Continental Sector Research Articles

GEOGRAPHIC REGULARITIES OF MODERN DEFLATION IN STEPPES OF EASTERN EUROPE AND WEST SIBERIA

Modern relief of the steppe zone is subject to imposed process of natural and man-induced wind erosion. Chernozems and chestnut soils of the steppes are potentially prone to destruction by wind to a different degree which may be attributed to their genesis. While dust storms occur at about the same frequency, soils of European steppes are much more actively eroded, which may be explained by specific features of atmospheric circulation. Modern semiarid belt of the continental sector in Eurasia may be considered as an area of active wind erosion of the surface, with products of erosion being involved in global migration of matter.

Characterization of the sub-continental lithospheric mantle beneath the Cameroon volcanic line inferred from alkaline basalt hosted peridotite xenoliths from Barombi Mbo and Nyos Lakes

We carried out detailed petrographic, major and trace element geochemical, microstructural and FTIR analyses on eight characteristic ultramafic xenoliths from Nyos and Barombi Mbo Lakes in the continental sector of the Cameroon Volcanic Line (CVL). The studied xenoliths are spinel lherzolites showing lithologies similar to the other xenoliths reported previously along the CVL. They have protogranular and porphyroclastic textures. One of the Barombi xenolith contains amphibole, which had not been previously reported in this locality. Amphibole is common in the Nyos xenoliths suite. Peridotite xenoliths from both localities show some chemical heterogeneity, but Barombi xenoliths generally are less depleted in basaltic elements with respect to Nyos xenoliths. Trace element compositions of Nyos spinel lherzolites show a moderately depleted initial (premetasomatic) composition and variable enrichment in REE. Evidence for both modal and cryptic metasomatism is present in Nyos xenoliths. Rare earth element patterns of clinopyroxene suggest that interaction between mafic melts and the upper mantle occurred beneath the Nyos locality. Barombi Mbo xenoliths, on the other hand, record a small degree of partial melting. The Barombi Mbo xenoliths have weak, dominantly orthorhombic olivine crystal preferred orientations, whereas Nyos ones have strong axial-[010] patterns, which may have formed in response to transpression. Nominally anhydrous mantle minerals (NAMs) of the Barombi Mbo xenoliths show generally higher bulk concentrations of ‘water’ (70–127 ppm) than Nyos xenoliths (32–81 ppm). The Barombi Mbo xenoliths could originate from a juvenile segment of the lithospheric mantle, which had been originally part of the asthenosphere. It became a part of the lithosphere in response to thermal relaxation following the extension, forming a weakly deformed lower lithospheric mantle region along the CVL. The Nyos xenoliths, however, represent a shallow lithospheric mantle bearing imprints of several depletion and enrichment events probably prior or following the extension (at ∼30 Ma).

Vegetation and climate change, fire-regime shifts and volcanic disturbance in Chiloé Continental (43°S) during the last 10,000 years

Disentangling the roles of paleofires and explosive volcanism from climatic drivers of past vegetation change is a subject insufficiently addressed in the paleoecological literature. The coastal region of the Chiloé Continental sector of northwestern Patagonia is ideal in this regard considering its proximity to active eruptive centers and the possibility of establishing comparisons with more distal, upwind sites where volcanic influence is minimal. Here we present a fine-resolution pollen and macroscopic charcoal record from Lago Teo with the aim of documenting the local vegetation and climate history, and assessing the role of disturbance regimes as drivers of vegetation change during the last ∼10,000 years.The Lago Teo record shows a conspicuous warm/dry interval between ∼7500 and 10,000 cal yrs BP followed by a cooling trend and increase in precipitation that has persisted until the present, in agreement with previous studies in the region and interpretations of past southern westerly wind activity at multi-millennial scales. The presence of 26 tephras throughout the record allows examination of the relationship between explosive volcanism and vegetation change under contrasting climatic states of the Holocene. We found consistent statistically significant increases in Tepualia stipularis after tephra deposition over the last 10,000 years, in Eucryphia/Caldcluvia between 7500 and 10,000 cal yrs BP and in Hydrangea over the last 7500 years. Our results indicate a primary role of climate change as driver of long-term vegetation change and as a modulator of vegetation responses to volcanic disturbance at multidecadal and centennial timescales.

