Palaeosol Layers Research Articles

The late Quaternary loess record of Tokaj, Hungary: Reconstructing palaeoenvironment, vegetation and climate using stable C and N isotopes and biomarkers

Large areas of Southeast Europe are covered by Pleistocene loess deposits with interbedded palaeosol layers, reflecting interglacials or -stadials. These loess-palaeosol sequences are valuable terrestrial archives for continuous palaeoclimatic and palaeoenvironmental information. In recent years, the traditional paradigm of treeless full-glacial palaeoenvironments has increasingly been called into question, as there is palynological, anthracological and malacological evidence for the continuous presence of trees from loess-palaeosol sequences in the Carpathian Basin. This paper aims at contributing to this discussion by applying biogeochemical, geophysical and sedimentological methods to a late Quaternary loess-palaeosol profile at Tokaj, NE Hungary. Previous studies at the same location have focused on molluscs and charcoal, while this study concentrated on stable C and N isotopes and lipid biomarkers. Long-chain n-alkanes (C 23–C 33), which form major constituents of plant leaf waxes, have successfully been used as biomarkers over the last years and allow for a reconstruction of palaeovegetation based on the differentiation of grassland vs. shrubs and trees. Stable carbon isotopes were used to further differentiate between C 3 and C 4 palaeovegetation, and stable N isotopes to reconstruct the N cycle and climatic variations. Magnetic susceptibility (MS), organic carbon and grain size distribution, which may serve as proxies for e.g. the intensity of pedegenesis, weathering and palaeowind strength, were measured to complement the biogeochemical results and to establish a pedo– and biostratigraphy. Although the results do not agree in every detail with previous studies, there are surprising matches and similarities in the data. Alkane and stable N isotopic data indicate that, pedogenesis occurred under productive, fertile steppe-grasslands, probably in an intensified seasonal climate, and during reduced sedimentation. Two short treeless-steppe periods were found between 26 and 12 ka BP. However, periods characterised by a cooler climate in which trees and shrubs expanded were also found. This interpretation is also supported by previous mollusc data that provide evidence for several periods of favourable conditions for tree growth.

Similarities among the Plio–Pleistocene terrestrial aeolian dust deposits in the World and in Hungary

The widely distributed Plio–Pleistocene aeolian dust deposits are one of the most important terrestrial archives of past climate and environmental changes. The alternation of loess and palaeosol layers is regarded as evidence of the cyclic nature of Pleistocene climate changes. The loess succession is underlain by aeolian red clay, which has been formed under warm–humid climates. According to the studies of red clay–loess–palaeosol sequences from China, Central Asia, Alaska, South America and Central Europe, the mineral dust deposition has shown similar patterns since the Pliocene. The typical loess of the last one million years reflects glacial–interglacial conditions, whereas the old loess–palaeosol sequences could be the product of shorter arid–humid climate cycles. The similarity of the sedimentary structures could be caused by the similar global climate conditions, controlled by orbital forcing. The investigated sections from Hungary (Central Europe) have been affected by local and regional geomorphological and climate factors. Even so, they can be correlated fairly well with the major global climate changes. The Hungarian aeolian dust deposits consist of three main groups of sedimentary formations: (1) Pliocene–Early Pleistocene aeolian red clays, (2) the oldest loess–palaeosol sequences, formed from the almost continuous Early Pleistocene dustfall, and (3) the typical, glacial–interglacial loess deposits of the last one million years, without remarkable dust deposition in the warmer periods.

