Bohdan Matvienko

Carbon dioxide and methane emissions and the carbon budget of a 10-year old tropical reservoir (Petit Saut, French Guiana)

emissions, 0.07 ± 0.01) the first 3 years after impounding (1994–1996) and then decreased to 0.12 ± 0.01 Mt yr 1 C( CO2, 0.10 ± 0.01; CH4, 0.016 ± 0.006) since 2000. On average over the 10 years, 61% of the CO2 emissions occurred by diffusion from the reservoir surface, 31% from the estuary, 7% by degassing at the outlet of the dam, and a negligible fraction by bubbling. CH4 diffusion and bubbling from the reservoir surface were predominant (40% and 44%, respectively) only the first year after impounding. Since 1995, degassing at an aerating weir downstream of the turbines has become the major pathway for CH4 emissions, reaching 70% of the total CH4 flux. In 2003, river carbon inputs were balanced by carbon outputs to the ocean and were about 3 times lower than the atmospheric flux, which suggests that 10 years after impounding, the flooded terrestrial carbon is still the predominant contributor to the gaseous emissions. In 10 years, about 22% of the 10 Mt C flooded was lost to the atmosphere. Our results confirm the significance of greenhouse gas emissions from tropical reservoir but stress the importance of: (1) considering all the gas pathways upstream and downstream of the dams and (2) taking into account the reservoir age when upscaling emissions rates at the global scale.

Gas release from a reservoir in the filling stage

Concern about the greenhouse effect due to methane and carbon dioxide emissions has prompted a series of measurements of anthropogenically caused release. Our group has focused on such emissions from man-made lakes. A set of hydroelectric reservoirs was chosen which lies in the range of latitudes spanned by Brazil, and here we present the results from one of these reservoirs. From previous experience and other work (DucHEMIN et al. 1995, GALYLACAUX et al. 1999) we know that )atitude is the predominant factor influencing emissions in reservoirs mainly because it determines the ambient temperature, however, dimate type is of additional importance because it causes specific types of vegetation to grow and thus determines the type and density of the biomass which is eventually flooded by the reservoir.

Greenhouse gases and initial findings on the carbon circulation in two reservoirs and their watersheds

the overlying water. Bubbles containing mainly CO2, and CH4 also rise from this fresh sediment layer. We draw the lower boundary at the surface below which all carbon is permanent (i.e. not susceptible to mobilization and on its way to fossilization). We estimate that this boundary is somewhere between 5‐20 cm below the water-sediment interface. At this depth humic substances are already resistant to further carbon decomposition, as can be seen from the constant C/Si ratio starting at about this depth. In fact, the lower boundary could be described as lying in the two-dimensional region where this ratio is constant from this point down. Reservoir carbon inputs into the black box come through rivers, underground water, rainfall, and occasional diffusive absorption. Carbon outputs are effluent outflow, permanent sedimentation, and diffusive and bubbling emissions such as CH4 and CO2.

Gross Greenhouse Gas Emissions from Brazilian Hydro Reservoirs

This paper presents the results of gross carbon dioxide and methane emission measurements in several Brazilian hydro reservoirs. The term ‘gross emissions’ means gas flux measurements from the reservoir surface without correcting for natural pre-impoundment emissions by natural bodies such as the river channel, seasonal flooding and terrestrial ecosystems. The net emissions result from estimating pre-existing emissions by the reservoir. Measurements were carried in the Miranda, Barra Bonita, Segredo, Tres Marias, Xingo, Samuel and Tucurui reservoirs, located in two different climatological regimes. Additional data were used here from measurements taken at the Itaipu and Serra da Mesa reservoirs. Emissions of carbon dioxide and methane in each of the reservoirs selected, whether through bubbles or diffusive exchange between water and atmosphere, were assessed by sampling, with subsequent extrapolation of results to obtain a value for the reservoir. A great variability was found in the emissions, linked to the influence of various factors, including temperature, depth at the point of measurement, wind regime, sunlight, physical and chemical parameters of water, the composition of the local vegetation and the operational regime of the reservoir.

A method for the study of N2O evolution in tropical wetlands

Simple gas handling equipment is presented and its use is described in biological denitrification potential measurements. Preliminary results are given for denitrification potentials at latitude 22° in South America. For the upper 5 cm of the sediment layer a first order kinetic process is found with a nitrate concentration decrement rate of 12% per day.

An experiment in lithium carbide production for radiocarbon dating

Abstract A small scale vessel for reacting metallic lithium (3 g) with Co 2 , under pressure, was built. It has an internal heater and reaches reaction temperature (600°C) in 5 min. Reaction is virtually instantaneous. With a 30% Li excess the yield was about 77% of available carbon. The main advantage is time economy.

A combustion bomb for carbon dating with a new ignition system

Abstract A description is given of a large sample (10 g) combustion bomb with an internal heater which allows the sample to be brought to the spontaneous ignition point. The pre-heated sample burns smoothly upon admission of oxygen.

Estimate of degassing greenhouse gas emissions of the turbined water at tropical hydroelectric reservoirs

Biogenic gases generated underwater in hydroelectric reservoirs come from organic matter decomposition by microorganisms. There are 3 different pathways of emitting greenhouse gases (GHG). Gas emissions by impoundments are mainly methane (CH4), carbon dioxide (C02) and nitrous oxide (N20) transported through bubbles formed by decomposing organic matter on the lake bottom, as well as rising up the lake gradient through molecular diffusion. Another gas emission pathway is the degassing of these gases at turbined water sites and along the river below the dam. These different gases (C02, CH4, and N20) have distinct warming effects in the atmosphere; their instantaneous radiative forcing are different and so are their atmospheric life spans. This is particularly true in the case of power generation. The use of fossil fuels in thermal power plants and bacterial decomposition of biomass in hydroelectric reservoirs produce greenhouse gases. Debate continues regarding the amount of GHG emitted from freshwater reservoirs, especially the question of effects of impoundment on degassing emissions downstream of the power house. These emissions were observed for the first time by French researchers at Petit Saut reservoir in French Guiana in 1995 (GALY-LACAUX et al. 1997). The water coming out of the turbine was anoxic (O% o f oxygen); to allow the survival o f fish in the ri v er between the dam and the estu-

A combustion bomb for carbon dating with reduced dry distillation effect

Abstract A combustion bombs with hot sample ignition, some organic samples produce during the heating cycle, previous to ignition, a tar-like deposit on the bombs wall (wall effect). An internal catalytic converter is described which oxidizes these products of dry distillation on passage and reduces the wall effect by a factor of 5. The converter consists of an electrically heatable body, which has a porous bottom and a cavity filled with cupric oxide.