Oxygen fugacity constraints on the southern African lithosphere

Oxygen fugacities are calculated for olivine—spinel ±orthopyroxene assemblages recovered from diamonds and the concentrate of the Dokolwayo kimberlite, Swaziland. In addition thermobarometric oxygen fugacities are obtained for chrome spinel-garnet peridotites and diamonds from several other southern African kimberlites. The southern African lithosphere appears to be laterally homogeneous with respect to oxygen fugacity. Vertically the oxygen fugacity of the lithospheric upper mantle decreases with an increase in pressure. Locally, oxygen fugacities calculated for Dokolwayo mineral assemblages are indicative of an upper mantle characterised by diverse redox conditions within the range FMQ-IW. Reduced oxygen fugacities, calculated for the majority of the Dokolwayo samples, suggest that CH₄ may be the dominant carbon volatile species in the lower lithosphere. These reduced conditions also suggest that the Dokolwayo kimberlite is unlikely to be a product of redox melting, but may be the product of a thermal anomaly. Calculated equilibrium temperatures for olivine-spinel pairs from Dokolwayo diamonds and concentrate indicate that the upper mantle in the vicinity of Dokolwayo was characterised by cool subsolidus conditions.     

African, southern Indian and South American cratons were not part of the Rodinia supercontinent: evidence from field relationships and geochronology

We discuss the question whether the late Mesoproterozoic and early Neoproterozoic rocks of eastern, central and southern Africa, Madagascar, southern India, Sri Lanka and South America have played any role in the formation and dispersal of the supercontinent Rodinia, believed to have existed between about 1000 and 750 Ma ago. First, there is little evidence for the production of significant volumes of ˜1.4–1.0 Ga (Kibaran or Grenvillian age) continental crust in the Mozambique belt (MB) of East Africa, except, perhaps, in parts of northern Mozambique. This is also valid for most terranes related to West Gondwana, which are made up of basement rocks older than Mesoproterozoic, reworked in the Brasiliano/Pan-African orogenic cycle. This crust cannot be conclusively related to either magmatic accretion processes on the active margin of Rodinia or continental collision leading to amalgamation of the supercontinent. So far, no 1.4–1.0 Ga rocks have been identified in Madagascar. Secondly, there is no conclusive evidence for a ˜1.0 Ga high-grade metamorphic event in the MB, although such metamorphism has been recorded in the presumed continuation of the MB in East Antarctica. In South America, even the Sunsas mobile belt, which is correlated with the Grenville belt of North America, does not include high-grade metamorphic rocks. All terranes with Mesoproterozoic ages seem to have evolved within extensional, aulacogen-type structures, and their compressional deformation, where observed, is normally much younger and is related to amalgamation of Gondwana. This is also valid for the Trans-Saharan and West Congo belts of West Africa.Third, there is also no evidence for post-1000 Ma sedimentary sequences that were deposited on the passive margin(s) of Rodinia. In contrast, the MB of East Africa and Madagascar is characterized by extensive structural reworking and metamorphic overprinting of Archaean rocks, particularly in Tanzania and Madagascar, and these rocks either constitute marginal parts of cratonic domains or represent crustal blocks (terranes or microcontinents?) of unknown derivation. This is also the case for most terranes included in the Borborema/Trans-Saharan belt of northeastern Brazil and west-central Africa, as well as those of the Central Goíás Massif in central Brazil and the Mantiqueira province of eastern and southeastern Brazil.Furthermore, there is evidence for extensive granitoid magmatism in the period ˜840 to <600 Ma whose predominant calc-alkaline chemistry suggests subduction-related active margin processes during the assembly of the supercontinent Gondwana. The location of the main Neoproterozoic magmatic arcs suggests that a large oceanic domain separated the core of Rodinia, namely Laurentia plus Amazonia, Baltica and West Africa, from several continental masses and fragments now in the southern hemisphere, such as the São Francisco/Congo, Kalahari and Rio de La Plata cratons, as well as the Borborema/Trans-Saharan, Central Goiás Massif and Paraná blocks. Moreover, many extensional tectonic events detected in the southern hemisphere continental masses, but also many radiometric ages of granitois that are already associated with the process of amalgamation of Gondwana, are comprised within the 800–1000 age interval. This seems incompatible with current views on the time of disintegration of Rodinia, assumed to have occurred at around 750 Ma.     

