Chemical Sedimentation Research Articles

137Cs and 210Pb derived sediment accumulation rates and their role in the long-term development of the Mkuze River floodplain, South Africa

Wetlands are well known to act as sinks for sediment and chemicals in the landscape. Within the Mkuze River floodplain in northern KwaZulu-Natal, South Africa, deposition of clastic sediment occurs predominantly in close proximity to the river channel, resulting in the formation of levees. Over time, this leads to channel avulsion, a process that may be accelerated by the activities of hippopotami whose trails create hydraulically favourable pathways. Sedimentation rates, determined using the isotopes 210Pb and 137Cs, indicate that the Mkuze River floodplain is a relatively rapidly aggrading system that should experience frequent avulsion, with average short-term rates in the order of 0.25 to 0.50 cm/y. Sediments on the floodplain are also an important sink for solutes, which concentrate in the groundwater and precipitate out because of evapotranspiration. Over long timescales, chemical sedimentation affects the landscape by influencing salinity, vegetation distribution, hydrological flows, and local topography. The Mkuze River floodplain is an actively evolving system, which continues to aggrade as a result of the combination of clastic and chemical sedimentation. In a region characterised by a strong annual water deficit, temporal patterns of clastic and chemical sedimentation are likely to exert influence on the long-term development of wetland systems elsewhere in southern Africa. This study, the first on sediment accretion rates using 210Pb and 137Cs dating for a southern African wetland, demonstrates that radioisotopic methods are an important tool that can be applied toward fully understanding wetland formation, evolution, and functioning in the region.

응집침전에 의한 제철공장 냉각수질향상

This study was initiated to improve the cooling water quality by chemical coagulation and sedimentation in steel mills. Due to the inefficient flocculation in the settling tanks of blast furnace cooling water systems, the solid particles in the cooling water overflow accumulate and clog the cooling system. To protect the cooling water system from such fouling, proper flocculants must be continuously used. Laboratory tests were performed for the indirect cooling water system of a plate mill. The batch test in the gas scrubbing cooling water system of a blast furnace showed that the proposed coagulant was more effective for the improvement of coagulation and sedimentation than the existing one. During the tests, cationic flocculants were more effective than use of only an anionic flocculant. The suggested combination of anionic and cationic flocculants can probably improve the turbidity removal efficiency of the cooling water. Proper control of the overflow rate by the designed residence time would help turbidity removal efficiency in the settling tank. In addition, the settling can be enhanced by adopting rapid and slow mixing alternatively. Scale problems in blast furnace cooling water system were reduced to some extent by efficient settling.

Preparation of lead sulfate powder directly from galena concentrates

A new approach to prepare PbSO 4 powder is studied. Using the methods of the leaching of galena concentrates in the ferric chloride media, selective purification and chemical sedimentation, PbS concentrate can be converted into PbSO 4. The conversion recovery is 97.39%, the purity of PbSO 4 powder is above 99% and its average crystallite size is about 42 nm. In this process, emission of lead vapour and SO 2 cannot occur. The experimental results demonstrate the feasibility of realizing a green route to prepare the lead sulfate powder.

Petrology and geochemistry of the banded iron formation in the Eastern Sierras Pampeanas of San Luis (Argentina): Implications for the evolution of the Nogolí Metamorphic Complex

