Viscosity of the Martian mantle and its initial temperature: Constraints from crust formation history and the evolution of the magnetic field

The rheology of the Martian mantle and the planet's initial temperature is constrained with thermal evolution models that include crust growth and test the conditions for magnetic field generation in the core. As observations we use the present-day average crustal thickness of 50-120 km as estimated from the Mars Global Surveyor gravity and topography data, the evidence for the crust being produced mostly early, with a rate declining from the Noachian to the Hesperian, and the evidence for an early magnetic field that likely existed for less than a billion years. We use the fact that the rate of crust growth is a function of temperature, which must be above the solidus in the sub-lithosphere mantle, and the mantle convection speed because the latter determines the rate at which melt can be replenished. The convection speed is a strong function of viscosity which, in turn, is a strong function of temperature and also of the water content of the mantle. We use a viscosity parameterization with a reference viscosity evaluated at 1600 K the value of which can be characteristic of either a dry or a wet mantle. We further consider the Fe-FeS phase diagram for the core and compare the core liquidus estimated for a sulphur content of 14% as suggested by the SNC meteorite compositions with the core temperatures calculated for our cooling models. Two data sets of the Fe-FeS eutectic temperature have been used that differ by about 200 K [Böhler, R., 1996. Fe-FeS eutectic temperatures at 620 kbar. Phys. Earth Planet. Inter. 96, 181-186; Fei, Y., Bertka, C.M., Finger, L.W., 1997. High-pressure iron-sulphur compound, Fe₃S₂, and melting relations in the Fe-FeS system. Science 275, 1621-1623] at Martian core-mantle boundary pressure and in the eutectic composition by 5 wt%. The differences in eutectic temperature and composition translate into a difference of about 400 K in liquidus temperature for 14 wt% sulphur.We find it premature to rule out specific mantle rheologies on the basis of the presently available crustal thickness and crust growth evidence. Rather a trade-off exists between the initial mantle temperature and the reference viscosity. Both a wet mantle rheology with a reference viscosity less than 10²⁰ Pas and a dry mantle rheology with a reference viscosity of 10²¹ Pas or more can be acceptable if initial mantle temperatures between roughly 1700 and 2000 K are allowed. To explain the magnetic field history, the differences in liquidus temperatures matter. For a liquidus temperature of about 1900 K at the Martian core-mantle boundary as calculated from the Böhler et al. eutectic, a dry mantle rheology can best explain the lack of a present-day dynamo. For a liquidus temperature of about 1500 K at the core-mantle boundary as calculated from the Fei et al. eutectic all models are consistent with the observed lack of dynamo action. The reason lies with the fact that at 14 wt% S the Martian core would be close to the eutectic composition if the Fei et al. data are correct. As inner core growth is unlikely for an almost eutectic core, the early field would have been generated by a thermally driven dynamo. Together with the measured strength of the Martian crustal magnetization this would prove the feasibility of a strong thermally driven dynamo.     

Variation of hydrogeochemical characteristics of water in surface flows,shallow wells,and boreholes in the coastal city of Douala (Cameroon)

ABSTRACT

Groundwater is used by 3?million inhabitants in the coastal urban city of Douala, Cameroon, but comprehensive data are too sparse for it to be managed in a sustainable manner. Hence this study aimed to (1) assess the potability of the groundwater; (2) evaluate the spatial variation of groundwater composition; and (3) assess the interaction and recharge mechanisms of different water bodies. Hydrogeochemical tools and methods revealed the following results in the Wouri and Nkappa formations of the Douala basin, which is beneath Douala city: 30% of water samples from hand-dug wells in the shallow Pleistocene alluvium aquifer were saline and highly mineralized. However, water from boreholes in the deeper (49–92 m depth) Palaeocene aquifer was saline-free, less mineralized and potable. Water in the shallow aquifer (0.5–22 m depth) was of Na⁺-K⁺-Cl^?-NO₃^? type and not potable due to point source pollution, whereas Ca⁺-HCO₃^? unpolluted water dominates in the deeper aquifer. Water in the deep and shallow aquifers indicates the results of preferential flow pass and evaporative recharge, respectively. Possible hydrogeochemical processes include point source pollution, reverse ion exchange, remote recharge areas and mixing of waters with different chemical signatures.

