长江下游南漪湖沉积记录的全新世以来温度变化

了解全新世的温度变化能为理解目前日益突出的全球变暖、评估未来全球气候变化给出重要的参考。在这项研究中,基于长江下游南漪湖沉积岩芯深度为0~450cm中161个样品的brGDGTs代用指标,对过去12.0ka的大气温度进行重建,以进一步深化对全新世温度变化的理解。发现湖泊周边土壤与湖泊沉积物brGDGTs分子组成存在显著差异：土壤以brGDGTs-Ⅰ系列为主,占到总比重的80%以上,计算得的MBT'_5ME平均值为0.81;湖泊表层和柱状沉积物的brGDGTs分子组成更相似,其brGDGT-Ⅰ和brGDGT-Ⅱ分别为43%、48%和62%、35%,对应的MBT'_5ME平均值分别为0.44和0.62,因此认为湖泊沉积物brGDGTs主要为自生来源,进而选用基于MBT'_5ME的湖泊温度经验计算式进行古温度的重建。重建的南漪湖年均大气温度自12.0 ka B.P.以来变化范围为13.8~22.4℃,根据变化趋势,可以分为4个阶段：①阶段,早全新世（约12.0~8.2 ka B.P.）,温度变化范围为15.1~20.6℃,属低温阶段;②阶段,中全新世（约8.2~6.0 ka B.P.）,温度为16.8~20.0℃,为稳定高温阶段;③阶段,中晚全新世（约6.0~3.0 ka B.P.）,温度为13.8~19.4℃,快速降温阶段;④阶段,晚全新世（约3.0 ka B.P.以来）,温度在17.4~22.4℃,快速升温阶段。通过对比其他古气候记录,可以得到以下结论：长江下游地区在约12.0~8.2 ka B.P.时期温度变化主要受高纬度冰川残留的影响,为低温时期;在约8.2~6.0 ka B.P.时期的温度变化主要受到较强的太阳辐射量控制,属稳定高温期,对应全新世大暖期;约6.0 ka B.P.后,温度受到6.0~3.0 ka B.P.中低纬度冷事件以及上升温室气体辐射强迫共同影响,呈现先降后升的"V"型变化趋势。本研究表明长江下游地区自12.0 ka B.P.以来温度变化主要受全球温度变化控制,自晚全新世以来温室气体辐射强迫是影响其温度变化的主要因素。

Variation of temperature in Lake Nanyi sediments from the lower Yangtze River region since the last 12.0 ka B. P.

The Holocene temperature variation is critical for better understanding the global warming and predicting the future global climate change. In this study, we reconstructed the temperature over the past 12.0 ka B.P. based on the branched-GDGTs(brGDGTs) index of 161 samples from 0 to 450cm in length sediment core taken from the Nanyi Lake(31°03'~31°10'N, 118°50'~119°02'E; 2.2m a.s.l.). The lake is hydrologically-open and located in the plain area of the lower Yangtze River region, belonging to the north subtropical zone. Large differences in the distributions of brGDGTs are found between our surface lake sediments(n=9) and soils(n=8). brGDGTs of the soils is dominated by the Ⅰ series compounds, accounting for more than 80%, and the average value of MBT'5ME is 0.81. The brGDGTs distribution of lake surface and core sediments samples are both dominated by Ⅰ and Ⅱ series compounds, accounting for 43%, 48% and 62%, 35%, respectively. The corresponding MBT'5ME average values are 0.44 and 0.62, respectively. This indicates that the brGDGTs in the lake are mainly produced in situ in the sediment or water column. Therefore, the brGDGT-based calibration formula to lakes is chosen for the temperature reconstructions. The reconstructed temperature(13.8~22.4℃) can be divided into four stages. First stage, the Early Holocene(ca.12.0~8.2 ka B.P.), the temperature is relatively lower, varying 15.1~20.6℃. Then during the first half of Middle Holocene(ca.8.2~6.0 ka B.P.), the temperature becomes relatively high and stable, fluctuating 16.8~20.0℃. In the Middle-Late Holocene(ca.6.0~3.0 ka B.P.), the temperature shows a rapid decrease trend, ranging 13.8~19.4℃. Last, the temperature exhibits an increase trend during the rest Late Holocene(since ca. 3.0ka B. P), varying 17.4~22.4℃. After comparing with multiple records, we get the following conclusions. The low temperature in the Early Holocene and the high temperature in the first half of Middle Holocene are response to the high latitude large glacier cover area and the strong solar radiation, respectively. After ca.6.0 ka B.P., the temperature is mainly influenced by the cold event in low-medium latitude during 6.0~3.0 ka B.P. and then by the rising greenhouse gas pressure since 3.0 ka B.P., and therefore presenting the "V" type variation trend. Collectively, this paper shows that during Holocene the temperature in the lower Yangtze River region is response to the variation of global temperature, and greenhouse gas radiative forcing has become the main controlling factor of temperature in the Late Holocene.