Weathering regime associated with subsurface circulation on volcanic islands

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

Hidden erosion on volcanic islands

International audienceWe have studied rivers from 3 volcanic tropical Islands (Guadeloupe and Martinique in Lesser Antilles and Reunion) in order to evaluate chemical weathering rates. These islands are constituted of very porous pyroclastic deposits, andesitic or basaltic lava flows, allowing significant water infiltration. From hydrological budgets, we estimate that infiltration represents 10% in Guadeloupe and 40% in Martinique. Element fluxes transported by subsurface water to the ocean have been quantified by coupling the hydrological budget with the chemical composition of the sampled subsurface waters. This is the first estimate of chemical weathering rates from volcanic islands, which takes into account the high temperature weathering contribution to the chemical weathering rates. Subsurface waters are impacted by high-temperature water–rock interactions and present therefore higher concentrations (4 to 100 times) of major and trace elements compare to surface waters, representing thus respectively 90%, 60% and 75% of the total (surface + subsurface) weathering dissolved flux to the ocean for Martinique, Guadeloupe and Reunion. This shows the importance of these fluxes, which until now were not estimated in the basalt weathering geochemical budgets. Chemical weathering rates from subsurface water are 2 to 5 time higher than the rates from surface waters. The total average chemical rates calculated are 290 t/km2/yr in Guadeloupe, 1080 t/km2/yr in Martinique and 270 t/km2/yr in Réunion. These values are among the highest worldwide. Extrapolating this budget to the surface of all volcanic islands world-wide, we estimate that these could contribute between 23% and 31% of the global flux of dissolved load transported to the oceans by major rivers draining silicate rocks whereas their surface area represent only 9% of the silicate rock total surface are

Effect of natural organic matter on thallium and silver speciation

International audienc

River dissolved and solid loads in the Lesser Antilles: New insight into basalt weathering processes

International audienceWe present here the first available estimations of chemical weathering and associated atmospheric CO2 consumption rates as well as mechanical erosion rate for the Lesser Antilles. The chemical weathering (100-120 t/km2/year) and CO2 consumption (1.1- 1.4 106 mol/km2/year) rates are calculated after subtraction of the atmospheric and hydrothermal inputs in the chemical composition of the river dissolved loads. These rates thus reflect only the low-temperature basalt weathering. Mechanical erosion rates (approx. 800-4000 t/km2/year) are estimated by a geochemical mass balance between the dissolved and solid loads and mean unaltered rock. The calculated chemical weathering rates and associated atmospheric CO2 consumption rates are among the highest values worldwide but are still lower than those of other tropical volcanic islands and do not fit with the HCO3 concentration vs. 1/ T correlation proposed by Dessert et al. (2001). The thick soils and explosive volcanism context of the Lesser Antilles are the two possible keys to this different weathering behaviour; the development of thick soils limits the chemical weathering and the presence of very porous pyroclastic flows allows an important water infiltration and thus subsurface weathering mechanisms, which are less effective for atmospheric CO2 consumptio

Chemical weathering and erosion rates in Lesser Antilles: an overview in Guadeloupe, Martinique and Dominica

International audienceGuadeloupe, Martinique and Dominica islands, alike numerous tropical environments, present extreme weathering regimes. Physical denudation is mainly controlled by landslide, which reflects the torrential dynamics of the rivers. In Guadeloupe, the mechanical weathering rates are comprised between 800 and 4000 t/km2/yr. The lithology is very porous with high infiltration rates, which induces that most of the elements fluxes are produced on subsurface with chemical erosion rates 2 to 5 times higher than the rates from surface water. We show how kinetic of chemical weathering rates depends on the age of the lava and subsurface circulation. In addition, timescale of erosion have been calculated from U-series analyses sediments from rivers. Our results show a large range: from 0 to 150 ka in Martinique and from 0 to 60 ka in Guadeloupe. From analyses of the dissolved loads, we propose to evaluate residence times in the river water. It appears that water circulation is globally 3 times longer for subsurface water than for surficial water (Rad et al. 2011). Moreover these islands are highly impacted by agriculture, however our result show that human activity brings no disturbance on Critical zone processes contrary to what one might think. Indeed, we show that among the combined impact of all parameters (climate, runoff, slopes, vegetation...), the basins age seems to be the control parameter on chemical weathering and land use: the younger the basin, the higher the weathering rates. We could observe a combined effect between the higher erodibility and a higher climate erosivity of the younger reliefs

Weathered profiles in tropical volcanic islands by combined geochemical and geophysical approaches

International audienceTropical volcanic islands commonly have pronounced relief and high runoff rates, and consist of easily weathered volcanic material. Intense mechanical and chemical weathering in volcanic terrains has been recognized as being an important component in the transport of the global dissolved load to the oceans [1]. High chemical weathering rates are mainly due to the impact of hydrothermalism inputs with subsurface water circulations [2]. A helicopter-borne TDEM (Time Domain ElectroMagnestism) and magnetic survey was conducted by BRGM (French Geological Survey) over 3 islands of Guadeloupe, Martinique and Reunion in 2012 and 2013 for a total of 20,000 km of flightlines. TDEM method uses the diffusion of a transient EM field to determine the electrical resistivity versus depth. Erosion timescales were calculated from U-series analyses of river sediments. Our results show a broad range: 0 to150 ka in Martinique, 0 to 60 ka in Guadeloupe and 55 to 90 ky for Piton de la Fournaise (Réunion). At watershed scale, the estimated weathered profile (WP) depth obtain by using U-series method are consistent with TDEM helicopter-borne geophysical imagery method, ranging from 0 to 70 m. WP are locally impacted by hydrothermal circulations with associated secondary minerals (halloysite, tridymite…). Among the combined impact of all parameters (climate, runoff, slopes, hydrothermalism inputs, vegetation etc.), basin age seems to be the key control parameter: the younger the basin, the higher the weathering rate is