The metamorphosed banded iron formation from the Nogolí Metamorphic Complex of western Sierra de San Luis, Eastern Sierras Pampeanas of Argentina (Nogolí area, 32°55’S–66°15’W) is classified as an oxide facies iron formation of Algoma Type, with a tectonic setting possibly associated with an island arc or back arc, on the basis of field mapping, mineral and textural arrangements and whole rock geochemical features. The origin of banded iron formation is mainly related to chemical precipitation of hydrogenous sediments from seawater in oceanic environments. The primary chemical precipitate is a result of solutions that represent mixtures of seawater and hydrothermal fluids, with significant dilution by mafic–ultramafic volcanic and siliciclastic materials. Multi-stage T DM model ages of 1670, 1854 and 1939 Ma and positive, mantle-like ξ Nd (1502) values of +3.8, +1.5 and +0.5 from the banded iron formation are around the range of those mafic to ultramafic meta-volcanic rocks of Nogolí Metamorphic Complex, which are between 1679 and 1765 Ma and +2.64 and +3.68, respectively. This Sm and Nd isotopic connection suggests a close genetic relationship between ferruginous and mafic–ultramafic meta-volcanic rocks, as part of the same island arc or back arc setting. A previous Sm–Nd whole rock isochron of ∼1.5 Ga performed on mafic–ultramafic meta-volcanic rocks led to the interpretation that chemical sedimentation as old as Mesoproterozoic is possible for the banded iron formation. A clockwise P–T path can be inferred for the regional metamorphic evolution of the banded iron formation, with three distinctive trajectories: (1) Relict prograde M 1–M 3 segment with gradual P and T increase from greenschist facies at M 1 to amphibolite facies at M 3. (2) Peak P–T conditions at high amphibolite-low granulite facies during M 4. (3) Retrograde counterpart of M 4, that returns from amphibolite facies and stabilizes at greenschist facies during M 5. Each trajectory may be regarded as produced by different tectonic events related to the Pampean? (1) and the Famatinian (2 and 3) orogenies, during the Early to Middle Paleozoic. The Nogolí Metamorphic Complex is interpreted as part of a greenstone belt within the large Meso- to Neoproterozoic Pampean Terrane of the Eastern Sierras Pampeanas of Argentina.

Optimization of Biochemical Treatment of Tannery Wastewater

The present work is concerned with the finding of the optimum conditions for biochemical wastewater treatment for a local tannery. The water samples were taken from outline areas (the wastewater of the chrome and vegetable tannery) in equal volumes and subjected to sedimentation, biological treatment, and chemical and natural sedimentation treatment.The Box-Wilson method of experimental design was adopted to find useful relationships between three operating variables that affect the treatment processes (temperature, aeration period and phosphate concentration) on the Biochemical Oxygen Demand (BOD5).The experimental data collected by this method were successfully fitted to a second order polynomial mathematical model. The most favorable operating conditions for the treatment are;Temperature 32.5oC,Aeration period 10 hours andphosphate concentration 16.8 mg/L.On using the optimum conditions a mathematical model simulating the operation for the treatment was obtained.

Next-Generation Fibrous Media for Water Treatment

AbstractFibrous media in the form of nonwoven filters have been used extensively in water treatment as pre-filters or to support the medium that does the separation. Nonwoven media are composed of randomly oriented micron-size fibers and provide a one step separation as a substitute for conventional processes comprising chemical addition, flocculation, sedimentation, and sand filtration. At present the use of nonwoven filter media is limited to pre-filters and is not used further downstream as high performance filters. However it is expected that by reducing the fiber size in the nanometer range, higher filtration efficiency can be achieved. With the advent of nanotechnology, the ease of producing high quality nano scaled fibers is now a reality. Recent advancements in nanofibrous media through surface modifications have shown that nonwoven media can be used beyond the prefilter stage. Furthermore the pore size of the filter media can be controlled through modification of fiber size and thickness of membranes. These nanofibrous membranes possess high surface area and large porosity leading to high flux, low pressure membranes. This article highlights important opportunities and challenges associated with developing nanofibrous media for water treatment. In addition, we have attempted to capture a snapshot of this rapidly developing new area of fibrous media for water treatment for the benefit of the wider membrane community.

Seawater-like trace element signatures (REE + Y) of Eoarchaean chemical sedimentary rocks from southern West Greenland, and their corruption during high-grade metamorphism