Mesozoic thermal evolution of the southern African mantle lithosphere

The thermal structure of Archean and Proterozoic lithospheric terranes in southern Africa during the Mesozoic was evaluated by thermobarometry of mantle peridotite xenoliths erupted in alkaline magmas between 180 and 60 Ma. For cratonic xenoliths, the presence of a 150–200 °C isobaric temperature range at 5–6 GPa confirms original interpretations of a conductive geotherm, which is perturbed at depth, and therefore does not record steady state lithospheric mantle structure.

Xenoliths from both Archean and Proterozoic terranes record conductive limb temperatures characteristic of a “cratonic” geotherm (40 mW m⁻²), indicating cooling of Proterozoic mantle following the last major tectonothermal event in the region at 1 Ga and the probability of thick off-craton lithosphere capable of hosting diamond. This inference is supported by U–Pb thermochronology of lower crustal xenoliths [Schmitz and Bowring, 2003. Contrib. Mineral. Petrol. 144, 592–618].

The entire region then suffered a protracted regional heating event in the Mesozoic, affecting both mantle and lower crust. In the mantle, the event is recorded at 150 Ma to the southeast of the craton, propagating to the west by 108–74 Ma, the craton interior by 85–90 Ma and the far southwest and northwest by 65–70 Ma. The heating penetrated to shallower levels in the off-craton areas than on the craton, and is more apparent on the southern margin of the craton than in its western interior. The focus and spatial progression mimic inferred patterns of plume activity and supercontinent breakup 30–100 Ma earlier and are probably connected.

Contrasting thermal profiles from Archean and Proterozoic mantle result from penetration to shallower levels of the Proterozoic lithosphere by heat transporting magmas. Extent of penetration is related not to original lithospheric thickness, but to its more fertile character and the presence of structurally weak zones of old tectonism. The present day distribution of surface heat flow in southern Africa is related to this dynamic event and is not a direct reflection of the pre-existing lithospheric architecture.     

Neoproterozoic geodynamic evolution of SW-Gondwana: a southern African perspective

Our current understanding of the tectonic history of the principal Pan-African orogenic belts in southwestern Africa, reaching from the West Congo Belt in the north to the Lufilian/Zambezi, Kaoko, Damara, Gariep and finally the Saldania Belt in the south, is briefly summarized. On that basis, possible links with tectono-stratigraphic units and major structures on the eastern side of the Río de la Plata Craton are suggested, and a revised geodynamic model for the amalgamation of SW-Gondwana is proposed. The Río de la Plata and Kalahari Cratons are considered to have become juxtaposed already by the end of the Mesoproterozoic. Early Neoproterozoic rifting led to the fragmentation of the northwestern (in today??s coordinates) Kalahari Craton and the splitting off of several small cratonic blocks. The largest of these ex-Kalahari cratonic fragments is probably the Angola Block. Smaller fragments include the Luis Alves and Curitiba microplates in eastern Brazil, several basement inliers within the Damara Belt, and an elongate fragment off the western margin, named Arachania. The main suture between the Kalahari and the Congo-S?o Francisco Cratons is suspected to be hidden beneath younger cover between the West Congo Belt and the Lufilian/Zambezi Belts and probably continues westwards via the Cabo Frío Terrane into the Goiás magmatic arc along the Brasilia Belt. Many of the rift grabens that separated the various former Kalahari cratonic fragments did not evolve into oceanic basins, such as the Northern Nosib Rift in the Damara Belt and the Gariep rift basin. Following latest Cryogenian/early Ediacaran closure of the Brazilides Ocean between the Río de la Plata Craton and the westernmost fragment of the Kalahari Craton, the latter, Arachania, became the locus of a more than 1,000-km-long continental magmatic arc, the Cuchilla Dionisio-Pelotas Arc. A correspondingly long back-arc basin (Marmora Basin) on the eastern flank of that arc is recognized, remnants of which are found in the Marmora Terrane??the largest accumulation of oceanic crustal material known from any of the Pan-African orogenic belts in the region. Corresponding foredeep deposits that emerged from the late Ediacaran closure of this back-arc basin are well preserved in the southern areas, i.e. the Punta del Este Terrane, the Marmora Terrane and the Tygerberg Terrane. Further to the north, present erosion levels correspond with much deeper crustal sections and comparable deposits are not preserved anymore. Closure of the Brazilides Ocean, and in consequence of the Marmora back-arc basin, resulted from a change in the Río de la Plata plate motion when the Iapetus Ocean opened between the latter and Laurentia towards the end of the Ediacaran. Later break-up of Gondwana and opening of the modern South Atlantic would have followed largely along the axis of the Marmora back-arc basin and not along major continental sutures.     

