47
adsorptive interactions with the mineral surface, such that microbial hydrolytic
enzymes may not easily access the substrates, resulting in inhibition or extensive
retardation of degradation (Kaiser and Guggenberger 2007). Regardless of the internal mechanism, the adsorption process enhances preservation of OC in sediment,
which apparently leads to a strong correlation between OC and SSA in clastic
coastal sediments. The mechanism assumed to drive this correlation is often referred
to as sorptive preservation of organic matter. Because SSA is generally greater for
fine-grained sediments, ecosystems like seagrass meadows with a high capacity to
accumulate fine-grained sediment also have a high potential to sequester OC.
2.5 Influence of Exposure to Molecular Oxygen
on the Preservation of Organic Matter
Another major factor that influences the burial efficiency of OC in sediment is the
availability of metabolic electron acceptors, especially O 2 . This is because the
remineralization rate of OC by sediment microorganisms depends strongly on the
availability of electron acceptors, and O 2 is the most energetically favorable electron
acceptor for the microbial electron transport system. Moreover, O 2 is a precursor of
highly reactive intermediates such as reactive oxygen species, peroxides, and Mn
3+
,
which are produced by certain microbial enzymes, initiating autocatalytic oxidation
of OC and degrading even inherently recalcitrant organic matter such as lignin
(Raghukumar et al. 1999; Hammel et al. 2002; Galeron et al. 2018).
When the overlying seawater contains O 2 , the surface layer of sediment contains
O 2 in its pore water (oxic layer). The concentration of O 2 in the pore water decreases
exponentially as sediment depth increases due to consumption by the respiration of
microorganisms. The O 2 concentration eventually drops below the detection limit at
a certain depth, referred to as the oxic/anoxic interface (Fig. 2.2). The thickness of
the oxic layer, also referred to as the oxygen penetration depth, varies widely
depending on the balance between supply and consumption of O 2 . The supply rate
of O 2 is constrained by the O 2 concentration in the overlying water and the advective
or diffusive supply rate of O 2 into sediment, with the latter depending on sediment
texture and the intensity of turbulence in the overlying water. The consumption rate
of O 2 depends principally on the availability of degradable organic matter as
microbial substrates and on water temperature. In the fine-grained sediment of
sheltered eutrophic embayments, the oxic layer is generally very thin, often <1 mm.
In contrast, the oxic layer of carbon-limited, oligotrophic open ocean sediment is
much thicker, sometimes reaching several meters (Wilson et al. 1985; Emerson and
Hedges 2003).
The time required for newly deposited surface sediment to reach the oxic/anoxic
interface through burial is referred to as the oxygen exposure time (OET). It is
evaluated based on the thickness of the oxic layer (in mm) divided by the
sedimentation rate (in mm year
−1
) and ranges geographically from <1 to >1000 years.
The OET is one of the principal factors constraining the OC burial efficiency in
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows
adsorptive interactions with the mineral surface, such that microbial hydrolytic
enzymes may not easily access the substrates, resulting in inhibition or extensive
retardation of degradation (Kaiser and Guggenberger 2007). Regardless of the internal mechanism, the adsorption process enhances preservation of OC in sediment,
which apparently leads to a strong correlation between OC and SSA in clastic
coastal sediments. The mechanism assumed to drive this correlation is often referred
to as sorptive preservation of organic matter. Because SSA is generally greater for
fine-grained sediments, ecosystems like seagrass meadows with a high capacity to
accumulate fine-grained sediment also have a high potential to sequester OC.
2.5 Influence of Exposure to Molecular Oxygen
on the Preservation of Organic Matter
Another major factor that influences the burial efficiency of OC in sediment is the
availability of metabolic electron acceptors, especially O 2 . This is because the
remineralization rate of OC by sediment microorganisms depends strongly on the
availability of electron acceptors, and O 2 is the most energetically favorable electron
acceptor for the microbial electron transport system. Moreover, O 2 is a precursor of
highly reactive intermediates such as reactive oxygen species, peroxides, and Mn
3+
,
which are produced by certain microbial enzymes, initiating autocatalytic oxidation
of OC and degrading even inherently recalcitrant organic matter such as lignin
(Raghukumar et al. 1999; Hammel et al. 2002; Galeron et al. 2018).
When the overlying seawater contains O 2 , the surface layer of sediment contains
O 2 in its pore water (oxic layer). The concentration of O 2 in the pore water decreases
exponentially as sediment depth increases due to consumption by the respiration of
microorganisms. The O 2 concentration eventually drops below the detection limit at
a certain depth, referred to as the oxic/anoxic interface (Fig. 2.2). The thickness of
the oxic layer, also referred to as the oxygen penetration depth, varies widely
depending on the balance between supply and consumption of O 2 . The supply rate
of O 2 is constrained by the O 2 concentration in the overlying water and the advective
or diffusive supply rate of O 2 into sediment, with the latter depending on sediment
texture and the intensity of turbulence in the overlying water. The consumption rate
of O 2 depends principally on the availability of degradable organic matter as
microbial substrates and on water temperature. In the fine-grained sediment of
sheltered eutrophic embayments, the oxic layer is generally very thin, often <1 mm.
In contrast, the oxic layer of carbon-limited, oligotrophic open ocean sediment is
much thicker, sometimes reaching several meters (Wilson et al. 1985; Emerson and
Hedges 2003).
The time required for newly deposited surface sediment to reach the oxic/anoxic
interface through burial is referred to as the oxygen exposure time (OET). It is
evaluated based on the thickness of the oxic layer (in mm) divided by the
sedimentation rate (in mm year
−1
) and ranges geographically from <1 to >1000 years.
The OET is one of the principal factors constraining the OC burial efficiency in
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows