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.     

Vegetation response to intensive commercial horticulture and environmental changes within watersheds in central highlands,Kenya, using AVHRR NDVI data

Expansion and heterogeneous clustering of commercial horticulture within the central highlands of Kenya after the mid-1990s impact watersheds and the sustainable resource management. This is distressing since climate conditions for world horticultural regions are projected to change, making such farming extremely difficult and costly to the environment. To understand the scope of impact on vegetation, the study evaluated (1) interannual variability in averaged normalized difference vegetation index (NDVI); (2) trends in average annual NDVI before and after 1990 – the presumed onset of rapid horticulture; and (3) relationship between the average annual NDVI and large-scale commercial farms, population density, and mean annual rainfall in subwatersheds. Time-series analysis of long-term Global Inventory Modeling and Mapping Studies NDVI data were analyzed as indicator of vegetation condition. NDVI trends before 1990s (1982–1989) and after 1990s (1990–2006) were evaluated to determine the slope (sign), and the Spearman’s correlation coefficient (strength). Overall, results show considerable variations in vegetation condition due largely to mixed factors including intensive farming activities, drought, and rainfall variation. Statistical analysis shows significant differences in slopes before 1990 and after 1990 (p < 0.05 and p < 0.1 respectively). Negative (decline) trends were common after 1990, linked to increased commercial horticulture and related anthropogenic disturbances on land cover. There was decline in vegetation over densely populated subwatersheds, though low NDVI values in 1984 and 2000 were the effect of severe droughts. Understanding the linkage between vegetation responses to the effects of human-induced pressure at the subwatershed scale can help natural resource managers approach conservation measures more effectively.     

Peralkaline syenite autoliths from Kilombe volcano, Kenya Rift Valley: Evidence for subvolcanic interaction with carbonatitic fluids

Mineral chemistry, textures and geochemistry of syenite autoliths from Kilombe volcano indicate that they crystallized in the upper parts of a magma chamber from peralkaline trachytic magmas that compositionally straddle the alkali feldspar join in the “residuum system” (ne = 0–1.03; qz = 0–0.77). Mineral reaction and/or overgrowth processes were responsible for the replacement of (i) Mg–hedenbergite by aegirine–augite, Ca–aegirine and/or aegirine, (ii) fayalite by amphiboles, and (iii) magnetite by aenigmatite. Ti–magnetite in silica-saturated syenites generally shows ilmenite exsolution, partly promoted by circulating fluids.

By contrast, the Fe–Ti oxides in the silica-undersaturated (sodalite-bearing) syenites show no signs of deuteric alteration. These syenites were ejected shortly after completion of crystallization. Ilmenite–magnetite equilibria indicate fO₂ between − 19.5 and − 23.1 log units (T 679–578 °C), slightly below the FMQ buffer. The subsequent crystallization of aenigmatite and Na-rich pyroxenes suggests an increase in the oxidation state of the late-magmatic liquids and implies the influence of post-magmatic fluids.

Irrespective of silica saturation, the syenites can be divided into (1) “normal” syenites, characterized by Ce/Ce ratios between 0.83 and 0.99 and (2) Ce-anomalous syenites, showing distinct negative Ce-anomalies (Ce/Ce 0.77–0.24). “Normal” silica-saturated syenites evolved towards pantelleritic trachyte. The Ce-anomalous syenites are relatively depleted in Zr, Hf, Th, Nb and Ta but, with the exception of Ce, are significantly enriched in REE.

The silica-saturated syenites contain REE–fluorcarbonates (synchysite-bastnaesite series) with negative Ce-anomalies (Ce/Ce 0.4–0.8, mean 0.6), corroded monazite group minerals with LREE-rich patches, and hydrated, Fe- and P-rich phyllosilicates. Each of these is inferred to be of non-magmatic origin. Fractures in feldspars and pyroxenes contain Pb-, REE- and Mn-rich cryptocrystalline or amorphous material. The monazite minerals are characterized by the most prominent negative Ce-anomalies (Ce/Ce_mean = 0.5), and in the most altered and Ca-rich areas (depleted in REE), Ce/Ce is less than 0.2.

It is inferred that carbonatitic fluids rich in F, Na and lanthanides but depleted in Ce by fractional crystallization of cerian pyrochlore, percolated into the subvolcanic system and interacted with the syenites at the thermal boundary layers of the magma chamber, during and shortly after their crystallization.

Chevkinite–(Ce), pyrochlore, monazite and synchysite-bastnaesite, occurring as accessory minerals, have been found for the first time at Kilombe together with eudialyte, nacareniobsite–(Ce) and thorite. These latter represent new mineral occurrences in Kenya.     

Ground Thermal Profiles from Mount Kenya, East Africa

This paper presents and compares ground thermal regimes at 4200 and 4800 m a.s.l. on Mount Kenya's southern aspect. Temperatures were recorded using Tinytalk? data loggers, installed at the ground surface and at depths of 1 cm, 5 cm, 10 cm and 50 cm. Temperatures were logged at 2‐hour intervals over a period of 12 months (August 1998 to July 1999). The study is designed to demonstrate near‐surface freeze conditions, which would have implications for contemporary periglacial landform production. Although ground freeze at 4200 m a.s.l. occurs during most nights (c. 70% at 1 cm depth), freeze penetration is restricted to the top 2 to 3 cm, such that no freeze was recorded at 5 cm depth. At 4800 m a.s.l., the diurnal frost frequency at the surface is 365 days (100%), whilst that at 10 cm depth is 165 days (45%). The paper demonstrates that a greater longevity of contemporary thin snow cover at 4800 m a.s.l. permits progressive sub‐surface cooling with depth. However, the near‐surface ground temperature profiles suggest that conditions are not conducive to permafrost development at the sites.     

Detecting and mapping slums using open data: a case study in Kenya

ABSTRACT

The worldwide slum population currently stands at over one billion, with substantial growth expected in the coming decades. Traditionally, slums have been mapped using information derived mainly from either physical indicators using remote sensing data, or socio-economic indicators using census data. Each data source on its own provides only a partial view of slums, an issue further compounded by data poverty in less-developed countries. To overcome such issues, this paper explores the fusion of traditional with emerging open data sources and data mining tools to identify additional indicators that can be used to detect and map the presence of slums, map their footprint, and map their evolution. Towards this goal, we develop an indicator database for slums using open sources of physical and socio-economic data that can be used to characterize slum settlements. Using this database, we then leverage data mining techniques to identify the most suitable combination of these indicators for mapping slums. Using three cities in Kenya as test cases, results show that the fusion of these data can improve the mapping accuracy of slums. These results suggest that the proposed approach can provide a viable solution to the emerging challenge of monitoring the growth of slums.     

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。     

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.