Cyanobacterial toxins in Lake Baringo, Kenya

This paper presents data on the first identification, characterization and quantification of hepatotoxic microcystins and neurotoxic anatoxin-a in water samples of Lake Baringo, Kenya. The shallow turbid Lake Baringo was investigated five times between June 2001 and May 2002. The phytoplankton community was mainly dominated by the cyanobacterium Microcystis aeruginosa. Due to the high turbidity the phytoplankton biomass was low, ranging between 1.5 and 8.2 mg L⁻¹. High mean total phosphorus concentration (1.0 mg L⁻¹) and mean total nitrogen concentration (2.8 mg L⁻¹) typical for hypertrophic lakes were found. Using HPLC technique the hepatotoxins microcystin-LR, -RR and -YR and the neurotoxin anatoxin-a were detected in the water samples. The microcystin concentrations varied from 310 to 19800 μg microcystin-LR equivalents g⁻¹ DW and the anatoxin-a concentration ranged from 270 to 1260 μg g⁻¹ DW. To our knowledge this is the first evidence of cyanobacterial toxins in Lake Baringo.     

Distribution gradients of ichthyoplankton in an East African mangrove creek

As part of an investigation into the role of East African mangrove creeks as fish spawning and nursery areas, nocturnal ichthyoplankton sampling was carried out at five stations in Tudor Creek, Mombasa, Kenya, from September to November 1985. Throughout the creek, the salinity was close to that of seawater at about 35‰, but from the mouth to the upper creek there were gradients of increasing temperature and turbidity, and decreasing depth and dissolved oxygen concentration. Pelagic eggs and larvae of fish considered to be creek residents and creek non-residents were distinguished. Gradients of decreasing egg and larval abundance (particularly of non-residents), and of decreasing diversity of recognized types, were apparent from the mouth towards the upper creek. Such a distribution suggests that the mangrove system is not an important spawning or development area for the earliest life-history stages of non-resident species. The existence of the gradients also has implications both for sampling at fixed stations in tidal waters, and for investigating the existence of circatidal and semilunar spawning cycles in fish.     

Crystallization and Breakdown of Metasomatic Phases in Graphite-bearing Peridotite Xenoliths from Marsabit (Kenya)

Mantle-derived xenoliths from the Marsabit shield volcano (easternflank of the Kenya rift) include porphyroclastic spinel peridotitescharacterized by variable styles of metasomatism. The petrographyof the xenoliths indicates a transition from primary clinopyroxene-bearingcryptically metasomatized harzburgite (light rare earth element,U, and Th enrichment in clinopyroxene) to modally metasomatizedclinopyroxene-free harzburgite and dunite. The metasomatic phasesinclude amphibole (low-Ti Mg-katophorite), Na-rich phlogopite,apatite, graphite and metasomatic low-Al orthopyroxene. Transitionalsamples show that metasomatism led to replacement of clinopyroxeneby amphibole. In all modally metasomatized xenoliths melt pockets(silicate glass containing silicate and oxide micro-phenocrysts,carbonates and empty vugs) occur in close textural relationshipwith the earlier metasomatic phases. The petrography, majorand trace element data, together with constraints from thermobarometryand fO₂ calculations, indicate that the cryptic and modal metasomatismare the result of a single event of interaction between peridotiteand an orthopyroxene-saturated volatile-rich silicate melt.The unusual style of metasomatism (composition of amphibole,presence of graphite, formation of orthopyroxene) reflects lowP –T conditions (850–1000°C at < 1·5GPa) in the wall-rocks during impregnation and locally low oxygenfugacities. The latter allowed the precipitation of graphitefrom CO₂. The inferred melt was possibly derived from alkalinebasic melts by melt–rock reaction during the developmentof the Tertiary–Quaternary Kenya rift. Glass-bearing meltpockets formed at the expense of the early phases, mainly throughincongruent melting of amphibole and orthopyroxene, triggeredby infiltration of a CO₂-rich fluid and heating related to themagmatic activity that ultimately sampled and transported thexenoliths to the surface. KEY WORDS: graphite; peridotite xenoliths; Kenya Rift; modal metasomatism; silicate glass     

