中國甘肅省黃爺洞1450年來之石筍碳氧同位素紀錄：氣候、植被以及人類活動的關聯

Abstract

本研究報導2005年採自位於中國甘肅省武都區的黃爺洞(33o35'N, 105o07'E)三根石筍的碳氧同位素記錄。三根石筍分別為HY05-2 (長14 cm)、 HY05-3 (長23.6 cm) 和HY05-4 (長19 cm)。經過鈾系定年，三根石筍生長範圍都在全新世以內。石筍在老於1400年前皆有沉積間斷，而1400年以來三根石筍的沉積都是連續的。因此，本文著重討論這個石筍連續沉積期間的同位素記錄。在這段時間內，HY05-2，HY05-3和HY05-4分別有340個，695個和468個樣品進行了碳氧同位素分析。 首先，在年齡的誤差範圍內三根石筍的氧同位素曲線有較好的重合，顯示石筍氧同位素可以反映當地降水的變化：氧同位素偏輕的時段，表示季風降雨強度較強；氧同位素偏重的時段，指示氣候較乾旱。其次，在較長時間尺度上（>50年）碳氧同位素的變化呈現相同的趨勢，說明植被受氣候的影響很大，即濕潤氣候下（氧同位素值偏輕），植物較發育，植被密度增加，C3/C4植物種類的比例增加，石筍碳同位素值變輕。黃爺洞石筍記錄顯示：西元600~1000年間，氧同位素值由平均值逐漸變重，碳同位素值比平均值重大約1‰，指示氣候變乾旱，植被發育較差。在西元1100~1200年，1300~1360年，1420~1500年，1600~1780年和1850~1900年這幾個時間段內，氧同位素值都比平均值偏輕，指示這幾個時間段內的氣候相對濕潤，而在這些時間段之間，氣候變乾旱。除了在西元1200- 1300年之間，碳同位素在西元1000年以來的變化基本上與氧同位素一致，而且比西元1000年前的碳同位素值要輕。這個西元1000年前後的碳同位素值的變化，也可能包含人類活動對植被的影響，如種植農作物等。碳同位素在宋末元初期間不隨氧同位素變重而保持較輕，可能是人口大量往南方遷移，人類活動的影響大幅減少，植物與生態得以恢復。在西元1360 ~1410年，1580~1600年，1790~1840年有三次碳同位素明顯變重的短暫時期。這三個碳氧同位素同時變重的時期基本上可以與萬象洞石筍氧同位素記錄對應，但並不能說明氣候變化是導致中國朝代更替的主因。 對比黃爺洞、萬象洞和大禹洞的石筍記錄、以及青海湖沉積物岩心中碳氮比（指示降雨）記錄和太陽輻照記錄，在許多事件上呈現一致，但也存在不少差異。在進入小冰期第二階段(西元1550年至1850年)的時候，黃爺洞和大禹洞以及青海湖三個地方的記錄都指示降雨相對增強，與萬象洞記錄相反。黃爺洞氧同位素值變得最輕的西元1100~1200年，剛好對應於太陽黑子活動的Medieval Maximum，同地區的萬象洞與太陽黑子活動在長時間尺度上變化極為相似，表明太陽黑子活動對東亞夏季風的強弱有明顯的影響。

This study presents carbon and oxygen isotopic records from three stalagmites collected in 2005 from Huangye cave (33o35'N, 105o07'E) in Wudu County, Gansu Province, China. Three stalagmites with lengths of 14 cm for HY05-2, 23.6 cm for HY05-3, and 19 cm for HY05-4, have been dated with ICP-MS 230Th/U in HISPEC at NTU. The 230Th/U dating result shows that the three stalagmites grow within Holocene and contain hiatuses in the parts of older than age 1400 years. However, the three stalagmites had grown continuously during the last 1400 years, for which we focus on. In the 1400-yr part, there are 340, 695 and 468 isotope data sets in HY05-2, HY05-3 and HY05-4, respectively. First of all, within dating uncertainties the three stalagmite δ18O records are comparable each other, suggesting that the δ18O records of the stalagmites is a reliable paleoclimate proxy. Periods with lighter δ18O values reflect wet climate, and vice versa. Secondly, the δ13C co-varied with the δ18O on longer time scale ( >50 years), demonstrating climatic control on vegetation change with wet climates (lighter δ18O) resulting in better vegetation coverage, and/or rise in vegetation C3/C4 ratio that gives lighter δ13C. Huangye Cave record shows that during AD 600-1000, δ18O values gradually became heavier from the average value and δ13C values were heavier than its average about 1‰, reflecting climate were drier and poor vegetation coverage. During the intervals of AD 1100-1200, AD 1300-1360, AD 1420-1500, AD 1600-1780 and AD 1850-1900, the δ18O values are lighter than the average, illustrating relatively wet climates. Dry climates occurred among the intervals between the above these time periods. Except the interval of AD 1200-1300, the δ13C record basically followed the δ18O record since AD 1000 and the values are lighter than that before AD 1000. The change of δ13C value before and after AD 1000 might contain the human impact on vegetation, such as crop cultivation. Diminished human impact on vegetation around the cave might occur as the population decrease during the late Song Dynasty and early Yuan Dynasty, shown by relatively light δ13C values and not following the δ18O trend. The δ13C record was obviously heavier during AD 1360-1410, AD1580-1600 and AD 1790-1840. These heavier δ13C and δ18O periods generally agree with Wanxiang Cave stalagmite δ18O record. However, these correlations do not support that climate was the main cause of Chinese dynastic change. Comparisons among stalagmites records (Huangye Cave, Wanxiang Cave, Dayu Cave), C/N ratio of cores in Lake Qinghai, and solar irradiance record reveal consistence in many events, but many discrepancies also exist. Unlike Wanxiang Cave record, the records from Huangye Cave , Dayu Cave, Lake Qinghai indicate that relatively wet climates were prevailed during the 2nd half of the Little Ice Age. The δ18O of the Huangye cave record has lightest values during AD 1100-1200 which were corresponding to the Medieval Maximum of sunspot activity. In the same region, Wanxiang Cave record shows the similar trend with sunspot activity on long time scale, demonstrating that solar forcing is one of influential role in modulating variability of Asian monsoon strength.