A influência dos modos de variabilidade climática na temperatura em superfície na Antártica

Este trabalho analisa a variação sazonal da temperatura do ar à superfície (Tar) na Antártica, e sua relação com os modos de variabilidade climática El Niño-Oscilação Sul (ENOS) e Oscilação Antártica (OA). Os resultados indicam que para os 4 períodos sazonais as Tar nas estações localizadas no setor oriental da Antártica apresentaram significativas correlações negativas (nível de significância de 1% na maioria das estações) com a OA. Todavia o único período sazonal, em que o setor continental (Amundsen-Scott, Dome C) apresentou forte correlação negativa com a OA, foi durante o inverno (significativa a 1%). Correlações positivas foram encontradas (significâncias entre 1% e 5%) entre as Tar da Península Antártica e Shetland do Sul com a OA para os períodos de outono, inverno e primavera. Os resultados demonstram a influência que a OA exerce sobre a climatologia da Tar na Antártica. É importante notar um dipolo entre as regiões oriental da Antártica e da Península Antártica, no que concerne as influências da OA. O modo de variabilidade ENOS apresentou correlação negativa significativa com as três estações da Península Antártica somente durante a primavera, enquanto que para a estação Rothera, à oeste da Península, a correlação foi significativa também para o outono e inverno. A correlação entre ENOS e Tar foi positiva e significativa no setor do Mar de Ross durante a primavera, e com o setor oriental da Antártica durante o verão, com nível de significância chegando a 5%.

Resolving mantle and magmatic processes in basalts from the Cameroon volcanic line using the Re–Os isotope system

This study presents major-, trace element and Re–Os isotope and elemental data for young alkaline basalts (<10Ma) from oceanic (Annobon, S. Tomé, Principe), continental (Manengouba) and continent–oceanic boundary (COB, Mt. Cameroon) sectors of the Cameroon volcanic line (CVL). The CVL is a chain of Tertiary to recent, transitional to strongly alkaline intraplate volcanoes extending from the south Atlantic island of Annobon to the continental interior of West Africa (Biu Plateau).The basalts from the oceanic sector display a range of initial 187Os/188Os ratios between 0.128 and 0.190 and those from the COB and continental sector range between 0.142 and 0.560. The samples with high 206Pb/204Pb (e.g. ratios >20) possess 187Os/188Os isotope compositions between 0.14 and 0.18 (e.g., basalts from Mt Cameroon and Sao Tomé) which reflect the chemical characteristics that are more likely to be primary features of CVL, and are close to the value of 0.153 attributed to the HIMU end-member (Tubuai–Mangaia). However, most of the lavas from the continental sector show highly radiogenic initial 187Os/188Os ratios (0.36 to 0.56) that are outside the range previously observed for ocean island basalts, with shifts to radiogenic Os isotope compositions accompanied by less radiogenic 206Pb/204Pb and increasing SiO2 contents. The increase in 187Os/188Os is also associated with the decrease of Os, Ni, MgO and phenocryst abundances. These data can be explained by fractional crystallisation and assimilation of continental crust by the ascending magma.The systematic shift to unradiogenic lead isotope compositions from the COB into the oceanic sector is positively correlated with variations in 187Os/188Os isotope composition (from 0.140 to 0.128). At first sight this covariation might be attributed to the mixing of HIMU material with the ambient upper mantle (DMM). However, there is a clear covariation of the Os isotope and elemental composition, best explained with contamination of the oceanic basalts by the physical entrainment of xenoliths and xenocrysts of mantle origin. Overall, these results indicate that Os in CVL basalts is highly susceptible to contamination from both oceanic and continental lithospheres, under these circumstances covariations with other isotopes and elements must be interpreted with caution.

Tchabal Gangdaba massif in the Cameroon Volcanic Line: a bimodal association

Tchabal Gangdaba (TG) volcanic massif, which is a part of the continental sector of the Cameroon Volcanic Line (CVL), is dated between 34.4 and 25.1 Ma. It displays mafic lavas (picrobasalt and basanite, 41–43 wt % SiO2) and felsic lavas (rhyolite, 68–73 wt % SiO2). The lack of intermediate rocks evidences a pronounced Daly gap between 43 and 68 wt % SiO2, which corresponds to an important time span of 3.4 Ma. It is interpreted as due to extensive fractional crystallization under peculiar thermodynamical conditions. Felsic lavas yield strong negative anomalies in Ba, Sr and Eu (0.1 < Eu/Eu* < 0.3) reflecting alkali feldspar and plagioclase fractionation and in Ti reflecting titanomagnetite and ilmenite fractionation. All TG rocks are enriched in LREE and mildly fractionated, which suggests enriched mantle sources. Sr initial isotopic ratios ranging from 0.7033 to 0.7059, eNd from +2.89 to +4.64 and Pb isotopic ratios of 18.988 < 206Pb/204Pb < 19.998, 15.592 < 207Pb/204Pb < 15.673 and 38.948 < 208Pb/204Pb < 39.648 are typical of an enriched mantle source close to HIMU and FOZO. A discrete Sr crustal leaching is evidenced.

