BURIAL AND DECOMPOSITION OF PARTICULATE ORGANIC MATTER IN A TEMPERATE, SILICICLASTIC, FRESHWATER WETLAND

Understanding the role of freshwater wetlands in the global carbon cycle has become more important as evidence of climate change increases. In this paper, we examine the burial and decomposition of particulate organic matter (POM) in a temperate, siliciclastic, freshwater wetland. High POM abundances are found in silt layers, while sand units preserve very little POM. The POM distribution with depth is compared to the biogeochemistry of sediment porewater with depth. POM acts as an electron donor source in the wetland system. Porewater biogeochemistry and POM decomposition are controlled by seasonal water table changes, which cause seasonal shifts in the oxic/anoxic boundary. At the oxic/anoxic boundary, reoxidation of iron sulfide minerals in the sediment causes increased POM decomposition. In the late summer and early fall, when water levels in the slough generally drop below the sediment surface, reoxidation of iron sulfide minerals is particularly important and the resulting POM decomposition occurs at the level of the water table. The average minimum water table in the slough during the dry season is -16.5 cm below the sediment surface and POM decomposition appears to be focused at this depth, which results in a pattern of variable but high POM abundance in the upper 16 cm of the sediment, and lower POM abundance below 16 cm depth. Variation in the minimum depth of the water table from year to year may account for high, yet fluctuating POM abundance in the upper silt layers. The results from this study can be used to infer the presence of seasonal water level fluctuations in ancient wetland and to explain recurrence horizons in peat, which are relatively undecomposed layers within highly decomposed peat deposits.