EDITOR D. Koutsoyiannis ASSOCIATE EDITOR M.D. Fidelibus     

Mobility of arsenic in West Bengal aquifers conducting low and high groundwater arsenic. Part II: Comparative geochemical profile and leaching study

Aquifer sediments from areas of low- and high-As groundwater were characterized mineralogically and geochemically at a field site in the Nadia district of West Bengal, India. Leaching experiments and selective extraction of the sediments were also carried out to understand the release mechanism of As in the sub-surface. The correlation between measured elements (major, minor and trace) from low- and high-As groundwater areas are only significant for As, Fe and Mn. The borehole lithology and percentage of silt and clay fraction demonstrates the dominance of finer sediments in the high-As aquifer. Multivariate analysis of the geochemical parameters showed the presence of four different mineral phases (heavy-mineral fraction, phyllosilicates/biotite/Fe-oxyhydroxides, carbonates and sulphides) in the sediments. Selective extraction of sediment reveals that amorphous Fe-oxyhydroxide acts as a potential sink for As in the sub-surface. The result is consistent with microbially mediated redox reactions, which are controlled in part by the presence of natural organic matter within the aquifer sediments. The occurrences of As-bearing redox traps, primarily formed of Fe- and Mn-oxides/hydroxides, are also important factors that control the release of As into groundwater at the study site.     

Mars

Mars is the fourth planet out from the sun. It is a terrestrial planet with a density suggesting a composition roughly similar to that of the Earth. Its orbital period is 687 days, its orbital eccentricity is 0.093 and its rotational period is about 24 hours. Mars has two small moons of asteroidal shapes and sizes (about 11 and 6 km mean radius), the bigger of which, Phobos, orbits with decreasing semimajor orbit axis. The decrease of the orbit is caused by the dissipation of tidal energy in the Martian mantle. The other satellite, Deimos, orbits close to the synchronous position where the rotation period of a planet equals the orbital period of its satellite and has hardly evolved with time. Mars has a tenous atmosphere composed mostly of CO with strong winds and with large scale aeolian transport of surface material during dust storms and in sublimation-condensation cycles between the polar caps. The planet has a small magnetic field, probably not generated by dynamo action in the core but possibly due to remnant magnetization of crustal rock acquired earlier from a stronger magnetic field generated by a now dead core dynamo. A dynamo powered by thermal power alone would have ceased a few billions of years ago as the core cooled to an extent that it became stably stratified. Mars' topography and its gravity field are dominated by the Tharsis bulge, a huge dome of volcanic origin. Tharsis was the major center of volcanic activity, a second center is Elysium about 100° in longitude away. The Tharsis bulge is a major contributor to the non-hydrostaticity of the planet's figure. The moment of inertia factor together with the mass and the radius presently is the most useful constraint for geophysical models of the Martian interior. It has recently been determined by Doppler range measurements to the Mars Pathfinder Lander to be (Folkner et al. 1997). In addition, models of the interior structure use the chemistry of the SNC meteorites which are widely believed to have originated on Mars. According to the models, Mars is a differentiated planet with a 100 to 200 km thick basaltic crust, a metallic core with a radius of approximately half the planetary radius, and a silicate mantle. Mantle dynamics is essential in forming the elements of the surface tectonics. Models of mantle convection find that the pressure-induced phase transformations of -olivine to -spinel, -spinel to -spinel, and -spinel to perovskite play major roles in the evolution of mantle flow fields and mantle temperature. It is not very likely that the -spinel to perovskite transition is present in Mars today, but a few 100 km thick layer of perovskite may have been present in the lower mantle immediately above the core-mantle boundary early in the Martian history when mantle temperatures were hotter than today. The phase transitions act to reduce the number of upwellings to a few major plumes which is consistent with the bipolar distribution of volcanic centers of Mars. The phase transitions also cause a partial layering of the lower mantle which keeps the lower mantle and the core from extensive cooling over the past aeons. A relatively hot, fluid core is the most widely accepted explanation for the present lack of a self-generated magnetic field. Growth of an inner core which requires sub-liquidus temperatures in the core would have provided an efficient mechanism to power a dynamo up to the present day. Received 10 May 1997     

