Chemical Weathering Conditions Research Articles

Lacustrine clay mineral assemblages as a proxy for land-use and climate changes over the last 4 kyr: The Amik Lake case study, Southern Turkey

Abstract Lake sediments are sensitive to landscape changes and most of these changes seem to be modulated by land-use (anthropogenic factors) coupled to palaeoenvironmental/palaeoclimatic changes. In its detrital fraction, the lacustrine sediments record the history of soil erosion within its catchment via the inputs of clays and others detrital products. Within a Mediterranean context, the study investigates the upper sediments infilling the central part of the Amik basin in southern Turkey. This tectonic basin was occupied and exploited by modern human at least since 6000–7000 BCE. We focus on the clay mineralogy (x-ray diffraction on oriented aggregates) and magnetic susceptibility measurements (Bartington) of the sedimentary record in the area over the last 4000 years, to assess environmental changes in relation with the different land uses and/or weathering during the successive Bronze, Iron, Roman, Islamic/Ottoman and Modern civilizations. The clay fraction of Amik Lake sediments comprises smectite, kaolinite, illite, chlorite and chlorite/smectite mixed layers that are the inherited clay phases. A relative change in abundance and crystallinity and chemistry of illite attests that environmental conditions evolved in the Amik Plain from the Bronze to Modern Age in relation with climates and/or land-use changes. The history of the Amik Lake reveals different soil erosion episode. The most intense erosion phase occurred during the Bronze/Iron Ages as indicated by the clay and magnetic susceptibility proxies. The Roman period was an exceptional period with soil erosion products arriving from the watershed, probably due the water channelization. A reduction of soil erosion occurred during the post Roman period until nowadays. Significant pedogenesis transformations are evidenced, especially during the Islamic/Ottoman periods suggesting intense chemical weathering conditions related to climate change.

Reconstructing chemical weathering during the Lower Mesozoic in the Western-Central Mediterranean area: a review of geochemical proxies

AbstractThe Triassic–Jurassic rift-valley stage of Tethyan rifting in the Western-Central Mediterranean area is characterized by a development of a puzzle of plates and microplates with the deposition of continental redbeds (in the internal domains of the Gibraltar Arc and Calabria–Peloritani Arc) that can be considered a regional lithosome. This paper aims to reconstruct the chemical weathering conditions of the Triassic–Jurassic boundary in the Western-Central Mediterranean area using the geochemical and mineralogical composition of continental redbed mudrocks of Mesozoic age. The mudrocks from the Calabria–Peloritani Arc show higher values of weathering (mobility) indices (αMg=(Al/Mg)sed/(Al/Mg)UCC;αK=(Th/K)sed/(Th/K)UCC;αBa=(Th/Ba)sed/(Th/Ba)UCC) than the Gibraltar Arc samples. Furthermore, the CIA (Chemical Index of Alteration) and MIA (Mineralogical Index of Alteration) values and the ‘Rb-type indices’ (e.g. Rb/Sr and Rb/K ratios) are higher for the Calabria–Peloritani Arc mudrocks than the Gibraltar Arc samples. All these geochemical proxies closely resemble each other and show similar variations suggesting climatic changes towards humid conditions through the Uppermost Triassic to Lowermost Jurassic that favoured chemical weathering conditions. This period is probably characterized by seasonal climate alternations corresponding to an increase in palaeoclimatic humidity. The mineralogical compositions of the Mesozoic mudrocks further confirm these indications as shown by a higher abundance of kaolinite, related to warm–humid conditions, in the Calabria–Peloritani Arc mudrocks than in those of the Gibraltar Arc.

Chemical weathering east and west of the emerging Caledonides in the Silurian – Early Devonian, with implications for climate

The late Llandovery (Silurian) – early Lochkovian (Devonian) climate of the tropical zone is tracked considering orogenesis, global glaciation events, and cratonic drift. Mineral and chemical compositions of clay fractions of Canadian (the Franklinian Basin) and Estonian (the Baltoscandian Basin) sedimentary rocks from different sides of the emerging Caledonides were studied, using clay as an index of climatic conditions of the provenance area. Illite is the main clay mineral, with addition of orthoclase, muscovite, and chlorite in both regions. Authigenic chert is present in some Canadian sections. The similarity of clay minerals and elemental composition in the two regions suggest common chemical weathering conditions. Nevertheless, the Al/Ti ratio of the <1 μm clay fraction, the proxy for climate’s aridity–humidity and weathering intensity, reveals differences. The Franklinian Basin in Laurentia shows a shift of climate from humid to semi-humid in pace with the uplift of the Caledonides. The sub-meridional mountain chain in the east reduced the amount of rainfall and, therefore, caused drier climate than could have been expected from the low latitudinal position of the area. The material from the Baltoscandian Basin reveals semi-arid and arid climate for most of the Silurian. Although the southern part of the Fennoscandian Shield was situated in desert latitudes, the evidence from the provenance of clay suggests that the climate was not fully arid. In Baltoscandia, covariation between low Al/Ti of clay and negative shifts of δ13C of marine carbonates is recorded at some stratigraphical levels, suggesting a link between humid climatic episodes in mainland and waning of glaciers at the South Pole.

