Holocene soils and relict paleosols, southern coast ranges,California, USA

Thirty-nine soils were studied on Holocene and late Pleistocene geomorphic surfaces. Granodiorite, sandstone, and alluvium derived from these rocks are the parent materials. Climate is Mediterranean. Chamise (Adenostoma faciculatum) is on the drier sites and redwood (Sequoia sempervirens) on the moister sites. Our objectives are twofold, (1) Find if today's two-season wet- dry, subhumid climate explains the general noncalcic nature of the soils, or if not (2) accept that their noncalcic nature results from more moist past climates and define some indicator soil properties. The depth to carbonate in the soils formed in calcareous materials on Holocene surfaces corresponds roughly to the average annual depth of water movement, not to the predicted wettest years. We accept then, that the relict paleosols formed under one or more pluvial cycles because they are free of carbonate below their B horizons. Defined levels of pedon clay accumulation, dithionite-citrate extractable Fe (Fe_d) accumulation in the B horizons, cation exchange capacity at pH 7 (CEC_7) to clay ratios, and the minimum base saturation at pH 7 (BS_7) in the pedons are useful properties for separating these relict paleosols from the Holocene age soils. A further evidence of the relict nature of the soils on the Pleistocene surfaces is the weathering reversal noted in these previously weathered materials.     

东亚夏季风变迁的灵敏的指标:黄土碳酸盐的~(87)Sr/~(86)Sr比值

对130ka以来陕西洛川黑木沟剖面的黄土和古土壤碳酸盐中银同位素进行的系统测定显示．在末次间冰期期间87Sr／86Sr平均值为0．71136,末次冰期期间87Sr／86Sr平均值为0．71079．古土壤碳酸盐的87Sr／86Sr比值明显高于黄土碳酸盐的87Sr／86Sr比值．导致这种差别的主要原因是化学风化强度的变化．研究表明,黄土和古土壤碳酸盐87Sr／86Sr比值的变化是现出一个由本次间冰期到未次冰期的约10万年的周期,在这个周期变化的总趋势之上又覆盖了一种波动周期约在2．5ka～2．7ka的高频波动变化．它与小冰期年龄（LittleIceAge）大致吻合．黄土碳酸盐87Sr／86Sr比值的这种高振幅和周期的变化．表明它可能是一种反映东亚夏季风变化的高灵敏度的替代指标．     

Paleosols and climates in the quaternary of south America

Numerous paleosols have been described in the Quaternary as well as a few paleosols in pre-Quaternary rocks of South America. A broad ordering of the more important occurrences of paleosols show four main regions for pedogenesis: A) The lowlands adjacent to the South Atlantic (the Pampa and surrounding areas) with typical loess-paleosol and similar sequences. B) The Andes, characterized by paleosols intercalated with moraines, as high as 4000 m.a.s.l., which are frequently developed in pyroclastic materials. C) The Caribbean region, where soil-forming processes underwent the following succession: Oxisols in the Lower Quaternary → Ultisols → Alfisols → Vertisols → Entisols in the Holocene. D) The Amazonian region located almost permanently under udic moisture regimes of different types. The soils are typically Oxisols and Ultisols.     

Two different illites in Luochuan loess,Northern Shaanxi Province

Illite is the dominant clay mineral in loess and pa-leosol of China accounting for over 50% in content. Ac-cording to the latest mica group nomenclature issued bythe International Mineral Association (IMA), illite is de-fined as a dioctohedron-textured mi…     

A chemical index of weathering without effect of wind sorting: Fe/Mg ratios in the acid-insoluble phases of loess deposits

Chemical analysis of acid-insoluble fractions in loess and paleosols shows that concentrations of Fe and Mg were under control of wind sorting and post-depositional weathering-pedogenesis. The former caused Fe and Mg concentrated in the finer grain-size fractions, displaying synchronous variations, while the latter made Fe and Mg separated, leading to Fe retained in the weathered section and Mg leached out. Therefore, Fe/Mg ratios in the acid insoluble fraction of loess and paleosols can eliminate the effect of wind sorting and serve as an excellent proxy record on intensity of weathering-pedogenesis. Based on calculation, leaching percentage of Mg in the paleosol S1 from the Luochuan, Xifeng and Huanxian sections is 15%, 11% and 2%, respectively, and on average 9% for the paleosols S₂—S₁₄ from the Luochuan section, with the highest value amounting to 22% in S_5-1, suggesting the strongest weathering-pedogenesis.     

