青藏高原东北缘武山盆地中中新世炭屑记录及其古气候意义

doi:10.12118/j.issn.1000-6060.2021.279

摘要/Abstract

摘要：

火是生态系统中的重要因子,是反映古气候和环境变化的重要标志。因此,重建火活动历史可以帮助我们理解过去的气候变化和火活动的机制,但是目前在全球范围内十分缺少对中新世时期高分辨率的火活动记录的研究。炭屑已被证明是重建火活动历史的有效替代性指标,基于青藏高原东北缘武山盆地中中新世时期高分辨率的炭屑记录,重建了研究区天然火活动历史,结合现有资料,探讨了火-植被-气候之间的关系以及研究区火活动对全球变化的响应。结果表明：（1） 15.30~13.60 Ma时期,炭屑总浓度变化范围为59~4324粒·g^-1,平均浓度为835粒·g^-1。炭屑形状以次圆形为主,且几乎所有的炭屑粒径都小于50 μm,反映出研究区天然火活动是以乔木植物燃烧的森林火活动为主,主要是区域性火活动。根据炭屑总浓度的变化趋势,将研究区天然火活动历史分为2个主要阶段。阶段Ⅰ（15.30~14.00 Ma）：炭屑总浓度逐步增加,平均浓度为866粒·g^-1。其中,阶段Ⅰ又可以细分为3个次要阶段,Ⅰa（15.30~14.38 Ma）：炭屑总浓度最低,平均浓度为693粒·g^-1;Ⅰb（14.38~14.20 Ma）：炭屑总浓度快速减少,平均浓度为1140粒·g^-1;Ⅰc（14.20~14.00 Ma）：炭屑总浓度急剧增加,平均浓度为988粒·g^-1。阶段Ⅱ（14.00~13.60 Ma）：炭屑总浓度急剧减少,平均浓度为777粒·g^-1。（2）孢粉数据重建的研究区的植被和气候变化结果表明,15.30~14.38 Ma时期为开阔的森林植被,湿度较低;14.38~14.00 Ma时期乔木增加,湿度增加;14.00~13.60 Ma时期乔木属种显著减少,湿度降低。（3）经过对比分析,炭屑总浓度变化趋势与乔木类花粉百分比趋势相近,次圆形炭屑浓度趋势与阔叶类植物花粉的百分比趋势显著正相关,认为武山盆地中中新世的天然火活动与森林植被的盖度（尤其是阔叶林的盖度）有较强联系,在气候温暖湿润的时期,炭屑浓度高。此外,通过对比炭屑总浓度趋势和深海底栖有孔虫氧同位素的变化,可以推测,全球温度变化可能通过影响研究区植被变化来对天然火活动产生重要影响。

关键词: 武山盆地, 中中新世, 炭屑, 天然火, 古气候

Abstract:

Fire activity is an important part of ecosystem dynamics which can be indicative of paleoclimatic and paleoenvironmental change. Reconstructing the history of fire can help us to understand wildfire mechanisms and past climatic patterns. However, globally, there are few high-resolution fire records for the Miocene. Micro-charcoal preserved in sediments can provide a basis for reconstructing the history and variations of wildfire activity. Micro-charcoal concentration (MC) can be used as a proxy for the intensity of fire activity, and particle size can, to a certain degree, indicate the extent of fire activity. Micro-charcoal particle shape can also be usefully classified into two distinct shapes according to length/width ratio (L/W): sub-round (L/W<2.5) and sub-long (L/W>2.5); these represent woody and herbaceous sources, respectively. We use high-resolution charcoal records to reconstruct fire history in the Wushan Basin (a semi-aid and semi-humid region in China) during the Middle Miocene (15.30-13.60 Ma). We discuss fire-climate-vegetation relationships, and possible links to global changes, by comparing our findings with complementary data (specifically pollen from the Nanyu section of the Wushan Basin, and global deep-sea foraminiferal δ¹⁸O records). Our results show that: (1) Total micro-charcoal concentration (MC_total) varies from 59 to 4324 grains·g^-1 (averaging 835 grains·g^-1); particle sizes finer than 50 μm (MC<50 μm) form the main component; and sub-round particles (MC_R) are much more abundant than sub-long (MC_L) ones. Together, these findings suggest that fire activity was dominated by the burning of trees, which mainly occurred on a regional scale as fires in the forest area. From variations in MC_total over time, we divide the Middle Miocene fire history of the study area into two phases: Phase I (15.30-14.00 Ma), during which MC_total gradually increased (averaging 866 grains·g^-1); and Phase II (14.00-13.60 Ma), characterized by a dramatic decrease in MC_total (averaging 777 grains·g^-1). (2) Analysis of sporopollen assemblages has shown that there was open forest with low humidity during the period 15.30-14.38 Ma; humidity and forest density increased during the period 14.38-14.00 Ma; then tree cover and humidity decreased significantly during the period 14.00-13.60 Ma. (3) Variations in MC_total through the Middle Miocene show similar trends to arboreal pollen percentage, and the MC_R trend correlates strongly with changes in broad-leaved plant pollen percentage. Therefore, we propose that the intensity of fire activity in the Wushan Basin during the Middle Miocene was strongly linked to changing forest coverage, especially changes in the extent of broad-leaved forest. Additionally, by comparing the MC_total trend with global deep-sea δ¹⁸O records (from benthic foraminifera), we propose that global temperature changes may have influenced the intensity of fire activity through effects on vegetation in the study area.

Key words: Wushan Basin, Mid Miocene, charcoal, fire, paleoclimate

周学文,魏晓,陈鹏,石天宇,惠争闯. 青藏高原东北缘武山盆地中中新世炭屑记录及其古气候意义[J]. 干旱区地理, 2022, 45(3): 826-835.

ZHOU Xuewen,WEI Xiao,CHEN Peng,SHI Tianyu,HUI Zhengchuang. Charcoal records during the Middle Miocene and its paleoclimatic significance in the Wushan Basin, northeastern Tibetan Plateau[J]. Arid Land Geography, 2022, 45(3): 826-835.

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