Eric Duchemin

Production of the greenhouse gases CH4 and CO2 by hydroelectric reservoirs of the boreal region

The emission fluxes and the distribution of dissolved methane (CH4) and carbon dioxide (CO2) were determined for 11 sampling stations in two hydroelectric reservoirs (flooded since 1978 and 1993) located in the James Bay territory of northern Quebec. The measured benthic fluxes for the two greenhouse gases were found to be either higher or similar to those determined at the water-air interface during the ice-free sampling periods. For the 2 year duration of the study, emission fluxes of CH4 to the atmosphere generally varied between 5 and 10 mg m−2 d−1, while those for CO2 ranged from 500 to 1100 mg m−2 d−1. Furthermore, through the use of static chambers at the water-air interface, we determined that the emission fluxes for the gases are controlled by molecular diffusion. Our calculated fluxes have been separated into two groups: (1) regular emission fluxes and (2) above-average emission fluxes. The first type comprises the majority of fluxes measured during the sampling periods (i.e., 88% for CH4 and 87% for CO2). The second group reflects unusual sampling conditions (e.g., strong winds, water column depths of less than 1 m, or flooded peatland mats floating at the surface). Although data for this group are limited, our preliminary results suggest that they may be an important component in an atmospheric emissions budget for large reservoirs. Concentration profiles for CH4 and CO2 dissolved in the water column clearly show that oxidation and/or horizontal advection of these gases are controlling factors in their subsequent release to the atmosphere. Most of the CH4 is oxidized within the first 25 cm above the flooded soil-water interface. Consequently, neither benthic emissions of CH4 and CO2 nor the type of flooded soil appear to control atmospheric emissions of these gases from hydroelectric reservoirs.

Greenhouse gas emissions from reservoirs of the western United States

= 0.81; p < 0.0001). All other correlations between GHG diffusivefluxes and independent variables tested were weak and/or not significant. Finally,while attempting to resolve the spatial variability in diffusive fluxes, we were able tocluster reservoirs neither according to geological nor ecological criteria.

EARLY DIAGENETIC PROCESSES IN RECENT SEDIMENTS OF THE GULF OF ST-LAWRENCE:PHOSPHORUS, CARBON AND IRON BURIAL RATES

Abstract Selective extraction procedures were used to quantify different forms of solid-phase phosphorus, carbon and iron in marine sediments, and to evaluate the impact of authigenic formation of mineral forms such as carbonate fluorapatite (CFA), calcium carbonate (CaCO3) and pyrite (FeS2) on major elemental cycles during early diagenesis. Detrital P and Fe phases were successfully used as indicators of the constancy or variability of detrital inputs to several sedimentary environments from the deep channels of the Gulf of St-Lawrence. In cores characterized by near steady state influx rates, solid-phase P, C and Fe data and sediment burial rates indicate that CFA, CaCO3 and probably FeS2 are currently forming in the sediments of the Gulf. However, high concentrations and/or formation of CaCO3 in marine sediments appear to inhibit the formation of authigenic CFA. On the other hand, the formation of FeS2 does not influence authigenic CFA precipitation. In the deep troughs of the Gulf of St-Lawrence, total P burial rates range from ≈ 50 to 500 mgP/m2/yr. Truly authigenic precipitation of CFA, when observed, may represent up to ≈ 25% of the total burial rate of P. Bioturbation of sub-surface sediments reduces the potential for authigenic precipitation of CFA and CaCO3, thus affecting immobilization reactions that have a strong impact on the global oceanic cycles of C and P. The spatial heterogeneity of diagenetic reactions precludes the establishment of an accurate quantification of P removal on the scale of a continental shelf such as the Gulf of St-Lawrence.

Comparison of greenhouse gas emissions from an old tropical reservoir with those from other reservoirs worldwide

Flooding of forest soils, following the creation of reservoirs, worldwide is a significant anthropic source of GHG emissions. In fact, since 1993, i t has been observed that the impoundment of large forest areas in the boreal region leads to an increase in CO, and CH4 evasion to the atmosphere (Ruoo et al. 1993, DucHEMIN et al. 1995, 1998, VAisANEN et al. 1996). Up to now there have been only a few data sets available to describe anthropic greenhouse gas (GHG) emissions from tropical reservoirs (GALY-LACAUX et al. 1997, KELLER & STALLARD 1994). However, these measurements tend to demonstrate that tropical reservoirs are also important sources ofGHG. Furthermore, FEARNSIDE (1995) concluded, from extrapolations based on these results, that tropical reservoirs could emit larger quantities of GHG than boreal reservoirs. The purpose of this paper is to present a first data set of GHG evasion measurements, from a 21-year-old tropical reservoir in the Brazilian Amazon, and to compare it with data sets published for other reservoirs of the tropical and boreal region. According to the geochemical dynamics of tropical environments, i t is expected that the GHG emissions from the tropical reservoir would be greater than those from boreal and temperate ones.