The southern extension of the East African Rift System and related igneous activity

New details of the late- and post-Karroo fracture pattern in eastern southern Africa are shown on a map together with associated igneous rocks. The pattern is remarkably like that of the East African Rift Valleys with which it is continuous.There is a close parallelism between the Rift fractures and intracratonic orogenic belts, and also locally between the fractures and foliation trends of the gneisses in the belts. Relief differences across the faults range up to 2000 feet; many of the Rifts are asymmetrical with displacement only along one side.Although earth movements were intermittent over a long period of time, two main episodes occurred during post-Karroo to Cretaceous and Tertiary to recent times. Forty-nine isotopic ages of igneous material are taken from the literature and indicate eruptive activity over a 100 m. y. period.

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Zusammenfassung Neue Einzelheiten über das Bruchmuster in der Spät- und Post-Karroo im Osten des südlichen Afrikas sowie über die damit verbundenen Magmagesteine sind auf einer Karte angegeben. Das Bruchmuster hat eine außerordentliche Ähnlichkeit mit dem Ostafrikanischen Graben, welcher eine Fortsetzung davon ist.Ein enger Parallelismus besteht zwischen den Brüchen des tektonischen Grabens und den intrakratonischen, orogenetischen Zonen sowie auch örtlich innerhalb der Zonen zwischen den Brüchen und dem Schieferungsstreichen der Gneise. Der Unterschied im Relief über die Verwerfungen hin ist bis zu 2000 feet; viele dieser tektonischen Gräben sind asymmetrisch, und Verschiebungen geschehen nur entlang einer Seite.Obgleich Erdbewegungen durch lange Zeiträume unterbrochen waren, konnten jedoch zwei Hauptepisoden von der Post-Karroo bis zur Kreide und von dem Tertiär bis zur heutigen Zeit festgestellt werden. Neunundvierzig isotopische Alter für Magmagestein sind in der Literatur zitiert und deuten auf eine eruptive Tätigkeit von über 100 M. J. hin.

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Résumé Nouveaux détails concernant le dessin de fracture pendant le Karroo Supérieur et le Post-Karroo dans l'est de l'Afrique méridionale ainsi que les roches ignées associées sont indiqués sur une carte. On y observe une similarité très remarquable entre le dessin de fracture de l'Afrique méridionale et celui du fossé tectonique de l'Afrique-Est, du quel il est un prolongement.Il y a un parallélisme étroit entre les fractures de fossé tectonique et les zones orogéniques ainsi que localement entre les fractures et la direction de foliation des gneisses dans les zones orogéniques. La différence en relief à travers les failles se range près de 2000 feet; beaucoup de fossés tectoniques sont asymmétriques; ils montrent un déplacement sur un côté seulement.Les mouvements de terre étaient intermittents pendant une longue période de temps; mais deux épisodes principaux se présentaient pendant le Karroo Supérieur jusqu'au Crétacé et pendant le Tertiaire jusqu'au Quaranteneuf âges isotopiques de roches ignées sont cités de la littérature consultée et les âges indiquent une activité eruptive plus de 100 m. a.

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Sedimentation,tectonism and volcanism relationships in two Andean basins of southern Peru

The intermontane basins of the Central Andes, formed from the first episode of the andine tectonic (peruvian phase, Santonian), are characterized by a clastic continental sedimentation, settled in a compressive tectonic context and containing volcanic intercalations. The two basins presented in this paper, have distinct ages and a different geodynamic context, each:

1. The Cuzco-Sicuam Basin, Maestrichtian-Paleocene in age, is a back-arc basin. A coarse grained red bed sedimentation appeared at the same time as the synsedimentary tectonic and volcanic activity increased in the source area.
2. The Moquegua Basin, Oligocene in age, is a fore-arc basin. After a compressive phase, the northeastern border of the basin shows high mobility, characterized by superposed alluvial fans. Within the basin, the coarse grained sediments are associated with volcanic tuffs.

