Pierre Genthon

Thermal convection in a vertical permeable slot: Implications for hydrothermal circulation along mid‐ocean ridges

Hydrothermal vents along unsedimented portions of mid-ocean ridges are fed by flow confined to a 1–3 km high percolating system of fissures subparallel to the spreading center. We have solved the coupled thermal and flow equations in such a system. We assume that hydrothermal circulation occurs in a vertical porous slot 1.5 km high, 2.3 km long, and at most 100 m wide. In this model we take into account the heat transported by the hydrothermal flow inside the porous slot as well as the heat conducted through the surrounding impermeable crustal layer. The fluid is free to enter or leave the top of the slot. The fluid enters the slot at a temperature of 2°C, while the bottom of the porous slot is held at a constant temperature of 420°C. We first consider a vertical slot with a horizontal base. Our calculations show the development of tall and narrow unsteady convective cells. Because of the nonlinear relationship of the viscosity and density of seawater with temperature, the upper thermal boundary layer is much thicker than the lower one. The fluid advected in the hot plumes tends to remain confined inside the slot because the upper boundary is thick and viscous. In high Rayleigh number experiments, some very active hot plumes are able to thin and pierce the upper cold boundary layer, and vent hot fluids at the seafloor. The maximum temperature at the exit of these plumes reaches a value at most equal to 270°C. The presence of an axial magma chamber along most of the length of the East Pacific Rise may justify the use of a horizontal base to model hydrothermal systems along fast spreading ridges. However, where the hydrothermal systems develop above a short-lived magmatic intrusion a few kilometers long, the lower interface of the percolating system of fissures is inclined. In that case our calculations show that the convective flow tends to be unicellular and steady state with a wide region of downwelling and a narrow upwelling zone. The temperature of the fluid exiting the slot at the top reaches a maximum value of 360°C. These results lead us to suggest that hydrothermal plumes generated inside a fracture with a horizontal base give rise to vents which wander along the fracture plane with a maximum temperature of about 270°C. However, when the base of the fracture plane is inclined, a large hydrothermal steady plume is generated which can give rise, at its exit, to a black smoker with fluid temperatures in excess of 350°C.

Two- and three-dimensional modeling of hydrothermal convection in the sedimented Middle Valley segment, Juan de Fuca Ridge

The heat flow map derived from 550 measurements collected in a the southern portion of the sedimented rift in Middle Valley, northern Juan de Fuca Ridge, displays kilometer-sized quasi-circular regions of very high heat flow. Some of these domains, explored during Ocean Drilling Program (ODP) leg 139, are thought to be discharge zones of large-scale hydrothermal plumes. To understand this unique data set, we modeled the kilometer-scale hydrothermal circulation within both the sedimentary and the igneous crust, using a set of two- and three-dimensional models that use a numerical technique based on horizontal spectral decomposition of the flow equations. These models include variations in the viscosity and density of the hydrothermal fluids with temperature. We examine the variations in flow patterns due to different permeability-versus-depth distribution within sediment and pillow layers. Models with the same permeability in both layers do not match the seafloor heat flow field in Middle Valley. When the permeability decreases from the bottom to the top of the simulation domain by a factor greater than 20, convection assumes a plume pattern to produce surface heat flow comparable to that observed in Middle Valley. Within the models the ratio of the heat flux above the recharge and discharge domains is directly related to the vertical harmonic mean of the permeability field. A value of 7×10−16 m2 provides a good match to the heat flow observations. The Darcy velocities of the hydrothermal fluids in the discharge areas approach 16 cm/yr while in the recharge areas they are lower than 3 cm/yr. These rates and the temperature inside the plumes are sufficiently high to produce the observed massive sulfide deposits and mineral alterations in 1–2 × 105 years. The dynamic pressure produced by the hydrothermal flow matches the pressure measured in drill sites. This process may play a major role in compaction, fracturing, and uplift of the sediment cover. For example, the dynamic pressure in the ascending plume equals the lithostatic pressure at a depth of 50 m. Resulting hydrofracturing could explain the genesis of the vent fields associated with the hydrothermal discharge.

