Evidence of reworking and resuspension of carbonates during last glacial maximum and early deglacial period along the southwest coast of India

A gravity core collected from the upper slope of southwest of Quilon at a water depth of 776 m (Lat: 8°12′263″N, Long: 76°28′281″E) was analysed for texture (carbonate free), calcium carbonate and organic carbon. Variation in silicic fraction seems to be controlled by silt, i.e., enrichment from 15 ka BP to 10 ka BP and then constant in Holocene. Below 15 ka BP, the silicic fraction gets depleted compared to the Holocene section with a minimum around 21 ka BP. Clay content remains nearly constant except in the Holocene where it shows an enrichment. Carbonate content of less than 63 micron when computed by subtracting coarse fraction content from the total carbonate suggests that the total carbonates are mainly concentrated in the finer fraction. All these carbonate phases show an inverse relationship with silicic fraction except in Holocene. Below 15 ka BP, CaCO₃ dominates in sediments comprising more than 65%, such an increase is also seen in the coarse fraction. Coarse fraction from these sections contains abundant nodular type aggregates encrusting small forams. This period is marked by a high sedimentation rate comparable to Holocene. These parameters suggest that the productivity and precipitation have increased in the Holocene due to the intensification of the southwest monsoon. During the last glacial maximum and early deglacial period the high sedimentation rate indicates redeposition of the carbonates from the existing carbonate lithofacies situated between Quilon and Cape Comorin probably due to the slope instability.     

冰芯记录中末次间冰期-冰期旋回气候突变事件的研究进展

冰芯记录中的气候变化是古气候研究中的重要组成部分. 极地、中低纬度和热带地区的冰芯记录表明, 在冰期间冰期旋回大尺度气候变化背景下, 全球经历了一系列数百年至千年时间尺度的快速气候突变事件, 诸如末次间冰期的干冷事件、末次冰期的DansgaardOeschger事件、 Heinrich事件和Younger Dryas事件等, 虽然这些穿插在冰阶中的暖湿气候事件、间冰阶中的干冷气候事件的成因、机制和影响范围还存在明显的不确定性. 主要介绍不同区域冰芯记录中末次间冰期冰期旋回这些气候突变事件发生的时间、过程和机制等的研究进展.     

Rates of Deglaciation during the Last Glaciation and Holocene in the Cordillera Vilcanota-Quelccaya Ice Cap Region, Southeastern Perú

Moraine chronology is combined with digital topography to model deglacial rates of paleoglacier volumes in both the Huancané Valley on the west side of the Quelccaya Ice Cap and the Upismayo Valley on the northwest side of the Cordillera Vilcanota. The fastest rates of deglaciation (39×10⁻⁵ to 114×10⁻⁵ km³ yr⁻¹ and 112×10⁻⁵ to 247×10⁻⁵ km³ yr⁻¹ for each valley, respectively) were calculated for the most recent paleoglaciers, corresponding to the last few centuries. These results are consistent with observations in the Venezuelan Andes showing high rates of deglaciation since the Little Ice Age. These rates also fall within the range of 20th century rates of deglaciation measured on the Quelccaya Ice Cap (29×10⁻⁵ to 220×10⁻⁵ km³ yr⁻¹, Brecher and Thompson, 1993; Thompson, 2000). These results imply that rates of deglaciation may fluctuate significantly over time and that high rates of deglaciation may not be exclusive to the late 20th century. Equilibrium line altitude (ELA) depressions for the ice volumes of the last glaciation modeled here were computed as 230 m for the Quelccaya Ice Cap and 170 m for the Cordillera Vilcanota. Maximum ELA depressions are lower than previously published: <500 m for the Cordillera Vilcanota and <400 m for the Quelccaya Ice Cap. These lower values could imply a topographic control over paleoglacier extent.     

