Eolian Component Research Articles

Quaternary Magnetic Stratigraphy of Deep‐Sea Sediments in the Western North Pacific: Influences of Paleomagnetic Recording Efficiency and Lock‐In Delay

AbstractUnderstanding past environmental changes in the western North Pacific is rather limited due to the general paucity of robust chronology for carbonate‐lacking pelagic clay sediments. Here, we present a new magnetostratigraphy of Quaternary deep‐sea sediments in the northern Mariana Basin. A comprehensive set of paleomagnetic, rock magnetic, and microscopic results reveal that paleomagnetic signals are mainly carried by detrital magnetic minerals of Asian eolian dust and volcanic origins. The volcanic materials, most likely from the Mariana Arc, are characterized by low coercivity (∼30 mT) interacting single domain (SD) and pseudo‐SD grains, associated with abundant dendritic magnetic crystals/inclusions. This volcanic component is a suitable remanence carrier but is less efficient in recording geomagnetic intensity than the higher coercivity eolian component. The relative paleointensity record derived with minimum rock magnetic artifacts shows comparable patterns with global templates. At the Matuyama‐Brunhes transition, a ∼2‐cm remanence lock‐in depth offset is identified from beryllium isotope records, and the estimated magnetostratigraphic age delay is small (∼8 kyr). Our results provide an improved understanding of remanence recording processes in carbonate‐poor deep‐sea clays and demonstrate magnetic stratigraphy as a valuable chronology in the western North Pacific region.

Aeolian sediments in paleowetland deposits of the Las Vegas Formation

AbstractThe Las Vegas Formation (LVF) is a well-characterized sequence of groundwater discharge (GWD) deposits exposed in and around the Las Vegas Valley in southern Nevada. Nearly monolithologic bedrock surrounds the valley, which provides an excellent opportunity to test the hypothesis that GWD deposits include an aeolian component. Mineralogical data indicate that the LVF sediments are dominated by carbonate minerals, similar to the local bedrock, but silicate minerals are also present. The median particle size is ~35 μm, consistent with modern dust in the region, and magnetic properties contrast strongly with local bedrock, implying an extralocal origin. By combining geochemical data from the LVF sediments and modern dust, we found that an average of ~25% of the LVF deposits were introduced by aeolian processes. The remainder consists primarily of authigenic groundwater carbonate as well as minor amounts of alluvial material and soil carbonate. Our data also show that the aeolian sediments accumulated in spring ecosystems in the Las Vegas Valley in a manner that was independent of both time and the specific hydrologic environment. These results have broad implications for investigations of GWD deposits located elsewhere in the southwestern U.S. and worldwide.

The aeolian component inferred from lake sediments in China

Lake sediments preserve different components transported by wind, runoff, and glaciers; therefore, it is important to distinguish the aeolian components in lake sediments. In this study, we used the sediment grain-size characteristics, the end-member modelling algorithm (EMMA), and the thickness of coarse-grained layers in varves to identify aeolian sediments in the KS-2010 core from Kusai Lake. The results indicate that the sand component in the lake sediments can be considered to be related to aeolian processes. Then, we used the sand contents of the surface sediments from 67 lakes to infer the distribution of aeolian activity in China, and the results correlate well with the spatial distribution of the annual maximum wind speed, mean annual blowing sand weather, and mean annual sandstorm. Therefore, the sand component in lake sediments is transported by aeolian processes and can be used as an indicator of wind activity.

Zircon provenance of Quaternary cover beds using U–Pb dating: Regional differences in the Southwestern USA

