孢粉、炭屑揭示的黔西高原MIS3b期间古植被、古气候演变

Abstract

将孢粉与炭屑结合应用于贵州盘县坪地晚更新世剖面,对比高分辨率的NGRIPδ¹⁸O、石笋记录分析,用以揭示黔西高原MIS3b期古植被、古气候演变。结果显示:45.16-44.44 ka BP期间,孢粉缺失,炭屑少,说明此时植被稀少,气候寒冷,可能指示H₅事件。44.44-44.04 ka BP期间,以禾本科、百合科为主的草本及喜冷湿的蕨类占优势,乔、灌木稀少。炭屑浓度低,但末期出现峰值,说明气候开始由冷湿向温干过渡。44.04-42.5 ka BP期间,禾本科草类继续增加,成为草原植物的主要成分;乔木种类增加,但含量不高。炭屑浓度高,峰值频繁出现,指示气侯温干。42.5-41.29 ka BP期间,草原萎缩,森林面积扩展。针叶、常绿及落叶阔叶林迅速扩展。炭屑浓度总体偏低稳定,指示该时期气候温凉湿润。总之,MIS3b整体较冷,干湿交替出现。;Climate and vegetation changes during marine isotope stage 3b (MIS3b) are a major focus of Quaternary research. The vegetation on the western Guizhou plateau, part of the Yungui Plateau in China, is highly sensitive to climate changes and thus represents a potential environmental archive.<br> We obtained a 13-m-long sediment sequence from Pan County of Guizhou Province and analyzed the range corresponding to 45-41 ka(10-12.85 m). Pollen date, charcoal concentration, and four accelerator mass spectroscopy (AMS) ¹⁴C dates (only three are available) were obtained. We used the three AMS¹⁴C dates (without calibration) to establish an age model and compared pollen and charcoal data with oxygen isotope records from ice cores from North Greenland and the Tibetan plateau and with a stalagmite from Hulu Cave, China. The results indicated that pollen zone A (12.85-12.50 m, 45.16-44.44 ka) had sparse vegetation and a cold climate, as reflected by the low pollen and charcoal concentration. This period may correspond to the H5 event. Pollen zone B (12.50-12.2 m, 44.44-44.04 ka) was characterized by pollen of Gramineae, Cupressaceae, Liliaceae, and Davalliaceae and acted as a transient period. Herbs and ferns were dominant in this zone, indicating that the temperature was warmer than in zone A, but still cold. The charcoal concentration in zone B was low, but a peak implied fire activity (fire suggests dry conditions). Pollen of these taxa, especially Gramineae, herbs, and ferns, increased significantly in abundance in zone C (12.20-11.15 m, 44.04-42.5 ka). Pollen of trees and shrubs, including <em>Pinus</em>,<em> Quercus</em>,<em> Abies</em>,<em> Ulmus</em>,<em> Salix</em>,<em> Corylus</em>, and<em> Betula</em>, were increasing in abundance, but their percentages were still low. Frequent peaks of micro-charcoal concentration indicated regional fire activity. Hence, the climate was warmer and drier in zone C than in zone B. The composition of the ecosystem changed substantially in pollen zone D (11.15-10.45 m, 42.5-41.29 ka). Trees and shrubs continued to increase in abundance compared to zone C and became major components of the ecosystem; in particular, pine proliferated rapidly and became the dominant tree type. Coniferous, evergreen, and deciduous broad-leaved forests expanded quickly as grasslands diminished in area. The low and stable charcoal concentration indicated cool and wet conditions during this period. To summarize, this area during MIS3b was generally cold and alternated frequently between wet and dry conditions.<br> Comparing our data to other climate vectors, we found that climate changes on the western Guizhou plateau during MIS3b corresponded extremely well to indices from loess and lake sediments and to δ¹⁸O in ice cores from the Tibetan plateau and North Greenland. Thus, Guizhou sediment sequences can reflect regional and even global climate changes. We inferred that the low temperatures and high precipitation during MIS3b were caused by the solar precession cycle, which decreased solar radiation and allowed the polar ice sheet to expand southward, helping cool the global climate. As the globe cooled, the southwest Asia monsoon strengthened, bringing heavy precipitation and creating large lakes in southwest China. With limited records, the historical patterns of the southwest Asia monsoon as well as global circulation remain unclear, so further research is needed.