Evidence for Non-Conservative Behaviour of Chlorine in Humid Tropical Environments

The knowledge of the biogeochemical cycle of chlorine (Cl)is important since this element is used as a tracer of geochemical and hydrological processes in oceanic or continental environments. More specifically, Cl can be used to correct surface water composition from atmospheric contribution in order to calculate precise chemical weathering rates in watersheds. Beyond the problem of potential Cl sources in a given watershed, which is directly related to the lithology, vegetation, and industrial activities, the Cl normalization is based on the assumption that this element behaves conservatively during surface processes (e.g., chemical weathering, adsorption/desorption processes).The purpose of the present study is to forecast the geochemical behavior of Cl in a forested ecosystem located under humid tropical environment.For this reason, we have analyzed the Cl (and also Ca and Na) concentrations ofsurface waters (rainwater, groundwater, river water) over a two-year period in the Nsimi–Zoetele watershed (Cameroon).The Cl mass balance for the watershed appears to be equilibrated over the studied period (1995–1996) but Cl behavior in Mengong River draining the watershed suggests a non-conservative behavior. Indeed, Cl concentrationsin the Mengong River are low during dry seasons and high during wet seasons, which is the reverse tendency to what is usually observed taking into account dilution and evaporation processes. As Cl concentrations in the Mengong River are lower than those measured in all the feeding reservoirs, Cl should be adsorbed onto the soils of the watershed. However, as the Cl mass balance is equilibrated over the whole-year, Cl should be adsorbed and releasedat a seasonal scale. The results we obtained for this small watershed were not generalized for a larger studied basin (i.e., Nyong River basin). Even if these results should be followed by further investigations, this study suggests that Cl normalization should be used with caution to avoid under- or over-estimation of chemical weathering rates.     

辽东地区沉积变质硼矿床及硼同位素研究

辽东地区元古界硼矿床属于沉积变质硼矿。根据现代盐湖沉积 ,认为硼矿物沉积通常为硼砂 ,然后在区域变质作用过程中 ,硼砂矿物转变为硼镁矿物或硼铁矿等硼酸盐矿物 ,但辽东硼矿床中不同硼矿物的硼同位素有一定差别。电气石富10 B ,δ11B值较低或呈负值 ;而硼镁石与硼镁铁矿类矿物明显富集11B ,δ11B值较高 ,为 +2 3‰～ +17 4‰。根据水岩作用过程中硼同位素分馏特征的研究 ,11B与10 B比较有下列 4个特征 :( 1) 11B属于极不相容元素 ,优先进入水相 ,因此在变质残余矿物相中形成低的δ11B值 ;( 2 )在蚀变和交代变质反应中硅优先替代11B进入矿物晶格 ,因此在硅化交代中可以降低δ11B值 ,而脱硅反应中可以提高δ11B值 ;水化作用中 ,OH-带入11B ,使富水矿物具有较高的δ11B值 ;( 3)热水沉积及热液交代成因电气石均具有较低的δ11B值或者为负值 ;( 4)通过最近的研究表明 ,在热蒸馏过程中 ,硼同位素会发生明显的分馏 ,11B倾向于进入蒸汽相 ,而使残余相或后蒸馏相亏损11B ,形成较低的δ11B值。因此可以认为辽东地区元古界硼矿床属于热水沉积电气石岩在后期区域变质或热变质过程中分解出硼酸气水溶液交代镁碳酸盐形成硼镁石或硼镁铁矿矿物 ,而非蒸发沉积变质矿床。     

Ecological influences of Thule Inuit whalers on high Arctic pond ecosystems: a comparative paleolimnological study from Bathurst Island (Nunavut,Canada)

