古风力恢复研究进展:利用介质的搬运能力

摘要

古风力的恢复是古大气环流研究的重要组成部分,对古气候重建具有重要意义.但在古气候研究中,有关古风力的研究较少,原因是古大气流场活动遗留下来的有效信息极少,难以从地质记录中加以恢复.沉积物特征反映介质的搬运能力,据此可以为古风力的恢复提供思路.其中,古风力的定性恢复相对容易,替代性指标有地质记录中风尘沉积物的粒度、成分,以及风暴沉积的厚度等,有关古风力的定性恢复报道相对较多.比较而言,古风力的定量恢复难度大、报道少.通过文献调研,本文重点介绍两种古风力定量恢复的方法,旨在强调古风力定量恢复的可操作性,完善古气候研究的薄弱环节.(1)构成风成沙丘沉积物的搬运方式(即滚动、跳跃、悬浮)是颗粒属性(包括沉积物的密度、粒径等)和风力的函数,以沙丘为研究对象,可以计算风力;(2)风作用于水体会产生波浪,风浪关系可以通过经验公式表达出来,基于湖泊砾质滩的砾石粒度分析能够计算波浪的搬运能力,定量获得古波况信息,进而通过风浪关系定量恢复古风力.两种方法有各自的适用条件与优缺点,希望本文对这两种方法的介绍,能够为沉积学家在古气候恢复研究中提供借鉴,并在以后的研究中不断完善.%The paleowind strength is an important component in the study of paleo-atmospheric circulation,and is of great significance in the reconstruction of paleoclimate conditions.However,paleowind strength is less well studied in paleoclimatology,mainly because ancient wind has left little trace of its passing,making it difficult to reconstruct paleowind strength from geological records.Thankfully,transport capacities of fluid medium may be derived from sediment properties,which could provide clues for paleowind strength reconstruction.Qualitative restoration of paleowind strength is relatively easy,as particle size and composition of wind-blown sediments,and thickness of tempestites,directly reflect the wind power during the periods of sediment deposition.By comparison,quantitative restoration of paleowind strength is relatively difficult with scant research publications.In this paper,two independent methods,uncovered from our literature research,are introduced to illustrate how quantitative restoration of paleowind strength can be performed-a technique much needed in paleoclimate studies.Method 1:For eolian-dune constituting sediments,the modes of transport including creep,saltation and suspension,are functions of grain properties (such as grain size and density) and wind shear stress.Based on this principle,grain size analysis of eolian-dune sediments could be used to decipher paleowind strength.Method 2:Wind blowing over water can effectively transfer energy to form waves,and such process is governed by quantitative wind-wave relationships;and gravels deposited along shoreline by waves can record the critical wave power.Therefore,particle-size distribution analysis of gravel beach deposits could be used to reconstruct paleowave and subsequently paleowind conditions.Although both methods are limited by application conditions,we believe they are useful new tools in paleoclimate reconstruction and can be optimized.