Climate and soil moisture environment during development of the fifth palaeosol in Guanzhong Plain

Based on weathering characteristics of the fifth palaeosol layer (S5) of four sections in Guanzhong Plain, the thickness of the weathered profile of the paleosol is determined to be greater than the ordinary soil, a weathered and leached loess layer thicker than 2 m. The distribution depth of the red argillans, the weathered and leached loess layer, Fe2O3, CaCO3 and Sr content under the S5 all indicate that the precipitation in Guanzhong Plain was over 900 mm at that time. The distribution depth of gravity water zone reached 4.2 m at least, and the soil moisture content was generally more than 20% within the range of 4.2 m. At that time there was sufficient soil moisture and no dried earth layer developed in Guanzhong Plain, suitable for the forest to develop. When this soil developed, the mean annual precipitation was more than the annual soil moisture evaporation. The value of soil moisture balance was positive and the atmospheric precipitation could supply the underground water normally. Soil water was weak acidic in the middle and late stages when S5 developed in Guanzhong Plain. It was a kind of subtropical climate and even more humid and warmer than the northern edge of the subtropical climate zone in Guanzhong Plain when the S5 developed. At that time the subtropical climate was prevailing over the northern side and southern side of Qingling Mountains, showing the Mountains no longer to be the boundary between the subtropical zone and the temperate zone in China. The summer monsoon acted intensely and could go over Qingling Mountains frequently bring abundant precipitation.

The causes of apparent overconsolidation in the Namak marine deposit, Korea

An intensive experimental study was carried out to investigate the causes of the overconsolidation phenomena in the Namak New City site, Korea. Geological events in conjunction with sea level changes, groundwater fluctuation, weathering, leaching and cementation were examined as possible causes of overconsolidation. In addition to oedometer and triaxial tests, radiocarbon dating, geochemical components and pore water chemistry were analysed. It was shown that the overconsolidation in the upper marine clay deposit was not caused by stress reductions such as erosion and groundwater fluctuation. Furthermore, weathering or leaching seemed unlikely. The apparent overconsolidation can be explained by cementation, which is indicated by profiles of the sulphate and potassium ion concentrations in the pore water. At the early stage of the Holocene marine deposition, the rate of deposition was 4·4 mm/yr; thereafter it was reduced to 1·4 mm/yr, controlling chemical processes such as sulphate reduction and ion exchanges and, in turn, apparent overconsolidation. Some mechanical and chemical properties of the apparent overconsolidated clay were compared with the geologically overconsolidated Palaeosol layer in the site.

Denitrification capacity in the vadose zone at three sites in the Lake Taupo catchment, New Zealand

Land use in the Lake Taupo catchment is under scrutiny, as early signs of deteriorating water quality in Lake Taupo have been observed. Although the fate of contaminants in soil and groundwater are comparatively well studied, the transformations in the lower vadose zone, i.e. the zone between the soil and the groundwater, are less well understood. The capacity for NO3-N removal via biological denitrification, based on utilising the resident C substrate, in the vadose zone of the Lake Taupo catchment is quantified in this work. Complete vadose zone profiles were sampled at 3 sites (Rangiatea, Waihora, and Kinloch), from the soil surface down to the watertable in approximately 0.5-m depth increments. Texture, allophane content, pH, and concentrations of extractable NO3-N, NH4-N, and dissolved organic carbon were determined. Incubations were undertaken to determine the denitrification capacity of the vadose zone materials amended with NO3-15N, but no added carbon substrate, and maintained under anaerobic conditions at 28°C. Gas samples were taken from the headspace after 48 h and analysed for N2 and N2O. In soil depths down to about 1.2 m, the denitrification capacity ranged from 0.03 to 9.18 kg N/ha.day, and below this depth it ranged from <0.01 to 0.09 kg N/ha.day. A palaeosol layer in the Waihora profile had an enhanced denitrification capacity compared with the other samples in deeper zones of the profiles. In the surface sampling, at least 99.9% of the gas recovered from the 15N applied was in the form of N2. In contrast, no N2 gas production could be detected in any sample from below the second sampling depth, with only N2O detected. Denitrification capacities of all vadose zone materials were low when compared with other studies. Thus, careful land management is required to avoid groundwater contamination by nitrate leaching from the root-zone of the pasture.

