臺灣西南地區沖積平原土壤碳酸鈣結核的形成及其在古氣候變遷上的意涵

Abstract

土壤碳酸鈣結核屬於化育型碳酸鹽，其形成是土壤CO2溶解成的可溶解無機碳及母質來源的Ca2+，在土壤中重新分佈後再沉澱的碳酸鈣。過去研究認爲碳酸鈣結核可能富集稀土元素 (rare earth elements, REEs)，而REEs可以作爲化育示蹤劑 (pedogenic tracer)。碳酸鈣結核的穩定碳同位素受到土壤CO2影響，土壤CO2則是由植物代謝所產生的，包含C3及C4植物。碳酸鈣結核的穩定氧同位素受到土壤中水及溫度的影響，因此可以藉由穩定氧同位素推估碳酸鈣結核形成時的溫度。碳酸鈣結核在形成後能穩定存在於土壤中，因而適合作爲推估古氣候的化育特徵。化育型碳酸鹽常見於蒸發散量大的地區，然而在臺灣西南地區沖積平原上卻有碳酸鈣結核的出現。現代濕潤的氣候條件可能不利於碳酸鈣結核的生成。因此，本研究目的為探討臺灣西南地區沖積平原上碳酸鈣結核的形成及土壤形成過程中REEs的分化，並利用碳酸鈣結核中的穩定碳與氧同位素來推估植被種類及溫度，以進一步瞭解過去的氣候條件變化。過去西南地區沖積平原在全新世時受到海進及海退影響造成的海岸線變化，可能形成了不同化育程度的土壤，為時間序列土壤。研究假設土壤在海退後開始穩定化育。研究依據海進及海退期間，於高雄採集五個同爲砂頁岩母質但不同可化育時間的土壤剖面，包括10000 years Before Present (yr BP)、8000 yr BP、5000 yr BP、3000 yr BP及2000 yr BP。除了2000 yr BP的土壤剖面 (分類為Dystrustept) 外，其他土壤剖面 (分類為Haplustalfs) 皆具有黏粒膜及碳酸鈣結核。碳酸鈣結核具有較總體土壤低的REEs (12.7-108 v.s. 132-315 mg kg-1)。REEs會相對富集於黏聚層，且在碳酸鈣結核形成的過程中REEs會產生分化 (fractionation)，並會相對富集重稀土元素 (heavy REEs, HREEs)，這表示土壤中REEs的分化主要受到淋洗作用及鈣化作用的影響。碳酸鈣結核之放射性碳定年的介於4063 yr BP 至690 yr BP之間。穩定碳同位素顯示，碳酸鈣結核形成時的植被同時具有C3及C4植物，並以C4植物爲主。穩定氧同位素顯示平均年溫度為12.3-14.0°C；地球化學氣候函數顯示當時的平均年雨量介於1036至1342 mm之間，顯示碳酸鈣結核的生成環境較現代涼 (cool) 且乾，這可能是受到當時較微弱的東亞夏季季風影響。碳酸鈣結核中穩定碳與氧同位素的分析表明，其形成時的氣候條件與土壤鈣化作用的條件相符。

Soil calcium carbonate nodules are pedogenic carbonates formed by the dissolution of inorganic carbon from soil CO2 and calcium from parent materials, followed by the redistribution and reprecipitation as calcium carbonates. The enrichment of rare earth elements (REEs) in calcium carbonate nodules has been identified in previous studies, and REEs can serve as a pedogenic tracer. The main source of stable carbon isotopes in calcium carbonate nodules is the carbonate ions from the dissolution of soil CO2. The CO2 mainly originates from the metabolism of higher plants, including C3 and C4 plants. The stable oxygen isotopes in calcium carbonate nodules are mainly controlled by soil water and temperature. Hence, the temperature for the formation of calcium carbonate nodules can be estimated by the stable oxygen composition. Calcium carbonate nodules can be preserved in the soils after formation, and thus can be used for paleoclimate interpretation as a pedogenic feature. Pedogenic carbonates are commonly found in regions with high evapotranspiration rates, but can be found in the alluvial soils in southwestern Taiwan. However, the modern humid climate may be unfavorable for the formation of calcium carbonate nodules. Therefore, this study aimed to elucidate the formation of calcium carbonate nodules in alluvial plains from southwestern Taiwan, the fractionation of REEs during soil formation, and to interpret the types of vegetation and temperature using stable carbon and oxygen isotopes in calcium carbonate nodules, in order to evaluate past climatic conditions. The alluvial plains in the southwestern part were impacted by marine transgressive and regressive processes associated with shoreline changes during the Holocene, resulting in chronosequence soils with varying degrees of pedogenesis. This study hypothesized that the soils formed stably after marine regression. This study sampled five soil profiles with similar parent materials, namely sandstone and shale, and varying ages of shoreline changes from Kaohsiung, including 10000 years before present (yr BP), 8000 yr BP, 5000 yr BP, 3000 yr BP, and 2000 yr BP, according to the marine transgression and regression phases. Except for the pedon with an age of 2000 yr BP (classified as Inceptisol), clay coatings and calcium carbonate nodules were present in other pedons (classified as Alfisols). REEs contents were lower in calcium carbonate nodules than in the bulk soils (12.7-108 v.s. 132-315 mg kg-1), implying that REEs would not be concentrated in calcium carbonate nodules. The relative enrichment of REEs was found in the argillic horizon. The fractionation of REEs and relative enrichment of HREEs occurred during the formation of calcium carbonate nodules. This implied that the fractionation of REEs was influenced by leaching and calcification in the soils. The radiocarbon dating of the calcium carbonate nodules were between 4063 yr BP to 690 yr BP. The stable carbon isotopes revealed the coexistence of C3 and C4 plants, with a predominance of C4 plants. The stable oxygen isotopes indicated a mean annual temperature of 12.3 to 14.0°C, and the mean annual precipitation was estimated to be 1036 to 1342 mm using the geochemical climofunction. This study suggested that the climatic conditions during the formation of calcium carbonate nodules were cooler and drier than those of the present, and were likely due to a weaker East Asian Summer Monsoon (EASM). The stable carbon and oxygen isotopes of calcium carbonate nodules further demonstrate that the climatic conditions during their formation are consistent with the pedogenic conditions for calcification.