肯尼亚基图伊地区铜矿地质特征及找矿潜力分析

肯尼亚大地构造位于非洲地块东部,东非裂谷的东支裂谷沿维尔利亚湖东侧贯穿肯尼亚中央高地进入坦桑尼亚境内。基图伊铜矿区处于东非裂谷在肯尼亚段的南东部。主要以近南北向、北西向和近东西向断裂构造为主。铜矿区由于所处构造特殊性,岩浆活动强烈,铜矿主要赋存于斜长角闪片麻岩和花岗片麻岩中,矿体产状与岩石片麻理产状大致一致。主要矿石矿物有孔雀石、蓝铜矿,少量辉铜矿,脉石矿物以碱性长石、斜长石、石英、云母为主,矿石结构主要为放射状变晶结构,矿石构造主要为变胶状构造、块状构造、脉状构造、网脉状构造等。矿体走向延伸长达几十千米,矿体品位较富,铜品位1.0×10-27.5×10-2,矿体厚度较稳定,初步估算铜资源,矿床达大型规模,具有较大的找矿潜力。     

肯尼亚矿产资源及矿业投资环境分析

肯尼亚是东非第一大经济体,也是非洲东部的重要出海口,战略位置突出。境内矿产资源丰富,有钛铁矿、金、猛、铜等金属矿产,以及纯碱、萤石和重晶石等非金属矿产,主要赋存于太古代绿岩带和莫桑比克构造带上,其中钛铁矿储量全球第五。中肯两国关系稳定,且近年矿业发展迅速,其资源与我国有较强的资源互补性,加之肯尼亚地质工作程度较低,有大部分陆域国土面积尚未进行勘探,具有良好的投资前景。本文在系统梳理肯尼亚主要矿产资源基本情况、矿业开发现状的基础上,深入研判肯尼亚矿业投资环境及主要投资风险与机遇,进而对肯尼亚矿业投资提出建议。     

Multi‐basin depositional framework for moisture‐balance reconstruction during the last 1300 years at Lake Bogoria,central Kenya Rift Valley

Multi‐proxy analysis of sediment cores from five key locations in hypersaline, alkaline Lake Bogoria (central Kenya Rift Valley) has allowed reconstruction of its history of depositional and hydrological change during the past 1300 years. Analyses including organic matter and carbonate content, granulometry, mineralogical composition, charcoal counting and high‐resolution scanning of magnetic susceptibility and elemental geochemistry resulted in a detailed sedimentological and compositional characterization of lacustrine deposits in the three lake basins and on the two sills separating them. These palaeolimnological data were supplemented with information on present‐day sedimentation conditions based on seasonal sampling of settling particles and on measurement of physicochemical profiles through the water column. A new age model based on ²¹⁰Pb, ¹³⁷Cs and ¹⁴C dating captures the sediment chronology of this hydrochemically complex and geothermally fed lake. An extensive set of chronological tie points between the equivalent high‐resolution proxy time series of the five sediment sequences allowed transfer of radiometric dates between the basins, enabling interbasin comparison of sedimentation dynamics through time. The resulting reconstruction demonstrates considerable moisture‐balance variability through time, reflecting regional hydroclimate dynamics over the past 1300 years. Between ca 690 and 950 AD , the central and southern basins of Lake Bogoria were reduced to shallow and separated brine pools. In the former, occasional near‐complete desiccation triggered massive trona precipitation. Between ca 950 and 1100 AD , slightly higher water levels allowed the build‐up of high pCO ₂ leading to precipitation of nahcolite still under strongly evaporative conditions. Lake Bogoria experienced a pronounced highstand between ca 1100 and 1350 AD , only to recede again afterwards. For a substantial part of the time between ca 1350 and 1800 AD , the northern basin was probably disconnected from the united central and southern basins. Throughout the last two centuries, lake level has been relatively high compared to the rest of the past millennium. Evidence for increased terrestrial sediment supply in recent decades, due to anthropogenic soil erosion in the wider Bogoria catchment, is a reason for concern about possible adverse impacts on the unique ecosystem of Lake Bogoria.     

