Evaluation of oxygen isotopes in carbonate as an indicator of lake evolution in arid areas: The modern Qinghai Lake, Qinghai–Tibet Plateau

The oxygen isotopic composition of carbonate in lakes has been used as a useful indicator in Palaeolimnological research, and has made some important contributions to our understanding of lacustrine systems. For modern lakes in arid or cold areas, however, there are few data available to test the effect of lake salinity and temperature on the oxygen isotopic composition of various carbonate sources such as ostracod, bulk carbonate, and fine-grained carbonate (< 60 μm). Here we examined the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonates, as well as that of coexisting water from Lake Qinghai and the smaller surrounding lakes and ponds on the Qinghai–Tibet Plateau. Our investigation highlights three key effects. First, the oxygen isotopic composition of ostracods, bulk carbonate, and fine-grained carbonate in the lakes and ponds shows a clear response to lake water δ¹⁸O values, and these vary with water salinity. The relationship between lake water δ¹⁸O and salinity is not only dominated by the evaporation/freshwater input ratios, but is also controlled by the distance to the mouth of the major rivers supplying to the lake. Second, the ostracod, bulk carbonate, and fine-grained carbonate show similar isotopic change trends in the study area, and oxygen isotopic differences between ostracods and authigenic carbonate may be explained by the different water temperatures and very small ‘vital offsets’ of ostracods. Finally, the effect of water depth on temperature leads to increasing δ¹⁸O values in carbonates as water depth increases, both in benthic ostracods living on the lake bottom, as well as in bulk carbonate precipitated at the water surface.For arid, high-altitude Lake Qinghai, our results suggest that variations in the δ¹⁸O values of carbonate in Lake Qinghai are mainly controlled by the oxygen-isotope ratio of the lake water changing with water salinity. As a secondary effect, increasing water depth leads to cooler bottom and surface water, which may result in more positive δ¹⁸O values of ostracod and bulk carbonate.     

洞穴滴水、石笋中元素及元素比值对气候环境变化响应的研究进展

滴水/石笋元素是除δ18O和δ13C,研究气候环境变化的又一重要代用指标。外界气候环境变化通过改变表层岩溶带和岩溶含水层中的水文环境,甚至洞穴环境,进而影响元素的溶解、运移和沉淀过程,从而使得滴水/石笋中元素表现一定的变化规律。本文通过分析影响洞穴滴水元素及元素比值变化的因素：元素来源、水—岩相互作用和滞留时间、差异性淋滤、先期碳酸盐沉淀及分配系数的基础上,从元素对岩溶区“二元结构”和极端天气/气候事件响应的角度,探讨了滴水/石笋中元素的气候环境指示意义。取得了以下认识：① 强降水带来的冲刷作用和溶解作用,促进土壤和基岩中元素、胶体和天然有机质（NOM：Natural Organic Matter）等物质在短时间内快速溶解和运移,使滴水中元素含量增加;但随着降水增多带来的稀释作用,使得滴水/石笋中Mg,Sr和Ba等元素含量降低,因此,单一元素的解译较为复杂;② 基岩/溶液中元素溶解和沉积的差异,导致元素相对含量的变化,使得元素X/Ca值对外界环境的响应具有一致性,尤其是Mg/Ca和Sr/Ca值：在干旱条件下,降水减少导致方解石先期沉积（PCP：Prior Calcite precipitation）作用增强,使Mg/Ca和Sr/Ca值增大。但目前存在着一些问题：① Mg/Ca和Sr/Ca值变化对强降水事件的响应并不明显,可能与新、老水混合及元素溶解过程中的溶解比例差别不大有关;② 多源、多期混合水源会导致洞穴滴水元素对极端事件响应减弱;③ Mg/Ca和Sr/Ca的变化为解释δ18O的“雨量效应”及“源效应”提供了见解,但元素变化能否反映季风强度的变化,仍有待进一步的研究。基于以上认识,笔者提出开展更加系统的大气—土壤—包气带—洞穴的监测;开展更高分辨率、更长时间尺度的洞穴监测;开展多区域、多洞穴系统对比研究来更加深入地开展洞穴石笋元素研究。     

Thrombolite fabrics and origins: Influences of diverse microbial and metazoan processes on Cambrian thrombolite variability in the Great Basin,California and Nevada

