be explained by 1:1 mixing of DIC from carbonate dissolution and aerobic respiration. In contrast, pore water DIC in dense seagrass sediments was more enriched in 13C than predicted by the simple mixing model. A carbonate dissolution/reprecipitation model was proposed to explain these observations. In Chapter III, a series of closed-system sediment incubation experiments was carried out under controlled oxygen input rates (i) to further test the carbonate dissolution/reprecipitation model, (ii) to calculate reprecipitating carbonate phases, and (iii) to examine the relationship between the rates of oxygen consumption and carbonate dissolution in the these carbonate sediments. The carbonate reprecipitation model adequately explained pore water DIC 13C enrichment when dissolution and reprecipitation occur. Furthermore, using pore water data and solid phase analyses and assuming a high magnesium calcite (HMC) phase with ∼12 mole% Mg dissolved in these sediments, the reprecipitated carbonates had only a slightly lower Mg content than the starting material. Chapter IV presents the investigation of carbonate reprecipitation and dissolution mediated by seagrass based on an extensive pore water data set on the Bahamas Bank scale. A numerical advection-diffusion-reaction (ADR) model was used to calculate depth integrated reaction rates (i.e., fluxes at the sediment-water interface). The carbonate dissolution flux was then further examined as a function of seagrass density and sediment permeability. Based on the model results, a positive linear correlation was found between carbonate dissolution and leaf area index (LAI), while carbonate dissolution and sediment permeability showed no significant correlation. Carbon dissolution was found to be the likely dominant carbonate removal mechanism that accounts for ∼50% of gross carbonate production.

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DOI

10.25777/gn9t-c273

ISBN

9780549041016

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