In the two basins, the tectonism, the volcanism and the coarse grained sedimentation occured simultaneously. The coarse grained sedimentation does not appear to be linked with climatic changes, but with the increasing mobility of the source area. The relationship between tectonic regime and volcanic activity in the basin and in the source area is considered.     

A petrogenetic model of the relationships among achondritic meteorites

The basaltic achondrite, shergottite, nakhlite, and chassignite meteorites appear to define a petrological and geochemical sequence. Assuming that they developed from basaltic liquids produced by low pressure partial melting of plagioclase peridotites, their petrological and chemical distinctions can be understood in terms of the compositional differences between their source periodites. The source regions of basaltic achondrite magmas were alkali-poor, metal-bearing peridotites in which pigeonite and/or orthopyroxene was the only pyroxene. By simultaneously increasing the ratio of high-Ca pyroxene to low-Ca pyroxene, the alkali content of the feldspar, the oxidation state, and the overall volatile content of the basaltic achondrite source peridotite, peridotites capable of yielding the parent liquids of the shergottites can be produced. Further increases can produce peridotites capable of yielding the parent liquids of the nakhlites and chassignites.Addition of a volatile-rich component to the volatile-poor type of peridotite required for the source regions of the eucrites appears to be capable of producing the required series of peridotites. Alternatively, progressive volatile-loss from a volatile-rich material, possibly of roughly cosmic composition, could have produced this sequence of peridotites. A simple two-component model of planetary compositions is, to a first approximation, consistent with the petrology and chemistry of these igneous meteorite groups.     

贺兰山南部构造特征及其与固原-青铜峡断裂的关系

贺兰山南部的变形从经历的时间及方式上与贺兰山中北部有比较明显的区别.从构造和地层等方面论证了固原-青铜峡断裂通过贺兰山的具体地点,该断裂通过贺兰山后逐渐转为东西向;宁夏中部地区东西向构造是古生代弧型构造的一部分,由于后期的改造而成为现今的形式.黄河断裂和固原-青铜峡断裂控制了贺兰山南部的构造发育,由这两断层夹持的块体(卫宁北山)在新生代向东运动,在该块体的东部由于东西向的挤压形成了许多构造,一些山体隆起的原因可能是来自西部卫宁北山向东的挤压.     

Lake-groundwater relationships and fluid-rock interaction in the East African Rift Valley: isotopic evidence

The assessment of water resources in the Rift Valley environment is important for population, agriculture and energy-related issues and depends on a good understanding of the relationship between freshwater lakes and regional groundwater. This can be hampered by the amount of fluid-rock interaction which occurs throughout the rift, obscuring original hydrochemical signatures. However, O and H stable isotope ratios can be used as tracers of infiltration over sometimes considerable distances, while showing that the volcanic edifices of the rift floor have varying effects on groundwater flow patterns. Specific cases from Kenya and Ethiopia are considered, including Lakes Naivasha, Baringo, Awasa and Zwai.In addition to their physical tracing role, stable isotopes can reveal information about processes of fluid-rock interaction. The general lack of O isotope shifting in rift hydrothermal systems suggests a high water:rock ratio, with the implication that these systems are mature. Carbon isotope studies on the predominantly bicarbonate waters of the rift show how they evolve from dilute meteoric recharge to highly alkaline waters, via the widespread silicate hydrolysis promoted by the flux of mantle carbon dioxide which occurs in most parts of the rift. There appears to be only minor differences in the C cycle between Kenya and Ethiopia.     

Graphite vein mineralization in the ultramafic rocks of southern Spain: Mineralogy and genetic relationships

Geological and mineralogical characteristics of the two largest graphite vein deposits associated with the ultramafic rocks of the Serranía de Ronda (Málaga, southern Spain) are described for the first time in this paper. The deposits are hosted in the spinel-lherzolite facies of the ultramafic sequence forming in single dikes, pockets and stockworks. Veins are composed of graphite, chromite and Fe-Ni-Cu sulfides. Graphite crystallinity reflects their fully ordered nature, and the c₀ parameter indicates temperatures at the time of formation ranging from 770 °C to 820 °C. According to the mineralogy and structural features of graphite veins, a magmatic origin involving assimilation of biogenic carbon from crustal rocks is proposed.     

