THE TAPHONOMY OF PLANT MESOFOSSILS FROM A TEMPERATE SILICLASTIC WETLAND

Plant mesofossils (100 µm - 1000 µm) are an important data source for understanding plant evolution and paleoecology. However relatively few studies address the taphonomy of Recent plant mesofossils. In order to understand how mesofossils form and the relationship between plant mesofossils and the surrounding plant communities, we quantified the coarse (> 1000 µm) and fine (1000 - 500 µm) particulate organic matter (POM) in a 66 cm core from the Norman Landfill Slough, which is a seasonally dry, siliclastic wetland in the temperate zone, colonized primarily by grasses. The redox biogeochemistry of this site was studied in May of 2003, using Capillary Electrophoresis (CE) of water samples taken from passive diffusion samplers (peepers), which can collect a vertical profile of closely spaced pore water samples (at 5 -10 mm intervals) for biogeochemical characterization. The slough sediments consist of alternating organic-rich silts, which accumulated in quiet-water marshes, and medium to coarse sands, which represent flood deposits. Porewater pH ranges from 6.9 to 7.5. Most of the POM found in slough sediments consisted of plant fibers; however wood, seeds, insect parts and snail shells occurred throughout the core. Most of the seeds encountered derived from plants growing in the wetland. POM was most common in the upper 15-18 cm of the core, which was also the maximum depth of viable seeds, determined by exposing the cut core to light and noting the distribution of sprouts. This depth (18 cm) corresponds to the level at which fermentation products (acetate, propionate, and butrate) began to accumulate in sediment pore waters, suggesting a shift in sediment biogeochemistry associated with the decrease in POM. There may be a connection between the presence of fermentation products and the loss of seed viability: crop residues inhibited seed germination in fields due to high levels of phenolic acid in the soil. However, few data exist concerning the influence of simple organic acids on seed viability in wetland soil. Although the total amount of POM decreased with depth in slough sediments, the importance of wood relative to seeds and fibers increased with depth. Wood has a relatively high C:N ratio, thus the C:N ratio of POM probably increases with depth in the Norman Landfill Slough.