Spatial heterogeneity of the species composition of a clavarioid fungi’s compex in the Eurasian Arctic

Using the example of a model group of macromycete (clavarioid fungi), a large-scale investigation into the mycological complex of the Eurasian Arctic is conducted. The species composition of clavarioid fungi’s complex is revealed in all longitudinal sectors and latitudinal subzones, and a comparative analysis is carried out. It has been determined that, among groups of aphyllophoroid fungi, the clavarioid life form is the most adapted to the extremally psychrophilic conditions of the Arctic. It is shown that the near-oceanic sectors are the richest, whereas the continental sectors are much poorer. The distribution of the species composition of fungi conforms to the similar distribution of flowering plants, especially hemicryptophytes. The average annual quantity of atmospheric precipitation is the leading climatic factor. The differences make it possible to subdivide the Eurasian Arctic into four mycogeographical regions: Atlantic (European), Siberian, Chukotian (Beringian), and High Arctic.

English

&nbsp; Sr-Nd-Pb isotopic studies have been carried out on the lavas of Mt. Oku, the central part of the continental sector of the Cameroon volcanic line (CVL). The lavas define a considerable range of Sr-Nd-Pb isotopic compositions characterized by&nbsp;143Nd/144Nd = 0.512626 &ndash; 0.512937,87Sr/86Sr = 0.699411 - 0.704279 and more radiogenic isotopic ratios&nbsp;206Pb/204Pb = 18.3800 &ndash; 19.4480.&nbsp;207Pb/204Pb = 15.4216 &ndash; 15.6448,&nbsp;208Pb/204Pb = 37.1114 &ndash; 39.7507, with radiogenic Pb similar to the HIMU of the continent ocean boundary of the Cameroon volcanic line. These isotopic data display a linear array on the&nbsp;143Nd/144Nd vs&nbsp;87Sr/86Sr diagrams trending from MORB towards bulk earth.&nbsp;143Nd/144Nd and&nbsp;87Sr/86Sr vs.&nbsp;206Pb/204Pb show a continuum of compositions that suggest differing contributions to the parental magmas from three end-members. One end-member corresponds to a mixture of high uranium high lead (HIMU) and depleted mantle (DM) source components while the other end-member corresponds to an enriched mantle source thought to be the MORB source or sub-lithospheric source. The enriched geochemical signatures (EM) of the Oku magmas are unlikely to be the result of crustal assimilation during magma ascent. Rather, they are thought to be derived from a sub-continental lithospheric mantle enriched in incompatible trace elements by ancient metasomatic processes. The HIMU component is thought to have been inherited from the fossil plume that underlay the Equatorial Atlantic during the period 130 to 100 Ma before migrating to its present site. The radiogenic initial&nbsp;206Pb/204Pb ratios of the samples (~18.3800 &ndash; 19.44804) furthermore require the involvement of an ancient HIMU mantle plume in the magmatism and this may have been formed by metasomatism of fluids derived from St. Helena hot plume ca 120 Ma. &nbsp; Key words:&nbsp; Oku, isotopes, metasomatism, HIMU, enriched geochemical signatures (EM), mantle, enrichment.

