若尔盖高原湖沼沉积剖面饱和烃组分特征及地球化学意义

Geochemical Significance of the Saturated Hydrocarbons of Limnological Sediments from Ruoergai Plateau

利用GC—MS技术对若尔盖高原一湖沼沉积剖面(深346 cm)沉积有机质的饱和烃馏分进行了分析,结合已有的AMS14C年代、剖面岩性和TOC资料,探讨了该剖面分子化石分布特征及其相关参数对有机物来源、微生物活动和气候变化等环境信息的指示意义。分析表明,中高碳数占绝对优势的正构烷烃反映了有机物主要来源于高等植物,且草本高等植物输入丰富;岩相、TOC和Pr/Ph比值很好地指示了该剖面沉积物从早全新世(约11.4～7.3 cal kaBP,深度346～203 cm)强还原深湖条件到中全新世(约7.3～4.5 cal ka BP,>203 cm)弱还原沼泽条件的演化过程,正构烷烃参数奇偶优势(OEP1和OEP2)、C25/C31和轻烃/重烃比值(∑C21-/∑C22+)详细反映了这种水文演变过程中生物源的变化,尤其剖面120 cm以上很可能有泥炭藓(Sphagnum)的重要输入,而且敏感地指示了剖面120 cm以上有机物遭受了明显的微生物改造作用,从而引起正构烷烃组分的改变。另外,在约9.8～8.6 cal ka BP(深度286～240cm)期间剖面岩性及其以上各项代用指标发生明显快速变化,说明早全新世若尔盖高原气候相对暖湿阶段具有不稳定性。

Biomarker n-alkanes have been demonstrated as a useful tool in paleolimnological reconstruction since different types of plants have distinctive carbon chain lengths of n-alkanes.Peat deposits from Ruoergai Plateau are excellent archives for Holocene paleoenvironmental records.For example,Hong et al.（2003） successfully reconstructed paleoclimatic changes over the last 12 ka based on δ13C series of both Carex mulieensis（Carex m.） remains cellulose and peat cellulose from Hongyuan peat sequence.Zheng et al.（2007a） and Yamamoto et al.（2010） investigated the lipid compositions and n-alkane δ13C values of another Hongyuan peat profile over the 13 ka,respectively.However,Holocene paleoenvironmental information has been not obtained from lacustrine sediments there.Therefore,there is a need for additional records of environmental change to improve the accuracy of organic geochemical tools in paleoecological reconstruction.In this work,we carry out preliminary research on the saturated hydrocarbons of a 346-cm-long limnological profile from Northern Ruoergai Plateau by gas chromatography and mass spectrometry（GC-MS）,together with 14C dating,lithology and total organic carbon content（TOC） to evaluate the paleoenvironmental information of molecular fossil compositions and their parameters to organic sources,microbial activity and paleoclimate changes.The n-alkanes（C16~C35） dominated by the middle-long chain homologues and high OEP1 and OEP2 values indicate that the organic matter of the sedimentary section are mainly derived from higher plants,especially herbaceous plants.Lithology,TOC and pristane/phytane（Pr/Ph） ratios indicate the basin sedimentary evolution process from strong reducing deep lake conditions in the early Holocene（ca.11.3~7.3 cal ka BP,346~203cm） to weak reducing swamp environments during the mid-Holocene（ca.7.3~4.5 cal ka BP,203cm）.In the sedimentary period,variations in the proportions of algal,land-plant,and aquatic macrophyte inputs of organic matter delivered to the sedimentary profile is well recorded through four n-alkanes proxies including middle-chain and long-chain odd over even carbon number predominance（OEP1 and OEP2,respectively）,C25/C31 and ∑C21-/∑C22＋ n-alkane ratios.In particularly,there is likely important input of Sphagnum to the organic matter in the 120cm depth upwards resulting in the exceptional distribution of these above proxies.In the same sedimentary period,there also is relatively strong microbial degradation resulting to alter the n-alkane compositions.Furthermore,the records of the section confirm that Ruoergai Plateau have apparent climatic instability during the period from 9.8 to 8.6 cal ka BP.Of course,there is a clear need to do more work not only on the profile but also on modern vegetation lipid compositions from Ruoergai Plateau in the future exploitation of molecular proxies as tools for the generation of palaeoenvironmental information from the wealth of limnological profiles available.