A buried palaeosol and late Pleistocene unconformity in coastal deposits of the eastern Yellow Sea, East Asia

Deep-drill core sampling with a high-resolution seismic survey was carried out to identify a Holocene–late Pleistocene boundary in the eastern coast of the Yellow Sea, East Asia. Analysis of core sections revealed the existence of an oxidized and desiccated sedimentary layer, lying directly below a Holocene horizon (Unit I) and being developed at the top of a late Pleistocene (Unit II), possibly last interglacial tidal deposit. The buried oxidized-sedimentary layer is characterized by semi-consolidated, yellowish sediments showing signs of desiccation and alteration such as periglacial cryogenic structures, lithified trace fossils, depletion of smectite, and high geochemical weathering index (Ba/Sr ratio). These features, together with radiocarbon ages, suggests that the layer, defined as a palaeosol developed in marine sediments, has formed as a result of prolonged subaerial exposure of Unit II sediments during the Wisconsinan sea-level lowstand, producing a regional unconformity (late Pleistocene unconformity). This unconformable bounding surface corresponds to a prominent near-surface reflector (R), which is observed in seismic profiles obtained across the drilled-core sections and further is traced ubiquitously across the Yellow Sea shelf. Consequently, the buried palaeosol layer associated with the seismic reflector presents a key horizon for the understanding of late Quaternary environmental changes and integrated stratigraphical correlation in the East Asian marginal seas.

A buried palaeosol and late Pleistocene unconformity in coastal deposits of the eastern Yellow Sea, East Asia

Deep-drill core sampling with a high-resolution seismic survey was carried out to identify a Holocene–late Pleistocene boundary in the eastern coast of the Yellow Sea, East Asia. Analysis of core sections revealed the existence of an oxidized and desiccated sedimentary layer, lying directly below a Holocene horizon (Unit I) and being developed at the top of a late Pleistocene (Unit II), possibly last interglacial tidal deposit. The buried oxidized-sedimentary layer is characterized by semi-consolidated, yellowish sediments showing signs of desiccation and alteration such as periglacial cryogenic structures, lithified trace fossils, depletion of smectite, and high geochemical weathering index (Ba/Sr ratio). These features, together with radiocarbon ages, suggests that the layer, defined as a palaeosol developed in marine sediments, has formed as a result of prolonged subaerial exposure of Unit II sediments during the Wisconsinan sea-level lowstand, producing a regional unconformity (late Pleistocene unconformity). This unconformable bounding surface corresponds to a prominent near-surface reflector (R), which is observed in seismic profiles obtained across the drilled-core sections and further is traced ubiquitously across the Yellow Sea shelf. Consequently, the buried palaeosol layer associated with the seismic reflector presents a key horizon for the understanding of late Quaternary environmental changes and integrated stratigraphical correlation in the East Asian marginal seas.

Highly Negative δ13C Values in Organic Carbon in the Mt. Roe #2 Palaeosol: Terrestrial Life at 2.765 Ga?

The ~13C value of organic carbon at the top of the Archaean Mt. Roe #2 palaeosol is l km. Near the sonthern end of the palaeosol exposure there is a thin (c. 50 cm) lens (c. 20 m x 20 m) of black material interposed between the normal white palaeosol and the overlying basalt. Petrographic microscopy and EPMA show that the white palaeosol layer and the black palaeosol lens both are nearly pure sericite with a small amount of quartz. In thin section the black palaeosol appears to be made up of palaeosol fragments c. 1 mm in diameter. The fragments have no preferred orientation, and probably represent the fragile upper surface of the soil. Some of the palaeosol fragments in the black palaeosol contain sheaves and layers of black material which have the optical properties of graphite. These sheaves and layers are reminiscent of algal/bacterial mat fragments. The 513C values of organic carbon in three samples from the black layer are -41.1%o, -40.8%o, and -40.3%0 PDB. These values are consistent with values found by Hayes (1994) for organic carbon in some late Archaean sediments and suggest that methanotrophic microorganisms colonized the Mt. Roe #2 palaeosol 2.765__.0.010 Ga. The methane these organisms needed could have been generated at the base of the mats during the bacterial degradation of organic matter. It might also have been derived from the ambient atmosphere. The isotopic and morphological observations presented above suggest that methanogens and methanotrophs lived on or near the surface of soils during the Archaean. If there was a methanotrophic

