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Résumé :
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This study examines spatial variability in sedimentary organic matter (OM). A total of 25 surface sediment samples were analyzed from two transects across a 0.5-km(2) salt marsh island; one transect traverses relatively old marsh and the other a mix of old and new marsh. We use elevation and distance to creek head as geomorphic proxies and associate them with the abundance (carbon, nitrogen) and the isotopic/molecular composition of organic constituents (i.e., lignin). Statistically significant linear correlations were observed between elevation and distance from the creek head with OM content, suggesting a likely geomorphic influence. However, the trend was positive for the older marsh and negative for the new marsh. Hence, sediment sampling at or near levees can give rise to a factor of 5-8 difference in OM content. Also, opposing trends were observed in OM provenance with old marsh having greater contributions of marine algae or allochthonous OM and new marsh having a predominantly autochthonous OM signature. Further, these data reveal two scales of variability in OM content and composition, one at similar to 10(1) m and the other at similar to 10(3) m. We ascribe the 10(1)-m scale variability to the effect of salt marsh geomorphic structure, while the 10(3)-m scale variability is likely related to the differences in local nutrient input and OM preservation and due to sediment source and accumulation rates and processes between sites.
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