Diversity trends of bryophytes in continental Antarctica

Bryophytes exhibit a decline in species richness with latitude across the sub-Antarctic islands, Antarctic Peninsula and Antarctic continent, but not within the Antarctic continent itself. We analyzed diversity and biogeographic patterns of bryophytes at intra-regional scale across the Ross Sector of continental Antarctica, also comparing the “coast” and “slope” provinces within this region, and placed these patterns in the context of bryophyte biogeography across Antarctica. Our study area included 63 sites along a transect through Victoria Land and the Transantarctic Mountains. Distributions of bryophyte species were collated from recent field surveys, the Antarctic Plant Database and the literature. Data analyses included rarefaction, hierarchical classification, multivariate analyses and description of richness trends by latitude bands. Despite an almost linear climatic gradient, bryophyte diversity in the Ross Sector is not influenced by latitude, and patterns differ depending on the scale of analysis. At local scale, diversity “hot spots” appear to be related to favorable local microclimatic conditions. At intra-regional scale, site location in the coast or slope province is the most effective predictor of bryophyte diversity. The site clustering within each province is consistent with precipitation and biogeographic separation of two sub-regions due to important dispersal barriers, as also reported for the microarthropod fauna. At continental scale, bryophyte diversity patterns among sectors suggest a continent–Antarctic Peninsula separation, consistent with the Gressitt Line, suggesting a common feature in the evolutionary history of the vegetation and invertebrate fauna. The high similarities of the floras of adjacent continental sectors suggest a potential route for bryophyte dispersal along the coast of continental Antarctica.

Mineralogy, geochemistry and petrogenesis of the recent magmatic formations from Mbengwi, a continental sector of the Cameroon Volcanic Line (CVL), Central Africa

The Mbengwi recent magmatic formations consist of volcanics and syenites belonging to the same magmatic episode. Lavas form a bimodal basanite-rhyolite alkaline series with a gap between 50 and 62 wt.% SiO2. Mafic lavas (basanite-hawaiite) are sodic while felsic rocks (trachyte-rhyolite-syenites) are sodi-potassic, slightly metaluminous to peralkaline. The geochemical and isotopic characteristics (0.7031 < (87Sr/86Sr)initial < 0.7043; 1.03 < eNdi < 5.17) of these rocks are similar to those of other rocks from the CVL. The main differentiation process is fractional crystallization with two trends of fractionation. Their Rb/Sr isochron age of 28.2 Ma, almost similar to 27.40 ± 0.6 Ma K/Ar age obtained in a trachyte from neighboring Bamenda Mountains system, precludes any local age migration of an hypothetic hotspot. Mafic lavas have OIB features displaying an isotopic signature similar to that of HIMU mantle source different from FOZO known as source of most parental magmas along the CVL.

Petrology of spinel lherzolite xenoliths and host basaltic lava from Ngao Voglar volcano, Adamawa Massif (Cameroon Volcanic Line, West Africa): equilibrium conditions and mantle characteristics

Spinel-bearing lherzolite xenoliths have been recovered from Mio–Pliocene alkaline basalt flows of the Ngao Voglar volcano, 35 km northwest of Ngaoundere in the Adamawa volcanic Massif (Cameroon). They have been examined to characterize the petrography, mineralogical composition, and equilibrium conditions of the upper mantle beneath the Ngaoundere region. The xenoliths exhibit protogranular textures and consist of four main minerals: olivine (Fo89–90), Mg-enstatite (En89–91Wo1Fs8–10), Cr-diopside (En49–52Wo44–49Fs1.5–5) and spinel (Mg# ~79.2, Cr# ~10.7). Thermobarometric calculations show equilibrium temperatures ranging between 850 and 950 °C and pressures of 8 to 17 kbar consistent with the spinel lherzolite stability field. These data suggest that the xenoliths come from a depth of 28–31km in the uppermost mantle situated just below a thinned crust; they are in agreement with the geophysical data previously determined in the Adamawa Massif. On the basis of these features, and considering the evidence for textural, mineralogical, and chemical equilibrium in the studied xenoliths prior to their entrainment in the host magma, we conclude that the source of these xenoliths was a chemically and petrographically homogeneous, spinel lherzolite lithospheric mantle. But the occurrence of mantle-derived xenoliths of various types (dunite, lherzolite, wehrlite, harzburgite, websterite, clinopyroxenite, and orthopyroxenite) in alkali basalts from many other localities of Cameroon (Oku Massif, Lake Nyos area and Mount Cameroon) is consistent with upper-mantle heterogeneities on a regional scale and implies that the nature of the upper mantle beneath the continental sector of the Cameroon Volcanic Line varies under its different volcanic centres.

Talk about the Focus about Continental Architecture Sector's Attention on the Theory of Building Skin

The theory of building skin is a topic of great concern in domestic architectural theory community in recent years. In this paper, through to retrieve the keywords from the Chinese journal full-text database, China Ph.D. Thesis full text data, China's excellent master's degree thesis full-text database and WANFANG DATA(ten thousand party database), analysis and deduced that architecture ontology, architecture techniques, architecture arts and culture are the top three themes that the architect concerned about in the domestic, and the skin space, ecological skin and skin culture are the hot spots in these three themes, and they can supply scientific data.for further in-depth study of building skin theory.