AbstractU–Pb dating of detrital zircons (DZ) in Quaternary slope deposits (cover beds) and examination of the surface structure of single zircon grains may enable reconstruction of the provenance of their aeolian components and help to reconstruct sedimentary palaeo‐transportation cascades. We distinguish several layers of slope deposits (cover beds) using a palaeosol‐supported sequence‐stratigraphic approach. In a pilot study, we demonstrate the usability of using end‐member‐modelled analyses of grain sizes to identify the aeolian matter, multi‐dimensional scaling and density functions (probability density functions (PDF) and Kernel density estimations (KDE)) of the U–Pb data, and the grain morphology of DZ, and show that the age and appearance variation of DZ in the Great Basin differ remarkably from those of the Colorado Plateau.All samples contain aeolian matter. Density functions of the DZ ages show similarities within each of the two regions. The Great Basin samples are dominated by late Cretaceous and Paleogene zircons, which are assumed to derive from volcanism. In contrast, the Colorado Plateau samples are almost free of these contributions. Presumably, the difference is due to palaeolakes, which were frequent in the Great Basin only, whose sediments contain a similar spectrum of DZ ages. Zircon morphology indicates that most grains have a history of aeolian transportation, whereas the grains assumed to be of volcanic origin often do not show similar transportation marks; this indicates that their transportation was dominated by the eruptive process but fewer steps of a transportation cascade. Multidimensional scaling of age data defines clusters of samples with similar DZ ages and – compared with published ages from possible sources – allows first interpretations of provenance.Quaternary aeolian sediments have not been studied in, and the applied mix of methods has not been applied to, the southwestern USA before. We demonstrate their feasibility for this study area.

Lead Distribution in Ground-Level Atmospheric Aerosol of Zaporizhia City

The results of long-term observations on the geochemical peculiarities of technogenic lead distribution in atmospheric aerosol of Zaporizhia, as well as in soils, bottom sediments, suspended matter from Dnipro river and substance from wet gas filters of open-hearth furnaces PJSC Zaporizhstal are presented. The actual material was obtained on the basis of comprehensive field monitoring during 2015-2019. Simultaneous involvement of data on hydrometeorological conditions (seasonality, precipitation, wind speeds, directions and duration) in the study area provided a integral approach to the processing and generalization of research results. A number of peculiarities of sedimentary substance distribution in air, interrelations of its anthropogenic and natural components are defined. Correlations were revealed between lead concentrations changes and aeolian mineral component distribution, seasonality, and a number of other factors. The highest concentration of lead was found in solid aerosol particles accumulated by the sedimentary trap at the monitoring site. It was revealed that the average concentration of investigated element in river suspension of Dnipro exceeds significantly background values, and for soils exceeds more than 3.5 times the permissible norms. At the same time, the fixed form of lead in atmospheric aerosol, in contrast to other environmental objects, loses its domination, giving the way to easily soluble forms, so the evidence of environmental hazard. Monitoring of seasonal patterns of the element distribution in the city air showed excess concentrations in the cold period of the year, which, according to the authors, is due to both meteorological conditions and redistribution of natural and anthropogenic factors that serve the driving force in processes of arriving and conversion of lead in atmosphere during different seasons of the year.

Review of recent developments in aeolian dust signals of sediments from the North Pacific Ocean based on magnetic minerals

AbstractThe North Pacific Ocean (NPO) has received abundant aeolian dust transported by westerlies from the Asian inland. The aeolian components preserved in NPO sediments record information on palaeoclimatic and palaeoenvironmental changes in Asian source areas at different timescales. Previous studies have systematically investigated the source–sink effect of aeolian dust using the sedimentology, geochemistry, isotope and magnetic methods. In this study, we focus more on recent developments of aeolian signals in NPO sediments obtained by magnetic approaches. Generally, aeolian components contain a mixture of magnetite, maghemite, hematite and goethite of different origins. Magnetic properties (mineral category, concentration and particle size) of these minerals are modulated primarily by climatic/environmental conditions in source areas and sorting effects during the transportation process. Compared with the other methods, magnetic measurements have the advantages of non-sample destruction, high sensitivity and high efficiency. Finally, future studies are also discussed to address the importance of magnetism for tracing the dynamic transportation processes of the aeolian dust.