Carbonate-to-biosilica transition at the Norian–Rhaetian boundary controlled by rift-related subsidence in the western Tethyan Lagonegro Basin (southern Italy)

The Lagonegro Basin, located in the Southern Apennines, Italy, records a transition from carbonate sediments of the Calcari con Selce Fm. (“Cherty Limestone”) to biosiliceous sediments of the Scisti Silicei Fm. (“Siliceous Shale”) around the Norian–Rhaetian boundary (NRB), Upper Triassic. Sedimentologic and chemostratigraphic analysis of three sections (Pignola-Abriola, Monte Volturino, and Madonna del Sirino) representing a proximal-to-distal transect across the basin document marine productivity, redox, and chemical weathering conditions. The basin was characterized by low to moderate productivity and mainly oxic deep waters, with no systematic secular variation throughout the ~10-Myr-long study interval. A weathering proxy, the chemical index of alteration (CIA*), shifts toward higher values (from ~0.70 to ~0.80) in all three study sections, suggesting development of warmer and/or more humid conditions just before the NRB. The lack of evidence for changes in marine productivity or redox conditions and the time-transgressive nature of the carbonate-biosilica transition suggests that the latter was caused not by changes in planktonic communities but, rather, by a rise in the CCD, the most likely cause of which was seafloor subsidence within the actively rifting Ionian Ocean (i.e., the westernmost branch of the Tethys Ocean). The tectonically active character of the Lagonegro Basin is also reflected in evidence of synsedimentary paleoseismic activity, e.g., numerous debris flows around the basin margins. The present study thus documents sedimentologic changes within a marginal basin in a geologically young ocean.

Caracterización de las arenas y arcillas minerales de los depósitos de canal y planicie de inundación del río Portuguesa, Venezuela

Portuguesa river floodplain is one of the largest systems in the region of central western Venezuelan llanos. This sub-Orinoco River basin covers an area of 59 500km2 which means about 6.5% of the country. The basin includes two tectonic provinces, and the river and its tributaries flow through a geological mosaic composed of rocks different ages and mineralogical composition. The interest of this paper is to examine the mineralogical composition of sands and clays in the former and present sediments of the lower basin of the river and document the relationship between the mineralogy of river sediment source area and climate variations in this region during the Late Quaternary. Most of the tributaries of the Portuguese river transport sediments from the southern and southeastern ranges of the Andes and the Interior cordillera from the Caribbean range; other small rivers originate and transport sediments from the floodplain and from the oldest foothill terraces. The methodology consisted of collecting ancient sediments deposited in the floodplain and the current channel. For this study, 40 samples of sands associated with different pools of load current bed Portuguesa River and 23 samples of clay from the oldest sediments were collected; by core drilling on bar meanders and paleomeanders in the flood plain of the river. The sand fraction was separated by dry sieving and clays by the Bouyoucos (1962) method with some modifications. Mineralogical analyzes of both fractions was determined by X-ray diffraction for the case of sand the analysis was performed only on whole sample and clays were subjected to three treatments (total, ethylene glycol and heated sample at 550°) to detecting the presence of swelling clays. The results indicate that the sands are mainly composed of quartz, forming mature sand or quartz-arenites with lesser amounts of minerals such as chlorite, muscovite, calcite and albite, among others. Which identified peaks in the diffractograms quartz infer high crystallinity of quartz (3.3°) and a peak of lower intensity (4.2°). Meanwhile, in the clay fraction 13 crystalline mineral phases were determined mainly predominating well crystallized quartz (3.3°) and the well-crystallized kaolin (7.1°), illite (10 A°) and muscovite (5.0 A°). The origin of the sands is mainly related with the physical-mechanical disintegration of the rocks of the Venezuelan South Andean Cordillera orogen and the Caribbean range during arid climatic conditions that prevailed at the end of the Late Pleistocene and factors such as transport distance, residence time, pedogenesis, chemical weathering intensity and the source area. One detrital origin for quartz, illite and muscovite reported in the clay fraction, the origin of minerals kaolinite can be inherited from older or soil deposits associated with these fluvial sediments is postulated. However, as kaolinite and montmorillonite, vermiculite, neoformation may have originated from the chemical change experienced by the mica and feldspar, associated with wetter weather conditions, environment and intensely washed acidity conditions that characterize this tropical region. The influence of climate highlighted in the mineralogical evolution of sedimentary deposits reported in this study and suggests that the sediments were deposited from the Early to Late Holocene, under conditions similar to today's most humid tropical climate. It is concluded that the origin and composition of the sands and clays analyzed mainly show a close relationship with the source area of the sediments and factors such as the distance of sediment transport, the residence time in the channel and floodplain, the intensity of chemical weathering and climatic conditions that prevailed during the Late Pleistocene-Holocene.