Modern chemical sediments display a distinctive rare earth element + yttrium (REE + Y) pattern involving depleted LREE, positive La/La*SN, Eu/Eu*SN, and YSN anomalies (SN = shale normalised) that is related to precipitation from circumneutral to high pH waters with solution complexation of the REEs dominated by carbonate ions. This is often interpreted as reflecting precipitation from surface waters (usually marine). The oldest broadly accepted chemical sediments are c. 3,700 Ma amphibolite facies banded iron-formation (BIF) units in the Isua supracrustal belt, Greenland. Isua BIFs, including the BIF international reference material IF-G are generally considered to be seawater precipitates, and display these REE + Y patterns (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004). Greenland Eoarchaean BIF metamorphosed up to granulite facies from several localities in the vicinity of Akilia (island), display REE + Y patterns identical to Isua BIF, consistent with an origin by chemical sedimentation from seawater and a paucity of clastic input. Furthermore, the much-debated magnetite-bearing siliceous unit of “earliest life” rocks (sample G91/26) from Akilia has the same REE + Y pattern. This suggests that sample G91/26 is also a chemical sediment, contrary to previous assertions (Bolhar et al. in Earth Planet Sci Lett 222:43–60, 2004), and including suggestions that the Akilia unit containing G91/26 consists entirely of silica-penetrated, metasomatised, mafic rock (Fedo and Whitehouse 2002a). Integration of our trace element data with those of Bolhar et al. (Earth Planet Sci Lett 222:43–60, 2004) demonstrates that Eoarchaean siliceous rocks in Greenland, with ages from 3.6 to 3.85 Ga, have diverse trace element signatures. There are now geographically-dispersed, widespread examples with Isua BIF-like REE + Y signatures, that are interpreted as chemically unaltered, albeit metamorphosed, chemical sediments. Other samples retain remnants of LREE depletion but are beginning to lose the distinct La, Eu and Y positive anomalies and are interpreted as metasomatised chemical sediments. Finally there are some siliceous samples with completely different trace element patterns that are interpreted as rocks of non-sedimentary origin, and include metasomatised mafic rocks. The positive La/La*SN, Eu/Eu*SN and YSN anomalies found in Isua BIFs and other Eoarchaean Greenland samples, such as G91/26 from Akilia, suggests that the processes of carbonate ion complexation controlling the REE − Y patterns were already established in the hydrosphere at the start of the sedimentary record 3,600–3,850 Ma ago. This is in accord with the presence of Eoarchaean siderite-bearing marbles of sedimentary origin, and suggests that CO2 may have been a significant greenhouse gas at that time.

Wetlands in drylands: geomorphological and sedimentological characteristics, with emphasis on examples from southern Africa

Wetlands are poorly documented features of many landscapes, and there is often little understanding of the geomorphological controls on their origin, development and characteristics. This paper addresses the apparent paradox of wetlands in drylands, focusing particularly on the geomorphology and sedimentology of wetlands in southern Africa. Drylands are characterized by high (but variable) levels of aridity, reflecting low ratios between precipitation and potential evapotranspiration, so wetlands can only exist where there are locally positive surface water balances for all or part of the year. Most moderate to large wetlands in drylands are thus maintained by river inflows that combine with other factors that serve to impede drainage or reduce infiltration, including faulting, rock outcrops, swelling soils, and ponding by tributary or aeolian sediments. Together with variations in sediment supply, vegetation communities, and levels of animal activity, this promotes a diverse range of wetlands that span a continuum from permanently inundated, to seasonally inundated, to ephemerally inundated. In detail, every wetland has a unique range of geomorphological and sedimentological characteristics but, at a general level, the dryland setting can be shown to impart some distinctive features. By comparison with humid region (tropical and temperate) wetlands, we propose that many wetlands in drylands are characterized by: 1) more frequent and/or longer periods of desiccation; 2) channels that commonly decrease in size and even disappear downstream; 3) higher levels of chemical sedimentation; 4) more frequent fires that reduce the potential for thick organic accumulations and promote aeolian activity; and 5) longer timescales of development that may extend far back into the Pleistocene. Additional studies of wetlands in different drylands may reveal other distinctive characteristics. Correct identification of the factors giving rise to wetlands, and improved understanding of the geomorphological and sedimentological processes governing their development, is vital for the design of sustainable management guidelines for these diverse yet fragile habitats.

Effective Coagulation Technology for Treatment of Grease Filter Washwater

The treatment of grease filter washwater by chemical coagulation and sedimentation using different dosages of aluminum sulfate was investigated. Pollutant removal efficiency was measured in terms of total solids, pH and optical density. The process was found to be effective at the room temperature and the filter washwater pH (9.5). The optimum aluminum sulfate dosage was 2 g/L. The treatment reduced the total solids of the wastewater by 89.6%, and produced a supernatant with a pH of 4.15 and an optical density of 0.194 nm. A fully automated prototype was then constructed for the treatment of grease filter washwater. Three distinct layers were formed in the system (fat, liquid and sludge) and each was removed separately. The system successfully recovered over 80% recyclable water with a quality comparable to that of tap water. The combined mixture of sludge and fat (20%) contained high levels of heavy metals and was not suitable for bioconversion into value added product. However, dewatering the sludge using vacuum filtration reduced its volume to 0.8% of the original volume of washwater. As a result, about 99.2% of the washwater (treated water) is recycled in the washing operation.