Chemical weathering and erosion rates in Lesser Antilles: an overview in Guadeloupe, Martinique and Dominica

International audienceGuadeloupe, Martinique and Dominica Islands alike numerous tropical environment present extreme weathering regimes. Physical denudation is mainly controlled by landside, This reflects the torrential dynamics of the rivers. For Guadeloupe, the mechanical weathering rates are 800-4000 t/km2/yr. The lithology is very porous with high infiltration rates, which induces that most of the elements fluxes are produced on subsurface as the chemical erosion rates are 2 to 5 time higher than the rates from surface water. We show how kinetic of chemical weathering rates depends on the age of the lava and subsurface circulation. In addition, timescale of erosion have been calculated from U-series analyses sediments from rivers. Our results show a large range: from 0 to 150 ka in Martinique and from 0 to 60 ka Guadeloupe. From analyses from the dissolved loads, we propose to evaluate residence times in the river water. It would appear that waters circulation is globally 3 times longer for subsurface water than for surficial water (Rad et al., 2011). Moreover these islands are highly impacted by agriculture. It is therefore interesting to assess the impact of such influence on the weathering rates. Our result show that human activity brings no disturbance on Critical zone processes contrary to what one might think. Indeed, we show that among the combined impact of all parameters (climate, runoff, slopes, vegetation...), the basins age seems to be the control parameter on chemical weathering and land use: the younger the basin, the higher the weathering rate. We could observe a combined effect between the higher erodibility and a higher climate erosivity of the younger reliefs

Sedimentary and diagenetic features of the Oolithe Blanche formation (Middle Jurassic): new contribution from Ca, Sr, C, O isotopic compositions

International audienceThe Oolithe Blanche formation is present in the Paris Basin at more than 1500 m depth in the middle of the basin and on outcrops at the basin edges. This Bathonian formation (Middle Jurassic) is composed of very shallow marine oolithic and bioclastic limestones, located within a shoreface depositional environmental (Casteleyn et al., 2010). Calcium, strontium, carbon and oxygen isotopes have been analysed both in ooids and interparticular cement directly extracted from the limestone in order to study sedimentary and diagenetic environment. The 44Ca/40Ca ratios (expressed as 44/40CaSW) were measured by TIMS using a 42Ca-48Ca double-spike. Carbon and oxygen isotopic ratios were measured by IRMS. Preliminary results show a range of 44/40CaSW from -0.74‰ to -1.09‰ in the cement and from -0.85‰ to -1.05‰ in ooids. Carbon isotopic signature show a range of 13Cvs PDB range from 1.5‰ to 2.7‰ in the cement and from 1.5‰ to 2.4‰ in ooids while 18Ovs SMOW vary from 21.5‰ to 23.9‰ in the cement and from 21.2‰ to 24.9‰ in ooids. This first multi-isotopic approach on Oolithe Blanche formation seems to be consistent with the diagenetic evolution of Paris Basin. Effects of diagenesis on the isotopic signatures will be discussed. Reference: Casteleyn, L., Robion, P., Collin, P.Y., Menendez, B., David, C., Desaubliaux, G., Fernandes, N., Dreux, R., Badiner, G., Brosse, E., Rigollet, C., 2010. Sedimentary Geology 230, 123-138

Impact of land use on weathering rates in Guadeloupe, Caribbean islands

International audienceGuadeloupe is located in Lesser Antilles with a tropical climate with very high precipitation, temperature, very dense vegetation (forest on the steepest slopes, agricultural on the lowlands) and sharp relief. Rivers present torrential hydrological regime with extreme erosion conditions. The tropical context contributes to important development of saprolitic profile, with extreme chemical weathering rates (e.g. 100 to 600 t/km2/yr). As for many volcanic islands erodible lithology such as pyroclastic flows with ashes or even massif lava flows involve important material transported during the erosion processes. The lithyology is also very porous with high infiltration rates, which induces that most of the elements fluxes are produced on subsurface as the chemical erosion rates are 2 to 5 time higher than the rates from surface water (Rad et al., 2007). Moreover kinetic of chemical weathering rates depends on the age of the lava flows (with a NS gradient of age) and subsurface circulation with local hydrothermal springs, which highly increases chemical weathering rates. It appears that first stage of erosion are characterized by high chemical denudation rates and high physical denudation rates, the erosion products chemical compositions are then close to the bedrock one. It is then followed in a second stage by constant chemical weathering rates and lower mechanical denudation rates. Moreover Guadeloupe islands is highly impacted by agriculture (banana and sugar cane plantations), which significantly influence the hydrological cycle. It is therefore interesting to asses the impact of such influence on the weathering rates on this island. Chemical and physical weathering rates will be correlated to the different land use to quantify the impact of human activities and explain its role in the evolution of the Critical Zon

The fundamental role of island arc weathering in the oceanic Sr isotope budget

International audienceWe have re-assessed the Sr isotopic budget of the modern ocean taking into account the high erosion rates of volcanic islands, and especially of island arcs, emphasizing important contribution from subsurface weathering to global budgeting. We propose that intensive weathering on volcanic islands, island arcs and oceanic islands, coupled with large surface and subsurface water fluxes is the missing source of mantlederived 87Sr/86Sr (0.703) in seawater Sr isotope balance. In our approach, it represents 60% of the actual mantle-like input of Sr to the oceans, the remaining 40% supplied by ridge-crest hydrothermal activity and sea-floor low-temperature alteration of basalts. The seawater Nd isotopic budget is consistent with this interpretation and explains well the regional contributions from ridge crest and island arc activity among the oceans