Sedimentary evolution of the siliciclastic Aptian–Albian Massylian flysch of the Chouamat Nappe (central Rif, Morocco)

Sandstone petrography and geochemistry (major, trace and rare earth elements) of the Aptian–Albian siliciclastic Massylian flysch from the Chouamat Nappe (central Rif Morocco) are used to highlight the chemical weathering conditions in the source area and to infer their provenance. Petrographic studies show that the studied samples are mostly quartzarenite, sublitharenite and subarkose generally composed of quartz (up to 99%), K-feldspar (less than 5%) and scarce fragments of sedimentary rocks. The samples plot in the continental block provenance field of the QtFLt diagram. Thus, the compositional maturity of analyzed sandstones is typical of cratonic environments. The sandstone samples shows high SiO2 content (up to 96%) and strong depletion in mobile components such as Na2O, CaO as well as in ferromagnesian minerals, which are mainly related to intense chemical weathering processes in the source area, as confirmed by high Chemical Index of Alteration values (mean=79.8). Recycling is shown by the Th/Sc vs. Zr/Sc plot, where the studied sandstones fall along a trend involving zircon addition and thus sediment recycling. Several geochemical ratios, such as La/Sc, Th/Sc, Th/Co and Th/Cr, of the studied samples are similar to those of Post-Archean Australian Shales and of the Upper Continental Crust, and suggest a provenance from source area(s) mainly composed of plutonic and felsic metasedimentary and sedimentary rocks, which were most probably the basement rocks of the adjacent African plate. These source rocks are related to a large cratonic source region of Eburnean and Pan-African belts, Precambrian (?) and/or Variscan basements of the southeastern margin of the African plate.

Using GIS to evaluate degree of karstification according to some important factors in carbonate rocks in Iran

This study has been carried out with the aim of mapping karst and evaluation of degree of karstification of carbonate rocks in Iran. Mapping of karstic regions of Iran has been performed by some authors previously, but scale and limited information availability have limited the accuracy of their works. In addition, rock type was considered as the main factor to make a karstification map, and other important factors such as tectonic and climate conditions were ignored. One objective of this paper is a spatial evaluation of karst development in Iran considering the role of the chief factors affecting karst development. In this study, a theoretical karst map was developed on the basis of what is considered to be a proper combination of these factors. The main considered factors are those classified by White (Geomorphology and Hydrology of Karst Terrains, Oxford University Press, Oxford 1988) into three driving forces (i.e., chemical driving force, physical driving force and hydrogeological setting). Precipitation, temperature, relief, rock type, tectonic setting and stratigraphic thickness of soluble carbonate rocks are the main factors classified to describe the mentioned driving forces. In this study, precipitation and temperature are represented by the chemical weathering conditions during the modern and glacial periods prepared on the basis of Peltier’s graphs. The required data were gathered from the updated maps with proper scale and detailed references. Then the data were mapped, classified, weighted and managed in separate layers in GIS environment. The karst index (K i ) is introduced to define the modeled degree of karstification computed by proper combination of six weighted layers, and the final theoretical karst map of Iran is prepared based on the ranked karst index. According to the combination method, values of K i could be in a range 0–400 in which higher values show more degree of karst development. The classified theoretical karst map shows that out of 205,589 km2 of carbonate units (12.62 % of the surface area of Iran) about 78,253 km2 (38.1 % of carbonate rocks) have very low to low degree of development of karstic features. About 98,145 km2 (47.7 % of carbonate rocks) have moderate degree and about 29,192 km2 (14.2 % of carbonate rocks) have high to very high degree of karstification. In other words, 7.81 % of Iran (62 % of the carbonate units) is underlain by carbonate rocks with moderate to high degree of karstification. Densities of caves and karstic springs are hydrogeological features used to check the calculated degree of karstification. For this purpose, complete inventories of caves and karstic springs in Iran were made. Out of 590 caves, 491 caves are located in carbonate rocks. From 1,074 karstic springs (with discharge above 10 l/s), 338 springs have discharges above 50 l/s; 146 caves and 330 springs (with discharges above 10 l/s) occur in rocks in areas of very low to low degree of karstification. About 243 caves and 430 springs fall in the moderate class and 102 caves and 314 springs in areas with high to very high degree of karstification. Cave densities (number of caves per area) are 0.0017, 0.0019, 0.0025, 0.0031 and 0.0046 caves per square kilometer for very low, low, moderate, high and very high classes, respectively. In addition, densities of springs are 0.0041, 0.0043, 0.0044, 0.0103 and 0.0122 springs per square kilometer for very low, low, moderate, high and very high classes, respectively. In other words, densities of caves and springs show a good correlation with the evaluated karstification. Final result is also checked by surface karstic landforms (such as karren and doline) on some examples of well-known karstic regions.