Macrophytes and phytobenthos as indicators of ecological status in German lakes — a contribution to the implementation of the water framework directive

A new assessment system for macrophytes and phytobenthos in German lakes according to the Water Framework Directive of the European Community is described. Based on biological, chemical and hydromorphological data from about 100 lake sites covering the main ecoregions, hydromorphological lake types and degradation forms, biocoenotic types could be defined. For developing a classification system the quality element macrophytes and phytobenthos was divided into two components: macrophytes and benthic diatoms. For macrophytes 4 and for benthic diatoms 4 lake types were identified. The benthic vegetation at reference conditions is described and degradation is characterised as deviation in benthic vegetation species composition and abundance from the reference biocoenosis. For classification in five ecological status classes, several metrics were developed and used in combination with existing indices. For a few of the described lake types further investigations are necessary before a classification can be developed.     

Chemical Composition of Urban Street Sediments and Its Sources

INTRODUCTIONBeijingfacesseveredustpollutionduetosuchsourcesasdustimportedfromwesterndeserts,industryandpowerplants,domesticcombustionprocessesandtraffic .Inthelastdecades ,theurbanpopulationhasgrowntomorethan10million .TheeconomicgrowthofChinahasalsoleftitsfootprintsinBeijing .Newindustrialzonesanddomesticar easweredeveloped ,inducinganincreasingtrafficcirculation(Shi,2 0 0 1) .Thedroughtclimateinalmostthewholeyear ,thestrongmonsoonsinwinterbringingdustsfromtheGobidesert,theheavytraffic…     

Managing food shortages in Central Tanzania

Famine prevention and mitigation strategies have become an established area of interest for researchers concerned with food insecurity. Studies often focus on one famine event and the coping strategies immediately preceding or accompanying it. By contrast, this case study adopts a long-term perspective and depicts the changes of indigenous and exogenous famine-coping strategies among the Gogo, an ethnic group in semi-arid central Tanzania, over a time-span of one and half centuries. By doing so, this paper tries to give insights into the continuity and adaptability of human reactions to a persistent problem under changing socio-economic circumstances.     

Measuring and monitoring persistent organic pollutants in the context of risk assessment

Due to growing concerns regarding persistent organic pollutants (POPs) in the environment, extensive studies and monitoring programs have been carried out in the last two decades to determine their concentrations in water, sediment, and more recently, in biota. An extensive review and analysis of the existing literature shows that whilst the vast majority of these efforts either attempt to compare (a) spatial changes (to identify "hot spots"), or (b) temporal changes to detect deterioration/improvement occurring in the environment, most studies could not provide sufficient statistical power to estimate concentrations of POPs in the environment and detect spatial and temporal changes. Despite various national POPs standards having been established, there has been a surprising paucity of emphasis in establishing accurate threshold concentrations that indicate potential significant threats to ecosystems and public health. Although most monitoring programs attempt to check compliance through reference to certain "environmental quality objectives", it should be pointed out that many of these established standards are typically associated with a large degree of uncertainty and rely on a large number of assumptions, some of which may be arbitrary. Non-compliance should trigger concern, so that the problem can be tracked down and rectified, but non-compliance must not be interpreted in a simplistic and mechanical way. Contaminants occurring in the physical environment may not necessarily be biologically available, and even when they are bioavailable, they may not necessarily elicit adverse biological effects at the individual or population levels. As such, we here argue that routine monitoring and reporting of abiotic and biotic POPs concentrations could be of limited use, unless such data can be related directly to the assessment of public health and ecological risks. Risk can be inferred from the ratio of predicted environmental concentration (PEC) and the predicted no effect concentration (PNEC). Currently, the paucity of data does not allow accurate estimation of PNEC, and future endeavors should therefore, be devoted to determine the threshold concentrations of POPs that can cause undesirable biological effects on sensitive receivers and important biological components in the receiving environment (e.g. keystone species, populations with high energy flow values, etc.), to enable derivation of PNECs based on solid scientific evidence and reduce uncertainty. Using the threshold body burden of POPs required to elicit damages of lysosomal integrity in the green mussel (Perna virvidis) as an example, we illustrate how measurement of POPs in body tissue could be used in predicting environmental risk in a meaningful way.