Methane release and the carbon cycle on Mars

It has been suggested that the present release rate of methane to the Martian atmosphere could be the result of serpentinization in the deep subsurface, followed by the conversion of H₂ to CH₄ in a CO₂-rich fluid. Making this assumption, we show that the cryosphere could act as a buffer storing, under the form of micron-size methane clathrate particles, the methane delivered from below by hydrothermal fluids and progressively releasing it to the atmosphere at the top. From an extrapolation of the present CH₄ release rate back to the past, we calculate that up to several hundred millibars (～200–2000 mbar) of CO₂, resulting from the oxidation of the released CH₄, in addition to the volcanic supply (～400 mbar), should have accumulated in the atmosphere in the absence of a CO₂ sink. We reassess the capability of escape to have removed CO₂ from the atmosphere by C non-thermal escape and show that it is not significant. We suggest that atmospheric carbon is recycled to the crust through an active subsurface hydrological system, and precipitates as carbonates within the crust. During episodic periods of magmatic activity, these carbonates are decomposed to CO₂ dissolved in running water, and CO₂ can react with H₂ formed by serpentinization to build CH₄. CH₄ is then buffered in the subsurface cryosphere, above the water table, and finally released to the atmosphere, before being recycled to the subsurface hydrological system, and converted back to carbonates. We propose a typical evolution curve of the CO₂ pressure since the late Noachian based on our hypothesis. Contrary to the steady state carbon cycle at work on Earth, a progressive damping of the carbon cycle occurs on Mars due to the absence of plate tectonics and the progressive cooling of the planet.     

Book reviews

Climate change in contrasting river basins: adaptation strategies for water, food and environment. Edited by J Aerts and P Droogers. Published in 2004 by CABI Publishing, 288 p., hardcover. ISBN: 0–85199–835–6. Price: US$100 (orders: services@dadirect.com.au).

Australian water bugs. Their biology and identification (Hemiptera‐Heteroptera, Gerromorpha &; Nepomorpha). By Nils M Andersen? and Tom A Weir. Published in 2004 by CSIRO Publishing/Apollo Books, P.O. Box 1139, Collingwood, VIC 3066, Australia. 344 p., hardcover. ISBN: 0–643–090517. Price: AU$135.00 (orders: publishing.sales@csiro.au).

The Lakes Handbook. Volume I: Limnology and Limnetic Ecology. Volume II: Lake restoration and rehabilitation. Edited by Patrick O'Sullivan and Colin Reynolds. Published by Blackwell Publishing Ltd., 109 Cowley Road, Oxford OX4 1JF, United Kingdom (http//www.blackwellpublishing.com). Volume I (published in 2003) 712 p., hardcover. ISBN: 0632047976. Price: £125. Volume II (published in 2004) 568 p., hardcover. ISBN: 063204795X. Price: £125.     

利用不对称的地下水位潮汐波动确定滨海含水层参数

在滨海地区,地下水水位受潮汐波动影响较大,使得传统的抽水试验、水位回复试验等方法确定含水层参数存在困难且花费较大。通过对广西北海市滨海含水层地下水位动态资料进行分析,发现其上升段和下降段是不对称的。基于海岸带承压含水层正弦潮汐波的传播理论,提出了确定含水层参数的分段法,并与振幅衰减法和滞后时间法进行对比,各种方法求出的储水系数与导水系数之比(S/T)很接近,说明分段法是有效的。对于北海市滨海含水层,上升段求出的S/T值比下降段要大,其成因机理还有待进一步分析。     

Automatic classification of dome instabilities based on Doppler radar measurements at Merapi volcano, Indonesia: Part I

Monitoring lava dome instabilities is crucial to efficiently monitor active dome building volcanoes. The Doppler radar technique provides a unique opportunity to gather information about the number of instability events occurring at the growing dome and about the dynamic processes that take place during different types of instabilities. So far, three different kinds of processes have been identified: sliding material, gravitational break-offs and explosive outbursts. In addition, Doppler radars provide rain measurements, which can be used to investigate possible correlations between rainfall and dome activity. Two radar systems have been installed at Merapi volcano in October 2001 and January 2005 to continuously monitor dome instabilities. Due to the large number of instability events that occur during times of high activity, manual processing and analysis of instability events is not practical for monitoring purposes. Therefore, an automatic classification system has been developed, which is capable of identifying different kinds of instabilities as well as rainfall. Two different kinds of classifier models have been applied: (1) neural network and (2) K-nearest-neighbour classifier model. Both classify Doppler spectra according to the underlying dynamic process, that is, rain, sliding material, gravitational break-off or explosive outburst. The classifiers are able to identify disturbances, which have no physical source, but are merely artefacts from the radar device itself. Because radar events are sequences of Doppler spectra, a rule set has been defined, which finally determines the event class. All classifiers have been trained and tested on independent data sets to estimate the classification performance. The overall classification rate is about 90 per cent. Discrimination of instabilities and non-volcanic events reaches about 98 per cent accuracy.     