Variations in the westerlies in Central Asia since 16 ka recorded by a loess section from the Tien Shan Mountains

Central Asia comprises a vast area of arid terrain which has an important impact on the global climate system. Although still debated, the pattern of climatic variations in central Asia is regionally distinctive and does not appear to be strongly related to changes in insolation. To understand the climatic evolution of central Asia, as well as its forcing mechanisms, it is necessary to characterize variations in the Westerlies, which have had a major influence on climate change in the region. Past variations in the Westerlies are commonly reconstructed from records of the composition, grain-size and mass accumulation rate (MAR) of loess and the eolian component of lacustrine sediments which are commonly mixed by the signal of surface atmospheric circulation. Here we present the results of grain-size analysis of a section of last glacial and Holocene loess deposits in the Tien Shan Mountains. The results indicate that the strength of the Westerlies was weakest during the last deglacial and early Holocene (16–6 ka), strongest during the mid-Holocene (6–3.1 ka), and moderate during the late Holocene (3.1–0 ka). By comparison with an independent record of humidity, we propose that the Westerlies were an important driver of moisture variations in central Asia, with strong Westerlies circulation resulting in high precipitation and vice versa during the past 16 kyr.

Cover beds older than the mid-pleistocene revolution and the provenance of their eolian components, La Sal Mountains, Utah, USA

We used uranium-lead (U-Pb) dating of zircons from a tephra layer deposited in the La Sal Mountains to assign an age of more than c. 1.3 Ma to underlying loess-mixed slope deposits (cover beds) and paleosols developed therein. For the first time, we show that properties of cover beds and soils before the Mid-Pleistocene Revolution were similar to those formed after the revolution. However, the deepest exposed carbonate-enriched horizon is much farther developed than younger ones, indicating that there was a period of enrichment by far exceeding intensities of younger calcic horizons some time before the revolution, possibly in Neogene times. Remarkable differences between age distributions of detrital zircons (DZ) within the cover beds allow reconstructing the regional provenance of mixed eolian matter with high accuracy: we were able to trace particular cover beds back to areas with outcropping Permian and Upper Cretaceous rocks.

Changes in diffuse reflectance spectroscopy properties of hematite in sediments from the North Pacific Ocean and implications for eolian dust evolution history

Eolian dust preserved in deep-sea sediments of the North Pacific Ocean (NPO) is an important recorder of paleoclimatic and paleoenvironmental changes in the Asian inland. To better understand changes in the dust provenances, in this study diffuse reflectance spectroscopy (DRS) was used to extract the eolian signal recorded in sediments of ODP Hole 885A recovered from the NPO. First, we systematically investigated sieving effects on the DRS data; then band positions of hematite (obtained from the second order derivative curves of the K-M remission function spectrum derived from the DRS) were used to distinguish different provenances of the eolian dust preserved in the pelagic sediments of this hole. Our results show that the sieving (38 μm) process can suppress effectively the experimental errors. Eolian signatures from Chinese Loess Plateau (CLP) sources and non-CLP-sources have been identified in the pelagic sediments of ODP Hole 885A from the late Pliocene to the early Pleistocene. The provenance differences account for the discrepancies in the eolian records recovered from the pelagic sediments in the NPO and profiles in the CLP. Temporal changes in dust provenances are caused by the latitudinal movement of the westerly jet mainstream. The hematite DRS band position is a useful tool to distinguish the provenance of eolian components preserved in pelagic sediments.

Coarse‐sand beach ridges at Cowley Beach, north‐eastern Australia: Their formative processes and potential as records of tropical cyclone history

AbstractStorm surges generated by tropical cyclones have been considered a primary process for building coarse‐sand beach ridges along the north‐eastern Queensland coast, Australia. This interpretation has led to the development of palaeotempestology based on the beach ridges. To better identify the sedimentary processes responsible for these ridges, a high‐resolution chronostratigraphic analysis of a series of ridges was carried out at Cowley Beach, Queensland, a meso‐tidal beach system with a >3 m tide range. Optically stimulated luminescence ages indicate that 10 ridges accreted seaward over the last 2500 to 2700 years. The ridge crests sit +3·5 to 5·1 m above Australian Height Datum (ca mean sea‐level). A ground‐penetrating radar profile shows two distinct radar facies, both of which are dissected by truncation surfaces. Hummocky structures in the upper facies indicate that the nucleus of the beach ridge forms as a berm at +2·5 m Australian Height Datum, equivalent to the fair‐weather swash limit during high tide. The lower facies comprises a sequence of seaward‐dipping reflections. Beach progradation thus occurs via fair‐weather‐wave accretion of sand, with erosion by storm waves resulting in a sporadic sedimentary record. The ridge deposits above the fair‐weather swash limit are primarily composed of coarse and medium sands with pumice gravels and are largely emplaced during surge events. Inundation of the ridges is more likely to occur in relation to a cyclone passing during high tide. The ridges may also include an aeolian component as cyclonic winds can transport beach sand inland, especially during low tide, and some layers above +2·5 m Australian Height Datum are finer than aeolian ripples found on the backshore. Coarse‐sand ridges at Cowley Beach are thus products of fair‐weather swash and cyclone inundation modulated by tides. Knowledge of this composite depositional process can better inform the development of robust palaeoenvironmental reconstructions from the ridges.

