Geochemistry of dissolved and suspended loads of the Seine river, France: Anthropogenic impact, carbonate and silicate weathering

This study focuses on the chemistry of the Seine river system, one of the m ajor rivers in Europe, and constitutes the first geochemical investigation of both suspended and dissolved loads of this river. The Seine river drains a typical Mesozoic-Cenozoic sedimentary basin: the Paris basin, constitute d of limestones mixed or interbedded with terrigenous sediments derived fro m the paleoreliefs bordering the Mesozoic and Cenozoic seas. In the context of quantifying the global influence of carbonate and silicate weathering o n atmospheric CO2 consumption, the Seine river offers the possibility of ex amining weathering rates in a flat sedimentary environment, under temperate climatic conditions. One of the major problems associated with the Seine r iver, as with many temperate rivers, is pollution. We propose, in this paper, 2 approaches in order to correct the dissolved l oad of the Seine river for anthropogenic inputs and to calculate weathering rates of carbonates and silicates. The first uses the dissolved load of ri vers and tries to allocate the different solutes to different sources. A mi xing model, based on elemental ratios, is established and solved by an inve rsion technique. The second approach consists in using the suspended load geochemistry. Unde r steady state conditions, we show that the geochemistry of suspended sedim ents makes it possible to estimate the amount of solutes released during th e chemical weathering of silicates, and thus to calculate weathering rates of silicates. The total dissolved load of the Seine river at Paris can be decomposed into 2% of solutes derived from natural atmospheric sources, 7% derived from an thropogenic atmospheric sources, 6% derived from agriculture, 3% derived fr om communal inputs, and 82% of solutes derived from rock weathering. During high floods, the contribution of atmospheric and agriculture inputs predom inates. The weathering rate of carbonates is estimated to be 48 t/km(2)/yr (25 mm/1 000 yr). Only 10% of carbonates are transported in a solid form, the rest b eing transported in solution. CO2 consumption by carbonate weathering appro aches 400 X 10(3) mol/km(2)/yr. In the Seine river at Paris, about 2-3 mg/l of dissolved cations are found to originate from the chemical weathering of silicates. By taking dissolved silica into accounts, the total dissolved load derived from silicate weath ering is about 6-7 mg/l. This value is minimal because biological uptake of silica Probably occur in the Seine river. The chemical weathering rate of aluminosilicates is estimated to be 2 t/km(2)/yr. The ratio of physical ove r chemical weathering of silicates range between 1 and 3 and the total (che mical and physical) erosion rates of sedimentary silicates are about 2-3 mm /kyr. The CO2 consumption by silicate weathering 15-24 X 10(3) mol/km2/yr and is independent of dissolved silica concentration. Silicate consumption is thus 20 times less than carbonate consumption in the Paris basin. Compared to the neighboring granitic areas, the sedimentary region drained by the Seine river has 2 to 3 times lower CO2 consumption rates. We attribu te this difference to the cation-depleted nature of the Seine basin alumino silicates, which are of sedimentary origin. At a world scale, the chemical denudation rates found for the Seine basin are very low and comparable to t hose given for tropical lowland rivers draining silicates, such as the rive rs of the Congo and Amazon basins, in spite of huge climatic differences. W e attribute this similarity to the low mechanical denudation that character izes these two types of regions. Copyright (C) 1999 Elsevier Science Ltd.