Silicon as a permanent-carbon sedimentation tracer

A procedure to quantify permanent carbon (C) sedimentation rates was required to compare these rates to methane (CH4) and carbon dioxide (CO2) water–air emission rates measured during reservoir C flux studies. Therefore, a new method to estimate C burial rates using silicon (Si) as a tracer was devised and applied. Burial rates in 8 tropical reservoirs were measured. Ages of these 8 reservoirs varied between 3.7 and 49 years. Each reservoir was surveyed 3 times during 1 year. Median burial rate was 78 (min 12, max 516; n = 66) mg C m-2 d-1. Trapped C (Ct) rates were also measured; the resulting median was 845 mg C m-2 d-1 (min 179, max 19 064; n = 40). Burial efficiency (comparison between C burial rate and Ct rate) was ~10%. Carbon burial efficiency of the 8 reservoirs showed strong dependence on bottom water temperature, efficiency being halved for each 3.4 °C increase in annual average temperature of reservoir bottom water. This finding strongly supported the adequacy of the Si-tracer method for rate measurements of carbon burial in sediments. Simultaneous with our new Si-tracer method we conducted traditional lead 210 isotope (210Pb) dating. The resulting median was 133 (min 11, max 441; n = 15) mg C m-2 d-1. Compared to the Si-tracer median, the 210Pb-dating technique resulted in a higher C median burial rate because the sampling sites that lacked sediment (and therefore contributed a null burial rate) were, in retrospect, erroneously disregarded

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

International audienceThe emissions of carbon dioxide (CO2) and methane (CH4) from the Petit Saut hydroelectric reservoir (Sinnamary River, French Guiana) to the atmosphere were quantified for 10 years since impounding in 1994. Diffusive emissions from the reservoir surface were computed from direct flux measurements in 1994, 1995, and 2003 and from surface concentrations monitoring. Bubbling emissions, which occur only at water depths lower than 10 m, were interpolated from funnel measurements in 1994, 1997, and 2003. Degassing at the outlet of the dam downstream of the turbines was calculated from the difference in gas concentrations upstream and downstream of the dam and the turbined discharge. Diffusive emissions from the Sinnamary tidal river and estuary were quantified from direct flux measurements in 2003 and concentrations monitoring. Total carbon emissions were 0.37 ± 0.01 Mt yr-1 C (CO2 emissions, 0.30 ± 0.02; CH4 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

Emissões de gases de efeito estufa por reservatórios de hidrelétricas

GREENHOUSE GAS EMISSIONS OF DIFFERENT BRAZILIAN HYDROELECTRIC DAMS. Over the 90s surged a growing concern about the participation of hydroelectric dams in the global warming through considerable emission of greenhouse gases. Recently, the results of analysis of some hydroelectric dams in the Amazon have become a matter of controversy, since some of them were based and obtained by poorly reliable scientifi c methods. One of the most controversial aspects are the greenhouse gases emission by the downstream face of the dams � i.e. just after the turbines � especially of methane, which is found in greater concentrations at greater depths. It has been proposed that the gas concentration that should be considered when calculating the ebullition fl ux should be that of the capitation area, instead of the usually employed methane concentration at greater depths. The greenhouse gas emissions of hydroelectric dams is not constant, and proved to vary in an irregular basis that depends on many factors, including temperature, exposure to sunlight, and water physicochemical and biological traits. There are no established models for the spatial and temporal variations of greenhouse gas emissions by hydroelectric dams, which heavily depend on results of additional controlled studies. There is a lack of data on the carbon cycle conditions in different situations (according with water level, water column position, before and after impoundment, etc). For instance, different types of environments naturally generate methane (swamps, fl ooded forests). These emissions ought to be discounted when calculating methane emissions after water impoundment within such regions, thus ensuring the obtained fi gures really refl ect the increase in methane emissions after the dam was constructed. Carbon dioxide emissions could be incorporated into the hydroelectric dam system through natural cycling within a short time span. Keywords: Hydroelectricity, carbon, methane, carbon dioxide, gas emissionDurante a década de 90 surgiram intensas especulações a respeito de que reservatórios de hidrelétricas poderiam estar contribuindo para a intensifi cação do efeito estufa através da emissão de Gases de Efeito Estufa (GEE). Muita polêmica tem sido estabelecida recentemente a partir de estudos realizados nos reservatórios amazônicos, especialmente a partir de estudos teóricos e baseados em extrapolações desprovidas de critérios científi cos estabelecidos. Uma das questões mais polêmicas é a estimativa de emissão de gases à jusante de represas, logo após a passagem da água pelas turbinas, em particular do metano (CH4) cujas concentrações são mais elevadas em maiores profundidades. Um dos problemas é que a concentração de CH4 a ser utilizada nos cálculos de fl uxo ebulitivo deveria ser a concentração média deste gás na faixa de captação de água e não a concentração deste gás na profundidade maior, como tem sido usado. A intensidade da emissão de GEE não é invariante no tempo, havendo fl utuações em períodos de duração irregular. Estas fl utuações são infl uenciadas por muitos fatores, como temperatura, regime de ventos, exposição ao sol, parâmetros físicos, químicos e biológicos da água. Existem difi culdades para se estabelecer uma extrapolação que realmente represente a heterogeneidade espacial dos reservatórios e que possa captar a variação temporal dos fl uxos. Estudos adicionais são indispensáveis para reduzir as dúvidas a respeito da emissão de GEE pelos reservatórios de hidrelétricas. A compreensão das diferentes formas de fl uxo de carbono, em diferentes escalas espaciais (nível do reservatório, nível da coluna d´água) e temporais (antes e depois da inundação) é indispensável para compreender a real contribuição do reservatório na emissão de GEE. Muitos ambientes naturais emitem CH4, especialmente pântanos e outras áreas úmidas ou habitats de fl orestas em climas tropicais. Estas emissões devem ser consideradas e descontadas em cálculos de emissões de CH4 posteriores a inundação do reservatório neste tipo de ambiente. Este método garante que os dados obtidos após a inundação representem realmente o aumento na emissão de CH4 provocado pela inundação da área pela represa. A emissão de CH4 pelas hidrelétricas é sempre desfavorável para a hidroeletricidade, pois mesmo que o carbono origine-se de fontes naturais, ele se torna um gás de maior Potencial de Aquecimento Global (Global Warming Potential ¿ GWP) no computo fi nal. Já a emissão de CO2 em parte pode ser originada da atmosfera e ser incorporada ao sistema do reservatório pela ciclagem natural do carbono em ciclo curto de tempo. Palavras-chave: Hidroeletricidade, carbono, metano, dióxido de carbono, emissão de gase