Siderophile and other geochemical constraints on mixing relationships among HED-meteoritic breccias

We have used neutron activation and electron-probe fused-bead techniques to analyze the bulk major and trace-element compositions of 104 named HED meteorites (about 100–102 distinct meteorites, depending upon pairings), including 32 polymict eucrites, 30 howardites and six diogenites. Most were not previously analyzed for siderophile trace elements; many not even for major elements. Our typical sample was 350 mg, and in some cases two separate chips were analyzed as a test of meteorite heterogeneity. Meteorites with extraordinary compositions include Bluewing 001, an unequilibrated eucrite that is rich in Ti, Sm and other incompatible elements; Y-791192, a cumulate-dominated polymict eucrite; and LEW 87002, an oddly Sm-rich howardite dominated by a ferroan variety of diogenite. The eucrite:diogenite mixing ratio is the single most important factor determining the compositions of polymict HEDs, but wide ranges in eucrite incompatible element contents, in diogenite Cr and V contents, and in Sc contents of both eucrites and diogenites, make for diversity among the polymict HEDs.As our new siderophile data help to show, the common practice of describing the entire class of howardites as regolith breccias is erroneous. Most howardites are fragmental breccias showing no sign of origin from true (in the lunar sense, i.e., soil-like) near-surface regolith. Howardites are highly diverse in Ni content, often remarkably Ni-poor, compared to lunar regolith breccias. However, the few (8) howardites with between 300 and 1200 μg/g Ni consistently show some combination of other traits suggestive of regolith origin. Most importantly, all four cases (or five if we include Malvern, which appears to have been altered by annealing) of howardites known to have enrichments in solar-wind noble gases belong to the >300 μg/g Ni group. In many cases, an abundance of glasses, particularly in spheroidal or turbid-brown form, provides additional evidence for regolith origin. We propose that the important subset of howardites that are regolith breccias be formally distinguished by the designation regolithic howardite.Apart from high siderophile levels, the regolithic howardites are compositionally distinctive in having Al₂O₃ consistently near 8–9 wt%; corresponding to a eucrite:diogenite mixing ratio of precisely 2:1. Assuming the HEDs are reasonably representative of the ancient (i.e., pre-vestoid-launch) surface of Vesta, this clustering of regolith composition is difficult to explain unless most of the ancient diogenite component was brought to the surface in a single early episode (i.e., probably a single great impact), after which smaller-scale cratering (with no further major excavations of diogenite until the vestoid-forming event), efficiently homogenized the surface. Such a single-excavation model may also help to explain why diogenites, in marked contrast with eucrites, are seldom polymict; and why Al₂O₃-poor (diogenite-dominated) howardites consistently lack major siderophile enrichments. The low siderophile contents of polymict eucrites are most enigmatic. Possibly in the HED-asteroidal context (low collision velocities, etc.), only materials blended by multiple impacts consistently acquire major enrichments in siderophile elements.     

Statistical relationships among some formation parameters for Sherwood Sandstone, England

Statistical analysis of laboratory data indicates that intergranular permeability of the Sherwood Sandstone is influenced mainly by its porosity and matrix conductivity. At weak electrolyte concentrations, matrix conductivity is the most significant factor influencing induced polarization of saturated Sherwood Sandstone.     

Determining the drivers of long‐term aridity variability: a southern African case study

This paper highlights the importance of differentiating between precipitation amount and moisture availability (‘humidity’/‘aridity’) when considering proxy records of climate change. While the terms are sometimes used interchangeably, moisture availability is determined by both (i) precipitation amount and (ii) temperature, through its influence on potential evapotranspiration. As many palaeoenvironmental proxies reflect changes in this water balance rather than purely precipitation amount, it is important to distinguish between the potential relative influences of precipitation and temperature if those records are to be interpreted in terms of climate mechanisms and/or compared with model outputs. As a case study, we explore how precipitation and temperature have determined moisture availability in South Africa's summer rainfall zone over the last 45 000 years. Using quantitative reconstructions of mean annual temperature, summer rainfall amount and an aridity index, our analysis reveals strong spatiotemporal variability in the relative influences of precipitation and temperature on aridity. Temperature is shown to have exerted a considerable and even dominant influence on moisture availability, resulting in elevated humidity during the last glacial period, despite significant reductions in precipitation amount.     

