斜度驱动第四纪冰期-间冰期转换——来自中国黄土的证据

第四纪气候冰期-间冰期旋回与地球轨道周期性变化之间的关系是古气候研究的重点。文章利用中国黄土沉积的粒度和次生碳酸盐微量元素记录,在同一剖面中同时获得了冰量信息和地球轨道斜度信息。黄土的平均粒径反映了冬季风的强度从而指示了北半球高纬冰盖的变化,而次生碳酸盐（生物微钙体）的Sr/Ca比值记录了东亚夏季降水的变化,反映了斜度对东亚夏季风的控制作用。通过分析冰消期时粒度快速变化所处的夏季降水指示的斜度变化的相对位置,发现过去1.5 Ma以来的冰消期总在地球轨道处于较高斜度时发生,从而验证了斜度对第四纪冰期-间冰期旋回的驱动作用。

     

Chronology of late Pleistocene Pampa loess from the Córdoba area in Argentina

In Argentina, loess/palaeosol sequences were studied from the two sections at Monte Ralo and Corralito, located near to the city of Córdoba in the east of the Pampean ranges. Infrared optically stimulated luminescence (IRSL) and thermoluminescence (TL) dating techniques were applied to set up a more reliable chronological framework for the Upper Pleistocene loess record. IRSL ages increase with depth and are stratigraphically consistent. The oldest loess exposed correlates at least to the penultimate glaciation. The luminescence dating study gives evidence for humid climate conditions with soil formation during the last interglacial period and the middle pleniglacial correlating to Marine Isotope Stage (MIS) 5 and 3, respectively. Three extended periods of soil formation are recorded in the loess/palaeosol sequences at the Corralito section. These buried soils most likely correlate with the last interglacial and early last glacial period. During the Middle Pleniglacial, two palaeosols are intercalated in the loess successions. In the Córdoba area, the periods of increased loess accumulation and deposition of loessic sediments correlate with more arid and glacial periods during the Early Pleniglacial, the Middle Pleniglacial and the Late Glacial. Copyright © 2009 John Wiley & Sons, Ltd.     

Late pleistocene and holocene loess deposits of the Southeastern Buenos Aires province,Argentina

The Late Pleistocene and Holocene loess deposits of the SE Buenos Aires province are composed of four allostratigraphic units that represent four episodes of loess deposition. The first and the second episodes occurred in Late Pleistocene times. The second episode was followed by a soil forming interval (Early Holocene to Mid-Holocene times). The third episode took place at about 5000 yr BP, after the Holocene sea-level maximum when marine regression began. The fourth episode constitutes a historical event of only local significance.Loess shows a fairly constant granulometric and mineralogical composition. The modal fraction consists of very fine sand and coarse silt (3 to 5 phi). They are classified as sandy silts or silty sands. Three grain-size subpopulations are differentiated: coarse, medium and fine. The medium-size subpopulation, which is the most important, consists of most of the very fine sand and coarse silt. It is thought to be transported by modified saltation and short-term suspension during local dust-storms.The mineralogical composition of loess consists of a volcaniclastic assemblage derived mainly from reworked pyroclastic deposits, primary tephras and volcaniclastic sediments. The source area of these materials was located in the lower Colorado river valley about 400 km SW of the studied area. There was also a direct supply by volcanic ash falls.     

Properties of magnetic mineralogy of Alaskan loess: evidence for pedogenesis

The in situ pedogenic enhancement of ferrimagnetic content provides the well-established patterns of magnetic susceptibility variation within mid- to low-latitude loess deposits such as those of China and Central Europe. However, this pattern of high magnetic susceptibility in palaeosols, and lower values in unweathered loess, is not replicated in the higher-latitude loess deposits of Alaska and Siberia. In these localities the relationship is inverted, with high values in loess, and low values in palaeosols. This inverse relationship has been explained by the idea that magnetic susceptibility is reflecting the magnitude of an aeolian ferrimagnetic component of consistent mineralogy, the grain size of which is related to average wind velocity. However, the results of the magnetic study presented in this paper suggest that there are differences in magnetic properties between Alaskan loess and palaeosols, not only in magnetic grain size and concentration but also in magnetic mineralogy. This complicates the simple hypothesis of a `wind velocity’ signal by introducing an additional factor into the climatic signal. In contrast to the enhancement of susceptibility observed in palaeosols of the Loess Plateau, China, we suggest that the low magnetic susceptibility values in the Alaskan palaeosol units are a reflection, at least in part, of the alteration of the ferrimagnetic content by post-depositional processes associated with waterlogging (i.e. gleying) of the soils.     

