Fluxes, Remineralization Rates, and Spatial Distribution of Dissolved Carbon and Nutrients in Nearshore Hawaiian Permeable Sediments.

Fogaren, Kristen E.

Fluxes, Remineralization Rates, and Spatial Distribution of Dissolved Carbon and Nutrients in Nearshore Hawaiian Permeable Sediments.

Files

2017-12-phd-fogaren.pdf (10.24 MB)

Date

2017-12

Authors

Fogaren, Kristen E.

Department

Oceanography

Abstract

Unlike fine-grained, diffusively dominated sediments, permeable sediments are subject to advective flows that can support high rates of organic matter remineralization through enhanced solute and particle exchange between the overlying water column and the sediment. While the importance of this enhanced exchange in marine permeable sediments has now been recognized, the effects of hydrodynamics on solute spatial distributions, sediment remineralization rates, and solute fluxes are not well constrained in these environments. This dissertation used a combination of in-situ experiments and modeling approaches in three studies to explore dissolved carbon, nutrient, and oxygen dynamics at three sites dominated by carbonate permeable sediments on Oahu, Hawaii. 1.) The fine-scale spatial variability of dissolved nutrients in fine-sand and coarse-sand permeable sediments was investigated using two field-based methodological approaches. These in-situ studies found greater spatial variability in the coarse sand than in the fine sand. Results of these experiments suggest that the dominant nutrient-regulating process in the upper sediment of the coarse sand was advective porewater circulation; however, other nutrient-regulating processes (e.g., organic matter deposition, bioturbation, oscillating redox conditions, benthic photosynthesis/respiration) control fine-scale spatial variability of nutrients in the fine sand. 2.) The rates of porewater transport in permeable sediments were estimated using transient ambient heat as a natural tracer. A statistical solution to the 1-D heat transport equation was used to estimate vertical front velocities and effective thermal diffusivities in the sediment from unfiltered temperature time-series measured using an array of buried thermistors. The method was successfully assessed using synthetic temperature datasets, freshwater streambed datasets from New York, and marine datasets from a nearshore sandy sediment in Hawaii. These ix field-constrained transport rates, and data from simultaneously collected discrete porewater samples, were then used in 1-D steady-state diagenetic models to calculate depth-dependent vertical fluxes and remineralization rates for dissolved nutrients and carbon in nearshore permeable sediment environments. 3.) Results from diagenetic models indicated that dissolved nutrient and carbon distributions in the upper sediment at the fine-sand and medium-sand sites were driven by organic matter remineralization. Stochiometric models were used to investigate potential geochemical processes responsible for the alteration of porewater nutrient inventories following organic matter remineralization. These models revealed that the majority of the regenerated dissolved nitrogen at the medium-sand site is removed, likely due to denitrification. Conversely, denitrification did not appear to be a significant nutrient-regulating process at the fine-sand site. Weak or non-existent relationships between hydrodynamic parameters measured in the overlying water column and calculated porewater transport indicate that we do not fully understand the mechanisms driving advective flows in these environments. Results in this dissertation highlight the complicated dynamics of permeable sediments in which distributions of dissolved carbon and nutrients are driven by advective flows and altered by biogeochemical processes post-remineralization. These results also suggest that these poorly understood environments may play a significant role in the coastal and global cycles of organic matter.

URI

http://hdl.handle.net/10125/62649

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Ph.D. - Oceanography

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