肯尼亚地质矿产资源特征及找矿远景

通过对肯尼亚地层、构造、岩浆岩及矿产资源分布与特征的研究,认为肯尼亚基础地质工作程度总体偏低,缺乏系统的成矿规律研究。地层单元大致可分为前寒武系变质基底、上古生—中生界沉积岩系和新生界火山岩沉积岩系3类,大地构造位于非洲中东部泛非构造运动所形成的莫桑比克带上,区内经历了多期次构造岩浆活动。优势矿产资源为金、铜、煤和铁矿,其中金矿主要赋存于肯尼亚西南部地区前寒武系绿岩带和西部地区的莫桑比克带内,铜矿主要在肯尼亚裂谷两侧莫桑比克带内,煤矿主要蕴含于东南部地区的古生界上石炭统—二叠系页岩中。此外,肯尼亚油气资源潜力巨大,陆域产油地层主要为中新统中深湖相厚层泥岩,图尔卡纳湖及周边地区为重点勘探区域。     

Interannual variability of NDVI and bird species diversity in Kenya

Species richness, or simply the number of species in a given area, is commonly used as an important indicator of biological diversity. Spatial variability in species richness has been postulated to depend upon environmental factors such as climate and climatic variability, which in turn may affect net primary productivity. The Advanced Very High Resolution Radiometer (AVHRR) derived Normalized Difference Vegetation Index (NDVI) has been shown to be correlated with climatic variables including rainfall, actual evapotranspiration and net primary productivity. To determine factors favoring high species richness, we examined the relationship between interannual NDVI variables and species richness of birds at a quarter degree scale (55 × 55 km). Results revealed a strong positive correlation between species richness and maximum average NDVI. Conversely, species richness showed negative correlation with standard deviation of maximum NDVI and the coefficient of variation. Though these relationships are indirect, they apparently operate through the green vegetation cover. Understanding such relationships can help in mapping and monitoring biological diversity, as well as in estimating changes in species richness in response to global climatic change.     

Size-dependent distribution and feeding habits of Terebralia palustris in mangrove habitats of Gazi Bay, Kenya

The gastropod Terebralia palustris often dominates the surface of muddy to sandy substrates of intertidal mudflats and mangrove forests, where they clearly destabilize the sediment. In the present study, it was investigated whether and to what extent the behaviour of juvenile and adult snails differs among habitats (mudflat vs. mangrove stand) in a Sonneratia alba mangal at Gazi Bay, Kenya. For this purpose we: (1) examined their distribution along three land–sea transects; and (2) applied stable isotope analysis to determine the feeding patterns of different-sized snails from the mangrove and mudflat habitats. Additionally, we investigated if these gastropods exert an impact on microphytobenthic (diatom) biomass, and whether this is size-dependent. The latter objective was met by either enclosing or excluding different-sized snails from experimental cages on the intertidal mudflat and the subsequent assessment of a change in pigment concentration of the sediment surface. In agreement with several previous studies conducted in other mangroves and geographical locations, a spatial segregation was demonstrated between juveniles (more common on the mudflat) and adults (more common in the mangrove forest). On the intertidal mudflat juveniles avoided sediment patches characterized by highly saline water in intertidal pools and a high mud content, while adults tended to dwell on substrates covered by a high amount of leaf litter. Stable carbon isotope analysis of the foot tissue of snails sampled from the S. alba stand and the mudflat indicated a transition in food source when a shell length of 51 mm is reached. Considering the δ¹³C value of juveniles, it seems they might be selecting for microphytobenthos, which might explain their preference for the mudflat. The diet of size classes found in both habitats did not differ significantly, although juveniles inhabiting the mangrove forest were slightly more depleted in ¹³C compared to those residing on the mudflat. Assuming juveniles feed on benthic microalgae and considering the lower microalgal biomass inside the mangrove forest, this may be a consequence of a higher contribution of other, more ¹³C depleted organic carbon sources, like phytoplankton, to their diet. Experimental results indicate a negative, but insignificant, impact on benthic diatom biomass by juveniles (due to grazing) and adults (due to physical disturbance). This finding seems to be in agreement with the results of the stable carbon isotope analysis, strongly suggesting the selective feeding of juvenile T. palustris on benthic diatoms.