Intensity Of Pedogenesis Research Articles

A Late Pleistocene palaeoclimate record based on mineral magnetic properties of the entrance facies sediments of Kulna Cave, Czech Republic

Kulna Cave is located in the Moravian Karst, a well-developed karstic region formed in Devonian limestones in the eastern part of the Czech Republic. The entrance facies sediments in the cave consist of interbedded layers of silts (loess) and clay-rich silts (loam) that were either directly blown into the cave entrance or redeposited in the cave by slope processes during the Last Glacial Stage. The layers of loess and loam overlie fluvial sands and gravels deposited during the Last Interglacial. Previous research at Kulna concentrated on the archaeology, palaeontology and dendrology of these entrance facies sediments. From these data, palaeoenvironmental conditions in the vicinity of the cave were reconstructed. Our results suggest that susceptibility variations and in particular variations in pedogenic susceptibility yield a more detailed record of the palaeoenvironmental conditions at the cave during the Last Glacial Stage. Magnetic susceptibility (χ) was measured on approximately 700 samples collected throughout three well-studied profiles in the cave entrance. The χ record is well defined and correlates from one profile to another. Mineral magnetic measurements [FD, ARM/SIRM, S-ratio, χ(T)] suggest that χ variations in the Kulna sediments from the Last Glacial Stage are controlled by the concentration of magnetite and/or maghemite formed during pedogenesis. After the removal of the effects of fine carbonate debris and detrital ferromagnetic minerals on the bulk χ record, we obtained a record of pedogenic susceptibility (χp) that serves to quantify the concentration of magnetic minerals formed during pedogenesis. Therefore, χp can be thought of as a proxy reflecting the intensity of pedogenesis, which in turn is controlled by climate. Our χp record is also in good agreement with the median grain size record of the Kulna sediments (another proxy for climatic change). We suggest that in the case of Kulna, χp is more sensitive to climate change than bulk χ. The Kulna pedogenic susceptibility record shows variations on both long and short timescales. The long-term trends are in good agreement with the deep-sea SPECMAP record, while the short-term oscillations correlate well with rapid changes in the North Atlantic sea surface temperatures. Our results suggest that Central European climate during the Last Glacial Stage was strongly controlled by the sea surface temperatures in the North Atlantic. Short-term warmer events and perhaps higher precipitation over the mid-continent increased the intensity of pedogenesis. Given the number of independent climate proxies determined from the entrance facies of the cave and their high resolution, Kulna is an extremely important site for studying Late Pleistocene climate.

Magnetic properties of the Tertiary red clay from Gansu

The study on magnetic properties of the red clay indicates that the red clay and loess-paleosol sequence have a common magnetic mineralogy, with magnetite, maghemite, hematite (and possibly goethite) contributing to the magnetic behavior. The red clay magnetic susceptibility is also found to have a positive relation with extrafine superparamagnetic grains. This suggests that, like the Quaternary loess-paleosols, an ultrafine ferrimagnetic component produced during pedogenesis in the red clay under humid conditions also plays an important role in susceptibility enhancement in the soil units. This is supported by the correlation between Rb/Sr ratio and magnetic susceptibility. This signifies that, like the above loess-paleosol sequence, the magnetic susceptibility of the red clay can be used as a general proxy paleoclimatic indicator, although whether its susceptibility in the red clay is comparable to pedogenesis intensity and requires further investigation. Magnetic susceptibility variation in the red clay thus also provides an eolian/pedogenic record of paleoclimatic evolution. Study of the background susceptibility indicates that, on average, the absolute scale of the paleoclimatic shift from red clay development to Quaternary loess deposition is similar to the climatic shift from stage 5 (S1) to stage 2-4 (L1). This may suggest that during the Quaternary there is an evident strengthening of the absolute wind intensity to bring more (about double) coarser and less weathered (non-SP fraction) eolian magnetic input from the source regions to the Loess Plateau than during the Pliocene. The presence of eolian red clay since 7.5 Ma BP in central-northern China implies an important environmental change from the underlying Cretaceous red sandstone. The red clay development was closely related to global drying and climate cooling since the Cretaceous and closely associated with the abrupt uplift of the Qinghai-Xizang Plateau at about that time. This uplift of the plateau intensified the East Asia monsoon system and started red clay deposition.

