48
pelagic sediments, with longer OETs allowing more extensive degradation of OC,
resulting in lower OC burial efficiencies. Previous studies have demonstrated that
the OC burial efficiency (Hartnett et al. 1998) and the OC/SSA ratio (Hedges et al.
1999) in continental margin sediments of the eastern North Pacific have negative
correlations with the logarithm of OET. These relationships imply that (i)
remineralization of OC (in particular, mineral-associated OC) proceeds during the
OET but not significantly after burial in anoxic layers, and that (ii) the rate constant
of OC remineralization during the OET does not depend significantly on the
thickness of the oxic layer or OET.
In terms of remineralization during early diagenesis, OC in sediment may be
conceptually categorized into three groups, as shown in Table 2.4; these are mineralfree labile (degradable), mineral-associated labile, and inherently refractory
(undegradable) OC. Based on this classification, the conditions for long-term
sequestration of OC in sediments can be summarized as follows. OC that can be
sequestered long term in marine sediment is carbon contained in either mineralassociated labile organic carbon (LOC) or inherently refractory organic carbon
(ROC). The supply rate of ROC is controlled primarily by the supply rate of woody
organic substances from land and mangroves and therefore depends on the
hydrological proximity to these habitats. For LOC to be sequestered in sediment,
the mineral surfaces required for adsorption of LOC must be supplied simultaneously
with LOC. In highly productive environments such as seagrass meadows, overall
sequestration of OC may be limited by the supply of available mineral surfaces
rather than the supply of organic matter. As mineral-adsorbed LOC may be
remineralized slowly in the oxic layer, the burial efficiency of LOC is constrained
by the OET.
2.6 Significance of Seagrass Meadows in Coastal Ecosystem
Function
The factors outlined in Sects. 2.4 and 2.5 have important implications in relation to
the significance of seagrass meadows in shallow coastal ocean functions.
Oceanographic conditions that prevail over shallow coastal sediments are generally
unfavorable for sequestration of OC (Aller 1998). In such environments, wave
action and tidal currents often reach the bottom, causing frequent resuspension and
removal of fine-grained sediment, such as clay, silt and organic detritus. As explained
in Sect. 2.4, fine-grained sediment has a greater capacity to store OC by sorptive
preservation per unit weight than does coarse-grained sediment. Therefore, removal
of fine-grained sediment results in net loss of OC from shallow coastal sediments.
Furthermore, penetration of wave-induced oscillation flows and tidal pumping
cause intrusion of O 2 into deeper sediment layers, resulting in elongation of the
OET. As noted in Sect. 2.5, longer OET may cause gradual loss of mineral-adsorbed
LOC, and as a result, the OC loading of sediment particles may decrease from
T. Miyajima and M. Hamaguchi
pelagic sediments, with longer OETs allowing more extensive degradation of OC,
resulting in lower OC burial efficiencies. Previous studies have demonstrated that
the OC burial efficiency (Hartnett et al. 1998) and the OC/SSA ratio (Hedges et al.
1999) in continental margin sediments of the eastern North Pacific have negative
correlations with the logarithm of OET. These relationships imply that (i)
remineralization of OC (in particular, mineral-associated OC) proceeds during the
OET but not significantly after burial in anoxic layers, and that (ii) the rate constant
of OC remineralization during the OET does not depend significantly on the
thickness of the oxic layer or OET.
In terms of remineralization during early diagenesis, OC in sediment may be
conceptually categorized into three groups, as shown in Table 2.4; these are mineralfree labile (degradable), mineral-associated labile, and inherently refractory
(undegradable) OC. Based on this classification, the conditions for long-term
sequestration of OC in sediments can be summarized as follows. OC that can be
sequestered long term in marine sediment is carbon contained in either mineralassociated labile organic carbon (LOC) or inherently refractory organic carbon
(ROC). The supply rate of ROC is controlled primarily by the supply rate of woody
organic substances from land and mangroves and therefore depends on the
hydrological proximity to these habitats. For LOC to be sequestered in sediment,
the mineral surfaces required for adsorption of LOC must be supplied simultaneously
with LOC. In highly productive environments such as seagrass meadows, overall
sequestration of OC may be limited by the supply of available mineral surfaces
rather than the supply of organic matter. As mineral-adsorbed LOC may be
remineralized slowly in the oxic layer, the burial efficiency of LOC is constrained
by the OET.
2.6 Significance of Seagrass Meadows in Coastal Ecosystem
Function
The factors outlined in Sects. 2.4 and 2.5 have important implications in relation to
the significance of seagrass meadows in shallow coastal ocean functions.
Oceanographic conditions that prevail over shallow coastal sediments are generally
unfavorable for sequestration of OC (Aller 1998). In such environments, wave
action and tidal currents often reach the bottom, causing frequent resuspension and
removal of fine-grained sediment, such as clay, silt and organic detritus. As explained
in Sect. 2.4, fine-grained sediment has a greater capacity to store OC by sorptive
preservation per unit weight than does coarse-grained sediment. Therefore, removal
of fine-grained sediment results in net loss of OC from shallow coastal sediments.
Furthermore, penetration of wave-induced oscillation flows and tidal pumping
cause intrusion of O 2 into deeper sediment layers, resulting in elongation of the
OET. As noted in Sect. 2.5, longer OET may cause gradual loss of mineral-adsorbed
LOC, and as a result, the OC loading of sediment particles may decrease from
T. Miyajima and M. Hamaguchi