Geochemical signatures of two different coastal depositional environments within the same catchment

The geochemistry of lake (Renstradtrasket) and estuarine (Pieni Pernajanlahti Bay) sediment was investigated in a medium sized watershed draining to the Gulf of Finland, Baltic Sea. Catchment land-use types were compared and found similar. Sediment cores were dated using 210Pb- and 137Cs-chronologies and analyzed for Al, K, Cu, Zn, Fe, Mn, phosphorus fractions, TN, TC and biogenic silica (BSi). Differences between the sediment cores were studied by using linear regression analysis and principal components analysis (PCA). Despite similarities in catchment land-use and history, the sediment geochemical profiles of the sites varied significantly. Some of the differences could be related to differences in chemical sedimentation environment (lacustrine versus estuarine). TP concentration was found to be positively correlated with sediment iron content in estuarine sediment but negatively correlated with Fe in lake sediment. In the estuarine core sedimentary iron was not correlated to lithogenic potassium and aluminum but in the lake core the iron seemed to be lithogenic in origin, as suggested by the strong positive correlations (r 2 = 0.95–0.96) between these three variables. Most similarities among the cores were found in Al concentrations. Estuarine nutrient profiles appeared relatively monotonous compared to the lake core. This is probably due to more vigorous mixing of the sediments that may ensure more rapid and complete consumption of the organic matter deposited on the bottom of the estuary. Therefore the lake sediment appeared to preserve the historical record of eutrophication better. Biologically less active and more particle-bound materials like the trace metals Cu and Zn seemed to retain good records of anthropogenic impact also in the estuarine core. The study highlights the need to take the sedimentation environment into account when interpreting geochemical record.

Structure and iron mineralisation of the Carajás Province

The Carajás Province presents several volcano-sedimentary sequences that comprise the Itacaiúnas Supergroup. The rocks represent bimodal volcanism, and clastic and chemical sedimentation in relatively unstable basins subject to recurrent structural events with subsidence and volcanism. The Grão Pará Group represents one of these sequences with mafic volcanic rocks that enclose discontinuous jaspilite lenses, with development of large high-grade orebodies (Fe>65%). This unit is named the Carajás Formation and presents peculiar characteristics compared with other iron districts in the world, thereby departing from the classic Lake Superior or Algoma iron formation types. Owing to the dense vegetation, lack of outcrops and harsh landscape, the structural analysis of the district can only be accomplished by combination of regional field work, detailed structural work on the open pits and remotely sensed image interpretation, in this case Landsat ETM7, JERS-1 images and the shuttle radar topography mission (SRTM) digital terrane model. The regional trend of the several sequences of the area is approximately N–S and the structure is dominated by a flattened flexural fold system with axes moderately dipping WNW, intersected by several strike-slip faults subparallel to their axial plane. The Serra dos Carajás represents an s-shaped synform-antiform pair, herein named the Carajás Fold. This regional structure is partially disrupted by the Carajás Shear Zone that divides it in the northern (Serra Norte) and the southern (Serra Sul) Ranges and also probably prepared the terrane with the development of pathways for mineralising hydrothermal fluids forming large high-grade massive iron bodies. Syntectonic granitic bodies played an important role in the structural evolution of the area as well. They caused localised ductile flattening deformation and thermal contact metamorphism in surrounding terranes. The Estrela pluton, for instance, was responsible for the discontinuity of the regional trend between the Serras do Rabo and Leste. The Carajás Province is regionally the upper crustal product of a very shallowly eroded dome-and-keel geometry of Neo-Archaean volcano- sedimentary units intruded by syntectonic calc-alkaline intrusives and overlying pre-existing infracrustal rocks. The lithostructural data suggest continental-margin back-arc basin development and closure in an oblique collision belt, with N–S shallow crustal shortening, parallel to the inferred pre-existing strike of the stratigraphy.

