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

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Title:Fluxes, Remineralization Rates, and Spatial Distribution of Dissolved Carbon and Nutrients in Nearshore Hawaiian Permeable Sediments.
Authors:Fogaren, Kristen E.
Contributors:Oceanography (department)
Date Issued:Dec 2017
Publisher:University of Hawaiʻi at Mānoa
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
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
Description:Ph.D. Thesis. University of Hawaiʻi at Mānoa 2017.
Rights:All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections: Ph.D. - Oceanography

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