北京地区15000年以来植被与环境变迁的初步研究

本文通过高时间分辨率的孢粉分析、14C测年、烧失量测定(600℃)及炭片统计分析，结合表土孢粉分析、古地磁特性等，对北京地区房山东甘池剖面（40.2ºN，116ºE)进行了较为系统的研究，从而揭示了距今15000年以来北京地区植被演替及环境变迁的关系，讨论了气候突变性事件及火因子对森林植被的影响等，同时借助于考古材料及历史地理文献对7000年以来人类活动导致植被的破坏及环境变化进行了讨论。 考虑到本剖面的特殊地貌单元，在分析流水地貌中花粉沉积和沉积物沉积的特殊性后，将北京地区15000年以来植被与环境变化划分为8个阶段： 1．约15780-14767aB.P.，属晚冰期的最盛期，乔木稀疏，草本类以蒿、藜为优势，缺乏反映隐域性环境的沼泽植被，气候冷干。 2．约14767-13482a B.P.，由于降雨增多，湿度加大，平原区形成小的沼泽环境，香蒲、眼子菜、黑三棱等沼生、水生植物得以在水体中生长繁盛，泥炭发育。由于山区生长的针阔叶混交林中盖度不大，并以针叶树种为主，反映气候以寒冷湿润为特征。此段中约14066aB.P.处出现的冷干环境与哥德堡漂移事件相对应，13520aB.P.，则反映出泥炭积累的暂时中断，为一干冷的气候突变点。 3．约13482-12625a B.P.，针阔叶混交林进一步发展，水生植物减少，沼泽逐渐消失，反映气候趋向暖干。 4．约12625-11418aB.P.，乔木减少，针阔叶混交林中针叶、阔叶出现优势交替现象。水生植物也有波动，反映气候趋于干凉，并有冷暖的波动。环境中湿度变化明显。 5．约11418-9607aB.P.，植被稀疏，气候呈现进一步干冷的特征。其中在 10619-10318aB.P.，孢粉及炭片浓度、年沉积率和烧失量均很低，出现了约相当于新仙女木期的冷干气候特征。 6. 9607—7270aB.P.，落叶阔叶树种有所增加，但其盖度不大。水生植物不繁盛，反映由草原植被向森林草原的演替。气候特征由冷向温干转变。 7、7270-3395aB.P.，阔叶树种明显增多，盖度增大，可视为温暖时期的特征，在5771aB.P.和4563aB.P.，曾出现乔木树种的下降，可视为高温期的短暂冷期。 8. 3395-lOOOaB.P.，气候变干变凉，加之人类活动的加剧，导致植被覆盖度的下降。考古学及史料记载，反映出这段发生了多次冷暖干湿交替出现的情形，加之人类对森林的砍伐、焚烧和拓荒，因而使北京山区的森林植被逐渐发生逆向演替、成为现存的次生植被类型。

Characterization of alluvial formation by stochastic modelling of paleo-fluvial processes: The concept and method

Modelling fluvial processes is an effective way to reproduce basin evolution and to recreate riverbed morphology. However, due to the complexity of alluvial environments, deterministic modelling of fluvial processes is often impossible. To address the related uncertainties, we derive a stochastic fluvial process model on the basis of the convective Exner equation that uses the statistics (mean and variance) of river velocity as input parameters. These statistics allow for quantifying the uncertainty in riverbed topography, river discharge and position of the river channel. In order to couple the velocity statistics and the fluvial process model, the perturbation method is employed with a non-stationary spectral approach to develop the Exner equation as two separate equations: the first one is the mean equation, which yields the mean sediment thickness, and the second one is the perturbation equation, which yields the variance of sediment thickness. The resulting solutions offer an effective tool to characterize alluvial aquifers resulting from fluvial processes, which allows incorporating the stochasticity of the paleoflow velocity.

Formation and preservation of pedogenic carbonates in South India, links with paleo-monsoon and pedological conditions: Clues from Sr isotopes, U-Th series and REEs