Chemical weathering over the last 1200 years recorded in the sediments ofGonghaiLake,LvliangMountains,NorthChina: a high‐resolution proxy of past climate

Increasing interest in global climate change has led to attempts to understand and quantify the relationship between chemical weathering processes and environmental conditions, especially climate. This interest necessitates the identification of new climate proxies for the reconstruction of two importantEarth surface processes: physical erosion and chemical weathering. In this study, anAMS14C‐dated 2.8‐m‐long sediment core,GH09B1, fromLakeGonghai in north‐centralChina was subjected to detailed geochemical analyses to evaluate the intensity of chemical weathering conditions in the catchment. Multivariate statistical analysis of major and trace elemental data of 139 subsamples revealed that the first principal component axisPCA1explained ∼53% of the variance in the assemblage of elements/oxides with significant positive correlations betweenPCA1 scores and the separation of mobile and soluble elements/oxides from the immobile and resistant elements/oxides, which is thus able to indicate the chemical weathering in the catchment. These results are supported by the down‐core trends of other major and trace elemental ratios of chemical weathering intensity as well as by pollen data from the same core. Variations inPCA1, chemical index of alteration (CIA),Rb/Sr ratio and other oxides ratios indicate stronger chemical weathering due to a wet climate during the Medieval Warm Period (MWP). However, theMWPwas interrupted by an interval of relatively weaker chemical weathering conditions fromAD940–1070. Weak chemical weathering under a dry climate occurred during theLittleIceAge (LIA), and increased chemical weathering intensity during the Current Warm Period (CWP). Our proxy records of chemical weathering over the last millennium correlate well with the available proxy records of precipitation fromGonghaiLake as well as with the speleothem oxygen isotope record fromWanxiangCave, but do not show a significant correlation with the temperature record inN China, suggesting that the chemical weathering intensity in the study area was mainly controlled by the amount of rainfall rather than by temperature. We conclude that high resolution lacustrine sediment geochemical parameters can be used as reliable proxies for climate variations at centennial‐decadal time scales.

Petrography and geochemistry of Jumara Dome sediments, Kachchh Basin: Implications for provenance, tectonic setting and weathering intensity

In the Kachchh Mainland, the Jumara Dome mixed carbonate-siliciclastic succession is represented by the Jhurio and Patcham formations and siliciclastic-dominating Chari Formation (Bathonian to Oxfordian). The Jumara Dome sediments were deposited during sea-level fluctuating, and were interrupted by storms in the shallow marine environment. The sandstones are generally medium-grained, moderately sorted, subangular to subrounded and of low sphericity. The sandstones are mineralogically mature and mainly composed of quartzarenite and subarkose. The plots of petrofacies in the Qt-F-L, Qm-F-Lt, Qp-Lv-Ls and Qm-P-K ternary diagrams suggest mainly the basement uplift source (craton interior) in rifted continental margin basin setting. The sandstones were cemented by carbonate, iron oxide and silica overgrowth. The Chemical Index of Alteration values (73% sandstone and 81% shale) indicate high weathering conditions in the source area. Overall study suggests that such strong chemical weathering conditions are of unconformity with worldwide humid and warm climates during the Jurassic period. Positive correlations between Al2O3 and Fe2O3, TiO2, Na2O, MgO, K2O are evident. A high correlation coefficient between Al2O3 and K2O in shale samples suggests that clay minerals control the major oxides. The analogous contents of Si, Al, Ti, LREE and TTE in the shale to PAAS with slightly depleted values of other elements ascribe a PAAS like source (granitic gneiss and minor mafics) to the present study. The petrographic and geochemical data strongly suggest that the studied sandstones/shales were deposited on a passive margin of the stable intracratonic basin.

Geochemistry of Mesozoic and Cenozoic sediments in the northern Qaidam basin, northeastern Tibetan Plateau: Implications for provenance and weathering

Whole-rock geochemical data of the Mesozoic and Cenozoic sediments in the northern Qaidam basin were used to reconstruct the provenance and chemical weathering history. Based on the fairly uniform REE patterns and trace element ratios, both the Mesozoic (LaCN/YbCN=10.37±1.75; Eu/Eu*=0.68±0.07; Th/Sc=1.11±0.38) and Cenozoic (LaCN/YbCN=9.77±0.62; Eu/Eu*=0.69±0.03; Th/Sc=0.97±0.14) mudstones were derived from a similar source area with acidic-intermediate rocks as dominant contributors, and two modeled mixtures composed of 60% granite, 35% quartzdiorite and 5% mafic rocks, and 55% granite, 40% quartzdiorite and 5% mafic rocks can be deduced as the potential source compositions for the Mesozoic and Cenozoic sediments, respectively. This conclusion is reinforced by the mudstone major element composition and sandstone petrography. The Early–Middle Jurassic mudstones have relatively high K-corrected CIA (84–93) and PIA (88–99) values, indicating intense chemical weathering conditions; while the Late Jurassic to Cenozoic sediments have variable corrected CIA (51–85) and PIA (50–92) values, implying predominantly mild to moderate weathering conditions. The chemical weathering history suggests that a warm and humid climate prevailed during the Early–Middle Jurassic, whereas a cool and semiarid to arid climate prevailed from the Late Jurassic up to now. This dramatic transition of climate may be attributed to multiple likely factors, including the restructuring of global atmospheric circulation patterns in response to the breakup of Pangea since Late Jurassic, and the orogenic rejuvenation and high elevation of the Qilian Mountains, northeastern Tibetan Plateau. Furthermore, the decreasing chemical weathering intensity over time indicates the increase of aridification and cooling of the climate culminating in the Late Cenozoic.

