甘青藏地区现代土壤中有机质类异戊二烯烃来源及地质意义

摘要

本文通过对海拔高度在2500～ 4500 m的甘肃南部和祁连山、青海东部、西藏东部林中表土、林间表土、森林上限表土、森林下限表土、森林上限高山草甸土,以及青海玉树冬给错纳地区亚高山荒漠—草甸土中可溶有机质正构烷烃分析,利用检测出的类异戊二烯烃的分布特征,对Pr(姥鲛烷)、Ph(植烷)的成因特征进行研究.结果表明,在不同地理环境土壤中均有明显的Pr、Ph存在,Pr/Ph(姥植比)主要为0.6～1.4,平均0.95;Pr/n-C17值主要为0.2 ～1.2,平均0.64;Ph/n-C18值主要为0.4～1.4,平均0.85.Pr和Ph不仅在水体中生成,在土壤环境中也可以形成.Pr/Ph值与不同生态环境土壤之间没有明确的相关性,但与不同地理区域有一定的联系.在较为温湿的土壤中Pr优势,可能与微生物活动强烈,采用有氧呼吸方式降解有机质有关;干旱、寒冷土壤中Ph优势,可能主要与厌氧发酵有关.由于生成Pr、Ph有机母质及演化途径的差异,以及热演化、成岩过程中的转变,其比值很大程度上影响对沉积盆地原始介质环境信息的判断.%Objectives:Organic isoprenoids pristine (Pr) and phytane (Ph) formed in the waterbodies under different redox condition,and the ratio of Pr/Ph so can be used to judge the redox characteristics of environments in the past researches.But whether or not they can form in the waterbodies only and how about the stability,there are disregardful.We want to determine Pr and Ph forming or not in the modem soil samples from different ecological environments to study the isoprenoids formation and characteristics,so to show the geological significance of Pr/Ph.Methods:The soluble organic n-alkanes in the modern surface soils from the mid-forests,inter-forests,upper limit forests,lower limit forests,upper limit alpine meadow,and Donggi Cona subalpine desert—meadow in the Gansu,Qinghai and Xizang(Tibet) had been analyzed in this paper.Isoprenoids had been determined based on the m/z85 from the total ion current,and the distribution characteristics and origin of Pr and Ph had been discussed.Results:It is shown that Pr and Ph save in the soils distributing in the different geographical environments not only formed in the waterbodies in the past researched.Pr/Ph ratio is in the range of 0.6 ～ 1.4,mean 0.95;Pr/n-C17 ratio is 0.2 ～ 1.2,mean 0.64;and Ph/ n-C18 is 0.4 ～ 1.4,mean 0.85.There is not a clear relative between Pr/Ph and the soils in the different ecological environments,but there is a mid degree relation between Pr/ Ph with geographical belts.Conclusions:Pr predominance formed by the microorganisms degrade organic matter under aerobic respiration in the relative warm and wet soils.And Ph predominance formed under the anaerobic fermentation in the relative dry and cold soils.Because different parent materials of Pr and Ph,and different sources from terrestrial and waterbodies,and maybe influenced by the forming of Pr and Ph in the process of thermal evolution and diagenesis,Pr/Ph palaeoenvironmental proxy must be testified to judge which kinds of environmental condition in the sediment basins.