Siltstones Across the Daptocephalus ( Dicynodon ) and Lystrosaurus Assemblage Zones, Karoo Basin, South Africa, Show No Evidence For Aridification

Abstract: Changes in the record of vertebrate-assemblage zones in the Karoo Basin, South Africa, serve as a model for the response of terrestrial ecosystems to the end-Permian biotic crisis. Vertebrate turnover from the Daptocephalus to Lystrosaurus Assemblage Zone is believed to have begun with the appearance of subtle siltstone-color variance and is coeval with a unique, laminated lithofacies, both of which are interpreted by other workers to represent evidence for an aridification trend. The laminated facies consists of interbedded greenish-gray and dark reddish-gray siltstone, with coloration of the latter reported as a function of an increasing eolian component. The current study combines stratigraphy, petrography, geochemistry, and rock magnetic properties of an interlaminated succession and lateral equivalents at Old Lootsberg Pass in the Eastern Cape Province to determine whether such subtle color variance is a function of aridification and, hence, climate change. Siltstones were evaluated from intervals both below and above a pedogenic nodule-bearing feldspathic wacke that is physically correlated over a > 0.5 km distance. The lower succession consists of interbedded greenish-gray ( sensu latu ) and reddish-gray ( sensu latu ) siltstone in which undisturbed and minimally disturbed primary sedimentary, along with few secondary biogenic structures, are interpreted to represent deposition in an abandoned channel-fill complex. Massive greenish-gray and dark reddish-gray siltstone above the channel complex demonstrate that coloration is variable across laterally equivalent strata. Both lithologies were assessed using X-ray fluorescence, X-ray diffraction, Mossbauer spectroscopy, scanning electron microscopy with EDS, and bulk magnetic susceptibility, stepwise acquisition of an isothermal remanent magnetization (IRM), and backfield direct field demagnetization. Our data indicate that there is no statistically significant difference in major-element concentrations between lithologies and that subtle color difference is a function of the presence of fine ( Lystrosaurus Assemblage Zone was not arid. In contrast, the landscape was wet, that allowed for the continued growth of a Permian-age flora under conditions of high water table that experienced temporal fluxes in the hydrological regime promoting spatial color modification of sediment.

An aeolian or a glaciolacustrine record? A case study from Mieļupīte, Middle Gauja Lowland, northeast Latvia

Abstract In the Middle Gauja Lowland, northeast Latvia, dunes are distributed over a vast glaciolacustrine plain that formed during the retreat of the Fennoscandian ice sheet. Such a direct contact between aeolian and glaciolacustrine sediments can be used to infer depositional settings and decipher to what extent these sediments bear an aeolian component. Our proxies, although preliminary, reveal a limited range of variation in grain-size parameters, a significant presence of quartz grains with silica precipitation and matt-surface grains of various rounding degrees and massive structure combined with horizontal lamination. These are indicative of periglacial-aeolian depositional conditions in the foreland of the Linkuva ice-marginal zone. Sedimentary characteristics do not match a single luminescence date of 9.2±0.6 ka, which significantly postdates the minimum age of the Linkuva ice-marginal zone with 10Be ages between 15.4 and 12.0 ka. Whether deposition started directly after drainage of the Middle Gauja ice-dammed lake or if there is a gap of 2.8–6.2 ka is a matter of debate; only future studies at higher OSL resolution could resolve this.