Preservation of Fe isotope heterogeneities during diagenesis and metamorphism of banded iron formation

We present the iron isotope composition of primary, diagenetic and metamorphic minerals in five samples from the contact metamorphosed Biwabik Iron Formation. These samples attained peak meta- morphic temperatures of 1& on the millimeter scale. Minerals in the lowest grade samples consistently show a sequence in which d 56 Fe of magnetite > silicate ‡ carbonate. The inter- mineral Fe isotope differences vary in a fashion that cannot be reconciled with theoretical temperature- dependent fractionation factors. Textural evidence re- veals that most, if not all, magnetite in the Biwabik Formation is diagenetic, not primary, and that there was tremendous element mobility during diagenesis. The short duration of contact metamorphism allowed diagenetic magnetite compositions to be preserved throughout prograde metamorphism until at least the appearance of olivine. Magnetite compositions there- fore act as an isotope record of the environment in which these sediments formed. Larger-scale fluid flow and longer timescales may allow equilibration of Fe isotopes in regionally metamorphosed rocks to lower temperatures than in contact metamorphic environ- ments, but weakly regionally metamorphosed rocks may preserve small-scale Fe isotopic heterogeneities like those observed in the Biwabik Iron Formation. Importantly, Fe isotope compositions that are charac- teristic of chemical sedimentation or hydrothermal processes are preserved at low grade in the form of large inter-mineral variations, and at high grade in the form of unique bulk rock compositions. This observa- tion confirms earlier work that has suggested that Fe isotopes can be used to identify sedimentary processes in the Precambrian rock record.

Application of (31P) NMR in Analyzing the Degradation Efficiency of Organic Phosphorus Degrading-Bacteria

HPLC and HPLC-MS are the fastest and most accurate techniques for analysis of organic phosphorus pesticide (OPP) at the present time. Using these techniques, 14 strains of methamidopho (MAP) degrading-bacteria from the area contaminated with MAP have been identified. The results from HPLC and HPLC-MS analyses showed that the highest degradation rate was 73% after 7 days. In order to determine what metabolites will be formed after degradation, a key issue that has been neglected for a long time, we used ((31)P) NMR to track the degradation process. The results showed that different strains produced different metabolites. Ten strains were divided into three groups (groups A, B and C) by their metabolic profiling. Strains in group A degraded MAP into phosphor acid by breaking down all P-N, P-O and P-S bonds in 7 days. Strains in groups B and C had only broken down partially P-N and P-S bonds at the same time. Therefore, the bacterial strains in group A had a greater application potential than the other two groups. In addition, most metal phosphates are unsolvable in water. The analysis of X-ray showed, that the phosphate radicals generated by bacterial degradation induce crystallogenesis of heavy metal salts in water phase and also cause the chemical sedimentation of their crystals. Furthermore, these crystals are hydrogen phosphates. The results suggested that the MAP-degrading bacteria could be used for cleaning up not only the organic phosphorous pesticide contamination but also the phosphorous and heavy metal contamination in water environment simultaneously.

Iron Formation in Neoarchean Deltaic Successions and the Microbially Mediated Deposition of Transgressive Systems Tracts

Abstract Neoarchean deltaic deposits present in Superior Province, Canada, contain an excellent record of shallow-water siliciclastic and iron formation (IF) deposition. The deltas were fed by braided fluvial systems, which commonly resulted in the development of small, active lobes that repeatedly cycled through progradation and abandonment, although a thick progradational deltaic assemblage was also investigated. Magnetite-rich chemical sediments interbedded with siltstones and slates are present on flooding surfaces overlying shore-proximal fluvial channels and distributary-mouth bars of the braid-delta deposits. Magnetite laminae are also interbedded with some distributary-mouth sediments draping reactivation surfaces in ripples to barforms. Thicker accumulations of IF (up to 73 m) form portions of transgressive systems tracts where chemical sedimentation dominated. Offshore equivalents of these successions do not contain chemical sediments. Sedimentologic data imply that IF deposition in this well-mixed, nearshore water mass was not compatible with slow rates of constant iron-compound precipitation in the world ocean combined with siliciclastic sediment starvation. The model presented for iron precipitation relies on an elevated flux of continentally derived nutrients in the nearshore, stimulating photosynthetically induced oxidation of Fe2+. Under these conditions, elevated accumulation rates precluded the development of condensed intervals characteristic of transgressive systems tracts.

