41
2.4 Factors Controlling the Stability of Organic Carbon
in Sediments: Degradability of Organic Matter
2.4.1 Concept of Burial Efficiency
Most of the OC deposited in marine sediment is eventually remineralized into CO 2
by the heterotrophic activities of benthic animals and microorganisms at the
sediment–water interface and in the surface reactive layer (Emerson et al. 1985).
Only a small fraction of OC withstands decomposition during early diagenesis and
is buried in deep sediment layers. This fraction is commonly referred to as the OC
burial rate or OC burial efficiency. There is some ambiguity in the definition of the
burial efficiency, i.e., how deep OC must be buried or how long OC must withstand
decomposition to be regarded as “buried”. In ideal pelagic sediment cores, the
concentration of OC in the top layer (C 0 ) decreases exponentially down toward
deeper layers, approaching a constant concentration (C ∞ ). C ∞ is theoretically
considered to represent permanently buried OC. The OC concentration (C; μmol C
(g dry weight)
−1
) at any depth (d; cm) can be approximated as follows:
C C C
k d C
k
= (
) ( )+ (
)
0 –
–
constant
¥
¥
exp
:
.
(2.1)
Then, the burial efficiency B can be defined as
B C C
= ¥ / 0 .
(2.2)
In the conventional definition of “reactive marine sediment” (Sundquist 1985), early
diagenesis is considered to occur primarily in the top 50-cm layer of coastal
sediment and the top 30-cm layer of deep pelagic sediment. These values may be
used as a practical definition of burial efficiency, such that burial efficiency is
defined as the ratio of the OC concentration at the top (C 0 ) to that at the bottom (C b )
of the reactive layer (Fig. 2.2):
B C C
b
=
/ 0 .
(2.3)
However, it is sometimes problematic to apply this definition to seagrass meadow
sediments, because the production of belowground tissues by seagrasses disturbs
the typical exponential vertical profile of OC represented by Eq. (2.1). The OC
concentration often exhibits a maximum in the subsurface layer where the rhizomes
and roots of seagrasses are abundant, while OC in the top layer is relatively depleted
(e.g., Fig. 2.1f). The typical exponential profile is recognizable in only one (Fig. 2.3a)
of the seven examples shown in Fig. 2.3. Multiple mid-layer peaks of OC are
sometimes observed (e.g., Fig. 2.3b, d, g), which presumably reflects changing
supply rates of both OC and mineral sediment. As a practical solution, seagrass
researchers often assume that OC equivalent to the average concentration in the top
20–30 cm of sediment represents the long-term stock of OC, without explicit
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows
2.4 Factors Controlling the Stability of Organic Carbon
in Sediments: Degradability of Organic Matter
2.4.1 Concept of Burial Efficiency
Most of the OC deposited in marine sediment is eventually remineralized into CO 2
by the heterotrophic activities of benthic animals and microorganisms at the
sediment–water interface and in the surface reactive layer (Emerson et al. 1985).
Only a small fraction of OC withstands decomposition during early diagenesis and
is buried in deep sediment layers. This fraction is commonly referred to as the OC
burial rate or OC burial efficiency. There is some ambiguity in the definition of the
burial efficiency, i.e., how deep OC must be buried or how long OC must withstand
decomposition to be regarded as “buried”. In ideal pelagic sediment cores, the
concentration of OC in the top layer (C 0 ) decreases exponentially down toward
deeper layers, approaching a constant concentration (C ∞ ). C ∞ is theoretically
considered to represent permanently buried OC. The OC concentration (C; μmol C
(g dry weight)
−1
) at any depth (d; cm) can be approximated as follows:
C C C
k d C
k
= (
) ( )+ (
)
0 –
–
constant
¥
¥
exp
:
.
(2.1)
Then, the burial efficiency B can be defined as
B C C
= ¥ / 0 .
(2.2)
In the conventional definition of “reactive marine sediment” (Sundquist 1985), early
diagenesis is considered to occur primarily in the top 50-cm layer of coastal
sediment and the top 30-cm layer of deep pelagic sediment. These values may be
used as a practical definition of burial efficiency, such that burial efficiency is
defined as the ratio of the OC concentration at the top (C 0 ) to that at the bottom (C b )
of the reactive layer (Fig. 2.2):
B C C
b
=
/ 0 .
(2.3)
However, it is sometimes problematic to apply this definition to seagrass meadow
sediments, because the production of belowground tissues by seagrasses disturbs
the typical exponential vertical profile of OC represented by Eq. (2.1). The OC
concentration often exhibits a maximum in the subsurface layer where the rhizomes
and roots of seagrasses are abundant, while OC in the top layer is relatively depleted
(e.g., Fig. 2.1f). The typical exponential profile is recognizable in only one (Fig. 2.3a)
of the seven examples shown in Fig. 2.3. Multiple mid-layer peaks of OC are
sometimes observed (e.g., Fig. 2.3b, d, g), which presumably reflects changing
supply rates of both OC and mineral sediment. As a practical solution, seagrass
researchers often assume that OC equivalent to the average concentration in the top
20–30 cm of sediment represents the long-term stock of OC, without explicit
2 Carbon Sequestration in Sediment as an Ecosystem Function of Seagrass Meadows