Thrombolites are a common component of carbonate buildups throughout the Phanerozoic. Although they are usually described as microbialites with an internally clotted texture, a wide range of thrombolite textures have been observed and attributed to diverse processes, leading to difficulty interpreting thrombolites as a group. Interpreting thrombolitic textures in terms of ancient ecosystems requires understanding of diverse processes, specifically those due to microbial growth and metazoan activity. Many of these processes are reflected in thrombolites in the Cambrian Carrara, Bonanza King, Highland Peak and Nopah formations, Great Basin, California, USA; they comprise eight thrombolite classes based on variable arrangements and combinations of depositional and diagenetic components. Four thrombolite classes (hemispherical microdigitate, bushy, coalescent columnar and massive fenestrated) contain distinct mesoscale microbial growth structures that can be distinguished from surrounding detrital sediments and diagenetic features. By contrast, mottled thrombolites have mesostructures that dominantly reflect post‐depositional processes, including bioturbation. Mottled thrombolites are not bioturbated stromatolites, but rather formed from disruption of an originally clotted growth structure. Three thrombolite classes (arborescent digitate, amoeboid and massive) contain more cryptic textures. All eight of the thrombolite classes in this study formed in similar Cambrian depositional environments (marine passive margin). Overall, this suite of thrombolites demonstrates that thrombolites are diverse, in both internal fabrics and origin, and that clotted and patchy microbialite fabrics form from a range of processes. The diversity of textures and their origins demonstrate that thrombolites should not be used to interpret a particular ecological, evolutionary or environmental shift without first identifying the microbial growth structure and distinguishing it from other depositional, post‐depositional and diagenetic components. Furthermore, thrombolites are fundamentally different from stromatolites and dendrolites in which the laminae and dendroids reflect a primary growth structure, because clotted textures in thrombolites do not always reflect a primary microbial growth structure.     

Larger Foraminiferal Biostratigraphy and Facies Analysis of the Oligocene–Miocene Asmari Formation in the Western Fars Sub-basin, Zagros Mountains, Iran

The Oligocene–Miocene carbonate record of the Zagros Mountains, known as the Asmari Formation, constitutes an important hydrocarbon reservoir in southern Iran. This marine carbonate succession, which developed under tropical conditions, is explored in terms of larger foraminiferal biostratigraphy, facies analysis and sequence stratigraphy in a new section at Papoon cropping out in the western Fars sub-basin, in the south-east of the Zagros belt. Facies analysis shows evidence of re-working and transport of skeletal components throughout the depositional system, interpreted here as a carbonate ramp. The foraminifera-based biozones identified include the Globigerina–Turborotalia cerroazulensis–Hantkenina Zone and Nummulites vascus–Nummulites fichteli Zone, both of Rupelian age, the Archaias asmaricus–Archaias hensoni–Miogypsinoides complanatus Zone of Chattian age and the ‘Indeterminate’ Zone of Aquitanian age. The vertical sedimentary evolution of the formation exhibits a progressive shallowing of the facies belts and thus the succession is interpreted as a high-rank low-order regressive systems tract. This long-lasting Rupelian–Aquitanian regressive event is in accordance with accepted global long-term eustatic curves. Accordingly, long-term eustatic trends would have been a factor controlling accommodation during the deposition of the Asmari Formation studied in the western Fars sub-basin.     

Theoretical analyses of acidization dissolution front instability in fluid‐saturated carbonate rocks

This paper presents an instability theory that can be used to understand the fundamental behavior of an acidization dissolution front when it propagates in fluid‐saturated carbonate rocks. The proposed theory includes two fundamental concepts, namely the intrinsic time and length of an acidization dissolution system, and a theoretical criterion that involves the comparison of the Zhao number and its critical value of the acidization dissolution system. The intrinsic time is used to determine the time scale at which the acidization dissolution front is formed, while the intrinsic length is used to determine the length scale at which the instability of the acidization dissolution front can be initiated. Under the assumption that the acidization dissolution reaction is a fast process, the critical Zhao number, which is used to assess the instability likelihood of an acidization dissolution front propagating in fluid‐saturated carbonate rocks, has been derived in a strictly mathematical manner. Based on the proposed instability theory of a propagating acidization dissolution front, it has been theoretically recognized that: (i) the increase of the mineral dissolution ratio can stabilize the acidization dissolution front in fluid‐saturated carbonate rocks; (ii) the increase of the final porosity of the carbonate rock can destabilize the acidization dissolution front, while the increase of the initial porosity can stabilize the acidization dissolution front in fluid‐saturated carbonate rocks; (iii) the increase of the mineral dissolution ratio can cause an increase in the dimensionless propagation speed of the acidization dissolution front; (iv) the increase of the initial porosity can enable the acidization dissolution front to propagate faster, while the increase of the final porosity can enable the acidization dissolution front to propagate slower in the acidization dissolution system. Copyright © 2012 John Wiley & Sons, Ltd.     