Lithium, magnesium and silicon isotope behaviour accompanying weathering in a basaltic soil and pore water profile in Iceland

This study presents lithium, magnesium and silicon isotope ratios from pore waters and soils from a well-characterised Histic Andosol in south-west Iceland. The soil δ7Li composition ranges between values slightly lighter than basalt, to those that are much heavier (−1.1‰ to +26.8‰), and are possibly influenced by sea salt. In contrast, precipitation-corrected dissolved (pore water) δ7Li values (1.8–10.0‰) appear to reflect preferential adsorption of 6Li onto secondary minerals, where allophane supersaturation results in high δ7Li values. Conversely low δ7Li together with high [Li] are probably due to destabilisation of allophane at low pH, and thus desorption of Li. When compared to Icelandic river values, it would appear that soil pore waters reflect an intermediate isotope composition between basalts and river waters. Precipitation corrected pore water Mg isotope ratios (δ26Mg) range between −0.46‰ and −0.12‰, and correlate with the amount of heavy Mg adsorbed onto the soil exchange complex. Silicon isotopes in the soils are isotopically lighter (δ30Si=−0.91‰ to −0.53‰) than basalt (−0.29‰), whereas pore waters are heavier (+0.13‰ to +1.03‰). Soil δ30Si values show a clear evolution between unweathered basalt and a hypothetical isotopically light endmember representing secondary minerals. Dissolved Si isotopes also respond to chemical weathering processes, and show that isotopically heavy δ30Si corresponds to high cation fluxes and high secondary mineral formation. However, comparison of all these proposed isotopic weathering tracers suggests that they respond differently to the same chemical weathering conditions. This indicates a differing behaviour during secondary mineral neoformation or adsorption depending on whether the incorporated element is a major or trace constituent. In turn, this behaviour can potentially yield important information on secondary mineral behaviour and destabilisation, and thus on the chemical weathering processes.

Weathering intensity in the Mesoproterozoic and modern large-river systems: A comparative study in the Belt-Purcell Supergroup, Canada and USA

The Mesoproterozoic Belt-Purcell Supergroup (BPS) preserves a thickness of 17km of dominantly siliciclastic rocks deposited between 1470 and 1400Ma. The total range of Chemical Index of Alteration (CIA) values, corrected for diagenetic K-addition, are 62–88 for argillites and 55–80 for sandstones, with averages of 72±6 and 68±7, respectively. More intense CIA values, in conjunction with low absolute contents of Sr, Ca and Na, and high Rb/Sr ratios (average 4), reflect an intensely weathered provenance in a hot-wet climate with hot-arid intervals resulting in evaporitic sediments. Covariations of CIA-Eu/Eu* and Sr-Eu/Eu* are consistent with a large catchment area including extensive provenance terranes of weathered recycled sedimentary rocks for the most extreme CIA and Eu/Eu* values, with smaller less intensely weathered juvenile terranes represented by lower CIA values. Accordingly, variations of CIA within the BPS stratigraphic sequence may record some combination of shifting catchment terranes and weathering intensity. Stratigraphic trends in CIA within the Appekunny and Grinnell Formations of 80±6 to 66±4 to 79±5 record the variation in this combination with time.Siliciclastic rocks record a first order trend of CIA values from CIA ∼80–100 in the Mesoarchean, through ∼80–90 in the Neoarchean, and ∼70–85 in the Proterozoic, to ∼72 for global Phanerozoic shales. These values reflect progressive drawdown of greenhouse gases that promote silicate weathering by their sequestration into carbonates and black shales, as preserved in the geologic record. Second order secular peaks in CIA values correlate in time with mantle plumes that emit greenhouse gases, which enhance silicate weathering. Some of the more intense CIA values in the BPS may also stem from release of volcanic gases during magmatism that accompanied rifting of Laurentia during breakup of the Supercontinent Columbia at ∼1.4Ga. Overall, CIA values are within the range of modern humid-temperate and humid-tropical climatic catchment areas drained by large river systems such as the Orinoco, Nile and Amazon rivers.Proterozoic rivers have been viewed as mostly braided systems due to the lack of influence of rooted vegetation, which resulted in fast channel lateral migration, high run-off rates, and low bank stability. Many large-scale Proterozoic siliciclastic basins have been preserved, formed by river systems up to pan-continental scale. However, their significance as archives of continental weathering intensity remains under-explored. This study suggests that BPS CIA values reflect more aggressive chemical weathering, since Proterozoic rivers had less sediment residence times due to a lack of vegetation cover, and therefore faster transport time than their modern counterparts. To achieve high CIA values in shorter periods of time without vegetation cover, more intense chemical weathering conditions must have been present.

