Search

The aeolian deposits in the coastal region of North China are important for reconstructing paleoclimate. In this paper, chronology, climatic proxies and chemical compositions of four loess sections were studied. Optically stimulated luminescence (OSL) dating results indicate that the coastal loess was accumulated during late Quaternary, with ages ranging from the penultimate glacial period to the latest Holocene. Both granulometric characteristics and geochemical properties indicate that the dust provenance of the coastal loess is partially different from that of the Chinese Loess Plateau, characterized by more proximal contributions. Except distal dust transported by northwesterly winter winds from the inland gobi and sand deserts, the coastal loess also contains proximal, local coarse particles, which were derived from the exposed shelf as well as neighboring hills. Multiple climatic proxies, including color, magnetic susceptibility and particle size parameters, correlate well with the marine SPECMAP δ18O record, indicating that climatic changes in the coastal region of North China during the late Quaternary was mainly controlled by variations of the East Asian monsoon, driven by changes in global ice volumes. • Luminescence dating of coastal loess in eastern North China. • Coastal loess contain both distal and proximal source materials. • Exposed shelf during glacial time serving as an important source for coastal loess. • Costal loess reflects orbital climatic fluctuations driven by East Asian monsoon. [ABSTRACT FROM AUTHOR]

Loess is a product of aeolian deposition during Quaternary glaciation cycles. Loess–paleosol sequences are rich in calcareous nodules (CNs). In the literature, two models are widely cited for the formation of CNs, namely “per descendum” and “per ascendum”. However, there has been no direct testing or monitoring to support either of these contradictory models. This paper reviews a large number of multidisciplinary literature to evaluate the consistency, reliability and rationality of these two models. Three main conclusions are drawn: (1) the causative factors (variation of pH value along loess–paleosol sequence, decrease of CO 2 partial pressure, and reduction of solvent water) that are used to support the per descendum model do not completely explain the supersaturation of infiltration solution with CaCO 3 , thereby making this model questionable; (2) the per ascendum model explains the formation of CNs along narrow horizons through upward evaporation; however, it fails to produce sporadic distributions and irregular shapes of nodules on loess slope faces and the frequent appearance of nodules around plant roots. In order to reconcile these deficiencies, we conducted an extensive field survey in various parts of Shanxi province. Based on this new set of observations, it was concluded that the “per ascendum” model can be extended to explain all occurrences of CNs. This extended model is called “per evapotranspiration”. [ABSTRACT FROM AUTHOR]

The Ordovician-Silurian (O S) transition witnessed dramatic tectonic, climatic, marine, and biological coevolution, during which organic-rich black shale was widely deposited and is considered an important source rock. However, the potential links between the differential enrichment pattern of organic matter and climate fluctuations and paleoenvironmental evolution during this period are still unclear. In this paper, the Late Ordovician-early Silurian black shale in a JieLong (JL) section in South China was identified by graptolitic fossils, total organic carbon (TOC) content, X-ray diffraction (XRD), scanning electron microscopy (SEM) and geochemical analysis. The results show that the provenances of sedimentary rocks are all granodiorite-like. According to the variations in the TOC content and geochemical indicators, four layers are delineated. Under warm and humid climatic conditions, sea level rise and unstable seasonal upwelling led to Unit 1 organic matter enrichment controlled by high primary productivity, anoxic conditions and effective smectite production. Unit 2 organic matter burial was controlled by cold, arid and oxic environmental conditions, but primary productivity was still relatively high. After the end of the glacial period, warm and humid climatic conditions and rapid transgression were the main factors affecting the burial of organic matter in Unit 3. Under hot and humid climate conditions, higher terrigenous detrital inputs and unfavourable preservation environments, the dilution of organic matter in Unit 4 was accelerated. The corrected chemical index of alteration (CIA corr) values show that the study area experienced a climate change of alternating cold and warm conditions in response to the climate fluctuation during the glacial-interglacial period. The CIA corr value is negatively correlated with the TOC content, while the generation of available clay minerals (smectite) in warm and humid climates is usually consistent with the enhanced burial flux of organic matter, supporting the important role of climate fluctuations in the accumulation of organic matter. Finally, a comparison of the data from the present study with geochemical data from different profiles in South China reveals the role of climatic fluctuations during the O S transition in controlling paleoenvironmental evolution and organic matter enrichment, while such a heterogeneous factor is usually associated with global or regional geological events. Our study highlights this differential enrichment mechanism as a combined response of multiple factors and provides important implications for glacial-interglacial organic matter enrichment mechanisms. • Rapid changes of weathering indices imply climatic fluctuations during the Late Ordovician-Early Silurian. • Four different patterns were established for organic matter accumulation across the O/S. • Climate fluctuations and the heterogeneity nature of ocean systems explain the regional variations of OM accumulation. [ABSTRACT FROM AUTHOR]

Abstract: In the regions surrounding the Qinghai Tibet Plateau, changes in erosion rates have been linked to the tectonics, climate and topography over different time scales. To understand the mechanisms governing the changes of erosion rates, it is important to study erosion rates by different methods and for different time scales. In inland drainage basins, deposition rates of terminal lake sediments can indicate basin-wide erosion rates at the millennial-scale. This paper presents three lake records of the Last Deglaciation and Holocene (Huahai Lake, Zhuye Lake and Yanchi Lake) from the Hexi Corridor, north of the Qilian Mountains, in arid China. Organic matter, terrestrial pollen concentrates, seeds, grasses and plant debris are used for conventional and AMS 14C dating. On the basis of 66 radiocarbon dates, lithology and grain-size, we infer relatively high basin-wide erosion rates during the Last Deglaciation and early Holocene in the three drainage basins, when the three lake sediments were seriously affected by reworking. The deposition rates were an order of magnitude or greater in these lakes during the Last Deglaciation and early Holocene than during the mid-to-late Holocene. During the transition period of the last glacial–interglacial cycle, significant climatic changes occurred in East and Central Asia, corresponding to the strengthening of the Asian summer monsoon and to increasing effective moisture in arid Central Asia, which can have strong impacts on basin-wide erosion rates north of the Qinghai Tibet Plateau. Moreover, melting glaciers in the Qilian Mountains probably also contributed to the high basin-wide erosion rates. At the same time, tectonic activity was not recognizable in the study area during that period. In the arid and semiarid regions surrounding the Qinghai Tibet Plateau, dramatic changes in erosion rates appear during the transition periods of the glacial–interglacial cycles, which illustrate the climatic controls on erosion rates at this time scale. [Copyright &y& Elsevier]