Hydrological deformation induced by the West African Monsoon: Comparison of GPS, GRACE and loading models

Three-dimensional ground deformation measured with permanent GPS stations in nWest Africa was used for investigating the hydrological loading deformation associated nwith Monsoon precipitation. The GPS data were processed within a global network for the n2003–2008 period. Weekly station positions were retrieved with a repeatability (including nunmodeled loading effects) of 1–2 mm in the horizontal components and between 2.5 nand 6 mm in the vertical component. The annual signal in the vertical component for nsites located between 9.6N and 16.7N is in the range 10–15 mm. It is consistent at the n3 mm-level with the annual regional-scale loading deformations estimated from GRACE nsatellite products and modeled with a combination of hydrological, atmospheric, and nnontidal oceanic models. An additional 6 month transient signal was detected in the vertical ncomponent of GPS estimates at most of the West African sites. It takes the form of an noscillation occurring between September and March, and reaching a maximum amplitude of n12–16 mm at Ouagadougou (12.5N). The analysis of in situ hydro-geological data revealed na strong coincidence between this transient signal and peak river discharge at three sites nlocated along the Niger River (Timbuktu, Gao, and Niamey). At Ouagadougou, a similar ncoincidence was found with the seasonal variations of the water table depth. We propose a nmechanism to account for this signal that involves a sequence of swelling/shrinking of clays ncombined with local loading effects associated with flooding of the Niger River.

Assessment of Digital Elevation Model (DEM) aggregation methods for hydrological modeling: Lake Chad basin, Africa

Digital Elevation Models (DEMs) are used to compute the hydro-geomorphological variables required by distributed hydrological models. However, the resolution of the most precise DEMs is too fine to run these models over regional watersheds. DEMs therefore need to be aggregated to coarser resolutions, affecting both the representation of the land surface and the hydrological simulations. In the present paper, six algorithms (mean, median, mode, nearest neighbour, maximum and minimum) are used to aggregate the Shuttle Radar Topography Mission (SRTM) DEM from 3 (90m) to 5 (10km) in order to simulate the water balance of the Lake Chad basin (2.5Mkm^2). Each of these methods is assessed with respect to selected hydro-geomorphological properties that influence Terrestrial Hydrology Model with Biogeochemistry (THMB) simulations, namely the drainage network, the Lake Chad bottom topography and the floodplain extent. The results show that mean and median methods produce a smoother representation of the topography. This smoothing involves the removing of the depressions governing the floodplain dynamics (floodplain area 14,000km^2 with the maximum or nearest neighbour) but results in anomalies concerning the drainage network. An aggregation procedure based on a variographic analysis of the SRTM data is therefore suggested. This consists of preliminary filtering of the 3 DEM in order to smooth spikes and wells, then resampling to 5 via the nearest neighbour method so as to preserve the representation of depressions. With the resulting DEM, the drainage network, the Lake Chad bathymetric curves and the simulated floodplain hydrology are consistent with the observations (3% underestimation for simulated evaporation volumes).

Land Water Storage Changes from Ground and Space Geodesy: First Results from the GHYRAF (Gravity and Hydrology in Africa) Experiment

This paper is devoted to the first results from the GHYRAF (Gravity and Hydrology in Africa) experiment conducted since 2008 in West Africa and is aimed at investigating the changes in water storage in different regions sampling a strong rainfall gradient from the Sahara to the monsoon zone. The analysis of GPS vertical displacement in Niamey (Niger) and Djougou (Benin) shows that there is a clear annual signature of the hydrological load in agreement with global hydrology models like GLDAS. The comparison of GRACE solutions in West Africa, and more specifically in the Niger and Lake Chad basins, reveals a good agreement for the large scale annual water storage changes between global hydrology models and space gravity observations. Ground gravity observations done with an FG5 absolute gravimeter also show signals which can be well related to measured changes in soil and ground water. We present the first results for two sites in the Sahelian band (Wankama and Diffa in Niger) and one (Djougou in Benin) in the Sudanian monsoon region related to the recharge–discharge processes due to the monsoonal event in summer 2008 and the following dry season. It is confirmed that ground gravimetry is a useful tool to constrain local water storage changes when associated to hydrological and subsurface geophysical in situ measurements.