Trace elements in ferromanganese concretions, gibbsite spots, and the surrounding terra rossa overlying dolomite: Their mobilization, redistribution and fractionation

The mobilization, redistribution and fractionation of trace elements during chemical weathering processes have been investigated on a 4.05 m thick terra rossa profile overlying dolomite on the Yunnan-Guizhou Plateau, in Southwest China. In this in situ weathering profile, the ferromanganese concretions and the gibbsite spots coexist in the terra rossa saprolite. The mass-balance evaluation reveals that titanium, Nb and Hf in the terra rossa matrix are conservative elements during chemical weathering compared to Zr. The elements of Li, Sc, V, Cr, Fe, Ga, As, Mo, Cs, Ce, Ta, Tl, Pb and Th in the terra rossa matrix include additions from external sources. Beryllium, Mn, Co, Ni, Cu, Rb, Ba and La are depleted in the shallow parts of the terra rossa profile and enriched in the deep parts. The elements of Zn, Sr, Y, Cd, Sn and U in the terra rossa profile are lost during weathering. Compared to the terra rossa matrix, the ferromanganese concretions are significantly enriched in most trace elements, especially Mn, Co, Cd, Ce, Tl and Pb. In contrast, the gibbsite spots are depleted in all trace elements, except for U. The results regarding specific inter-element relationships indicate that most trace elements have different inter-element relationships in the ferromanganese concretions, the gibbsite spots and the terra rossa matrix. This suggests that the behavior of many trace elements during mobilization and redistribution differs from their behavior during incorporation into secondary mineral phases, especially the Mn and Fe oxides and/or oxyhydroxides in the ferromanganese concretions. It is worthy to note that the fractionation between Ce and Mn occurs under intensive chemical weathering conditions. Correspondingly, beryllium exhibits a similar geochemical behavior as that of rare earth elements (except for Ce) and Y during surface weathering.

Textural and geochemical characteristics of the Ajali Sandstone, Anambra Basin, SE Nigeria: Implication for its provenance

This study presents the mineralogical, textural and geochemical characteristics of the regional Maastrichtian Ajali Sandstone in Anambra Basin, SE Nigeria. The intent is to highlight possible constraints on the chemical weathering conditions of the source materials on one hand, and to infer the provenance on the other hand. The investigation approach involved field studies and collection of samples from 12 different outcrop locations, followed by laboratory studies involving grain-size analysis (GSA), major and trace elements analyses using the X-ray fluorescence (XRF) method as well as thin section petrography. Field studies show that the sandstones are friable at all locations and range in color from white in freshly cut stone, to reddish brown on weathering. In addition, the sandstone units are cross-bedded and show graded bedding exemplified by fining upward sequence. Textural examination indicates that the sandstones range from fine to medium sands, constituting about 76 to 99% sand fraction, with graphic mean grain size of 0.23 to 0.53 mm. Standard deviation (sorting) ranges from 0.56 to 1.24 Ø and implies moderately well sorted sediments. Inferred from the textural indices, the depo-environmental discrimination of the Ajali Sandstone revealed a fluvial/river system-dominated sedimentary process. The sandstones are quartz arenite with quartz greater than 90% and less than 5% K-feldspar which indicate a predominant basement source as also revealed by the heavy mineral assemblages. In addition, major elemental oxides shows SiO 2 content greater than 96% for the fresh Ajali Sandstone samples with extreme depletion of mobile oxides such as Na 2O, CaO and the ferromagnesian minerals through weathering and sedimentary processes. Provenance and tectonic setting discrimination using geochemical data and compositional maturity revealed typical felsic igneous-dominated cratonic environment while inter-elemental ratios (such as Zr/Cr, Y/Ni, Th/Sc, La/Sc and La/Co) and ternary plots (e.g. Th–Sc–Zr; La-Th–Sc and Th-Co-Zr) reflect passive continental margin setting for the Ajali Sandstone. Consequently, the source area is constrained to the Precambrian basement rock units of Adamawa-Oban massif areas to the east of the Anambra Basin and the adjacent Abakaliki Anticlinorium.