Pleistocene chemical weathering history of Asian arid and semi-arid regions recorded in loess deposits of China and Tajikistan

Quaternary loess-soil sequences in China and Tajikistan are valuable archives of the chemical weathering history for arid and semi-arid regions of Asia. The development of new proxies independent of grain size are vital to reconstruct the long-term chemical weathering history of eolian deposits. In this study, we analyze major elemental and Rb, Sr, and Ba concentrations of decarbonated residue from representative loess and soil units along a north-south transect on the Chinese Loess Plateau and compare these concentrations among different grain-size fractions. Results show that most of the elemental abundances and ratios vary considerably among different grain-size fractions for both loess and paleosols, indicating transport-driven compositional differentiation of minerals during subaerial transport. However, (CaO + Na₂O + MgO)/TiO₂ ratios show little variation among different size fractions for the loess-soil transect. Loess-soil sequences at Lingtai (northern China) and Chashmanigar (southern Tajikistan) for the past 1.50-1.77 Ma show lower (CaO + Na₂O + MgO)/TiO₂ ratios in soil units than in adjacent loess horizons, in good agreement with greater weathering intensity in soils than in loess units. The (CaO + Na₂O + MgO)/TiO₂ ratio of the decarbonated residue is therefore a reliable proxy for chemical weathering of loess deposits that is independent of grain size. The (CaO + Na₂O+MgO)/TiO₂ ratios of the Lingtai and Chashmanigar sections display a relatively regular oscillation within a narrow range in the early Pleistocene, and a rapid increase in average values and in variance from 0.85-0.6 Ma to the present, indicating lowered chemical weathering intensity in both the dust source regions and the depositional areas. This event may be causally related to the expansion of northern hemisphere ice and/or the regional tectonic uplift of high mountains in Asia since the mid-Pleistocene.     

A valley crossing in Pleistocene deposits

Sauer, E. Karl, 1977. A valley crossing in Pleistocene deposits. Eng. Geol., 11:1–21.Proglacial meltwater channels are of considerable importance in geotechnical engineering in Southern Saskatchewan. Dams, roads, bridges and urban development are major projects involving these channels. The Arm River crossing illustrates the types of problems that can occur in highways constructed across these channels, including instability, groundwater control and excavation through a boulder pavement. This discussion demonstrates how these problems can be resolved by establishing the geology of the sites and designing to preclude geotechnical problems. The investigation involved rotary drilling, geophysical logging, installation of piezometers and engineering analysis.     

阿根廷金矿床时空分布规律

阿根廷金矿资源丰富,西部是安第斯金成矿带的重要组成部分,勘查开发和利用程度较低,具有较大的找矿和开发潜力。根据区域构造演化和金矿成矿规律,梳理了阿根廷金成矿地质背景,厘定了金成矿作用与构造-岩浆事件的关系。将阿根廷金成矿时代划分为4个集中时期,阐述了各成矿期的主要金矿床类型,分析了其空间分布规律,划分出5个成矿带和15个矿集区,总结了各矿集区的主控矿因素,为进一步确定阿根廷金矿资源潜力和找矿方向奠定了基础。     

武夷山紫色土磁学特征及其对成壤的指示意义

紫色土是非地带性土壤, 具有磁性本底值低的特征, 是研究成土初始阶段土壤磁性矿物与成土因素关系的良好材料。目前, 国内紫色土系统的环境磁学研究处于起步阶段。本研究以福建省武夷山市的紫色土剖面(简称WYS剖面, 地理坐标为27°44'24'N, 118°00'36'E; 海拔216.3m)为分析对象。该剖面发育于白垩系紫红色粉砂质泥岩之上, 厚度为0.9m。通过系统的环境磁学研究, 结合漫反射光谱一阶导数曲线、色度指标和常量地球化学元素指标, 分析其磁学特征的层次分异, 探讨磁学参数对成壤的指示意义。结果表明: 1)WYS剖面以反铁磁性矿物赤铁矿为主。A层的亚铁磁性矿物磁铁矿含量略有增加, 超顺磁(SP)颗粒含量更高, 磁颗粒相对更细。成壤作用是造成A层和C层的磁学特征差异的主导因素。2)WYS剖面母岩具有易于物理风化、不易化学风化的特征。由于成土时间短, 即便在湿热气候条件下, 母质中磁性矿物转化微弱。母质和成土时间是影响紫色土磁学性质的主导因素。3)成壤过程中, 少部分赤铁矿转化为磁铁矿。磁学参数的微小变化响应于弱成壤作用, 表明紫色土磁学特征可以非常敏感地指示成壤强度。