Formas extraíveis de ferro em solos do estado do Rio de Janeiro

No estado do Rio de Janeiro, destacam-se três feições geomorfológicas: as formações constituídas por rochas do Complexo Cristalino, expostas ou capeadas por seus produtos de alteração; os tabuleiros costeiros, compostos pelos sedimentos da Formação Barreiras, e, por último, as planícies aluvionares, que são formações quaternárias. As diferenças entre os materiais de origem e as classes de relevo e sua influência na formação dos solos têm constituído o tema principal dos estudos de genêse realizados no Rio de Janeiro. Este trabalho teve como objetivo avaliar o comportamento das formas extraíveis de ferro, Feo, Fed e Fes, e as relações entre elas em função do grau de pedogênese dos solos. Para estes solos, as formas de ferro e relações Fed/Fes e Feo/Fed expressaram variações na mineralogia do material de origem, grau de intemperismo dos solos e do material formador, ou classe de drenagem, mais do que a intensidade de pedogênese. Assim, as formas extraíveis de Fe e respectivas relações para avaliar o grau de pedogênese, nos solos do Rio de Janeiro, requerem uma interpretação conjunta das demais propriedades do solo, com ênfase para a morfologia e classes de drenagem.

Summary outline and recommendations on paleopedological issues

Paleosols are soils that were formed on an ancient landscape. They may be buried soils and/or soils incorporated into the sedimentary sequences, exhumed soils, or soils developed on ancient relief surfaces (relict soils). Exposed on the surface and influenced by later environmental changes, paleosols reveal ancient environments and contain records regarding the climate, vegetation, landforms, organisms, parent material, time, intensity of pedogenesis, and sedimentation rates in effect during their training. In the Marília Formation, the paleosols, exhumed and buried, developed on sandstones have argic (Btk, Bt and Bss), hydromorphic (Cg) and calcic (Bk) horizons with different degrees of cementation, forming mainly calcretes. The objective of this study was to characterize the genesis of the paleosols of the Marília Formation, addressing the dynamics of their formation factors and processes at local and regional levels in order to reconstitute the conditions of environmental evolution of the Maastrichtian of the Bauru Basin. Eleven sections (S1, S2, S3, S4, S5, S6, S7, S8, S9, S10 and S11) of the Marília Formation were described and collected on nine sample profiles. Micromorphological, mineralogical and geochemical analyses were performed according to the specialized literature. In the Marília Formation, sedimentation, erosion and pedogenesis are processes that occurred together, concomitantly or subsequently, in view of the fluvial paleoenvironmental dynamics. The paleosols indicate the end of depositional cycles, with differentiated rates, intensities and duration. The paleosols of the Marília Formation comprise well-drained (Bt-horizon) and poorly drained or hydromorphic (Cg-horizon) pedotypes, of drier conditions (Bkm-horizon) and pedotypes indicative of drier periods alternated with periods of higher humidity (Bss-horizon). The profiles were classified as Luvisols (Bt and Btk horizons), Calcisols (Bk and Ck horizons), Arenosols (C-horizons) and Vertisols (Bss-horizons). Paleosols developed on the flatter and lower portions of the slopes and floodplains and were commonly influenced by the oscillation of the groundwater and the seasonality of the climate. Sandstone with allochthonous and autochthonous carbonate cementation constituted the main parent material of the paleosols. The predominant climate during the Maastrichtian was semi-arid, with seasonality and variations in precipitation patterns and a precipitation-evapotranspiration relationship that provided conditions for the formation of paleosols with Bk, Btk, Bss, Bg and Bt horizons. The time of formation of these diagnostic horizons was dependent on climatic and edaphic factors. However, the time factor lasted until approximately 200,000 years, allowing paleosols to develop with Bt-horizons and prismatic and block structures. Calcification and argilluviation were the formation processes responsible for the genesis and evolution of paleosols during the Maastrichtian of the Bauru Basin. Weathering indexes have shown that paleosols with Bt-horizons are more weathered than are those with Btk and Bk horizons. The paleosols with Bk-horizons reflect periods with more arid conditions in the general context of the semi-arid paleoclimate of the Maastrichtian of the Bauru Basin.