Elasticity of antigorite,seismic detection of serpentinites,and anisotropy in subduction zones

Serpentinization of the mantle wedge is an important process that influences the seismic and mechanical properties in subduction zones. Seismic detection of serpentines relies on the knowledge of elastic properties of serpentinites, which thus far has not been possible in the absence of single-crystal elastic properties of antigorite. The elastic constants of antigorite, the dominant serpentine at high-pressure in subduction zones, were measured using Brillouin spectroscopy under ambient conditions. In addition, antigorite lattice preferred orientations (LPO) were determined using an electron back-scattering diffraction (EBSD) technique. Isotropic aggregate velocities are significantly lower than those of peridotites to allow seismic detection of serpentinites from tomography. The isotropic V_P/V_S ratio is 1.76 in the Voigt–Reuss–Hill average, not very different from that of 1.73 in peridotite, but may vary between 1.70 and 1.86 between the Voigt and Reuss bonds. Antigorite and deformed serpentinites have a very high seismic anisotropy and remarkably low velocities along particular directions. V_P varies between 8.9 km s^? 1 and 5.6 km s^? 1 (46% anisotropy), and 8.3 km s^? 1 and 5.8 km s^? 1 (37%), and V_S between 5.1 km s^? 1 and 2.5 km s^? 1 (66%), and 4.7 km s^? 1 and 2.9 km s^? 1 (50%) for the single-crystal and aggregate, respectively. The V_P/V_S ratio and shear wave splitting also vary with orientation between 1.2 and 3.4, and 1.3 and 2.8 for the single-crystal and aggregate, respectively. Thus deformed serpentinites can present seismic velocities similar to peridotites for wave propagation parallel to the foliation or lower than crustal rocks for wave propagation perpendicular to the foliation. These properties can be used to detect serpentinite, quantify the amount of serpentinization, and to discuss relationships between seismic anisotropy and deformation in the mantle wedge. Regions of high V_P/V_S ratios and extremely low velocities in the mantle wedge of subduction zones (down to about 6 and 3 km.s^?1 for V_P and V_S, respectively) are difficult to explain without strong preferred orientation of serpentine. Local variations of anisotropy may result from kilometer-scale folding of serpentinites. Shear wave splittings up to 1–1.5 s can be explained with moderately thick (10–20 km) serpentinite bodies.     

ESR chronology of alluvial deposits and first human settlements of the Middle Loire Basin (Region Centre,France)

Since 1970, many prehistoric sites have been discovered in the alluvial terraces of several rivers of the Middle Loire River basin (France). During the last decade, the stepped-terraces systems of three of the Loire tributaries, the Creuse, the Cher and the Loir Rivers, were the focus of multidisciplinary studies, involving Quaternary geology, prehistory and geochronology. The geochronological data now available for these systems were mainly obtained by electron spin resonance (ESR) method applied to optically bleached fluvial quartz. These results provide a chronological framework for the evolution of fluvial systems and the human settlements of this area during Lower and Middle Pleistocene. Two phases of human settlement could be distinguished, around 1.1 Ma (Early Palaeolithic – Mode 1) and around 0.7 Ma ago (Acheulean – Mode II).     

OSL and ESR studies of Aeolian quartz from the Upper Pleistocene loess sequence of Nussloch (Germany)

As part of a chronological study of the famous Upper Pleistocene Nussloch (Germany) loess sequence, three samples were collected to check the applicability of palaeodosimetric dating methods (OSL and ESR) to quartz grains. The ESR-multicentre method showed a partial bleaching of the ESR centers in aeolian sands. This partial bleaching was also observed by OSL. Laminated loess seemed to be sufficiently bleached but showed a large scatter of the doses, which we ascribed to heterogeneous responses of the luminescent grains to the SAR protocol. Ages could nevertheless be calculated for the three samples and were found to be somewhat older than the IRSL and ¹⁴C ages obtained for the same layers of the laminated loess.     

Data assimilation with the ensemble Kalman filter in a numerical model of the North Sea

Coastal management and maritime safety strongly rely on accurate representations of the sea state. Both dynamical models and observations provide abundant pieces of information. However, none of them provides the complete picture. The assimilation of observations into models is one way to improve our knowledge of the ocean state. Its application in coastal models remains challenging because of the wide range of temporal and spatial variabilities of the processes involved. This study investigates the assimilation of temperature profiles with the ensemble Kalman filter in 3-D North Sea simulations. The model error is represented by the standard deviation of an ensemble of model states. Parameters’ values for the ensemble generation are first computed from the misfit between the data and the model results without assimilation. Then, two square root algorithms are applied to assimilate the data. The impact of data assimilation on the simulated temperature is assessed. Results show that the ensemble Kalman filter is adequate for improving temperature forecasts in coastal areas, under adequate model error specification.