摘要
晚新生代C_4植物扩张是地球环境-生态系统演化的重要事件,重建C_4植物地质演化历史是认识新生代地球气候变化、全球植被演化和大气CO_2演化历史及区域陆地生态系统转变模式的关键环节。通过最近20多年来对全球不同区域地质剖面中的植物化石、动物牙釉质及成土碳酸盐碳同位素等的分析,研究者认识到晚新生代C_4植物在全球成规模出现和扩张过程并不同时,不同记录反映的C_4扩张信号有所不同,目前对C_4植物扩张的全球格局还不明晰。亚洲季风区C_4植物扩张历史是理解全球C_4植物地质演化过程和机制的重要方面。南亚地质记录揭示的C_4植物扩张过程表现为8～4Ma前后的迁跃式转折,黄土高原已有研究揭示在约6Ma,3Ma前后及1Ma之后各有一个δ^(13)C_(SC)高值出现,在7～4Ma期间成土碳酸盐δ^(13)C_(SC)呈现自北向南变轻的格局,表明在晚中新世到上新世早期黄土高原C_4植物出现从北到南的扩张过程。对于黄土高原C_4植物在更早时期的演化扩张过程,目前限于记录尚不明晰。在此,本文通过分析六盘山邻区的平凉白水黄土-红粘土和庄浪红粘土剖面,获得成土碳酸盐碳同位素记录,揭示出C_4植物在黄土高原扩张时间至少可以下推到中新世早期(约20Ma),C_4植物比率在中新世变化于15%～40%之间。这一结果为拓展我们对C_4植物在黄土高原乃至东亚季风区早期演化历史的认识提供了新证据。
The global expansion of C4 plants during the Late Cenozoic is an important event in the Earth environmentalecosystem evolution. Reconstructing the geological history of C4 plants holds a key in the understanding of the patterns and mechanisms of the Ceonzoic climate change, global floras evolution and the long-term variations in atmospheric CO2. During the last two decades, records of plant macrofossil, carbon isotopes of fossil tooth enamel and pedogenic carbonate from different sites in the world have revealed that the timing of C4 plants origin and expansion was not globally synchronous, and different proxy records documented different timing features of C4 expansion. The history of C4 plants in Asia is an important component in reconstructing the globally geographical patterns of C4 expansion. Records from South Asia indicate Ca plants expanded abruptly during ca. 8 - 4Ma, and several localities in the Chinese Loess Plateau document a δ13Cac peak at ca. 6Ma,3Ma and from 1 Ma, respectively. Previous studies have demonstrated that during ca. 7 - 4Ma, soil carbonate records （δ13Cac） in the Chinese Loess Plateau display a southern-ward negative trend,indicative of a southern-ward expansion of C4 plants during the Late Miocene and the Early Pliocene. It is remained unclear, however, that if the C4 plants expansion over the Chinese Loess Plateau occurred earlier than during the Late Miocene. Here we report a study based on δ13Cac of loess-red clay sequence from the Baishui and Zhuanglang sections near the Liupanshan region, and the result shows that C4 plants expansion may have initiated as early as at ca. 20Ma with C4 component up to 15% -40% during the Miocene. The Baishui section（35°24′10″N,106°54′43″E） is located in the southeastern part of the Liupanshan region,Central Loess Plateau. This section contains an upper 214m loess-paleosol sequence and a lower 82m red clay deposit. The Zhuanglang section （35°13′34″N, 106°4′11″E） lies at the southwestern part of the Liupanshan region,ca. 80km to the Baishui section,and consists of 183.4m red clay deposit. A total 2729 paleomagnetic samples （ 1026 samples for the Baishui section, 1703 for the Zhuanglang section） were analyzed by using thermal demagnetization at the Laboratory of Plaeomagnetism of the Institute of Geology and Geophysics,Chinese Academy of Sciences and at Lanzhou University. The specimens that exhibit stable component two vectors（ 91% for the Baishui section and 83.5% for the Zhuanglang section）were used to establish the magnetostratigraphic column. The observed magnetostratigraphic column can be correlated with the geomagnetic polarity timescale（GPTS）and the base of the Baishui section is placed at about 6. 5Ma while the Zhuanglang section is delimited between 9. 3 - 20. 0Ma. We carbonate samples（ 166 samples for the have analyzed the oxygen and carbon isotope Baishui section and 146 for the Zhuanglang compositions of 312 pedogenic section）. The results show that δ13Cac records from the Baishui section exhibit a similar variation pattern with the previously published results of the loess-red clay sequences（such as the Lingtai,Xifeng and Lantian sections）,and δ13Cac records from the Zhuanglang section reveal an early（ca. 20Ma）appearance of C4 ecosystem,with C4 plants composing up to 15% -40% of the plant community in the Chinese Loess Plateau. This result adds a new line of evidence for promoting our understanding of C4 plant emergence and expansion history over the Chinese Loess Plateau.
出处
《第四纪研究》
CAS
CSCD
北大核心
2011年第5期800-811,共12页
Quaternary Sciences
基金
国家重点基础研究发展规划项目(973项目)(批准号:2010CB950203)
国家自然科学基金项目(批准号:40972226)资助
关键词
C4植物
成土碳酸盐
碳同位素
中新世
季风
干旱化
C4 plant, pedogenic carbonate, carbon isotope ratio, Miocene, monsoon, aridification