     

延安晚更新世黄土颗粒微观各向异性特征及其成因分析

土颗粒的形状及其空间方位的择优取向是导致宏观土体各向异性的重要原因之一.以具有明显各向异性的晚更新世马兰黄土为研究对象,借助Quanta FEG型电子显微镜扫描照片所包含的相关信息,揭示其微结构在水平向和垂直向上的差异,并对该微观结构的各向异性成因进行了分析.发现:(1)水平向和垂直向切片中颗粒的圆形度R0具有正态分布...     

A study of Late Pleistocene loess deposits,South Canterbury,New Zealand: Part II. Paleosols and their stratigraphic implications

Examination of loess columns in coastal South Canterbury, New Zealand, and the recognition of paleosols with comparable morphology to surface soils allowed the division of the loess column into six members. The upper loess members 1–4 are grouped into a proposed Dashing Rocks Formation. This commonly overlies an erosion surface on Timaru Basalt, but in some situations is underlain by loess member 5 and mixed loess and weathered basalt member 6. The paleosol developed on loess member 5, on both morphological and chemical evidence, is indicative of a period of soil formation of longer duration or greater intensity than is indicated by the overlying paleosols or surface soils. Therefore members 5 and 6 are grouped into a separate formation.A radiocarbon chronology for loess members 1 and 2 of the Dashing Rocks Formation, suggests loess accumulation phases from 9900 to 11,800 and prior to 31,000 radiocarbon yr BP, followed by periods of soil formation and contemporary peat development in surface depressions.A hypothesis is presented suggesting that at least loess members 1 and 2 of the Dashing Rocks Formation accumulated during periods of glacial recession which in turn precipitated fluvial and eolian erosion, transport and redeposition of fluvioglacial deposits. The periods of soil development indicated by the paleosols were initiated during warm interstadial conditions and continued throughout the cooling of the following stadial. Such an interpretation has its parallels in the northern hemisphere but is in slight disagreement with previous glacial and fluvioglacial chronologies accepted in New Zealand.     

The evaporitic celestite-barite deposits of Neuquen,Argentina

Celestite-barite occurrences of early Cretaceous age were studied in the Extra-Andean region of Neuquén, West central Argentina. The ores are located in a Subandean Mesozoic belt and show a conspicuous stratabound character related to evaporitic facies. The study of the geologic evolution of the sedimentary sequence allows prediction of the temporal and spatial setting of the evaporitic facies. Stratigraphy is the main geologic control for the celestite-barite ores and their areal distribution depends on the structure of the region. Based on the geometry and texture of the mineralization three types of ores have been recognized: 1) celestite beds syngenetic with the evaporitic facies; 2) epigenetic stratabound ores formed by "in situ" mobilization of celestite with stalactite growth and cavity infilling; 3) epigenetic vein-type ores formed by mobilized barium enriched solutions.Lithofacies and isotopic ⁸⁷Sr/⁸⁶Sr analyses together with the geological evidence confirm an evaporitic origin for the celestite beds.     

High Resolution Element Records in the Late Pleistocene Xifeng Loess Profile

Eighty-one samples were selected from the Late Pleistocene Xifeng loess profile and analyzed for their element abundances using instrumental neutron activation analysis,and for ^10Be using the AMS method.The results show that the variations of element abundance and ^10Be with dept are similar to those of δ^18O in DSDP ,and may be considered as an indicator of climatic and environmental changes.     

