俄羅斯南烏拉爾山五千年氣候變遷：石筍記錄

Abstract

高緯度地區在地球氣候系統中扮演著重要角色，俄羅斯南烏拉爾山地區便是如此，因該地為西風帶和極地東風帶兩大氣候系統的交匯處，具有極高的古氣候重建價值。本研究聚焦於分析來自該地區洞穴的兩根石筍樣本，於2022年從俄羅斯南烏拉爾山區的Ikenskaya Cave（54o43’1’’ N, 57o18’25’’ E）和Oktyabrskaya Cave（54°10'09.7"N, 56°50'56.5"E）採得，分別命名為IKEN-1及OTAR-1；IKEN-1長11.5公分，OTAR-1則長20.1公分。 210Pb定年結果表明，IKEN-1至少在200年前停止生長，而OTAR-1則持續生長至今。230Th/U定年的結果由於樣本中238U含量較低且232Th含量較高，導致誤差較大。通過高解析度AMS 14C定年，IKEN-1與OTAR-1的年代均呈現出良好的層序排列，IKEN-1樣品顯示除了40-90毫米處受死碳效應影響外，AMS 14C年代與230Th/U年代整體一致。而OTAR-1從樣本表層到底部年代層序雖然沒有出現倒序，但顯示14C年齡老於210Pb和鈾系年齡，表明AMS 14C年代存在相對穩定的死碳影響。利用AMS 14C與230Th/U年代之間的差異對死碳效應進行校正，基於校正後的AMS 14C年代框架通過Bacon模型建立石筍的年齡模式。IKEN-1從3700 Cal yr BP生長至600 Cal yr BP；而OTAR-1從5600 Cal yr BP生長至今。 本研究採集0.1mm間距的樣本進行碳氧同位素分析，IKEN-1共有1155個樣本，OTAR-1則有2010個樣本。首先，δ18O記錄不存在明顯的長期趨勢，利用各自的平均值進行距平計算，得到十年-百年尺度上的距平變化，反映雨量效應的影響，Δδ18O值越輕，氣候越濕潤，Δδ18O值越重，氣候越乾燥。而δ13C記錄存在長期變化趨勢，可能表明洞頂植被種類的變化。去趨勢化後的Δδ13C值與Δδ18O值的變化大致相同，指示此區域Δδ13C值主要受到氣候影響。兩根石筍的Δδ18O記錄變化在年齡誤差範圍內可以進行對比，說明石筍的Δδ18O記錄反映降水變化，可以為該地區5600年以來的古氣候記錄提供基礎。石筍紀錄顯示，該地區明顯濕潤的時期為：5~4.6, 3.6~3.3, 2.8~2.6, 1.3~1.1, 0.5~0 ka BP；明顯乾旱的時期為：4.6~3.8, 2.6~2.4, 0.8~0.6 ka BP。

Climate in high-latitude regions plays a crucial role in the Earth's climate system, and the South Ural Mountain region in Russia is such a place. This region, located at the intersection of the Westerlies and the Polar Front, holds significant value for paleoclimate reconstruction. This study focuses on analyzing two stalagmites collected from two caves in the southern Ural Mountains, Russia, in 2022. The samples were obtained from Ikenskaya Cave ( 54o43’1’’ N, 57o18’25’’ E ) and Oktyabrskaya Cave ( 54°10'09.7"N, 56°50'56.5"E ), and were named IKEN-1 and OTAR-1, respectively. IKEN-1 is 11.5 cm long, while OTAR-1 measures 20.1 cm long. 210Pb dating results indicate that IKEN-1 stopped growth before 200 years, whereas OTAR-1 continuously grew to the present. The 230Th/U dating results with large uncertainties and reversed age sequences due to low 238U but high 232Th contents. High-resolution AMS 14C dating revealed a well-ordered chronological sequence for both IKEN-1 and OTAR-1. For IKEN-1, the AMS 14C dating ages are generally consistent with 230Th/U ages, except in the 40-90mm section, which was influenced by the dead carbon influence (DCI). For OTAR-1, although 14C ages were significantly older than 210Pb and 230Th/U ages, no chronological reversals were observed from the surface to the bases of the sample, indicating that the DCI in the AMS 14C ages was relatively stable. Corrections for the DCI were applied based on the differences between AMS 14C and 230Th/U ages, and the final chronological framework was constructed by using the Bacon model based on the corrected AMS 14C ages. IKEN-1 grew from about 3700 cal yr BP to about 600 cal yr BP; whereas OTAR-1 grew continuously during 5600~0 cal yr BP. In the carbon and oxygen isotope analysis, a total of 1,155 samples were analyzed for IKEN-1, and 2,010 samples for OTAR-1. Variations in δ18O and δ13C values provide insights into past climatic conditions. First of all, there is no apparent long-term trend in the δ18O records. One can use the average values of its own record to calculate the δ18O anomalies (Δδ18O). The Δδ18O values are primarily influenced by the rainfall effect: negative Δδ18O values correspond to wetter climatic conditions, while positive Δδ18O values indicate drier conditions. The δ13C record values show a trend broadly consistent with δ18O values, suggesting that δ13C is also primarily influenced by climatic factors in this region. The similar trends in Δδ18O values observed in both stalagmites provide a solid foundation for reconstructing paleoclimate variations over the past 5,600 years BP. Based on the two stalagmite records, climates in the study area were significantly wetter during 5~4.6, 3.6~3.3, 2.8~2.6, 1.3~1.1 and 0.5~0 ka BP; climates were drier during 4.6~3.8, 2.6~2.4 and 0.8~0.6 ka BP.