下辽河平原第四纪磁性地层的初步探讨

通过对下辽河地区古地磁样品分析。发现缺失松山反向时的磁性地层,即没有早更新世的沉积地层,讨论了此现象和郯庐大断裂的应力场的变化关系。古地磁分析对本区第四纪磁性地层的划分提供了依据。     

Clay minerals in paleosols of cretaceous age in Istria, Croatia

Greenish clays often occur as thin layers in Early Cretaceous shallow-water carbonate sediments of Istria where they mark the top of fining upwards sequences. When they show evidence of subaerial exposure and pedogenesis they are considered paleosols. The clay mineralogical composition is as follow: (1) randomly oriented I/S mixed layer, (2) regularly oriented I/S mixed layer, (3) illitic material, (4) kaolinite and (5) chlorite. The insoluble limestone residue situated immediately below the greenish clays is dominated by smectite, and also contains kaolinite and illitic material. The very high smectite content in the insoluble residue is probably the result of additions of volcanic dust during carbonate sedimentation. That clay mineral composition in the pelitic material clearly indicates the influence of both pedogenic and diagenetic processes. It may also indicate additional volcanic dust contribution before and during the subaerial exposure.     

Do multiple cycles of aeolian deposit-pedogenesis exist in the reticulate red clay sections in southern China?

Since the Quaternary scientists successfully dis-closed the paleoclimatic significance of the loess-paleo- sol sequence in the Loess Plateau, Northwest China, the loess-paleosol sequence has become well-known terres-trial materials revealing the multiple cyclic changes of Quaternary climates. As a carrier of paleoenvironmental information, it could compare beauty with the deep ocean deposit[8]. In recent years, however, the researchers began to put their sights to southern China. They try t…     

Variations in CaCO3 content and Sr isotopic composition of loess and records of paleoclimatic fluctuations

The carbonate Sr isotopic composition of the Heimugou loess-paleosol section in Luochuan of Shaanxi Province shows large variation, with the 87 Sr/86 Sr ratios ranging from 0.7104 to 0.7120. The carbonate in paleosol has in general higher 87Sr/86Sr than the carbonate in loess. In the loess-paleosol section, the Sr isotopic composition vary with an about 100 ka periodicity, corresponding to the change of global glacial cycle and comparable with the variation of Sr isotopic composition of paleosea water. This result indicates that the Sr isotopic composition of carbonate in loess can be an indicator of crustal weathering and paleoclimate change. The fluctuation in Sr isotopic composition of paleoseawater is mainly controlled by the continental riverine flux of Sr and its isotopic composition.     

Rock-magnetic investigation of Siberia loess and its implication

Multiple-rock magnetic investigations conducted on the loess-paleosol sequences at Kurtak in Southwestern Siberia reveal that the mass-normalized low-field magnetic susceptibility profiles reflect changes in lithology between relatively unweathered primary loess of glacial periods and the interglacial paleosols. Maxima in susceptibility values correspond with the least-weathered loess horizons, and minima with the humic horizons of soils. Frequency-dependent susceptibility of the loess-paleosol sequences at Kurtak is very low and practically uniform, indicating the dominance of non-SP ferrimagnetic minerals and negligible pedogenesis. The history of temperature-dependence of susceptibility (TDS) and stepwise acquisition of the isothermal remanent magnetization (SIRM) have confirmed that magnetite is predominant magnetic mineral, and only few maghemite and probably hematite are present within the studied section. Anisotropy of the magnetic susceptibility (AMS) can be used to monitor tilt and disturbance of the sedimentary layers, and also to provide information about the paleo-transport direction for Siberia loess.     