Variable lifetimes and loss mechanisms for NO&amp;lt;sub&amp;gt;3&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; during the DOMINO campaign: contrasts between marine, urban and continental air

Abstract. Nighttime mixing ratios of boundary layer N2O5 were determined using cavity-ring-down spectroscopy during the DOMINO campaign in Southern Spain (Diel Oxidant Mechanisms In relation to Nitrogen Oxides, 21 November 2008–8 December 2008). N2O5 mixing ratios ranged from below the detection limit (~5 ppt) to ~500 ppt. A steady-state analysis constrained by measured mixing ratios of N2O5, NO2 and O3 was used to derive NO3 lifetimes and compare them to calculated rates of loss via gas-phase and heterogeneous reactions of both NO3 and N2O5. Three distinct types of air masses were encountered, which were largely marine (Atlantic), continental or urban-industrial in origin. NO3 lifetimes were longest in the Atlantic sector (up to ~30 min) but were very short (a few seconds) in polluted, air masses from the local city and petroleum-related industrial complex of Huelva. Air from the continental sector was an intermediate case. The high reactivity to NO3 of the urban air mass was not accounted for by gas-phase and heterogeneous reactions, rates of which were constrained by measurements of NO, volatile organic species and aerosol surface area. In general, high NO2 mixing ratios were associated with low NO3 lifetimes, though heterogeneous processes (e.g. reaction of N2O5 on aerosol) were generally less important than direct gas-phase losses of NO3. The presence of SO2 at levels above ~2 ppb in the urban air sector was always associated with very low N2O5 mixing ratios indicating either very short NO3 lifetimes in the presence of combustion-related emissions or an important role for reduced sulphur species in urban, nighttime chemistry. High production rates coupled with low lifetimes of NO3 imply an important contribution of nighttime chemistry to removal of both NOx and VOC.

Italian black alder swamps: Their syntaxonomic relationships and originality within the European context

The aim of this study is to present a phytosociological survey of the Italian Alnus glutinosa-dominated swamps (Alnion glutinosae). The analysis highlights the occurrence of the following associations: Corno hungaricae-Alnetum glutinosae and Carici elatae-Alnetum glutinosae (northern Italy), Hydrocotylo-Alnetum glutinosae and Scrophulario umbrosae-Alnetum glutinosae (central and southern Italy). Carici elongatae-Alnetum glutinosae, which is widely distributed in central and northern Europe, has so far only been identified in a narrow area of the western continental sector of the Po plain; elsewhere, this association is replaced by communities generally characterized both by the lower expression of species whose main center of distribution is in the central-northern sector of Europe, and by the presence of differential entities. The comparison of data from southern, central and northern Europe confirms the syntaxonomic autonomy of most of the Italian Black Alder swamps. Moreover, the study reveals the existence of a north-to-south phytogeographic gradient, according to which the sub-boreal phytocoenoses are gradually being replaced by the central-European temperate communities as well as by the last southern European phytocoenoses occurring in the temperate and Submediterranean/Mediterranean contexts.

Origin of carbonatites of eastern taimyr deduced from an isotopic and geochemical study of zircons

Injection carbonatites of volcanic-plutonic origin with Au-bearing base-metal sulfide mineralization were established in 2006–2007 along the southern slope of the Byrranga Range in the Russian continental sector of the Arctic. According to thermobarogeochemical data, carbonatites were formed at a temperature not lower than 380–410°C from boiling gas-liquid H2O-CO2-salt fluids under decompression. The REE concentrations and patterns of sedimentary and magmatic carbonates are sharply distinct. Carbonatites are enriched in REE as compared to carbonates from limestone and are characterized by an REE distribution close to MORB. The geochronological and isotopic geochemical study of zircons has shown that carbonatite bodies (lavas?) were formed 238–219 Ma ago (Middle-Late Triassic); zircon crystallized at a high temperature. Detrital zircons from the Lower Carboniferous crinoid limestone are not younger than 594 Ma. The absolute predominance of Precambrian zircons, which are variable in age, indicates that they were captured from the crust, when carbonatitic magma ascended from subcrustal depths.