Mapping compositional diversity on the surface of Mars: The Spectral Variance Index

The advent of high spatial resolution instruments onboard orbiting Mars missions entails the need for identification of critical regions to be analyzed in further detail. We present the Spectral Variance Index (SVI), a method to detect regions of large surface materials diversity compared to the average of the planet. Such diversity may be indicative of significant active and past geological processes, such as hydrothermal activity, chemical and mechanical sedimentation, pyroclastic volcanism, erosion unveiling layering of diverse composition, and weathering, among others. The SVI was derived from statistical analysis of the Mars Global Surveyor Thermal Emission Spectrometer spectral data set for 5 × 5 degree cells covering the entire planet; global SVI maps depicting surface materials diversity were produced. Regions of elevated SVI occur clustered in low‐albedo, high‐thermal inertia regions, indicative of rock‐dominated surfaces. The surface geology of two regions of anomalously high SVI (Nili Fossae and Mare Tyrrhenum), and one region of low SVI (Amazonis Planitia), was investigated utilizing spectroscopic, thermophysical, and morphological data. The following data‐derived spectral end‐members were identified in all three regions: atmospheric water‐ice, surface dust (both in close spatial relationship), and instrument noise. The high SVI regions show also signatures characteristic of surfaces type 1 and 2, forsterite (Nili), phyllosilicates and/or high‐Si glass (Nili), and another end‐member most consistent with fayalite (or, alternatively, high‐Ca pyroxene) in volcanic materials (Tyrrhenum). The SVI method effectively identifies regions, both previously known and new ones, of surface material diversity on Mars; spectroscopic data of higher spatial resolution will improve our understanding of their compositional diversity.

Groundwater in the wetlands of the Okavango Delta, Botswana, and its contribution to the structure and function of the ecosystem

The Okavango Delta of northern Botswana is a large (40,000 km 2) alluvial fan located at the terminus of the Okavango River. The river discharges about 10 km 3 of water onto the fan each year, augmented by about 6 km 3 of rainfall, which sustains about 2500 km 2 of permanent wetland and up to 8000 km 2 of seasonal wetland. Interaction between this surface water and the groundwater strongly influences the structure and function of the wetland ecosystem. The climate is semi-arid, and only 2% of the water leaves as surface flow and probably very little as groundwater flow. The bulk of the water is lost to the atmosphere. The Okavango River also delivers about 170,000 tonnes of bedload sediment and about 360,000 tonnes of solutes to the Delta each year, most of which are deposited on the fan. Bedload is deposited in the proximal, permanent wetland, whilst much of the solute load is deposited in the seasonal wetland. Notwithstanding the high evapotranspirational loss, saline surface water is rare. Between 80 and 90% of the seasonal flood water infiltrates the ground, recharging the groundwater beneath the flood plains and the many islands on the flood plains. The remainder is lost by evaporation. This groundwater reservoir is transpired into the atmosphere by both aquatic vegetation on the flood plains and terrestrial vegetation on the islands, and the water table is steadily lowered following passage of the seasonal flood. Trees, which are almost exclusively confined to islands, are particularly important, as they lower the water table beneath islands relative to the surrounding wetlands. There is therefore a net flow of groundwater towards islands. Accumulation of dissolved salts in this groundwater leads to precipitation of solutes (mainly of silica and calcite) in the soils beneath island fringes and the islands grow by vertical expansion. Islands are thus an expression of the chemical sedimentation taking place on the fan. Sodium bicarbonate accumulates in the groundwater beneath island centres, and this impacts on the vegetation, leading ultimately to barren island interiors. Dense saline brine thus produced subsides under density-driven flow. This cycling of seasonal flood water through the groundwater reservoir thus plays a key role in creating and maintaining the biological and habitat diversity of the wetland, and inhibits the formation of saline surface water.

Effects of single amino acid substitutions at the predicted coiled-coil or hydrophobic region on the self-assembly of ϕ29 replication protein, gp1

Gp1, the product of one of the essential genes of ϕ29 replication, is an RNA binding protein and self-associates to form large complexes. Furthermore, gp1 suppresses the synthesis of ϕ29 DNA polymerase and primer protein in the post-transcriptional process. In this report, we have employed seven variants with single amino acid substitutions to analyze the self-assembly of gp1. Using chemical cross-linking and sedimentation assays, amino acid substitutions within the predicted coiled-coil or hydrophobic region were shown to strongly affect the formation of large complexes, suggesting that these two regions were required for the self-assembly of gp1. The self-association of gp1 was suggested to be necessary for the efficient binding to RNA and the translational repression.

