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Contrasting biogeochemical characteristics of the Oubangui River and tributaries (Congo River basin).

Bouillon S, Yambélé A, Gillikin DP, Teodoru C, Darchambeau F, Lambert T, Borges AV - Sci Rep (2014)

Bottom Line: Spectral analyses of chromophoric dissolved organic matter showed wide temporal variations in the Oubangui compared to spatio-temporal variations in the tributaries, and confirm that different pools of dissolved organic carbon are mobilized during different hydrological stages. δ(13)C of dissolved inorganic carbon ranged between -28.1‰ and -5.8‰, and was strongly correlated to both partial pressure of CO2 and to the estimated contribution of carbonate weathering to total alkalinity, suggesting an important control of the weathering regime on CO2 fluxes.All tributaries were oversaturated in dissolved greenhouse gases (CH4, N2O, CO2), with highest levels in rivers draining rainforest.The high diversity observed underscores the importance of sampling that covers the variability in subcatchment characteristics, to improve our understanding of biogeochemical cycling in the Congo Basin.

View Article: PubMed Central - PubMed

Affiliation: Department of Earth and Environmental Sciences, KULeuven, Leuven, Belgium.

ABSTRACT
The Oubangui is a major tributary of the Congo River. We describe the biogeochemistry of contrasting tributaries within its central catchment, with watershed vegetation ranging from wooded savannahs to humid rainforest. Compared to a 2-year monitoring record on the mainstem Oubangui, these tributaries show a wide range of biogeochemical signatures, from highly diluted blackwaters (low turbidity, pH, conductivity, and total alkalinity) in rainforests to those more typical for savannah systems. Spectral analyses of chromophoric dissolved organic matter showed wide temporal variations in the Oubangui compared to spatio-temporal variations in the tributaries, and confirm that different pools of dissolved organic carbon are mobilized during different hydrological stages. δ(13)C of dissolved inorganic carbon ranged between -28.1‰ and -5.8‰, and was strongly correlated to both partial pressure of CO2 and to the estimated contribution of carbonate weathering to total alkalinity, suggesting an important control of the weathering regime on CO2 fluxes. All tributaries were oversaturated in dissolved greenhouse gases (CH4, N2O, CO2), with highest levels in rivers draining rainforest. The high diversity observed underscores the importance of sampling that covers the variability in subcatchment characteristics, to improve our understanding of biogeochemical cycling in the Congo Basin.

No MeSH data available.


Related in: MedlinePlus

Relationship between (A) observed and modeled total alkalinity (TA) (see text for details), and (B) the carbon stable isotope composition of DIC (δ13CDIC) and the estimated contribution of TA derived from carbonate weathering to the observed TA.
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f10: Relationship between (A) observed and modeled total alkalinity (TA) (see text for details), and (B) the carbon stable isotope composition of DIC (δ13CDIC) and the estimated contribution of TA derived from carbonate weathering to the observed TA.

Mentions: For the tributaries, there is a good linear regression between the modeled TA (TAcarb + TAsil) and observed TA (r2 = 0.99, n = 36, Figure 10A). In the Oubangui mainstem, the modeled TA is well correlated to the observed TA but markedly deviates from the 1:1 line as the values increase. The stoichiometric model of37 assumes that the weathering of silicate rocks leads to a release of HCO3− and dissolved Si (DSi) according to a 1:2 ratio. This is true for common mineral forms such as olivine and albite, but the dissolution of some other mineral forms lead to a different HCO3−:DSi ratio. For instance, the weathering of plagioclase feldspar (NaCaAl3Si5O16) leads to the release of HCO3− and DSi in a 6:4 ratio. While no detailed information is available on the mineralogy of the underlying bedrock in our study catchments, they are underlain mostly by metamorphic rocks in which feldspars are likely to be common consituents. When we apply this ratio to the Oubangui mainstem data, the recomputed modeled TA in the high values range is in better agreement with observed TA, than based on the HCO3−:DSi ratio of 1:2 (Figure 10A). The contribution of carbonate rock weathering (%TAcarb) was estimated as the percentage of TAcarb to total modeled TA (TAcarb + TAsil), and ranged between 30% and 94%, encompassing the range of %TAcarb computed by Gaillardet et al.39 between 40% and 91%. %TAcarb is positively correlated to TA indicating a lower contribution of carbonate rock weathering in the basins draining humid forest than savannah.