The influence of the pedogenic and climatic contexts on the formation and preservation of pedogenic carbonates in a climosequence in the Western Ghats (Karnataka Plateau, South West India) has been studied. Along the climosequence, the current mean annual rainfall (MAR) varies within a 80 km transect from 6000 mm at the edge of the Plateau to 500 mm inland. Pedogenic carbonates occur in the MAR range of 500-1200 mm. In the semi-arid zone (MAR: 500-900 mm), carbonates occur (i) as rhick hardpan calcretes on pediment slopes and (ii) as nodular horizons in polygenic black soils (i.e. vertisols). In the sub-humid zone (MAR: 900-1500 mm), pedogenic carbonates are disseminated in the black soil matrices either as loose, irregular and friable nodules of millimetric size or as indurated botryoidal nodules of centimetric to pluricentimetric size. They also occur at the top layers of the saprolite either as disseminated pluricentimetric indurated nodules or carbonate-cemented lumps of centimetric to decimetric size. Chemical and isotopic (Sr-87/Sr-86) compositions of the carbonate fraction were determined after leaching with 0.25 N HCl. The corresponding residual fractions containing both primary minerals and authigenic clays were digested separately and analyzed. The trend defined by the Sr-87/Sr-86 signatures of both labile carbonate fractions and corresponding residual fractions indicates that a part of the labile carbonate fraction is genetically linked to the local soil composition. Considering the residual fraction of each sample as the most likely lithogenic source of Ca in carbonates, it is estimated that from 24% to 82% (55% on average) of Ca is derived from local bedrock weathering, leading to a consumption of an equivalent proportion of atmospheric CO2. These values indicate that climatic conditions were humid enough to allow silicate weathering: MAR at the time of carbonate formation likely ranged from 400 to 700 mm, which is 2- to 3-fold less than the current MAR at these locations. The Sr, U and Mg contents and the (U-234/U-238) activity ratio in the labile carbonate fraction help to understand the conditions of carbonate formation. The relatively high concentrations of Sr, U and Mg in black soil carbonates may indicate fast growth and accumulation compared to carbonates in saprolite, possibly due to a better confinement of the pore waters which is supported by their high (U-234/U-238) signatures, and/or to higher content of dissolved carbonates in the pore waters. The occurrence of Ce, Mn and Fe oxides in the cracks of carbonate reflects the existence of relatively humid periods after carbonate formation. The carbonate ages determined by the U-Th method range from 1.33 +/- 0.84 kyr to 7.5 +/- 2.7 kyr and to a cluster of five ages around 20 kyr, i.e. the Last Glacial Maximum period. The young occurrences are only located in the black soils, which therefore constitute sensitive environments for trapping and retaining atmospheric CO2 even on short time scales. The maximum age of carbonates depends on their location in the climatic gradient: from about 20 kyr for centimetric nodules at Mule Hole (MAR = 1100 mm/yr) to 200 kyr for the calcrete at Gundlupet (MAR = 700 mm/yr, Durand et al., 2007). The intensity of rainfall during wet periods would indeed control the lifetime of pedogenic carbonates and thus the duration of inorganic carbon storage in soils. (C) 2010 Elsevier Ltd. All rights reserved.

Sheltered coastal environments as archives of paleo-tsunami deposits: Observations from the 2004 Indian Ocean tsunami

The 2004 earthquake left several traces of coseismic land deformation and tsunami deposits, both on the islands along the plate boundary and distant shores of the Indian Ocean rim countries. Researchers are now exploring these sites to develop a chronology of past events. Where the coastal regions are also inundated by storm surges, there is an additional challenge to discriminate between the deposits formed by these two processes. Paleo-tsunami research relies largely on finding deposits where preservation potential is high and storm surge origin can be excluded. During the past decade of our work along the Andaman and Nicobar Islands and the east coast of India, we have observed that the 2004 tsunami deposits are best preserved in lagoons, inland streams and also on elevated terraces. Chronological evidence for older events obtained from such sites is better correlated with those from Thailand, Sri Lanka and Indonesia, reiterating their usefulness in tsunami geology studies. (C) 2014 Elsevier Ltd. All rights reserved.

India-Madagascar paleo-fit based on flexural isostasy of their rifted margins

The present study contributes new constraints on, and definitions of, the reconstructed plate margins of India and Madagascar based on flexural isostasy along the Western Continental Margin of India (WCMI) and the Eastern Continental Margin of Madagascar (ECMM). We have estimated the nature of isostasy and crustal geometry along the two margins, and have examined their possible conjugate structure. Here we utilize elastic thickness (Te) and Moho depth data as the primary basis for the correlation of these passive margins. We employ the flexure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elastic thickness. Gravity inversion and flexure inversion techniques are used to estimate the configuration of the Moho/Crust-Mantle Interface that reveals regional correlations with the elastic thickness variations. These results correlate well with the continental and oceanic segments of the Indian and African plates. The present study has found a linear zone of anomalously low-Te (1-5 km) along the WCMI (similar to 1680 km), which correlates well with the low-Te patterns obtained all along the ECMM. We suggest that the low-Te zones along the WCMI and ECMM represent paleo-rift inception points of lithosphere thermally and mechanically weakened by the combined effects of the Marion hotspot and lithospheric extension due to rifting. We have produced an India-Madagascar paleo-fit representing the initial phase of separation based on the Te estimates of the rifted conjugate margins, which is confirmed by a close-fit correlation of Moho geometry and bathymetry of the shelf margins. The matching of tectonic lineaments, lithologies and geochronological belts between India and Madagascar provide an additional support for the present plate reconstruction. (C) 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Sea surface temperature and paleo-El Niño events in Santa Barbara Basin, AD 1841-1941

Like pages of a "natural coastal diary", successive layers of anoxic varved sediment in the central Santa Barbara Basin have been used by paleoceanographers to reconstruct aspects of past coastal climate. This report focuses on the end of the "Little Ice Age" (15th to 19th century) and on the beginning of this century, a period known to encompass extreme climate excursions and weather events in the Santa Barbara Basin and other parts of Southern California. El Niño events are known to disrupt Southern California's coastal ecosystems and to cause anomalous weather conditions, but El Niño events in Southern California before 1990 have been largely undocumented.

Distinctive Paleo-Indian Migration Routes from Beringia Marked by Two Rare mtDNA Haplogroups

Background: It is widely accepted that the ancestors of Native Americans arrived in the New World via Beringia approximately 10 to 30 thousand years ago (kya). However, the arrival time(s), number of expansion events, and migration routes into the Western

The effects of the last glacial paleo-aeolian sands on desertification in northern China

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Use of Zr/Rb ratios in Chinese loess sequences to trace paleo-winter monsoon winds strength

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