Salt marsh sedimentary organic matter (SOM) is a mixture of organic carbon from several sources difficult to identify quantitatively. Geochemical analyses of sediment cores at 4 sites in salt marshes at North Inlet, South Carolina (USA), dominated by Spartina alterniflora, were used to investigate accumulation and diagenesis of organic matter in sediments. Stable carbon isotope ratios ( 6 C ) and concentrations of organic carbon in the fine fraction of SOM ranged from -22 to -17 %O and 2 to 9 %, respectively. 6° values were significantly more positive in sediments from a short-form Spartina zone than from intermediate or tall-form Spartina areas. Samples from the site dominated by short-form S. alterniflora also contained significantly higher amounts of organic carbon than sites closer to the tidal creek, and demonstrated a positive correlation between organic carbon content and isotopically more positive 6° values. Spartina litter buried for 1.25 yr and Spartina lignin had 613C values of -15.35 and -16.34 %o respectively and were significantly more depleted in "C than fresh S. alterniflora (-13.63 %o), but not as depleted as the fine fraction of SOM. However, litter harvested from the marsh surface after 1.25 yr of decomposition had a S1^C value of -13.75 %o. The S^C values of SOM appear to be influenced by a combination of processes, including the selective preservation of isotopically hght refractory carbon, aerobic and anaerobic decay processes, sedimentation of allochthonous carbon from plankton or terrestrial sources, and bioturbation.

We measured the physical charactenstics of 4 sedlment types commonly found in southeastern (USA) salt marshes and their relationship to sediment drainage and compressibility. Compressibility was found to be positively correlated with total silt-clay content (r2 = 0.953) and negatively correlated with total sand content (r2 = 0.942). A linear relationship (r2 = 0.832) was found between the square root of sediment percolation velocity and bulk density. Calculations of the rate of air entry and porewater turnover based on our measurements of drainage and compressibility are consistent with the supply of SO,' ' necessary to support the rates of SO,'-reduction found at North Inlet, South Carolina, USA, by previous researchers and indicate that compressibility may regulate the turnover of reduced end products such as pyrite. For incompressible sediments, it was demonstrated that the entry of air into sediments following water loss by evapotranspiration is quantitatively important in oxidizing reduced sulfur compounds, while drainage of compressible creek bank sediments is apparently sufficient to replace SO4'-utilized by dissimilatory Sod2-reducers.

The volume of water removed from marsh sediments by evapotranspiration and drainage during low tide is replaced at least in part by air drawn into the sediment. The volume of gas advected into salt marsh sediments was measured in situ by recording the displacement of air in manometers connected to the sealed headspace of core tubes placed in the sediment. Measurements, made during June and July in short form Spartina alterniflora marshes in South Carolina, indicated that as much as 4.0 1 m-, of gas was drawn into the sediment, depending on marsh location and climatic variables. Gas advection rates were equivalent to water losses of 1.5 to 4.0 % of the total sediment water above the water table and corresponded to water table depths that fell to as much as 34 cm during low tide. Transpiration rates of S. alterniflora accounted for about 22% of the gas volume drawn into the sediment. Analyses of gas bubbles expelled from the sediment during tidal inundation showed that O2 was consumed in excess of CO, advected. Excess O2 consumption may result because of sulfide oxidation; this possibility is supported by decreasing pore-water pH during low tide exposure. Alternatively, some CO, produced in the sediment may remain in the pore water in dissolved form. Considering both advective and diffusive gas fluxes, a total of 56.3 mm01 m-, of CO, (4.9 by advection, 51.4 by diffusion) was evolved from the sediment during low tide compared to a total 0, consumption of 32.9 mm01 m-,. The mean 0, consumption from gas drawn into the sediment during low tide was 10.9 mm01 m-,. This compares to a mean diffusive 0, consumption by surface sediment of 22.0 mm01 m-, during 10 h of low tide exposure.