Aeolian control on the deposition of high altitude lacustrine basins in the Middle East: The case of Lake Neor, NW Iran

Lake Neor is the largest lacustrine basin in the high mountains of the Middle East, at 2500 m altitude in the Alborz belt. This lake of Holocene age is a shallow, fresh water body of glacial origin with ca 4 km2 surface area and a 40 km2 drainage area. Its sedimentary sequence comprised of peat and gyttja consists of >10 m infill, which is fairly thick for such a lake. Autochthonous organic matter is limited in and around the lake except for small areas at the northern and southern ends. Inorganic constituents of the infill sediment include a significant amount of fine-grained calcite, dolomite and mica particles that are exotic to the catchment. They do not occur in the source rocks, as the drainage area of the lake is composed of only andesites and trachyandesites of Eocene age. In addition, the uppermost part of the infill includes abundant fine-grained charcoal particles derived from large forest fires perhaps derived as far as the Mediterranean region. As shown by the present meteorological records, the sediment of Lake Neor is under aeolian control during dust storms; this is leading to a considerable amount of deposition air-borne particles. The infill of some lakes, particularly those in highlands, may have contain an important aeolian component and thus they cannot be used directly for the interpretation of the evolution of the palaeogeography without taking into consideration the role of dust storms on deposition.

Extensive Quaternary aeolian deposits in the Drakensberg foothills, Rooiberge, South Africa

Deposits of aeolian sand are known to have accumulated in periglacial environments during the cold phases of the late Quaternary. In many instances, however, they form low-relief topographic units which may not be readily identified without detailed field survey. This study aims to use a multidisciplinary approach, combining remotely sensed data analysis and field survey, to investigate the extent and palaeoenvironmental significance of sand ramps in the Drakensberg/Rooiberge foothills of South Africa. Analysis of Google Earth™ imagery has demonstrated that gully systems are a common component of the landscape, and heterogeneously distributed across the landscape. Field investigation confirmed the hypothesis that the gullies are mainly eroding into sand ramps of fine sands and very coarse silts which mantle many of the lower hillslopes of the region. These sand units include palaeosols and occasional gravel lags, but are otherwise remarkable for their homogenous composition, cross-bedding and the complete absence of clasts. Much of the sediment is thus interpreted as aeolian in origin.The deposits are sufficiently similar in many respects to the Masotcheni Formation, a late Quaternary colluvium which outcrops abundantly in the Drakensberg, to propose an assignation to this unit. However, an aeolian component in the Masotcheni has not previously been described. The distribution of aeolian accumulation in the region is consistent with southward transport during late Quaternary cold phases from a source on the Highveld to the north of the study area. The low relief and complex fluvial network in this region would concentrate sediment eroded from the Drakensberg/Rooiberge, which would subsequently be available for deflation when the balance between fluvial flow regime, seasonally frozen ground and north-westerly trade winds were optimal for aeolian entrainment. Deposition is primarily topographically controlled, and is in places sufficiently extensive that it may be better described as a discontinuous coversand. This study suggests that aeolian deposits may be overlooked in other environments subject to past periglacial landscape development, and develops a potential methodology by which this problem may be overcome.

Paraglacial fluvial landscape change in a continuous permafrost environment around the ‘Twin Creeks’ catchment, Banks Island, western Canadian Arctic

Following Late Wisconsin glaciation of southwest Banks Island, four stages of landscape change are reconstructed for a small (c. 2km2) tundra catchment draining a south facing slope in an environment underlain by continuous permafrost. Initially, during deglaciation, glaciolacustrine silts and sands were deposited on a proximal slope underlain by till (Unit A). Subsequently a cyclic sub-aerial fluvial erosional–depositional system developed. First, an alluvial fan aggraded unconformably on Unit A with penecontemporaneous growth of ice and sand wedges (Unit B). Second, deep incision of the fan created a network of steep-sided flat bottomed channels. Third, wetland fens developed on the channel floors and peats with a sandy aeolian component aggraded and largely infilled the channels (Unit C). Synchronously, on the interfluves a soil catena developed under a warmer climate in the early Holocene. This was later buried by coversands (Unit D). Fourth, in the late Holocene, entrenchment recurred and currently fluvial processes are both exhuming and extending the first phase channel networks. A combination of climate and vegetation cover changes appears to control the pattern of geomorphological process responses.