脑动脉粥样硬化斑块中钙化物矿物学特征研究

脑动脉钙化与脑动脉系统病变有密切联系。本文从矿物学角度出发,使用偏光显微镜(POM),扫描电子显微镜(SEM)、透射电子显微镜(TEM)、显微红外光谱(micro-FTIR)、同步辐射微区X射线衍射(μ-SRXRD)以及同步辐射微区X射线荧光光谱(μ-SRXRF)对脑动脉粥样硬化斑块中的钙化样品进行了研究。结果表明,钙化主要有球状和块状两种集合体形态,球状钙化具有同心环状或同心放射状构造,直径约0.3~5μm;块状钙化中可见球状钙化被杂乱分布的柱状纳米晶体包裹。钙化主要矿物相为B型碳羟磷灰石(B-CHA),并含Na、Mg、Zn、Fe和Sr等元素,与主动脉钙化中的矿物组成类似。钙化在其形成初期呈分散的小球状,后期大量球体逐渐被CHA纳米晶体充填胶结形成了致密的块状钙化。     

柴达木盆地SG-1孔1.0Ma以来碳酸盐同位素记录的亚洲内陆干旱化及成因

亚洲内陆干旱区是连接赤道和中高纬地区的过渡地带,也是西风气候和季风气候的相互作用区,其形成演化与青藏高原的隆起和全球变化等因素密切相关.因此,揭示其干旱化过程和趋势具有重大的理论和现实意义.文章对来自柴达木盆地察汗斯拉图干盐湖的深钻SG-1孔沉积物进行了初步的碳酸盐碳氧同位素测试与分析,结果清晰地指示了柴达木盆地自1.0Ma以来的持续干旱化及约0.6Ma以来的加速干旱化过程,我们认为1.0Ma以来的持续干旱化可能是全球气候变化和构造活动共同作用的结果,而约0.6Ma以来的加速干旱化可能是昆黄运动所导致的高原北部强烈隆升和区域环流系统变化或强化所造成的.     

Study on the Isothermal Evaporation Process at 5℃ with the Brine of Laguocuo Salt Lake

正1 Introduction Tibet has nurmerous salt lakes.Laguocuo is one of the salt lakes,which is located to the sorthern of Ali Plateau,31°59′02″N-32°04′08″N,84°02′03″E-84°12′03″E.Its lake water is rich in potassium,magnesium,lithium,boron,rubidium,cesium and other resources.The study of     

Characteristics and Genetic Analysis of the Deep-buried Weathered-crust Karst Hydrocarbon Reservoirs of the Lower Paleozoic Group in the Tarim Basin

The genetic analysis of the deep-buried reservoirs of the Lower Paleozoic carbonate rocks in the Tarim basin is a difficult task involving many factors. Firstly, the object of study is carbonate rocks, which have undergone a long term of modification. Secondly, the rocks are deeply buried with depths of 3800-7000 m in the Tarim basin. The primary reservoir properties formed in the deposition have been strongly modified during the deep burial process. Concurrently, the different burial depths in different areas result in diversities of burial temperature, pressure, underground water, hydrochemistry and various physicochemical changes, which further lead to differences in the diagenetic type, diagenetic property, diagenetic degree and their impacts on the reservoir properties. The Lower Paleozoic Cambrian and Ordovician carbonate reservoirs in the Tarim basin can be grouped into four types, i.e., paleo-weathered-crust reservoirs, reef reservoirs, buried karst reservoirs and dolomite reservoirs. This paper