The significance of rock structure,lithology and pre‐glacial deep weathering for the shape of intermediate‐scale glacial erosional landforms

The main landforms within the glacially scoured Precambrian rocks of the Swedish west coast are closely connected to the principal structural pattern and have lately been explained as mainly a result of etch processes, probably during the Mesozoic and with a possible second period of etching during the Neogene. To explore the effect of multiple glacial erosion on the rock surfaces, an island with two different lithologies and with striae from different directions was selected for a detailed study, focusing on the shape of roches moutonnées. Air‐photo interpretation of bedrock lineaments and roches moutonnées combined with detailed field mapping and striae measurements are used to interpret the structural and lithological control on the resulting shape. The study reveals a significant difference in shape between roches moutonnées in augen‐granite and orthogneiss. Low elongated and streamlined roches moutonnées occur in the gneiss area, striated by a Late Weichselian ice flow from the NE. This ice flow is subparallel with both the local dominant trend of topographically well‐expressed joints and the schistosity of the gneiss. Frequently, there are no signs of quarrying on the lee‐sides of the gneiss roches moutonnées and hence they resemble the shape of whalebacks, or ruwares, as typically associated with the exposed basal weathering surface found in tropical areas. The granite roches moutonnées were formed by an older ice flow from the ESE, which closely followed the etched WNW–ESE joint system of the granite. Late Weichselian ice flow from the NE caused only minor changes of the landforms. On the contrary, marks of the early ESE ice flow are poorly preserved in the gneiss area, where it probably never had any large effect as the flow was perpendicular to both schistosity and structures and, accordingly, also to the pre‐glacial relief. The study demonstrates that coincidence between ice flow direction and pre‐glacially etched structures is most likely to determine the effects of glacial erosion. Copyright © 2002 John Wiley & Sons, Ltd.     

两个冰期-间冰期旋回的黄土记录及其古气候意义

北京邻区枣沟、斋堂和赤峰黄土－古土壤剖面粒度和磁化率变化在幅度和相位上都有明显的差异。在两个冰期－间冰期旋回，磁化率记录具有冰量变化模式，而粒度记录具有轨道周期特点，粒度和磁化率之间的相位关系在从冰期向间冰期转换和冰期向间冰期过渡的过程中，既有滞后也有超前的表现，很难单纯以成土作用来解释。由于黄土粒度主要反映粉尘源区环境和古风场强度的变化，而磁化率主要记录的是沉积区的成壤作用，反映夏季风的强度变化，因此剖面粒度和磁化率变化的不协调说明古风场和粉尘源区变化与粉尘沉积区气候变化之间不统一。这可能意味着控制夏季风强度变化和控制冬季风强度变化的机制存在差异。     

辽南滨海黄土粒度环境信息高分辨率研究

对辽南滨海黄土粒度研究证明 ,滨海黄土是在第四纪冰期寒冷气候下的产物 ,但是 ,由于滨海黄土所处靠近沿海的地理位置使得这种黄土的形成机制具有独特性。冰期时渤海湾西岸海面下降出露的冲洪积物和海底砂是滨海黄土除西北内陆地区以外的另一重要物源 ,搬运以风砂跃移方式进行。滨海黄土的粒度中包涵了丰富的有关渤海湾海面变化的信息。辽南滨海黄土剖面中的粒度拟合值、 2 - 3Ф粒级百分含量反映了渤海的海退、海侵过程和末次冰期蒙古高压的强弱 ,这种变化与滨海黄土 3个孢粉组合带所反映的古气候环境相吻合 ,同邻近地区的海面升降变化和深海岩芯氧同位素阶段也存在良好的对应关系 ,滨海黄土粒度的拟合度是本区古环境变化的重要的替代指标。     

灌溉对甘肃中部黄土台塬区地面塌陷及生态环境影响研究

甘肃中部黄土台塬地面塌陷实质上是由水利灌溉引起的新的生态环境问题。文章在深入调查、勘测和试验研究的基础上，综合分析了地面塌陷变形的成因机制，认为黄土台塬地面塌陷变形除了黄土母质理化性质和人类活动等因素外，主要是不合理灌溉引起的。过量大水漫灌使大量渗入的水体将土壤中起固结作用的盐分和粘粒矿物淋溶殆尽，使原始结构破坏，进而崩解、湿陷。随下渗水量的增加，犁底层及以下隔水层孔洞进一步发展为洞穴和陷缝，大面积塌陷变形随之出现，同时也成为台塬滑坡的诱导因素。研究为黄土台塬灌区突出的水利环境问题治理和生态重建方案提供了基础依据。