Seasonal and diurnal variations in dust characteristics on the northern slopes of the Tien Shan – Grain-size, mineralogy, chemical signatures and isotope composition of attached nitrate

A dust monitoring program was initiated in the summer of 2010 to (i) improve the informative value of aeolian components of varved lake sediments in Central Asia for palaeoclimatic interpretations, (ii) evaluate the impact of aeolian influx on modern lakes in the region and (iii) obtain data for a future comparison with dust east of the high mountain belt formed by the Alay, Pamir, Tien Shan and Altai mountains. We collected the coarse (>2.5μm) dust fraction on the northern slopes of the Tien Shan 42°40′49.69″N, (74°41′37.36″E, 1740m asl) using a high-volume slit-impactor at 3-day sampling intervals. We present data on the mineralogical composition, particle-size distribution, soluble salts and nitrate isotope composition of the collected dust. The short-term and seasonal changes in dust concentration and composition are discussed in the context of high temporal resolution measurements of meteorological parameters and particle counts for 31 grain-size bands.Throughout the study period, CaCO3 was a major dust constituent (average particle frequency 14%). Between July 2010 and October 2012, the average content of soluble salts was 10wt.%; mole percentages of water-leachable anions were 60% NO3, 30% SO4, 10% Cl. Ca was the dominant leachable cation (>90%). The collected dust comprised (i) gypsum which forms pedogenically in the topsoils of arid regions and (ii) secondary gypsum originating from the interaction of sulphuric acid aerosols with CaCO3 in the atmosphere. Variable proportions of (i) and (ii) and the extent of Ca(NO3)2 formation (verified by chemical mass budgets) were documented in the Ca/Sr and Ca/SO4 ratios of the aqueous leachates.The isotopic compositions of the dust nitrate in the majority of the samples clustered between −10 and +10‰ for δ15N[NO3] (VSMOW) and +50 and +100‰ for δ18O[NO3] (Air). The δ18O[NO3] values of a majority of the collected samples exhibited a weak positive correlation with the NO3 load of the collected dust (R2=0.148). The time series of the δ15N[NO3] values varied around a basement level of −6‰, which indicates that NOx from mobile sources is a major precursor of the dust-NO3. The distinct, positive departures from these values correlated with increases in the content of soot. We conclude that the related δ15N[NO3] peaks reflect increased NOx contributions from coal burning. The possible origin of the dust was evaluated using satellite images and backward trajectory calculations for a few example monitoring intervals. Data synthesis in the context of hemispherical atmospheric circulation models, including the data for 2013, the results from ongoing chemical and isotope analyses of the non-soluble solid residues of the collected materials and the statistical handling of an extended data set will further elucidate the relationships involving atmospheric circulation, atmospheric dust load, dust sources and the characteristics of the study region.

Geochemistry of rare earth elements in the mid-late Quaternary sediments of the western Philippine Sea and their paleoenvironmental significance

Based on a delta O-18 chronology, rare earth elements (REE) and other typical elements in sediments from core MD06-3047 in the western Philippine Sea were analyzed to constrain the provenances of the sediments and investigate quantitative changes in the Asian eolian input to the study area over the last 700 ka. Among the competing processes that might affect REE compositions, sediment provenance is the most important one. Provenance analysis suggests that the study sediments have two provenance end-members; local volcanic sources are dominant, and eolian dust from the Asian continent has a smaller contribution. During glacial periods, eolian input to the western Philippine Sea was enhanced. In contrast, material supply from local volcanics increased during interglacial periods. Changes in eolian input to the study area were probably related to the strength of the East Asian winter monsoon (EAWM) as well as aridity in the Asian continent on an orbital time scale, and were partly influenced by local control factors on shorter time scales. Therefore, we propose that the present study expands the application of the REE-based method for quantitatively estimating the eolian component from the mid-latitude northern Pacific to the low-latitude western Pacific. Additionally, the study preliminarily confirms the influence of EAWM-transported eolian material on sedimentation in the western Philippine Sea since 700 ka.