Sedimentological characterization of palaeo-soils in the northern part of the Basin of Mexico

Superficial or buried soils are important palaeostratigraphic indicators that have been poorly studied in the Basin of Mexico. This paper summarizes soil genesis and evolution based on the study of 15 soil profiles with palaeosol layers. An environmental morphogenetic and pedological sequence was established, based on granulometric, micromorphological, geochemical and field soil studies to identify the properties and characteristics of the pedogenetic environments in which they were formed. Results indicate two continuous and recurrent phenomena which altered the genesis of the soils, they are: (a) Torrential erosive processes under tectonic influences and arid conditions, and (b) Volcanic deposits that buried and fossilized soils. Both processes interrupt the evolution and sequence of soil profiles. Consequently, from the upper talus slopes to the lower portions stretching the length of the central lowest plains there are many palaeo-soil discontinuities, that have not been considered into stratigraphic studies of the lacustrine plains.

Rock Magnetic Properties and Palaeoclimate of Chinese Loess.

A typical loess stratigraphy in China consists of alternating loess and palaeosol layers, which potentially provide one of the best terrestrial records of palaeoclimate for the past 2.5 Ma particularly as the magnetic susceptibility record of the loess/palaeosol stratigraphy can be strongly correlated with the climatic fluctuations of oxygen isotope ratios from deep sea sediments. Rock magnetic measurements have verified that magnetite, maghaemite and haematite, are the three most important magnetic minerals. The magnetic susceptibility of Chinese loess varies with changes in both magnetite/maghaemite. concentration and grain size. Although the magnetic susceptibility of palaeosols is several times higher than loess, their magnetic fabric has not been enhanced but slightly weakened, which strongly suggests in situ pedogenesis to be the predominant cause of their increased susceptibility.

Magnetic susceptibility of loess in China

The low-field susceptibility of the loess deposits in the Chinese Loess Plateau is due to both paramagnetic and ferromagnetic minerals. Paramagnetism can be seen in the less- to unweathered loess horizons where superparamagnetic contributions which arise from ultrafine magnetite particles, are not as strongly developed as in the heavily weathered palaeosols. Frequency-dependent susceptibility and minimum unblocking temperatures of low-temperature thermoremanent magnetization generally indicate finer grain sizes in the palaeosols as compared to the loess beds. This grain size difference and the fact that susceptibilities of the same stratigraphic palaeosol layer vary by an order of magnitude between the western and central regions of the plateau, are considered as strong evidence that the ultrafine magnetite is not of detrital (aeolian) origin but was formed in situ. The pedogenetic processes are clearly related to palaeoclimatic changes. Therefore the various susceptibility properties may serve for a detailed reconstruction of the continental palaeoclimatic conditions for the last 2.5 Ma since the palaeomagnetically dated beginning of loess sedimentation in China.

ESR dating and trapped electrons lifetime of quartz grains in loess of China

This paper reports the preliminary application of ESR dating to loess strata. The samples were collected from the 7th palaeosol layer (S7) of the Luochuan section, Shaanxi province in China. The ESR age of S7 is 736 ka (total dose 2945 Gy, annual dose 4 mGy/year). This age represents the original eolian accumulation age. The result is consistent with the palaeomagnetic data (730 ka). We have also carried out thermal annealing experiments on quartz grains from the S7 sample. ESR intensities ( g = 2.0005) increase from 25°C to 320°C. It may be that trapped electrons transfer into the E′ centre site. ESR intensities decrease from 340°C to 460°C due to thermal annealing. We obtained a mean-life of E′ centre electrons at 20°C of 6.66 × 10 8 years. The activation energy is 1.35 eV and frequency factor is 3.7 × 10 8 min −1.