Hydrological response of a dryland ephemeral river to southern African climatic variability during the last millennium

A long-term flood record from the Buffels River, the largest ephemeral river of NW South Africa (9250 km²), was reconstructed based on interpretation of palaeoflood, documentary and instrumental rainfall data. Palaeoflood data were obtained at three study reaches, with preserved sedimentary evidence indicating at least 25 large floods during the last 700 yr. Geochronological control for the palaeoflood record was provided by radiocarbon and optically stimulated luminescence (OSL) dating. Annual resolution was obtained since the 19th century using the overlapping documentary and instrumental records. Large floods coincided in the past within three main hydroclimatic settings: (1) periods of regular large flood occurrence (1 large flood/~30 yr) under wetter and cooler prevailing climatic conditions (AD 1600–1800), (2) decreasing occurrence of large floods (1 large flood/~100 yr) during warmer conditions (e.g., AD 1425–1600 and after 1925), and (3) periods of high frequency of large floods (~ 4–5 large floods in 20–30 yr) coinciding with wetter conditions of decadal duration, namely at AD 1390–1425, 1800–1825 and 1915–1925. These decadal-scale periods of the highest flood frequency seem to correspond in time with changes in atmospheric circulation patterns, as inferred when comparing their onset and distribution with temperature proxies in southern Africa.     

VS crustal models of the Roccamonfina volcano and relationships with Neapolitan volcanoes (southern Italy)

Shear wave velocities of the crust and upper mantle are defined beneath the Roccamonfina volcano and surrounding Apennines (southern Italy) from the simultaneous nonlinear inversion of the local group velocity dispersion data, obtained from seismic events recorded in 1988–2004 at Roccamonfina station of the INGV-RSNC network, and regional dispersion data obtained in previous studies. The main features of the representative V_S models are a carbonatic basement and a low velocity zone at 6–10?km of depth. The sedimentary succession is ~5?km thick below the Roccamonfina volcano and lays above a high V_S (3.8?km/s) ascribable to solidified magma body, while it is ~10?km thick below the surrounding Apennines. A low velocity layer with an average thickness of 10?km is detected below the Roccamonfina volcano which can be associated with the presence of partial melting and interpreted as magmatic reservoir. Such low velocity layer, also found below the surrounding Apennines but with a reduced thickness of 2–3?km, extends to the Campanian Plain and to the Neapolitan volcanic area, from Campi Flegrei to Somma-Vesuvius.     

Plume dynamics beneath the African plate inferred from the geochemistry of the Tertiary basalts of southern Ethiopia

Southern Ethiopian flood basalts erupted in two episodes: the pre-rift Amaro and Gamo transitional tholeiites (45-35 million years) followed by the syn-extensional Getra-Kele alkali basalts (19-11 million years). These two volcanic episodes are distinct in both trace element and isotope ratios (Zr/Nb ratios in Amaro/Gamo lavas fall between 7 and 14, and 3-4.7 in the Getra-Kele lavas whereas ²⁰⁶Pb/²⁰⁴Pb ratios fall between 18-19 and 18.9-20, respectively). The distinctive chemistries of the two eruptive phases record the tapping of two distinct source regions: a mantle plume source for the Amaro/Gamo phase and an enriched continental mantle lithosphere source for the Getra-Kele phase. Isotope and trace element variations within the Amaro/Gamo lavas reflect polybaric fractional crystallisation initiated at high pressures accompanied by limited crustal contamination. We show that clinopyroxene removal at high (0.5 GPa) crustal pressures provides an explanation for the common occurrence of transitional tholeiites in Ethiopia relative to other, typically tholeiitic flood basalt provinces. The mantle plume signature inferred from the most primitive Amaro basalts is isotopically distinct from that contributing to melt generation in central Ethiopian and Afar. This, combined with Early Tertiary plate reconstructions and similarities with Kenyan basalts farther south, lends credence to derivation of these melts from the Kenyan plume rather than the Afar mantle plume. The break in magmatism between 35 and 19 Ma is consistent with the northward movement away from the Kenya plume predicted from plate tectonic reconstructions. In this model the Getra-Kele magmatism is a response to heating of carbonatitically metasomatised lithosphere by the Afar mantle plume beneath southern Ethiopia at this time.     