A thermoconvective interpretation of heat flow data in the area of Ocean Drilling Program Leg 116 in a distal part of the Bengal Fan

We present the results of a numerical simulation study of the thermo-convective circulation in a sedimentary pile, in the distal part of the Bengal Fan. The occurrence of faults and the variability of surface heat flow data led previous workers to suggest hydrothermal circulation in this area. According to the purpose of Ocean Drilling Program survey Leg 116, data were collected which are adequate to constrain a model for the comprehension and the quantification of the relationship between ocean basin heat flow and pore fluid circulation. In this paper the sedimentary pile is modeled as an anisotropic and inclined porous layer interrupted by faults with a periodic distribution. An important observation the model tries to reproduce is the shift of approximately 2–3 km between the surface expression of the heat flow maximum and the fault zone near site 718. The influence of permeability anisotropy and of permeability within those fault zones is analyzed. When the maximum vertical permeability in the fault zones is 5 times greater than the vertical permeability of the porous block, the circulation consists of longitudinal rolls (i.e., rolls with their axis oriented downslope), with more vigorous convection in fault zones. When the maximum permeability of the fault zone is 10 or more times greater than the vertical permeability of the porous layer, convection is primarily restricted to the fault plane. In all cases maximum heat flow values are located directly above faults. This paper shows that the best agreement between those data suggests a 1 m yr−1 forced convection and a permeability of the sediments of 0.5 darcy (5 × 10−13 m2) and 5 mdarcy respectively, in the horizontal and vertical derictions.

Fluctuations in the size of Lake Chad: consequences on the livelihoods of the riverain peoples in eastern Niger

Recent level changes in Lake Chad are associated with large area changes because the lakebed is almost flat. They have deeply impacted the lifestyle of local populations. The Mober are the main ethnic group living in the surroundings of Bosso, in eastern Niger near the Yobe River and in Lake Chad. They were initially herders and farmers who developed flood-retreat farming and sophisticated irrigation systems. But their main activity during high Lake levels was fishing: it provided food as well as substantial incomes, thanks to the export of smoked and dried fish. Since 1973, Lake Chad has shrunk, mainly because of the decrease in rainfall in its southern hydrological basin, on the border between Chad and the Republic of Central Africa. On the border between Niger and Nigeria, the Yobe River discharge was more stable but it provided a minor input to the Lake. Large areas with rich soils, termed here as polders by analogy with those of the Bol region in Chad, became available and allowed maize, cowpeas, sorghum and vegetables farming without irrigation or fertilizer. This system is governed by the “bulama,” chiefs of the villages and of the land that is still abundant. However, without any return of the Lake on the polders, there is a serious risk of soil exhaustion. Sweet pepper farming has been developed on the sandy Yobe borders since 1960, partly thanks to the local farmers’ ancient knowledge of irrigation techniques. It requires an investment of capital to buy fertilizers, pesticides, and gasoline for the motor pumps. This farming system may not only provide high incomes, but it also promotes inequality between men and women on the one hand and on the other hand, between poor farmers, who must borrow money and who have to sell their harvest immediately, and those who are able to store their products and wait for the best prices. The Mober of Bosso were able to adapt to rapid changes in the level of the Lake firstly without public intervention due to their long-lasting pluriactivity. However, their ability to cope with stronger changes induced either by climate or by large projects aiming at restoring high Lake Chad levels should be carefully monitored.

Comment on “Carbon uptake by karsts in the Houzhai Basin, southwest China” by Junhua Yan et al.