Material source analysis and element geochemical research about two types of representative bauxite deposits and terra rossa in western Guangxi, southern China

In this study, geologic field observations and geochemical indicators of the types of representative bauxite deposits, terra rossa and underlying carbonate rocks in western Guangxi, southern China, were investigated. Their interrelations, material sources, formation and evolution processes were discussed. The results show an inherited relationship between terra rossa and its underlying limestone. The relevant parameter diagrams of trace and rare earth elements suggest that the two types of bauxite deposits probably possess identical material sources. Both bauxite deposits and terra rossa have close affinities with each other, implying that carbonate rocks similar to the underlying limestone might be a major material source for their formation. However, the eolian component represented by Chinese loess and igneous rock genesis with respect to Emeishan basalt plume may not contribute so much to the material source of bauxite deposits and terra rossa. In the process of leaching and pedogenesis, the mobile elements, such as Ca, Mg and Na, were preferentially dissolved and removed from parent rocks, and the less mobile elements Al, Fe, Ti and Si were accumulated and enriched. During the period of diagenesis and bauxitization, elements K and Si were largely removed from the system. Especially, in transformation from primary bauxite to secondary bauxite deposit, the contents of many “less mobile” elements, such as Fe and Ti, decreased. This finding is possibly due to the strongly acidic environment caused by decomposition of pyrite in primary sedimentation-type bauxite deposit. This study systematically analyzed the inherited relations and evolution processes among carbonate rock, terra rossa, and two types of bauxite deposits, and gave a guide for the bauxite deposit prospecting in western Guangxi.

Climatic stages recorded in sediments of the Gunang Cave, South Korea

This paper presents and analyses lithological, stratigraphic and geochronological data on the Gunang cave, which is an important archaeological site in South Korea. It discusses problems of the age and origin of its sedimentary sequence, which consists of clastics and hardpan speleothems. The sediments compose a 4 m thick lens near the cave entrance. The lens formed during the Middle–Late Pleistocene and Holocene according to 14C and U–Th dating. The changes of cave environments are reconstructed based on sedimentological, palynological, mineralogical, magnetic and geochemical data. The clastic sediment unit consists of roof-fallen limestone debris cemented by fine-grained matrix. The matrix is dominated by silt (80%), which aeolian component is 60–65%. The amount of the wind-blown component is persistent throughout the clastic layers suggesting a relatively stable depositional process in dry environment. In contrast, three lime-cemented hardpans formed in a wet environment of groundwater seepage. The revised age model suggests that the hardpans formed in early MIS1, MIS 5.5, and MIS 7, and therefore correspond to the wetness maximums of these warm stages.

Influence of chemical weathering on the composition of the continental crust: Insights from Li and Nd isotopes in bauxite profiles developed on Columbia River Basalts

Mineralogical, chemical, and Li and Nd isotopic compositions of two drill cores (8–9m deep) through bauxites developed on the Miocene Columbia River Basalts document the changes associated with basalt weathering, provide insights into the processes involved, and allow us to examine the overall influence of chemical weathering on juvenile (basaltic) crust. Gibbsite, hematite, ±kaolinite, halloysite, goethite, and maghemite are the weathering products in the bauxites. Quartz is observed near the tops of the cores and its abundance decreases progressively with depth; no quartz is observed below five meters depth in either core. Most major and trace elements, including “mobile” and some “immobile” elements are severely depleted in the bauxites. Niobium is less mobile relative to the rare earth elements, thus chemical weathering attenuates the negative Nb anomaly in the continental crust. Li and Nd are strongly depleted relative to fresh basalt, and both increase systematically towards the surface in the quartz-bearing samples while δ7Li and εNd values decrease systematically towards the surface in these same samples. Both Li and Nd were likely lost from the bauxites through leaching. The systematic enrichment of Li, Nd, and quartz, as well as the less radiogenic Nd isotopic composition at the tops of both profiles reflects 20–60wt.% addition of an eolian component to the soils. The eolian dust is unlikely to have experienced significant post-depositional weathering due to the relatively high Li contents near the tops of the profiles, and, therefore, the low δ7Li and εNd values suggest that the dust came from an old, weathered region of the continent. Our results demonstrate that lithium isotopes are sensitive tracers of chemical weathering, particularly in extreme weathering settings, and support the hypothesis that chemical weathering influences the mass and composition of the continental crust.