Structure of the crust beneath Cameroon, West Africa, from the joint inversion of Rayleigh wave group velocities and receiver functions

The Cameroon Volcanic Line (CVL) is a major geologic feature that cuts across Cameroon from the south west to the north east. It is a unique volcanic lineament which has both an oceanic and a continental sector and consists of a chain of Tertiary to Recent, generally alkaline volcanoes stretching from the Atlantic island of Pagalu to the interior of the African continent. The oceanic sector includes the islands of Bioko (formerly Fernando Po) and Sao Tome and Principe while the continental sector includes the Etinde, Cameroon, Manengouba, Bamboutos, Oku and Mandara mountains, as well as the Adamawa and Biu Plateaus. In addition to the CVL, three other major tectonic features characterize the region: the Benue Trough located northwest of the CVL, the Central African Shear Zone (CASZ), trending N70 degrees E, roughly parallel to the CVL, and the Congo Craton in southern Cameroon. The origin of the CVL is still the subject of considerable debate, with both plume and non-plume models invoked by many authors (e.g., Deruelle et al., 2007; Ngako et al, 2006; Ritsema and Allen, 2003; Burke, 2001; Ebinger and Sleep, 1998; Lee et al, 1994; Dorbath et al., 1986; Fairhead and Binks, 1991; King and Ritsema,more » 2000; Reusch et al., 2010). Crustal structure beneath Cameroon has been investigated previously using active (Stuart et al, 1985) and passive (Dorbath et al., 1986; Tabod, 1991; Tabod et al, 1992; Plomerova et al, 1993) source seismic data, revealing a crust about 33 km thick at the south-western end of the continental portion of the CVL (Tabod, 1991) and the Adamawa Plateau, and thinner crust (23 km thick) beneath the Garoua Rift in the north (Stuart et al, 1985) (Figure 1). Estimates of crustal thickness obtained using gravity data show similar variations between the Garoua rift, Adamawa Plateau, and southern part of the CVL (Poudjom et al., 1995; Nnange et al., 2000). In this study, we investigate further crustal structure beneath the CVL and the adjacent regions in Cameroon using 1-D shear wave velocity models obtained from the joint inversion of Rayleigh wave group velocities and P-receiver functions for 32 broadband seismic stations. From the 1-D shear wave velocity models, we obtain new insights into the composition and structure of the crust and upper mantle across Cameroon. After briefly reviewing the geological framework of Cameroon, we describe the data and the joint inversion method, and then interpret variations in crustal structure found beneath Cameroon in terms of the tectonic history of the region.« less

FeO*‐Al2O3‐TiO2‐Rich Rocks of the Tertiary Bana Igneous Complex, West Cameroon

AbstractFeO*‐Al2O3‐TiO2‐rich rocks are found associated with transitional tholeiitic lava flows in the Tertiary Bana plutono‐volcanic complex in the continental sector of the Cameroon Line. These peculiar rocks consist principally of iron‐titanium oxides, aluminosilicates and phosphates, and occur as layers 1–3 m thick occupying the upper part of lava flows on the southwest (site 1) and northwest (site 2) sites of the complex. Mineral constituents of the rocks include magnetite, ilmenite, hematite, rutile, corundum, andalusite, sillimanite, cordierite, quartz, plagioclase, alkali feldspar, apatite, Fe‐Mn phosphate, Al phosphate, micas and fine mixtures of sericite and silica. Texturally and compositionally, the rocks can be subdivided into globular type, banded type, and Al‐rich fine‐gained massive type. The first two types consist of dark globule or band enriched in Fe‐Ti oxides and apatite and lighter colored groundmass or bands enriched in aluminosilicates and quartz, respectively. The occurrence of andalusite and sillimanite and the compositional relations of magnetite and ilmenite in the FeO*‐Al2O3‐TiO2‐rich rocks suggest temperatures of crystallization in a range of 690–830°C at low pressures. The Bana FeO*‐Al2O3‐TiO2‐rich rocks are characterized by low concentrations of SiO2 (25–54.2 wt%), Na2O + K2O (0–1%), CaO (0–2%) and MgO (0–0.5%), and high concentrations of FeO* (total iron as FeO, 20–42%), Al2O3 (20–42%), TiO2 (3–9.2%), and P2O5 (0.26–1.30%). TiO2 is positively correlated with Al2O3 and inversely correlated with FeO*. The bulk rock compositions cannot be derived from the associated basaltic magma by crystal fractionation or by partial melting of the mantle or lower crustal materials. In ternary diagrams of (Al2O3)−(CaO + Na2O + K2O)−(FeO*+ MnO + MgO) and (SiO2)−(FeO*)−(Al2O3), the compositional field of the rocks is close to that of laterite and is distinct from the common volcanic rocks, suggesting that the rocks are derived from lateritic materials by recrystallization when the materials are heated by the basaltic magmas. A hydrothermal origin is discounted because the rocks contain high‐temperature mineral assemblages and lack sulfide minerals. It is proposed that the FeO*‐Al2O3‐TiO2‐rich rocks of the Bana complex were formed by pyrometamorphism of laterite by the heat of basaltic magmas.