The magnetic susceptibility of modern soils in China and its use for paleoclimate reconstruction

Magnetic susceptibility of more than 160 modern silty soil samples in China was measured to assess the relationship between the magnetic susceptibility and modern climatic parameters. Correlation between magnetic susceptibility and mean annual temperature (MAT) or mean annual precipitation (MAP), shows a complex picture and no single function can be found to fit all the data on the national scale. In East China, where East Asian monsoon plays an important role for the climate conditions, magnetic susceptibility increases with the increase of MAT or MAP in temperate semi-arid regions of the Loess Plateau and surrounding areas. This can be attributed to increasing intensity of pedogenesis which would favor the formation of strongly magnetic minerals and/or reduce depositional rate of eolian dust. Magnetic susceptibility tends to decrease with the increase of temperature and precipitation in the tropical and subtropical warm and humid regions of the vast areas south of the Yangtze River. This may be explained by pedogenic transformation of iron-bearing minerals to weakly magnetic minerals. Between these two different correlation patterns, 15°C of MAT and/or 1200 mm of MAP seem to be the thresholds. In West China, the correlation becomes quite complex in the great mountains and vast sedimentary basins in the north-west. This may be due to the prevailing continental climate in this region and topographic contrast within short distance. The correlation for the Qinghai-Xizang (Tibetan) Plateau is not clear because very few samples were collected. Fluctuations of paleo-temperature and paleo-precipitation at Luochuan for the last 130 ka were estimated using the climofunction obtained from this study.

INFLUENCE OF GEOGENESIS AND PEDOGENESIS ON CLAY MINERAL DISTRIBUTION IN NORTHERN VIETAM SOILS

This is a study of a sequence of five soils developed in the Red River valley of Vietnam along a transect extending from the foothills to old terraces. Most of these soils are chemically depleted, having, in particular, a very low CEC. The mineralogical study shows that the <2-μm fraction is made up of a mixture of vermiculite, illite, kaolinite, quartz, and iron oxyhydroxides. Other minerals, such as pyrophyllite in the foothills and chlorite and gibbsite in the old terraces, are present in small quantities. The major mineralogical differences among these soils lie in the relative abundance of quartz in the <2-μm fraction, the vermiculite/illite ratio, and the degree of the intergrade character of vermiculites, which is variable not only within each profile but also from one region to another. The amount of quartz generally increases toward the foothills, indicating the greater weathering of foothill materials relative to old terraces, which have been less weathered during geogenesis. The vermiculite/ illite ratio, which also increases toward the foothills, indicates an increasing transformation of 2:1 minerals. The effects of pedogenesis are characterized by the difference in vermiculite/illite ratios between the surface and 1-m-deep horizon. Nevertheless, the intensity of pedogenesis cannot be evaluated in profiles developed on highly weathered sediments because illite of the parent material has been completely transformed into vermiculite during a preweathering stage. Alternate wet and dry conditions apparently lead to an increase in illite content by the collapse of vermiculite sheets in surface horizons. The creation of a plow pan in these hydromorphic conditions may have favored the dissolution of clay by ferrolysis and the lateral leaching of fine clay particles out of the profile, increasing quartz content in the <2-μm fraction of the plow pan.