Clay-mineral record in Lake Baikal sediments: The Holocene and Late Glacial transition

We analyse the clay-mineralogical composition of two 1-m long cores from elevated plateaus in the southern and northern basins of Lake Baikal. Our aim is to test the limits of classically used clay proxies for climate reconstructions. Mineralogical signature is determined by X-ray diffraction on oriented aggregates from Holocene and Late Glacial sediments. Mineralogical results are presented on a palaeomagnetic-derived time scale. Sampling at a centimeter resolution allows for a sub-millenial order reconstruction. The evolution of clay-derived climate proxies (smectite/illite peak intensity ratio, S/I) is compared between the two locations, with respect to other climate reconstructions for the Lake Baikal area and Siberia. During the Late Glacial punctual increases of S/I are related to changes of particle source rather than any climate change. At both sites the most intense chemical weathering conditions occured during the Subboreal, lagging the Eurasian middle Atlantic climate optimum. S/I follows a gradual but irregular increase through the Holocene, probably related to slow warming favourable to the development of the Siberian soils. The different sensitivity to hydrolysing conditions in northern and southern sites can be explained by a combination of local lithological and topographical parameters.

Chronology of Pleistocene weathering processes, southeast Queensland, Australia

40Ar/39Ar dating of Mn-oxides from the Upper Kandanga (UKA) Mn deposit, southeast Queensland, Australia shows that the weathering profile overlying the deposit is younger than 1 Ma. Seventy plateau ages obtained for 81 grains from 17 samples fall into three groups: I. 1000–800 ka (n = 2), II. 630–510 ka (n = 2), and III. 400–50 ka (n = 66). The last age group can be further resolved, using the mixture modelling solution by Sambridge and Compston [Sambridge, M.S., Compston, W., 1994. Mixture modeling of multi-component data sets with application to ion-probe zircon ages. Earth Planet. Sci. Lett. 128, 373–390.], into two major age clusters (sub-groups, IIIa and IIIb) with peak values at 313 ± 4 ka (n = 43, with an outlier) and 213 ± 7 ka (n = 22), respectively. The results suggest that a supergene blanket in a Mn deposit may develop relatively fast if weathering and erosional conditions are appropriate. The age peaks measured in this study correspond to warm periods identified in SPECMAP (oxygen isotope records of deep-sea sediments) and local paleoclimatic data. The correlation of weathering age peaks with oceanic climatic indicators suggests a climatic control on continental chemical weathering, where warm and humid conditions are conducive to increasing weathering rates. The clustering of supergene Mn-oxide precipitation ages suggests that weathering rates vary with time, supporting an episodic weathering history for the profiles investigated.

Quaternary geology, tectonics and distribution of palaeo- and present fluvio/glacio lacustrine deposits in Ladakh, NW Indian Himalaya—a study based on field observations

The Ladakh region of the northwestern Indian Himalaya is rich in Quaternary deposits but it has not received much attention. Previous Quaternary research in the region has focused on the glacial sequences and only some scattered data of the lacustrine deposits are available. This article addresses the reconstruction of the palaeo-lacustrine deposits and the present-day lakes and their distribution in the Ladakh region. The region was under the influence of tectonic activity and cold climate during the late Quaternary times. Tectonic activity at ∼50,000 years BP, ∼35,000 years BP and ∼25,000 years BP has been recorded. We report the presence of three major lakes in the region in the late Quaternary. These formed due to the damming of the Indus river and its tributaries by debris avalanches initiated mainly by tectonic activity along the Indus Suture, Shyok suture and the Karakoram Fault. These are the Spituk–Leh palaeolake formed ∼>50,000 years BP; the Lamayuru palaeolake dated to 35,000 years BP in the Indus valley and the Khalsar palaeolake > 60 km in length in the Shyok valley. Vast exposures of the palaeolake deposits ranging from >25 to 150 m in thickness are reported—Spituk section (>25 m); Lamayuru section (>110 m) and Khalsar (>150 m) and Hundri (∼100 m) and which have a wide lateral extent. A glacial lake basin at Bhaktpur city, north of Baralacha La, is now completely filled, the TsoKar lake has been subdivided into smaller units now and the water level is lowering. Other lakes in the region (e.g., Pangong Tso, Tso Morari, the twin Kyun Tso lakes) are confined to the western side of the study area. Limited chemical weathering, rapid erosion and cold climatic conditions in late Quaternary times are suggested by the stable illite values in the Lamayuru section. Four levels of palaeoseismic structures (convolute structures, sand dykes, intraformational folds, micro faulting) are present in the Khalsar section and at three levels in the Tirit section (at Shyok, Nubra confluence). Palaeoseismic activity was also prevalent after 25,000 years BP as is evident from the three levels of palaeoseismic structures in the younger sediments.

