In this paper we present chemical composition data for major elements in rivers from three islands of the Lesser Antilles. The Lesser Antilles are a tropical volcanic subduction arc and are characterized by steep gradients of relief, bedrock age and precipitation. They constitute a natural laboratory where the response of the weathering engine to large variations of runoff can be understood. Data indicate that the Lesser Antilles are characterized by extremely variable chemical weathering (40-430 t/km 2 /a) and CO 2 consumption (300-3500.10 3 mol/km 2 /a) rates, amongst the highest found on Earth and consistent with the previous studies on the weathering of volcanic rock. A noteworthy observation is that, along the runoff gradient, concentrations of rock-derived solutes do not follow a pure dilution law and that a buffering mechanism exists stabilizing solute concentrations. As a result concentrations vary much less than runoff and chemical weathering rates are mainly controlled by runoff. Precipitation patterns in the Lesser Antilles are essentially orographic and controlled by the adiabatic decompression of the water-saturated Atlantic air masses. The production of acidity by volcanic degassing is an additional factor that modulates the runoff effect. Two main conclusions can be drawn from this study. First, chemical weathering fluxes of oceanic islands are strongly dependent upon relief repartition, which cautions the use of regional mean values to compare volcanic islands. Second, volcanic activity in the Lesser Antilles subduction arc, by creating relief, promotes high orographic precipitation and/or infiltration regimes, that in turn results in elevated chemical weathering and atmospheric CO 2 consumption fluxes. This feedback mechanism, implying mainly precipitation and relief, is proposed to act in complement to the temperature-related feedback proposed by previous authors for stabilizing the atmospheric CO 2 content of the atmosphere in response to volcanic CO 2 degassing. This study highlights the importance of the water cycle in controlling chemical weathering of volcanic arc islands and associated CO 2 consumption rates.
International audienceIn this paper, we use carbon isotopes in the dissolved load of rivers from the Lesser Antilles volcanic arc (Guadeloupe, Martinique and Dominica islands) to constrain the source of the carbon dioxide (CO2) involved in the neutralization reactions during water-rock interactions. The d13C data span a large range of variations, from -19% to -5 _ 2% for DIC (dissolved inorganic carbon) concentrations ranging from 11 mM to 2000 mM. Coupled with major element concentrations, carbon isotopic ratios are interpreted as reflecting a mixture of magmatic CO2 (enriched in heavy carbon (d13C_-3 _ 5%) and biogenic CO2 produced in soils (enriched in light carbon (d13C<-17%)). Carbon isotopes show that, at the regional scale, 23 to 40% of CO2 consumed by weathering reactions is of magmatic origin and is transferred to the river system through aquifers under various thermal regimes. These numbers remain first-order estimates as the major uncertainty in using carbon isotopes as a source tracer is that carbon isotopes can be fractionated by a number of processes, including soil and river degassing. Chemical weathering is clearly, at least, partly controlled by the input of magmatic CO2, either under hydrothermal (hot) or surficial (cold) weathering regimes. This study shows that the contribution of magmatic CO2 to chemical weathering is an additional parameter that could explain the high weathering rates of volcanic rocks. The study also shows that a significant part of the carbon degassed from the Earth's interior is not released as CO2 to the atmosphere, but as DIC to the ocean because it interacts with the groundwater system. This study calls for a better understanding of the contributions of deep carbon to the hydrosphere and its influence on the development of the Critical Zone
Natural organic matter (NOM) is known to play an important role in the transport and binding of trace metal elements in aquatic and soil systems. Thallium is a pollutant for which the extent of the role played by NOM is poorly known. Consequently, this study investigates thallium(I) and its complexation to a purified humic substance as proxy for NOM. Experiments were performed with the Donnan Membrane Technique to separate, for the first time, the free Tl + ion from its complexed form in the bulk solution. Various pH and concentrations were investigated at constant ionic strength and constant NOM proxy concentrations in solution. Experimental results were described with NICA-Donnan model. Thallium complexation was compared to silver complexation using literature data and using the same NICA-Donnan formalism. Parameters for these two cations (Tl + and Ag + ) are reported in this article, for the first time. Results display low thallium complexation to the NOM proxy while silver competes with divalent cations for the NOM binding sites. Calculated speciation for dissolved thallium highlights the dominance of free thallium (Tl + ) in solution whereas Tl-NOM complexes contribute roughly 15 % to total Tl(I) species in river and lake type waters.Similar results are obtained for soil solutions, Tl-bound to NOM < 30 % of total, from UK soils with different land use and geochemistry.
International audienceVolcanic islands, being characterized by highly porous basaltic/andesitic lava flows and pyroclastic deposits, are subject to important chemical weathering by subsurface waters. Moreover, such subsurface weathering is impacted by hydrothermal springs in both active and non-active volcanic areas, thus increasing dissolved load concentrations. Here, we focus on the subsurface water chemistry in the volcanic islands of the Lesser Antilles and Re'union and on the origin of these subsurface flows. We are able, through the use of various isotopic tools (C, Sr, U-Th), to identify hydrothermal influences in river water. For example, Li concentrations show a positive correlation with temperature of hot and cold springs and also a relationship with d13C; from this, we can show that several sources of hydrothermal activity influence the rivers of the Lesser Antilles and that some rivers also reveal an important organic influence. As much as 20% of the subsurface hydrothermal springs go to feed the rivers. The increasing temperatures result in more dissolved elements being mobilized and an increase in chemical weathering rates. In addition, using the (230Th/238U) isochron for the well and river dissolved loads in Martinique, Guadeloupe and Re'union, we can evaluate residence times in the river water, i.e. the average residence time in the water along the circulation path to the sampling point. Alteration takes longer when the water circulates through thick soil, for example, 400-5,000 years when circulating under an ash profile and 1,200-15,000 years when circulating through a collapse zone. It would appear that waters circulation is globally three times longer for subsurface water than for surficial water. The weathering regime in tropical volcanic environments seems to be controlled mainly by such subsurface circulation with high chemical concentration from hydrothermal inputs. The origin of these compositions is varied and not controlled by a single hydrothermal spring. Consequently, it is subsurface circulation that determines the weathering regime in tropical volcanic islands with the main controlling parameters being temperature and residence time
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