Sources and genesis of the Pleistocene gravelly deposits in Northern Kuwait

The Dibdibba Formation is of Pleistocene age and consists of a fluviatile sequence of ungraded, often cross-bedded sand and gravels intercalated with minor sandy clay layers. It occurs in the northern desert of Kuwait as subparallel elongated ridges trending NE-SW. Structures and mineralogy of the Dibdibba deposits suggest that they have been derived from the basement complex of the northeastern area of Saudi Arabia as sheet floods and deposited in the alluvial fan of the Wadi Ar-Rimmah-Al Batin valley system. Floods transporting these deposits were intermittent; the gravelly bands were deposited during severe floods and the sandy beds during relatively quiet floods.     

Climatic implications of loess deposits from the Beijing region

Variations in magnetostratigraphy, pedostratigraphy, grain size and magnetic susceptibility of the loess deposits near Beijing have been studied at two sections. The sections are about 400 km east of the main loess deposits in China, have a maximum thickness of 100 m and extend back to 1.1 Ma. The sequence consists of 14 loess–palaeosol couplets (S0‐S14), which correlate well with sequences in the Loess Plateau. Susceptibility records from the sites near Beijing are comparable to the Xifeng, Luochuan and Baoji sections located in the middle part of the Loess Plateau; however, the down‐core variations in the grain size in the Upper Lishi Formation exhibit some differences. The median grain size increases by about 25–30 µm from L4 to L2, with the sandy grains (>63 µm) increasing from 10–20 wt% to 40–50 wt% . This implies that the depositional environment of the dust sources in the Beijing loess section is different in some aspects from the Loess Plateau. The Beijing loess may have had a different dust source than the Loess Plateau. Copyright © 2001 John Wiley & Sons, Ltd.     

The nature and provenance of Golestan loess deposits in northeast Iran

Mineralogical, textural, geochemical, and weathering characteristics of loess deposits in Golestan province of Iran suggest that they are mostly derived from felsic igneous rocks and are related to Quaternary palaeoclimate. Whole‐rock analyses indicate heavy minerals such as zircon, tourmaline and phyllosillicate minerals (e.g. muscovite, chlorite, illite) exert a significant control on the chemical composition. The loess samples display uniform chemical composition, indicative of similar alteration history. Chemical index of alteration suggests a weak to moderate degree of weathering in a felsic source area. Scanning electron micrographs of quartz grains reveal abundant silt‐sized quartz particles, a result of glacial grinding during the Late Pleistocene in Central Asia. Subsequently, these silt particles were transported from Central Asia to their depositional site by wind and paraglacial processes. Local topography of northeast Iran (Alborz Mountains) acted as a major barrier, entrapping the airborne particles on the plains of Golestan province. Copyright © 2012 John Wiley & Sons, Ltd.     

Climatostratigraphy of Upper Pleistocene Vertebrates and the conditions of loess formation in Hungary

Having revised some reference vertebrate faunas from Hungary and from Central Europe their biostratigraphic positions could be determined more accurately. By extending the vole thermometer method to the continental parts of Central Europe fairly well identifiable average July temperature curves could be obtained for the Upper Pleistocene. The main events shown by the climatic curve for Hungary can be well correlated with the soil and loess complexes of the Paks loess profile, and also with the global climatostratigraphic scale.     

New remains of Nothrotheriinae (Mammalia,Xenarthra) from Late Pleistocene fluvial deposits of Northern Pampa (Santa Fe Province,Argentina)

In this contribution the record of a Nothrotheriinae (Xenarthra, Tardigrada) ground sloth is reported from the Late Pleistocene of the Northern Pampa of Santa Fe Province, Argentina. The stratigraphic unit where the fossil was collected corresponds to fluvial-palustrine sediments of the Timbúes Formation, outcropping along the Carcarañá River valley. The relative stratigraphic position of this lithostratigraphic unit, observed in several sections mainly on the Paraná River cliffs near Carcarañá River distal area, suggests that it was deposited during the Last Interglacial Stage. The specimen (MPAHND-135), assigned to Nothrotherium cf. torresi, is represented by the proximal two-thirds of a left femur that shows a prominent lesser trochanter and no connection between the third trochanter and the ectepicondyle. The presence of Nothrotherium cf. torresi in sediments related to humid conditions supports the idea that Nothrotheriinae had great ecological tolerance and was capable of inhabiting climates ranging from cold and arid, as was previously proposed, to warm and humid.