Processes that influence carbon isotope variations in salt marsh sediments

Abstract

Salt marsh sedimentary organic matter (SOM) is a mixture of organic carbon from several sources difficult to identify quantitatively. Geochemical analyses of sediment cores at 4 sites in salt marshes at North Inlet, South Carolina (USA), dominated by Spartina alterniflora, were used to investigate accumulation and diagenesis of organic matter in sediments. Stable carbon isotope ratios ( 6 C ) and concentrations of organic carbon in the fine fraction of SOM ranged from -22 to -17 %O and 2 to 9 %, respectively. 6° values were significantly more positive in sediments from a short-form Spartina zone than from intermediate or tall-form Spartina areas. Samples from the site dominated by short-form S. alterniflora also contained significantly higher amounts of organic carbon than sites closer to the tidal creek, and demonstrated a positive correlation between organic carbon content and isotopically more positive 6° values. Spartina litter buried for 1.25 yr and Spartina lignin had 613C values of -15.35 and -16.34 %o respectively and were significantly more depleted in "C than fresh S. alterniflora (-13.63 %o), but not as depleted as the fine fraction of SOM. However, litter harvested from the marsh surface after 1.25 yr of decomposition had a S1^C value of -13.75 %o. The S^C values of SOM appear to be influenced by a combination of processes, including the selective preservation of isotopically hght refractory carbon, aerobic and anaerobic decay processes, sedimentation of allochthonous carbon from plankton or terrestrial sources, and bioturbation.

Heterogeneity of elemental composition and natural abundance of stables isotopes of C and N in soils and leaves of mangroves at their southernmost West Atlantic range

“…These values are more positive in general compared to those reported by Lacerda et al (1986b) for the Sepetiba bay in Río do Janeiro, and may indicate the influence of carbon from marine organisms (Bouillon et al, 2003). We expected differences in carbon isotopic signatures to appear in those mangroves where S. densiflora made up a significant fraction of the vegetation cover due to its C4-photosynthesis (Ember et al, 1987), but it was not the case.…”

“…Concentration of C in soils varied from about 3 to a little above 8%, within the range expected for wetlands (Ember et al, 1987). The δ 13 C ranged from about -22 to -24.6 ‰, with a tendency for more positive values in the lower soil layer.…”

Diagenesis of belowground biomass of Spartina alterniflora in salt‐marsh sediments

“…The polysaccharide components of Spartina and other plants are degraded by microorganisms at rates several-fold higher than the lignin component, leading to the gradual enrichment of lignin-derived C in remaining tissues (Benner et al 19843, 1986b(Benner et al 19843, , 1987. The observation that the stable C isotope composition of Spartina shifted to more negative values during decomposition as the relative concentration of lignin increased led to the hypothesis that Spartina may contribute significantly more to pools of sedimentary and particulate organic matter in salt marshes and adjacent coastal waters than would be predicted based on the C isotopic compositions of undegraded tissues (Benner et al 1987;Ember et al 1987;Fogel et al 1989).…”

“…Stable C isotope compositions of sedimentary organic matter in marshes dominated by Spartina are consistently depleted in 13C (613C of -16 to -20%0) relative to Spartina plant tissues (613C of -13%0) (Haines 1976;Ember et al 1987). The relatively large difference bctween the bulk isotopic composition of Spartina tissues and the isotopic signature of sedimentary organic matter is suggestive that other sources of organic matter with isotopic values similar to those of sedimentary organic matter, such as algae, are dominant contributors to the organic C in sediments or that alterations of isotopic compositions occur during decomposition of plant tissues or both.…”

“…Salt marshes on the southeastern seaboard of the U.S. are accreting sediments at rates ranging from 1.4 to 11 .O mm yr-' (Sharma et al 1987), and the organic matter contents of salt-1358 marsh sediments typically range from 5-20% (Ember et al 1987;Sharma et al 1987). The accretion of organic matter in salt-marsh sediments ranges from 70 to 225 g m-2 yr-' (assumes organic matter is 40 wt% C; Howes et al 1985;Wiegert and Pomeroy 198 1) and may be an important fate for a significant fraction of belowground production.…”