Abstract:

Palynofacies and pyrolysis were applied to four surface sediment samples (F2F5) from the Changjiang estuary and their 18Φ particle size fractions to discuss the characteristics of organic matter (OM) and their fate in carbon cycle. The palynofacies of bulk samples are mainly comprised of woody tissue, charcoal, and black mass OM, which indicates a large contribution of terrestrial organic matter input. After fractionation, the feature of palynofacies changed significantly. Fine particle size fraction (8Φ) dominated in amorphous OM, while other fractions (27Φ) were comprised of structural OM. Fine particle size fraction (8Φ) was characterized by high HI, S2 and low Tmax value, which suggests an aquatic organic matter origin. However, the OMs of other size fractions (27Φ) were characterized by terrestrial organic matter input and great oxidative degradation. In samples of F4 and F5, which are far from the Changjiang estuary, about 70% of the TOC was contributed by fine particle size fraction (8Φ)，while the TOC is homogeneous in different size fractions of samples of F2 and F3 which are near the estuary. The amorphous OM in fine size fraction (8Φ) was protected by clay minerals from biodegradation and oxidative degradation. These OM flocculate and precipitate rapidly, and shortened the process from biosphere, hydrosphere to lithosphere. As a result，this may prolong the period of carbon cycling. The OMs in other fractions (27Φ) underwent oxidative degradation and were preserved in sediments with  the longest period of carbon cycling. Consequently, the relationship between sedimentary OM and clay mineral and their influence on the time of carbon cycling should be concerned. This may help us to get some future understanding of carbon cycle which is still uncertain.

Key words: surface sediment, palynofacies, heterogeneity of OM, carbon cycle