Revisitando a Lapa do Sumidouro: marco paleo-antropológico do Quaternário americano

REVISITING LAPA DO SUMIDOURO, A CLASSIC PALEOANTHROPOLOGICAL SITE OF THE AMERICAN QUATERNARY. In order to gather new data and discuss the hypothesis related to this important site, the Lapa do Sumidouro was visited and detailed sedimentological and taphonomical studies have been performed. U-series and radiocarbon ages point towards two events of terrigenous sedimentation; older than 238,000 yr and after 11,000 yr. Two events of chemical sedimentation were identified, the earlier being around 240,000 yr and a younger one around 11,000-8,000 yr. Two erosion phases were also identified. We interpret human remains to have been emplaced via the dry southern entrance, while input of non human bones occurred through the northern swallet entrance. A minimum age of 8,000 yr was obtained for some human and animal bones, confirming previous view of considerable antiquity for these remains for the Lagoa Santa region and America..

The micro‐topography of the wetlands of the Okavango Delta, Botswana

AbstractThe surface of the 40 000 km2 Okavango alluvial fan is remarkably smooth, and almost everywhere lies within two to three metres of a perfectly smooth theoretical surface. Deviations from this perfect surface give rise to islands in the Okavango wetlands. This micro‐topography was mapped by assigning empirical elevations to remotely sensed vegetation community classes, based on the observation that vegetation is very sensitive to small, local differences in elevation. Even though empirical, the method produces fairly accurate results. The technique allows estimation of depths of inundation and therefore will be applicable even when high resolution radar altimetry becomes available. The micro‐topography has arisen as a result of clastic sedimentation in distributary channels, which produces local relief of less than two metres, and more importantly as a result of chemical precipitation in island soils, which produces similar local relief. The micro‐topography is, therefore, an expression of the non‐random sedimentation taking place on the fan. Volume calculations of islands extracted from the micro‐topography, combined with estimates of current sediment influx, suggest that the land surface of the wetland may only be a few tens of thousands of years old. Constant switching of water distribution, driven by local aggradation, has distributed sediment widely. Mass balance calculations suggest that over a period of c. 150 000 years all of the fan would at one time or other have been inundated, and thus subject to sedimentation. Coalescing of islands over time results in net aggradation of the fan surface. The amount of vertical aggradation on islands and in channels is restricted by the water depth. Restricted vertical relief, in turn, maximizes the distribution of water, limiting its average depth. Aggradation in the permanent swamps occurs predominantly by clastic sedimentation. Rates of aggradation here are very similar to those in the seasonal swamps, maintaining the overall gradient, possibly because of the operation of a feedback loop between the two. The limited amount of local aggradation arising from both clastic and chemical sedimentation, combined with constant changes in water distribution, has resulted in a near‐perfect conical surface over the fan. In addition to providing information on sedimentary processes, the micro‐topography has several useful hydrological applications. Copyright © 2004 John Wiley & Sons, Ltd.

The character and origin of lacustrine source rocks in the Lower Cretaceous synrift section, Congo Basin, west Africa

Lower Cretaceous synrift lacustrine shales from the Congo Basin, west Africa, have been analyzed with sedimentological and geochemical techniques to characterize source rock quality and identify triggers for deposition of intervals richest in organic carbon. The sequence includes a lower active rift section, deposited during active faulting and subsidence, overlain by an upper late rift section, deposited during reduced faulting and subsidence.Total organic carbon (TOC) averages 2–3 wt.% throughout the active rift siliciclastic shale section, 6% in marls in the lower part of the late rift section, and 1–2% in deltaic shales in the upper part of the late rift section. Organic matter consists of mixed types I and III kerogen in the active rift shales, pure type I kerogen in the late rift marls, and a type I and III mixture in the late rift deltaic shales.Redox proxies indicate that the deep lake was relatively reducing throughout deposition of the active rift and lower late rift sections. Therefore, enhanced anoxia did not trigger deposition of the richest source rocks. Decreased sedimentation rates in the late rift do not account for the full increase in TOC nor the shift on organic matter type. The richest source rocks are associated with high rates of organic productivity and chemical sedimentation, indicating that flux of dissolved components to the rift lake, including nutrients for algae growth, was critical. We propose that reduced topography associated with the late rift was necessary for efficient cycling of plant-derived carbon into soil carbonate and ultimately the rift lake, and for enhancing chemical weathering and nutrient flux.