[1] In a recent paper Yan et al. [2011] estimate the carbon uptake in the Houzhai Basin due to dissolution of carbonate by rainwater using dissolved carbon measurements on this basin. Since both the surface and the underground river networks are known to converge toward a single outlet, this could be achieved with one hydrological station, monitoring water discharge and providing water for chemical analyses. From a nearly monthly record over the 1986–2007 period, they were able to monitor the seasonal variations of dissolved carbon uptake and to compute a mean yearly uptake of 20.7 g C m 2 yr 1 for the Houzhai Basin, which is further extended to estimate the total carbon uptake by karst in south China. Since several karstic basins are located in China, the study by Yan et al. is of great interest for the scientific community. Our concern is that it neglects the gaseous component of the carbon budget. However, it is known that karstic voids contain a few percent volume of CO2, and we will present below evidence that this CO2 is drained downward with rainwater from organic soils, collected by karstic voids and finally advected toward the Earth’s atmosphere, which can significantly alter the carbon budget computed by Yan et al. As the conclusions presented by Yan et al. are designed to constrain global atmospheric models, we suggest that the gaseous CO2 should also be considered in the estimates of the budget of carbon flowing through the Houzhai Basin. [2] Our team has accumulated more than 20 years’ research experience on this topic. Our studies were mainly motivated by the conservation of caves, including prehistoric painted caves, which requires the stability of the inner atmosphere. It is well known that the CO2 concentration can reach a few percent volume in the cave atmosphere [James, 1977], and we monitored its concentration in a large set of French caves [Bourges et al., 2001, 2006]. We have strong indications that this CO2 is mainly biogenic and produced in soils. This inference is supported by numerous dC measurements in the Villard Cave (SW France) with a mean dC of CO2 = 22.7‰ (50 samples from 2005 to 2010), where its variation has been correlated with the CO2 air concentration [Genty, 2008], and in the Chauvet Cave (SE France) with a mean dC of CO2 = 22.7‰ (20 samples from 2000 to 2010). There is also growing evidence that CO2 is transported downward both as dissolved species and in biphasic flow with water seeping toward the phreatic zone [Atkinson, 1977; Bourges et al., 2006;Mudry et al., 2008;Milanolo and Gabrovšek, 2009]. One of our most original results is that the cave atmosphere is not at rest but is continuously flowing toward the Earth’s atmosphere through large openings [Bourges et al., 2001]. Our interpretation is that soil air enriched with CO2 is dragged downward both in biphasic flow and as dissolved species during infiltration of rainwater and collected as gaseous CO2 by the network of macroscopic voids including fissures and caves. Our results indicate that a significant part of this CO2 is directly drained to the outside atmosphere through open fissures and large cave openings. This interpretation is now widely accepted by scientists working on caves [Baldini et al., 2006; Serrano-Ortiz et al., 2010]. After reviewing CO2 data gathered by the flux tower community and showing an anomalously high CO2 rate escaping from karst terrains toward the atmosphere, SerranoOrtiz et al. [2010] proposed an explanation similar to ours, involving a downward infiltration of CO2 with rainwater followed by an outflow of gaseous CO2 toward the terrestrial atmosphere. In some instances, winds flowing on permeable soils can further enhance the exchanges of CO2 between the cave atmosphere and the Earth’s atmosphere [SanchezCañete et al., 2011; Cuezva et al., 2011]. [3] We were able to observe the outward flow of CO2 on several occasions and to estimate the annual flow of gaseous CO2 at one French cave: the Aven d’Orgnac network (Figure 1). In winter, the outside air is colder and denser than the cave atmosphere, and the cave is therefore ventilated by downward flow of the outside air; then, in most parts of the cave the CO2 content is similar to that of the terrestrial atmosphere [Bourges et al., 2006; Kowalski and SanchezCañete, 2010]. This was shown with temperature, CO2 concentration and Rn concentration profiling along the cave system. Due to this thermo-compositional venting, the CO2 derived from soils and conveyed to the cave would be efficiently transferred to the Earth’s atmosphere. However, direct measurements of CO2 flux were impossible in winter, GEConseil, St.-Girons, Ariège, France. IRD/HSM, University of Montpellier II, Montpellier, France. LSCE, UMR CEA/CNRS/UVSQ 1572, L’Orme des Merisiers CEA Saclay, Gif-sur-Yvette, France. EcoEx-Moulis, Moulis, Ariège, France.

Distribution des cavités de l'atoll soulevé de Walpole. Implications sur le processus de karstification par les eaux météoriques

The distribution of voids recorded along the cliffs bordering the karstified coral island of Walpole shows that numerous metric aperture joints, enlarged by dissolution, are present in the thinner part of the island. By contrast, along the wider parts of the island, karstic voids consist in a few large caves. We suggest that those observations may be compared to experiments of acid injection in a limestone slice that result firstly in numerous short length channels, while the final state of the system consists of only one long wormhole, developing at the expense of all others.

Suitability of a lumped rainfall–runoff model for flashy tropical watersheds in New Caledonia

ABSTRACT The GR4H lumped hourly rainfall–runoff model was assessed for its integration in a ridge-to-reef modelling framework. Particular attention was paid to rainfall representation, robustness of parameter estimates and ability to reproduce the main runoff features. The study was conducted in four tropical mountainous watersheds in New Caledonia, which are exposed to intense rainfall events, large annual climatic variations triggered by El Niño oscillation, and wildfires. The inverse distance and elevation weighting algorithm outperformed other classical rainfall interpolation methods under data-limited conditions. The time span of data needed for robust calibration was site specific and varied from 6–7 years to 10 years, which may be linked to El Niño events and to wildfires. With sufficient data, simulation quality was equivalent during the calibration and validation periods. The GR4H model was generally able to simulate both flash floods and large annual variations. The model was more reliable when simulating wet years and watersheds not subject to land-cover changes.