Until recently, major anthropogenic impacts on freshwater ecosystems were believed to be rare in North America prior to the period of European colonization. However, recent paleolimnological and archaeological data collected from the Canadian Arctic suggest that the whaling activities of Thule Inuit, who lived in small, nomadic communities, altered freshwater ecosystems centuries earlier. Using a comparative paleolimnological approach from two ponds situated adjacent to a former Thule winter settlement on south-eastern Bathurst Island (Nunavut, Arctic Canada), we record marked ecological changes in pond ecology due to eutrophication from the Thule’s activities. The geography of our study site provided an interesting and rare opportunity for a comparative paleolimnological study of long-term Thule impacts on polar limnology, because our two study ponds (only ~50 m apart) were nearly identical in size and in geological and climatic settings, but differed markedly in the magnitude of Thule influence. Here, we recorded striking changes in diatom species assemblages, spectrally-inferred primary production, and nutrient geochemistry, indicating eutrophication in a small pond draining 18 Thule whale houses. Input of marine-derived nutrients from sea mammal carcasses used by the Thule for both sustenance and the construction of winter settlements, as well as other anthropogenic activities, coincided with a notable increase in the eutrophic diatom taxon Stephanodiscus minutulus, whereas no comparable changes were recorded in the nearby control pond for the duration of the sedimentary record. Although the diatom changes recorded in the affected site persisted after the period of Thule occupation, the most recent sediments and water chemistry suggest that the pond has largely recovered to near pre-impact conditions.     

Do spectrally inferred determinations of chlorophyll <Emphasis Type="Italic">a</Emphasis> reflect trends in lake trophic status?

Visible reflectance spectroscopy (VRS) has been used to reconstruct lake sediment chlorophyll a concentrations. Despite good concordance between inferred and measured chlorophyll a values, questions remain as to whether this spectral technique is tracking past changes in aquatic primary production, or simply recording a diagenetic signal. In this study, we critically evaluate how well VRS chlorophyll a determinations track past trends in aquatic primary production using sediment cores from several lake systems with well-known trophic histories. Our study sites include Arctic, boreal and prairie lakes that encompass a gradient of trophic states. In general, our spectrally inferred chlorophyll a values tracked past trends in lake trophic status consistent with historical measurements of production, or as inferred by independent proxies of primary production. We conclude that VRS chlorophyll a inferences indeed track histories of lake production trends and that this method is widely applicable as a rapid, inexpensive and non-destructive alternative to wet-chemical analyses of sediment chlorophyll a concentrations.     

Comparing nitrogen isotopic signals between bulk sediments and invertebrate remains in High Arctic seabird-influenced ponds

The mass transport of nutrients by migratory animals can markedly alter the biogeochemistry and ecology of recipient ecosystems, particularly in nutrient-poor regions such as the Arctic. However, the role of biovectors in the global cycling of nutrients is often overlooked. Here we investigate nitrogen dynamics in two seabird-affected ponds in the Canadian High Arctic. The ponds lie at the base of a large seabird colony and have been greatly enriched in nutrients due to the input of guano and other wastes. Using sediment cores that span the last ~200 years, we measured stable isotopes of nitrogen (δ¹⁵N) in bulk sediments as well from the subfossil remains of chironomid (Diptera) head capsules and Daphnia ephippia. The bulk-sediment samples from our seabird-affected ponds had elevated δ¹⁵N values relative to seabird-free sites elsewhere in the Arctic. In general, the chironomid δ¹⁵N profiles roughly paralleled those of bulk sediments in both study ponds, while the Daphnia profile remained relatively stable in contrast to the considerable variation recorded in the bulk sediments and chironomids. Interestingly, no apparent pattern emerged among δ¹⁵N values recorded in the bulk sediments, chironomids, and Daphnia between the two study ponds. The stability recorded in the δ¹⁵N profiles from bulk sediments relative to the more variable invertebrate profiles point towards the complexity of nitrogen uptake by chironomids and Daphnia at these sites. These data suggest that the bulk sediments are integrating the different fractions of the overall δ¹⁵N pool and thus may be most appropriate for reconstructing overall trends in lake trophic status.     