Chronology of the stratum containing the skull of the Dali Man

The field investigations of geomorphology and Quaternary sediment sequences show that the sediment stratigraphic profile containing the skull of the Homo sapiens Daliensis (Dali Man) at Tianshuigou trench, Jiefang Village (109° 44′E, 34°52′N), Dali County, Shaanxi Province, is composed of the loess-paleosol sequence, 17.3 m thick, in the upper part, and the alluvial silts, fine sands and sandy gravels interbedded, 15.4 m thick, in the lower part. There are the well developed layers L₁, S₁, L₂ and S₂ in the loess-paleosol sequence of the profile, which are comparable; to those of the Luochuan loess section on the basis of magnetic succeptibility measurements and IRSL dating of the loess samples. An alluvial gravel layer, where the skull of the Dali Man was discovered, is in 13 m beneath the bottom of paleosol S₂, implying that, the age of the Dali Man must be older than about 247 kaBP when the paleosol S₂ began to be developed. The samples of mammal animal tooth and shell fossils from the gravel layer containing the skull of the Dali Man have been dated by uranium series and ESR methods respectively. Based on the multiple dating results and the stratigraphic chronology, the age of the Dali Man, therefore, would be estimated as older than 250 kaBP and younger than 350 kaBP, and reasonably estimated as 260–300 kaBP.     

泥河湾盆地井儿洼剖面磁性地层学初步研究

泥河湾盆地井儿洼剖面古地磁测年结果全部为正向极性,剖面顶面下3.08m处白垩质粘土层铀系不平衡法测年结果为(50600±6700)aBP,据此确定其磁性地层学时代为布容正向极性时．剖面底部未出露B/M界限,说明地层时代晚于78万aBP.依据磁性地层学特征,井儿洼剖面可与虎头梁剖面对比,并能与红崖南沟、大道坡、郝家台等剖面的上部或顶部对比,属泥河湾组Ⅲ段．井儿洼、虎头梁剖面下部的石膏沉积晚于红崖、铺路等剖面下部的石膏沉积,它们分属不同的石膏沉积旋回.井儿洼剖面出露的湖相地层记录了古湖的消亡过程,是研究泥河湾古湖晚期历史的理想场所.     

历史时期北洛河流域的植被状况

为了探明历史时期北洛河流域的植被状况,采用文献资料法对此做了一定的研究。结果表明,该流域曾是森林草原植被景观,吴起、志丹位于森林草原区,甘泉、富县、洛川及黄陵位于落叶阔叶林区,但洛川塬在全新世时期是典型的草原景观。     

Stage characteristics for the development of late Quaternary paleosols in the Yangtze Delta area

The late Quaternary paleosols in the Yangtze Delta area developed in the period of 25 000-12 000 aBP. Phytolith ratios (value A) of contents of fan, square and rectangle types indicating the warm and humid climate to those of bar, hat and point types indicating cold and dry climates decrease upwards overall on profiles of the paleosols, suggesting that the climates turned from warm and humid to cold and dry. So, the paleosols developed mainly during the regression before the last glacial maximum. Changes of value A indicate no trend of warming, suggesting that the transgression was rapid after the last glacial maximum. The development of the paleosols exhibited obvious stages, which were controlled remarkably by transgression and regression : Ⅰ . Period of alternation of deposition and pedogenesis, which was from the occurrence of regresson before the last glacial maximum to the time when sea levels fell close to the lowest; Ⅱ . Exposure period with continual pedogenesis, which was terminated when the paleosols were drown in the post-glacial transgression; Ⅲ . Period of being drown and buried, the paleosols were buried by marine sediments, and reworked by early diagenesis.     

Holocene vegetation cover in Qin’an area of western Chinese Loess Plateau revealed by n-alkane

Previous studies have found that wetlands prevailed in western Chinese Loess Plateau and pine pollen could reach up to 80% in Qin＇an area of the plateau during middle Holocene. It was then deduced that forest vegetation covered Qin＇an area in a warm and wet climate during middle Holocene. The proxies of molecule biomarkers from two Holocene sections, a swamp-alluvial loess section and a typical Ioess-paleosol section are used to reconstruct regional vegetation history. It is found that the heavy-molecular-weight （HMW） homologues of the n-alkanes in all samples exhibit a pronounced odd-over-even predominance, maximizing at C31 and the abundance of the nC27-alkanes is the lowest in nC27, nC29, nC31, i.e. C27〈C29〈C31, a typical grass n-alkanes model. The results are the same as those of model surface soil samples under grass cover but different from both forest cover and modern pine leaves, which yield preferentially nC29-alkanes peak. It could be inferred that the area was dominated by grass cover but not forest cover during whole Holocene epoch. The study shows that combining both molecule biomarkers and pollen analysis can avoid the disadvantage of pollen analysis in reconstruction of regional vegetation cover.     