The ‘Cameroon Hot Line’ (CHL): A unique example of active alkaline intraplate structure in both oceanic and continental lithospheres

The Cameroon Hot Line (CHL) is an active N30°E tectono-magmatic alignment, extending from Pagalu Island to Lake Chad. Its oceanic sector is comprised of six major volcanoes, and the continental sector of seven major volcanoes, numerous monogenic cones and more than sixty plutonic anorogenic ring complexes. The lavas and plutonic rocks have the typical mineralogy of the alkaline series. Nephelinites and alkaline lamprophyres are also present. The basalts from the oceanic and continental sectors have similar trace elements and Sr and Nd isotope compositions, attesting that the source is sublithospheric and that the continental crust plays no significant role in the magma genesis. Uncommon intermediate lavas originate through mixing between basaltic and felsic magmas. The evolution from basaltic to phonolitic magmas is explained through crystal fractionation. Compositions of most continental trachytes and of rhyolites require a crustal component. Metasomatism of carbonatitic affinity affected the lithospheric mantle during the magma ascent. DM and FOZO mantle components were involved in the genesis of the CHL parental magmas.

Aerosol light absorption in the North Atlantic: trends and seasonal characteristics during the period 1989 to 2003

Abstract. Aerosol light attenuation on quartz fibre filters has been measured since February 1989 at the Mace Head Atmospheric Research station near Carna, Co. Galway, Ireland, using an Aethalometer. The frequency of occurrence of the hourly averaged aerosol absorption data is found to be bimodally distributed. The two modes result from clean marine air and anthropogenically polluted continental air both being advected to the station dependent on the prevailing wind direction. The hourly averages of the marine portion of the aerosol light absorption are found to follow closely a lognormal distribution with a geometric mean of 0.310 Mm-1. The hourly averages of continental sector aerosol absorption are neither normally nor lognormally distributed and have an arithmetic mean of 6.36 Mm-1, indicating the presence of anthropogenic sources for BC east of the Mace Head station. The time series of the monthly averaged attenuation coefficient σatt of both marine and continental sector aerosol shows an increase from 1989 to 1997 and a levelling off thereafter. The monthly maximum of marine sector σatt is found in May. Trend and seasonal characteristics of the clean marine aerosol attenuation coefficients observed at Mace Head appear to be driven by meteorological factors, as indicated by rainfall data and by trends in the North Atlantic Oscillation (NAO) indices. The observed increasing trends of the continental sector σatt from 1989 up to 1997 are possibly related to changes in BC emissions over Ireland, calculated from UNSTAT (2002) fuel consumption data.

The role of seasonality in abrupt climate change

A case is made that seasonality switches dominated by wintertime were instrumental in abrupt climate changes in the North Atlantic region during the last glaciation and into the Holocene. The primary evidence comes from mismatches between mean annual temperatures from Greenland ice cores in comparison with snowline changes in East Greenland, northern Europe, and North America. The most likely explanation is a shutdown (or reduction in strength) of the conveyor. This allows the spread of winter sea ice across the North Atlantic, thus causing the northern region to experience much colder winters. Because they mimic the Greenland temperature rather than the snowline signal, changes in the Atlantic Intertropical Convergence Zone and the Asian monsoon may also share a winter linkage with Greenland. Thus the paleoclimate record is consistent with the notion that a huge continental sector of the Northern Hemisphere, stretching from Greenland to Asia, was close to an extreme winter threshold during much of the last glaciation. Winter climate crossed this threshold repeatedly, with marked changes in seasonality that may well have amplified and propagated a signal of abrupt change throughout the hemisphere and into the tropics.