Contrasting biogeochemical characteristics of the Oubangui River and tributaries (Congo River basin).

Bouillon S, Yambélé A, Gillikin DP, Teodoru C, Darchambeau F, Lambert T, Borges AV - Sci Rep (2014)

Relationship between (A) observed and modeled total alkalinity (TA) (see text for details), and (B) the carbon stable isotope composition of DIC (δ13CDIC) and the estimated contribution of TA derived from carbonate weathering to the observed TA.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4066439&req=5

f10: Relationship between (A) observed and modeled total alkalinity (TA) (see text for details), and (B) the carbon stable isotope composition of DIC (δ13CDIC) and the estimated contribution of TA derived from carbonate weathering to the observed TA.
Mentions: For the tributaries, there is a good linear regression between the modeled TA (TAcarb + TAsil) and observed TA (r2 = 0.99, n = 36, Figure 10A). In the Oubangui mainstem, the modeled TA is well correlated to the observed TA but markedly deviates from the 1:1 line as the values increase. The stoichiometric model of37 assumes that the weathering of silicate rocks leads to a release of HCO3− and dissolved Si (DSi) according to a 1:2 ratio. This is true for common mineral forms such as olivine and albite, but the dissolution of some other mineral forms lead to a different HCO3−:DSi ratio. For instance, the weathering of plagioclase feldspar (NaCaAl3Si5O16) leads to the release of HCO3− and DSi in a 6:4 ratio. While no detailed information is available on the mineralogy of the underlying bedrock in our study catchments, they are underlain mostly by metamorphic rocks in which feldspars are likely to be common consituents. When we apply this ratio to the Oubangui mainstem data, the recomputed modeled TA in the high values range is in better agreement with observed TA, than based on the HCO3−:DSi ratio of 1:2 (Figure 10A). The contribution of carbonate rock weathering (%TAcarb) was estimated as the percentage of TAcarb to total modeled TA (TAcarb + TAsil), and ranged between 30% and 94%, encompassing the range of %TAcarb computed by Gaillardet et al.39 between 40% and 91%. %TAcarb is positively correlated to TA indicating a lower contribution of carbonate rock weathering in the basins draining humid forest than savannah.

Bottom Line: Spectral analyses of chromophoric dissolved organic matter showed wide temporal variations in the Oubangui compared to spatio-temporal variations in the tributaries, and confirm that different pools of dissolved organic carbon are mobilized during different hydrological stages. δ(13)C of dissolved inorganic carbon ranged between -28.1‰ and -5.8‰, and was strongly correlated to both partial pressure of CO2 and to the estimated contribution of carbonate weathering to total alkalinity, suggesting an important control of the weathering regime on CO2 fluxes.All tributaries were oversaturated in dissolved greenhouse gases (CH4, N2O, CO2), with highest levels in rivers draining rainforest.The high diversity observed underscores the importance of sampling that covers the variability in subcatchment characteristics, to improve our understanding of biogeochemical cycling in the Congo Basin.

View Article: PubMed Central - PubMed

Affiliation: Department of Earth and Environmental Sciences, KULeuven, Leuven, Belgium.

ABSTRACT
The Oubangui is a major tributary of the Congo River. We describe the biogeochemistry of contrasting tributaries within its central catchment, with watershed vegetation ranging from wooded savannahs to humid rainforest. Compared to a 2-year monitoring record on the mainstem Oubangui, these tributaries show a wide range of biogeochemical signatures, from highly diluted blackwaters (low turbidity, pH, conductivity, and total alkalinity) in rainforests to those more typical for savannah systems. Spectral analyses of chromophoric dissolved organic matter showed wide temporal variations in the Oubangui compared to spatio-temporal variations in the tributaries, and confirm that different pools of dissolved organic carbon are mobilized during different hydrological stages. δ(13)C of dissolved inorganic carbon ranged between -28.1‰ and -5.8‰, and was strongly correlated to both partial pressure of CO2 and to the estimated contribution of carbonate weathering to total alkalinity, suggesting an important control of the weathering regime on CO2 fluxes. All tributaries were oversaturated in dissolved greenhouse gases (CH4, N2O, CO2), with highest levels in rivers draining rainforest. The high diversity observed underscores the importance of sampling that covers the variability in subcatchment characteristics, to improve our understanding of biogeochemical cycling in the Congo Basin.

No MeSH data available.


Related in: MedlinePlus