Comparison of dissolved inorganic and organic carbon yields and fluxes in the watersheds of tropical volcanic islands, examples from Guadeloupe (French West Indies)

International audienceOrganic matter is an important factor that cannot be neglected when considering global carbon cycle. New data including organic matter geochemistry at the small watershed scale are needed to elaborate more constrained carbon cycle and climatic models. The objectives are to estimate the DOC and DIC fluxes exported from small tropical watersheds and to give strong constraints on the carbon hydrodynamic of these systems. To answer these questions, we have studied the geochemistry of eleven small watersheds around Basse-Terre volcanic Island in the French West Indies during different hydrological regimes from 2006 to 2008 (i.e. low water level versus floods). We propose a complete set of carbon measurements, including DOC and DIC concentrations, δ13C data, and less commonly, some spectroscopic indicators of the nature of the organic matter. The DOC/DIC ratio varies between 0.07 and 0.30 in low water level and between 0.25 and 1.97 during floods, indicating that organic matter is mainly exported during flood events. On the light of the isotopic composition of DOC, ranging from -32.8 to -26.2 ‰ during low water level and from -30.1 to -27.2 ‰ during floods, we demonstrate that export of organic carbon is mainly controlled by perennial saprolite groundwaters, except for flood events during which rivers are also strongly influenced by soil erosion. The mean annual yields ranged from 2.5 to 5.7 t km-2 yr-1 for the DOC and from 4.8 to 19.6 t km-2 yr-1 for the DIC and exhibit a non-linear relationship with slopes of watersheds. The flash floods explain around 60% of the annual DOC flux and between 25 and 45% of the DIC flux, highlighting the important role of these extreme meteorological events on global carbon export in small tropical volcanic islands. From a carbon mass balance point of view the exports of dissolved carbon from small volcanic islands are important and should be included in global organic carbon budgets

Impact of climatic changes on organic carbon dynamic in wet tropical watersheds of Guadeloupe (FWI).

19th Annual VM Goldschmidt Conference, Date: JUN 21, 2009 Davos SWITZERLANDInternational audienceTo better understand the global carbon cycle and the impact of changing climatic conditions, it is important to constrain the different sources, sinks and fluxes of carbon. The soil organic matter (≈ 1500 Gt [1]) is a major pool of carbon at the Earth surface, it is therefore important to understand its dynamic. If the increase of the frequency and/or intensity of extreme meteorological events (storms, cyclones) is confirmed [2, 3], it could lead to an increase of the export of dissolved and suspended material derived from soils. Volcanic provinces under wet tropical climate present optimal conditions for the weathering [4], including high temperature and high runoff, and account for 30% of the total soil organic carbon [5] and could be disrupted by climatic changes. Guadeloupe is selected for its geographical and geologycal settings and high rates of chemical weathering and mechanical denudation [4, 6, 7]. High-temporal resolution sampling of three small watersheds was performed since 2007 and a biannual sampling, during low water level and floods, was made in rivers of the Basse-Terre island. We quantified concentrations and fluxes of dissolved and particulate organic carbon (DOC, POC) as well as dissolved inorganic carbon (DIC). Organic carbon was also characterised by its isotopic composition (δ13CDOC) and its Specific UV Absorbance at the wavelengh 254 nm (SUVA254). DOC values range from 0.4 to 5.0 mg/L and increase with increasing rivers discharge. δ13CDOC varies between -32.8 and -26.2 for whole rivers which emphasized different carbon input according to the type of hydrologic events. Similarly, SUVA254 values vary between 2 and 6. Indeed, the high SUVA254 values are observed during floods and indicate that the aromaticity of organic matter increases during flood events. The less negative δ13C values and the high aromaticity for flood event samples are interpreted as the result of an increase of SOM inputs during these events. [1] Gregory et al. (1999) Inter. Geosph.-Biosph. Prog. Book, 4. [2] Emmanuel (2005) Nature, 436. [3] Webster et al. (2005) Science, 309. [4] Dessert et al. (2003) Chem. Geol., 202. [5] Batjes (1996) Europ. Journ. of Soil Science, 47. [6] Louvat (1997) PhD. [7] Rad et al. (2006) Journ. of Geoch. Explo., 88