Satellite prediction of soil δC distributions in a southern African savanna

Stable carbon isotopes have been frequently used to indicate carbon pools and processes in soils, plants, and the atmosphere. Carbon isotope compositions are particularly useful in partitioning soil carbon sources between C₃ and C₄ vegetation because of the distinct δ¹³C distributions for C₃ and C₄ vegetation. Remote sensing is a powerful tool used to identify ecosystem patterns and processes at larger scales. A union of these two approaches would hold promise for spatially continuous estimates of carbon isotope compositions. In the current study, a framework is presented for using high spatial resolution remote sensing to predict soil δ¹³C distributions across a southern Africa savanna ecosystem. The results suggest that if the vegetation–soil δ¹³C relationship can be established, soil δ¹³C distributions can be estimated by high-resolution satellite images (e.g., IKONOS, Quickbird). Despite limitations remote sensing is a promising tool to expand estimates of terrestrial δ¹³C spatial patterns and dynamics.     

Organic carbon burial rates,and carbon and sulfur relationships in coastal sediments of the southern Baltic Sea

Organic C burial rates and C–S relationships were investigated in the Holocene sediment sequences of 3 shallow polymictic coastal lagoons in the southern Baltic Sea to better understand the biogeochemical cycling of C and S in these environmental systems. The results show that these lagoons may have a considerable influence on the environmental status of the southern Baltic Sea area in having the potential to act as a temporary sink or source for heavy metals. High organic C accumulation rates (C_org-AR) can be observed in the sediments due to a high organic matter supply from land and a high productivity of the water bodies as a result of eutrophication. However, organic C burial does not increase as a result of increasing sediment accumulation rates (SAR). Even when high sedimentation rates do occur, there appears to be a thorough recycling and resuspension of the sediment enhancing the biological decay of organic matter before burial or the removal of organic matter from the system by transport. That is why high SAR in the coastal lagoons do not enhance pyrite formation, and thereby permanent fixing of heavy metals in the sediments, to the extent that could be expected from their magnitude. Initially there is a high potential for a temporary binding of heavy metals, but the latter are likely to be subject to mobilization and redistribution within the sediments and the water column. The patterns of burial of organic and mineral matter are different from those observed in the present-day Baltic Proper, implying possible important links in deposition between the central and coastal areas of the Baltic Sea and implications for C cycling in the ecosystem of the Baltic Sea.     

Facies relationships in Pleistocene outwash gravels, southern Ontario: a model for bar growth in braided rivers

Facies relationships in Pleistocene braided outwash deposits in southern Ontario demonstrate the presence of a large braid bar with adjacent side channel. The core of the bar is up to 6 m high, and consists of crudely horizontally stratified gravels. Downstream from the core is the bar front facies, consisting of large gravelly foresets up to 4 m high, rounded off in many places by reactivation surfaces. Upstream from the core is the bar stoss side facies consisting of several sets (individually up to 35 cm thick) of tabular cross-bedding, arranged in coarsening-upward sequences. The stoss side—core—bar front relationships are continuously exposed in one 400 m long quarry face which is cut almost parallel to the palaeoflow direction. A transverse quarry face shows the side channel facies, which consists of trough cross-bedded sands. Gravel layers can be seen to finger from the main gravelly bar into the sandy side channel, but they do not reach the base of the channel. This surprising relationship indicates that gravel moved only in the topographically higher parts of the system. After deposition in the side channel, and growth upstream and downstream from the bar core, the entire system aggraded. Crudely horizontally stratified, and imbricated gravel sheets were laid down as a bar top facies. Grain size analyses indicate strongly bimodal distributions, implying that much of the sand in the spaces between pebbles and boulders filtered in after the gravel had been deposited. This interpretation is strengthened by velocity calculations—mean velocities in excess of 300 cm/s would be needed to roll the gravel as bed load, but at such a velocity, a large amount of sand would be transported entirely in suspension. In a final section of the paper, our results are combined with other work on braided systems in an attempt to formulate a more general facies model.