Variations of geochemical compositions and the paleoclimatic significance of a loess-soil sequence from Garzê County of western Sichuan Province, China

The West Sichuan Plateau is located in the southeast margin of the Tibetan Plateau, where the climate is mainly influenced by the Indian southwest summer monsoon and the Tibetan Plateau monsoon. In this study, detailed geochemical analysis has been carried out on Ganzisi loess-paleosol sequence in Ganzê County of western Sichuan Province. The results indicate that Ganzê loess and paleosol have experienced the incipient stage of chemical weathering in dust source regions, characterized by the decomposition of plagioclase which caused the depletion of mobile elements Na and Ca. The post-depositional chemical weathering is characterized by carbonate dissolution and oxidation of Fe2＋. The variations of some geochemical indexes （such as CIA values, Na/K and Fe2＋/ Fe3＋ ratios） in Ganzisi loess-paleosol sequence indicate a gradually decreased chemical weathering intensity in the dust source regions and deposition areas since 1.15 Ma BP consistent with the general increase of global ice volume, reflecting that the arid trend since 1.15 Ma BP in the southeast Tibetan Plateau is a regional response to the global climate change. The geochemical indexes in this section also reveal an obvious drying step occurred at about 250 ka BP in this region. We interpret this drying step as a result of decreased influence of the Indian southwest summer monsoon. This decrease in monsoon moisture is probably attributable to the uplift of the southeast margin of the Tibetan Plateau at about 250 ka BP.     

Paleoclimatic significance of mineral magnetic properties of loess sediments in northeastern Qinghai-Tibetan Plateau

Loess deposit in the northeastern Qinghai-Tibetan Plateau offers an excellent record of climate and environment changes in this region.We have conducted multiparameter mineral magnetic analyses of smaples of loess deposits from the Dongchuan,Lalakou and Panzishan sections.The methods used include magnetic susceptibility,fre-quency-dependent suceptibility,temperature-dependent susceptibility,isothermal remanent magnetization acquisibility.Maghemite and hematite is also present in the loess layers and paleosol horizons.The higher concentration of maghemite in paleosols suggests that the formation of maghemite occurred during in situ pedogenesis,which plays an important role in the enhancement of the magnetic susceptibitily.Similar to that in the Chinese Loess Plateau lying to the east of the Liupan Mountains,magnetic granulometry in the studied loess and paleosols is predominantly pseudo-single-domain(PSD). However,the mean grain-size of the ferrimagnetic minerals in loess is evidently coarser (larger PSD and multiomain-like) than in paleosols (mainly PSD).The magnetic susceptibility of loess and paleosols is positively correlated with the content of ultrafine magnet-ite/maghemite grains,and hence with the intensity of in situ pedogenesis.Therefore,the enhanced magnetic susceptibility in the studied loess-paleosol sequences can be interpreted as being due to climatically induced in situ pedogenesis.     

Magnetostratigraphy of borehole EY02-2 in the southern Yellow Sea and its paleoenvironmental significance

Various media are used for the study of paleoenvi- ronment. Among them, ice-core, loess and marine sediments are the most important for long time re- cording[1]. In all kinds of researches of paleoenviron- ment, age determination of the sediments is the m…     

Susceptibility variations of multiple origins of loess from the Ily Basin (NW China)

The magnetic susceptibility of loess from the Ily Basin,northwestern China shows maximum values in S0 paleosols but minimum values in other paleosols,the mechanism of which has been well debated.In this work,systematic magnetic measurements were made on a representative section from Neleke county.The results show that the loess horizons(L1,L2 and L3) have multi-domain magnetite grains of aeolian origin,S0 is characterized by production of pedogenetic ultrafine-grained ferrimagnetic minerals,and the other paleosols(S1,S2,and S3) are characterized by the formation of nonferrimagnetic minerals associated with waterlogging.The correlation between the low concentration of ferrimagnets,high paramagnetic content,high magnetic coercivity remanence,fine ferrimagnetic grain size and intensified pedogenesis suggest two competing processes of pedogenetic enhancement and pedogenetic depletion in the lower paleosols.Pedogenetic depletion dominates and is responsible for the low susceptibility.Changes in magnetic grain size distribution occur during pedogenetic depletion.The susceptibility variations are of multiple origins in the loess of the Ily Basin.Pedogenetic enhancement,pedogenetic depletion,and allochthonous input of magnetic minerals should all be taken into account to explain the variations of magnetic parameters.