湖泊沉积物C4植物含量重建方法的讨论——以新生代柴达木盆地大红沟剖面为例

Reconstruction of C4 plant content in Cenozoic lacustrine sediments: An example from the Dahonggou section, Qaidam Basin

关键词:  C₄植物  碳同位素  (古)土壤  成壤碳酸盐  牙釉质  植硅体  长链正构烷烃  单颗粒孢粉碳同位素

Key words:  C₄ plants  carbon isotopes  (fossil) soil  pedogenic carbonate  tooth enamel  phytolith  N-alkanes  carbon isotopes of the single pollen grain

摘要:

当前气候变暖是一个全球面临的重大问题，它对人类赖以生存的植被生态系统造成的影响已经在全球各地逐步显现出来。为了深入了解植物生态系统对环境和气候变化的响应机制，我们需要更好地借鉴地质历史时期气候环境和植物协同演化的重要事件。C₄植物作为陆地生产力较强的植物，在植物生态演化中占举足轻重的地位。目前研究表明C₄植物可能最晚起源于始新世-渐新世之交，但从它早期起源到随后在生态系统中的大规模扩张时间间隔长达20多个百万年。是什么因素导致了C₄植物的起源和扩张是一个悬而未决的重要问题，需要开展大量的调查研究来评估和重建C₄植物在过去生态系统中的相对生物量变化。重建C₄植物的含量目前主要的方法是建立在C₃/C₄植物碳同位素和植物内部结构形态差异基础之上。最常运用的研究材料包括(古)土壤有机质、成壤碳酸盐、陆地食草动物体组织、沉积物生物标志物、孢粉、植硅体等。这些方法在重建现代以及地质历史时期C₄植物相对生物量变化的研究中发挥了重要作用，但同时也存在很多无法避免的问题。本文介绍了C₄植物起源和扩张机制的主流观点以及不同研究材料的碳同位素所推算C₄植物生物量的基本原理，并以柴达木盆地大红沟剖面为例，针对新生代湖泊沉积物中陆生高等植物的长链正构烷烃特征和单体烃碳同位素的研究结果，详细讨论C₄植物含量重建的方法与缺点，为探讨C₄植物起源、演化及控制因素提供参考。我们通过分析前人研究的大红沟剖面长链正构烷烃及单体烃 δ¹³C_alk 值特征，推测在30~24 Ma、20~17 Ma和13~7 Ma期间δ¹³C_alk值显示相对正偏的原因，可能是干旱和C₄植物在当地生态系统中出现的双重因素叠加造成的。但这一推断还需要借助于新的研究方法，即单颗粒孢粉碳同位素的方法来提供C₄植物的确凿证据。

Abstract:

Global climate change has posed serious effect on the vegetation of our ecosystem. In order to understand the response of plant ecosystems to environmental and climate changes, we need to learn well from the earth’s past. C₄ plants play a pivotal role in the modern ecosystems. Previous studies have shown that C₄ plants originated prior to the Eocene-Oligocene transition, but the expansion of C₄ grasses had not happened until the Middle to Late Miocene. What caused the origin and expansion of C₄ plants is important in understanding the ecosystem evolution. The current method for estimating C₄ contribution is mainly based on the differences in C₃/C₄ plant carbon isotopes and their structures. The commonly used samples include (paleo-) soil organic matters, pedogenic carbonates, terrestrial herbivore tissues, biomarkers, pollen and phytolith. The application of isotopic difference in these different samples has played an important role in the reconstruction of the relative biomass of C₄ plants in modern ecosystems and geological records. This article summarizes the mainstream views related to the driving forces that triggered the origin and expansion of C₄ plants, and briefly introduces the basic principles of various materials archiving the biomass of C₄ plants evolution history. Finally, taking the Dahonggou section in the Cenozoic Qaidam Basin as an example, the methods and shortcomings of the reconstruction of C₄ plant content in lacustrine sediments are discussed in detail based on the long-chain n-alkanes and carbon isotope analysis of Cenozoic terrestrial higher plants. The δ¹³C_alk values of long-chain alkanes vary from~-30‰ to~-26‰, and the δ¹³C_alk values are positive. We believe that the double superposition effect of drought and C₄ plant expansion leads to the positive excursions of δ¹³C_alk values. The expansion of C₄ vegetation in the Cenozoic Qaidam Basin may be controlled by dry and wet conditions and climate change. But this hypothesis needs to be tested by carbon isotope analysis from single pollen grains.