Present weathering rates in a humid tropical watershed: Nsimi, South Cameroon

The study of biogeochemical and hydrological cycles in small experimental watersheds on silicate rocks, common for the Temperate Zone, has not yet been widely applied to the tropics, especially humid areas. This paper presents an updated database for a six-year period for the small experimental watershed of the Mengong brook in the humid tropics (Nsimi, South Cameroon). This watershed is developed on Precambrian granitoids (North Congo shield) and consists of two convexo-concave lateritic hills surrounding a large flat swamp covered by hydromorphic soils rich in upward organic matter. Mineralogical and geochemical investigations were carried out in the protolith, the saprolite, the hillside lateritic soils, and the swamp hydromorphic soils. Biomass chemical analyses were done for the representative species of the swamp vegetation. The groundwater was analysed from the parent rock/saprolite weathering front to the upper fringe in the hillside and swamp system. The chemistry of the wet atmospheric and throughfall deposits and the Mengong waters was monitored.In the Nsimi watershed the carbon transfer occurs primarily in an organic form and essentially as colloids produced by the slow biodegradation of the swamp organic matter. These organic colloids contribute significantly to the mobilization and transfer of Fe, Al, Zr, Ti, and Th in the uppermost first meter of the swamp regolith. When the organic colloid content is low (i.e., in the hillside groundwater), Th and Zr concentrations are extremely low (<3 pmol/L, ICP-MS detection limits). Strongly insoluble secondary thorianite (ThO₂) and primary zircon (ZrSiO₄) crystals control their mobilization, respectively. This finding thus justifies the potential use of both these elements as inert elements for isoelement mass balance calculations pertaining to the hillside regolith.Chloride can not be used as a conservative tracer of hydrological processes and chemical weathering in this watershed. Biogenic recycling significantly influences the low-Cl input fluxes. Sodium is a good tracer of chemical weathering in the watershed. The sodium solute flux corrected from cyclic salt input was used to assess the chemical weathering rate. Even though low (2.8 mm/kyr), the chemical weathering rate predominates over the mechanical weathering rate (1.9 mm/kyr). Compared to the Rio Icacos watershed, the most studied tropical site, the chemical weathering fluxes of silica and sodium in the Mengong are 16 and 40 times lower, respectively. This is not only related to the protective role of the regolith, thick in both cases, but also to differences in the hydrological functioning. This is to be taken into account in the calculations of the carbon cycle balance for large surfaces like that of the tropical forest ecosystems on a stable shield at the global level.     

Cartographie du potentiel en eau souterraine du bassin versant de la Lobo (Centre-Ouest,Côte d’Ivoire) : approche par analyse multicritère

RÉSUMÉ

Ce travail aborde le problème d’accessibilité en eau potable dans les zones fissurées de la Côte d’Ivoire notamment en saison sèche. Sur le bassin versant de la Lobo, le problème se pose avec plus d’acuité puisque les retenues aménagées pour l’alimentation en eau des populations ne sont pas correctement alimentées ces dernières années et sujettes à une très forte eutrophisation. L’approche pluridisciplinaire (télédétection, analyse multicritère et système d’information hydrogéologique à référence spatiale) a été adoptée. La cartographie des sites potentiels en eau montre que la quasi-totalité du bassin de la Lobo possède une bonne et excellente disponibilité en eau souterraine (93% de la superficie totale). Cette bonne disponibilité en eau souterraine est due à une faible pente et à une bonne densité de fracturation qui entrainent une bonne infiltration des eaux dans l’aquifère. L’identification des sites potentiels montre que près de 72% de la superficie du bassin est favorable à l’implantation d’ouvrages à gros débits. Dans l’ensemble, ces aquifères présentent une bonne accessibilité avec cependant, une exploitabilité médiocre.

Editeur Z. W. Kundzewicz; Edithur Associé M.D. Fidelibus     

细砂质含水合物沉积介质的非线性弹性力学模型

描述含水合物沉积介质的应力-应变关系,建立含水合物沉积介质的力学模型,预测水合物储层的变形规律,是有效开发水合物资源的基本前提。以经典邓肯-张模型为基础,结合细砂质含水合物沉积介质力学三轴试验数据,提出了适合细砂质含水合物沉积介质的非线性弹性力学模型及其参数,并验证了模型的准确性。结果表明,该模型及其参数可以很好地描述细砂质含水合物沉积介质的应力-应变的非线性弹性关系。