Diagenesis of organic matter in georgia salt marshes

Abstract

Organic matter preserved in sediments of Georgia salt marshes derives from Spartina alterniflora and planktonic and bacterial substances. Sources and diagenesis of sedimentary organic material were studied by means of stable C and N isotopes and gas chromatography of pyrolysates from plants, suspended sediments, and a series of muds (modern to 1400 years old) from St Catherines Island, Georgia. Suspended particulate matter ( > 20 μm) has isotopic compositions (δ 13C = −18 to −22), which are composed of an average 34–42% plankton carbon and 58–65% higher plant detritus, as determined by isotopic mass balance of the system. The δ 13C of bulk sediments and humic acids (δ 13C = −18 to −19) extracted from them remain relatively constant over time. The δ 13C of acid hydrolysates, however, changed from − 15‰ in surface marsh sediment to a δ 13C of −19 from sediment sampled at a depth of 125 cm. Higher-plant normal-hydrocarbons are present in muds and in particulate matter from the water column, and δ 13C in lipids and gas chromatography of pyrolysates of samples reflect this addition. With increasing age, δ 15N of total sediments and acid hydrolysates increases, and the ratio of odd-to-even hydrocarbons in the pyrolysates decreases. Initially, low density material in sediments consists primarily of plant fragments. With time, this material is decayed and diluted by microbial material, changes that are reflected by more negative δ 13C of −15 to −17·5, as compared to δ 13C of −12 for moden Spartina, and the addition of n-alkanes in the range of C 13–19. The isotopic heterogeneity in surface sediments with carbon as an example, ranged from a δ 13C of −15 (plant fragments) to −22 (sediment following lipid, humic acid, and plant fragment extraction). Nowhere was the C isotopic composition of total Spartina expressed even though microscopic plant pieces, lignocellulose pyrolysis fragments, and n-alkanes from C 22–27 were detected in suspended particulate matter, total sediments, and humic acids.