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Peatlands contain one-third of soil carbon (C), mostly buried in deep, saturated anoxic zones (catotelm). The response of catotelm C to climate forcing is uncertain, because prior experiments have focused on surface warming. We show that deep peat heating of a 2 m-thick peat column results in an exponential increase in CH4 emissions. However, this response is due solely to surface processes and not degradation of catotelm peat. Incubations show that only the top 20–30 cm of peat from experimental plots have higher CH4 production rates at elevated temperatures. Radiocarbon analyses demonstrate that CH4 and CO2 are produced primarily from decomposition of surface-derived modern photosynthate, not catotelm C. There are no differences in microbial abundances, dissolved organic matter concentrations or degradative enzyme activities among treatments. These results suggest that although surface peat will respond to increasing temperature, the large reservoir of catotelm C is stable under current anoxic conditions.

Submarine ground water discharge (SGD) is now recognized as an important water pathway between land and sea. It is difficult to quantitatively predict SGD owing to its significant spatial and temporal variability. This study focuses on quantitative estimation of SGD caused by tidally induced sea water recirculation and a terrestrial hydraulic gradient. A two-dimensional hydrogeological model was developed to simulate SGD from a coastal unconfined aquifer in the northeastern Gulf of Mexico, where previous SGD studies were performed. A density-variable numerical code, SEAWAT2000, was applied to simulate SGD. To accurately predict discharge, various influencing factors such as heterogeneity in conductivity, uncertain boundary conditions, and tidal pumping were systematically assessed. The tidally influenced sea water recirculation zone and the fresh water-salt water mixing zone under various tidal patterns, tidal ranges, and water table heights were also investigated. The model was calibrated and validated from long-term, intensive measurements at the study site. The percentage of fresh SGD relative to total SGD ranged from 4% to 50% under normal conditions. Based on simulations of two field measurements in summer and spring, respectively, the fresh water ratios were 9% and 15%, respectively. These results support the hypothesis that the SGD induced by tidally driven sea water recirculation is much larger than terrestrial fresh ground water discharge at this site. The estimates of total and fresh SGD are at the low and high ends, respectively, of the estimation ranges obtained from geochemical tracers (e.g., (222)Rn).

Rates of microbially mediated sedimentary organic carbon remineralization vary along the salinity gradient ofthe White Oak River estuary, N.C. A direct comnarison oftwo sites. one uoriver dominated by methanogenesis and one downriver dominated by &fate reduction, indicates a more rapid rate of remineralization upriver. Measurements of diffusive ZCOZ flux plus CH, diffusive (56%) and bubble (44%) fluxes at the upriver, freshwater site from June 1986 to March 1988 yield an average yearly flux of 1.35kO.35 mmol m-' h-l. At the downriver midestuarine site, the dominant X0, flux for the same period is 0.46kO.02 mmol m-* h-l. The uncertainties in these yearly fluxes are calculated from observed ranges in duplicate flux measurements. About 43% of the incoming organic C is remineralized at both sites. The greater remineralization upriver, of which 47% can be attributed to methanogenesis, appears to be supported by rapid decomposition of detritus derived from freshwater, emergent and submersed macrophytes, including Pontederia, Ceratophyllum, and Najas in surficial sediments. The high organic C content of these upriver sediments may, however, result from the long-term storage of slower degrading plants, such as Typha and Taxodium.

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