Geochemical characteristics of Mesoproterozoic clastic sedimentary rocks from the Chakrata Formation, Lesser Himalaya: implications for crustal evolution and weathering history in the Himalaya

To evaluate the changes in the chemistry of upper continental crust during Precambrian time in the Himalaya, the oldest and best-preserved Mesoproterozoic clastic sedimentary rocks (pelites/shales and quartzites) from the Chakrata Formation, NW Lesser Himalaya, Uttar Pradesh have been analysed for major elements and a number of trace elements, including rare earth elements (REEs). When compared to post-Archean shales (for e.g. North American Shale Composite), the Chakrata pelites have high SiO 2, Al 2O 3, K 2O, Rb, Ba contents and low CaO, Fe 2O 3 t , TiO 2, Sr, Sc, Ni and Cr concentrations. The REE abundances are high (up to 296 ppm) in pelites and reveal enriched LREE and depleted HREE patterns (La N /Yb N =7.9–11.1). Strong negative Eu anomalies are evident in the Chakrata pelites (Eu/Eu ∗=0.46–0.58). The quartzites of the Chakrata Formation have high silica contents and low trace element abundances when compared to Phanerozoic sandstones. The chemical index of alteration values of these rocks ranges from 70 to 76, indicating intense weathering conditions in the source areas. Most pelites of the Chakrata Fm plot near the A–K line in the A–CN–K diagram which also suggests intense chemical weathering conditions. Major and trace element abundances and elemental ratios critical of provenance (e.g. high SiO 2/Al 2O 3, K 2O/Na 2O, La/Sc, Th/Sc, La/Yb and low Eu/Eu ∗ and Cr/Zr) suggest that the source rocks of these pelites were remarkably felsic in nature. The paleocurrent analyses reveal that the Chakrata pelites may have been derived from the granitoid rocks from the southerly direction (i.e. from Peninsular India). The geochemical similarities favour granitoid rocks from the Aravalli and Bundelkhand regions as the main source rocks for Chakrata sedimentary rocks.

Composition and trace-element geochemistry of detrital clay and heavy-mineral suites of the lowermost Amazon River; a provenance study

ABSTRACT Compositions of clays and heavy minerals as well as trace-element geochemistry of the lowermost Amazon sediments reflect source regions and the intense chemical weathering conditions. Illite and montmorillonite indicate mountainous sources under physical weathering conditions, whereas kaolinite is related to the soil composition under humid tropical conditions, where chemical weathering predominates. The suite of unstable heavy minerals, on the other hand, reflects some sediment transport from the Andes, more supply from tributaries draining the basic and metamorphic rocks from the Southeastern Guiana Shield, and strongest influence of the upstream Amazon and Solimoes areas. These source signals are damped by transitional storage and intense lateral sediment exchange between channel and floodplain. The Brazilian Shield contribution (via the Xingu River) seems to be of minor importance, although the Alter do Chao and the Barreiras Formations, which are products of erosion of the shields, supply suites of stable heavy minerals that reflect recycling. Rare-earth and trace-element data from the clay and heavy-mineral fractions provide additional clues to the source areas: in the downstream direction, superimposed upon provenance of sediments from an undifferentiated arc, the Andes, is provenance from strongly weathered continental crust, the Precambrian shields, as well as sedimentary recycling. Small-scale provenance can be delineated by the most immobile elements. Heavy rare earth elements in the heavy-mineral fraction increase downstream toward Estreitos (to the Para River).

Implications of late Eocene to early Miocene clay mineral assemblages in McMurdo Sound (Ross Sea, Antarctica) on paleoclimate and ice dynamics

The clay mineral assemblages of upper Eocene to lower Miocene sediments recovered at the CIROS-1 and MSSTS-1 drill sites on the McMurdo Sound shelf, Antarctica, were analyzed in order to reconstruct the Cenozoic Antarctic paleoclimate and ice dynamics. The assemblages are dominated by smectite and illite, with minor amounts of chlorite and kaolinite. The highest smectite amounts and best smectite crystallinities occur in the upper Eocene part of CIROS-1, below 425–445 mbsf. They indicate that during their deposition, chemical weathering conditions prevailed on the nearby continent. Large parts of East Antarctica were probably ice-free at that time, but some glaciers reached the sea and contributed to the glaciomarine sedimentation. In contrast, only minor total amounts of smectite are present in Oligocene and younger sediments due to the shift to mainly physical weathering on an ice-covered Antarctic continent. However, relative smectite percentages rise to more than 60% during two late Oligocene intervals (ca. 27.5–26.2 and 25.0–24.5 Ma) and during one early Miocene interval starting at ca. 23.3 Ma. These intervals are characterized by ice masses coming probably from the south, where volcanic rocks acted as a source, as also indicated by the composition of the sand and gravel fractions. During the other intervals, the ice came from the west, where the physical erosion of basement rocks and sedimentary rocks of the Beacon Supergroup in the Transantarctic Mountains provided high illite concentrations. Because the two drill sites are only 4 km apart, their clay mineral records can be correlated. This led to a new interpretation of the Oligocene paleomagnetic data of the MSSTS-1 site and to a more detailed lithostratigraphic correlation of the Miocene parts of the cores.