階段式加熱碳十四定年法於台灣西部階地沉積物之應用

Radiocarbon dating of terrace sediments from western Taiwan using a temperature stepped-combustion procedure

王興麟(Shing-Lin Wang)

指導教授 ： 林立虹

國立臺灣大學/理學院/地質科學研究所/博士(2016年)

https://doi.org/10.6342/NTU201601315

摘要

台灣由於氣候變遷和板塊擠壓持續抬升，一連串的河階沿著河流生成，利用碳十四定年法分析在階地上找到的碳屑或是樹幹，即可得知這些河階的生成年代。然而台灣氣候溫暖潮濕，河階沉積層常有地下水通過，在氧化環境之下，碳屑或是樹幹會快速的被氧化降解，因此在野外，並不容易找到保存良好的碳屑或是樹幹。過去的研究，嘗試利用沉積物中的有機碳來做碳十四定年分析，但在沉積物中的碳循環是屬於開放系統，年代會因為不同碳來源進入這個系統而改變，直到目前為止，沒有一個方法可以完全分離沉積物中不同年代的碳來源。本研究嘗試修改過去的實驗步驟，主要包括兩個階段：1）在化學前處理之前將沉積物先做粒徑分選 2）改單一高溫燃燒為階段式加溫燃燒的方式，希望能分離出沉積物中的多個碳來源，並探討這些分離出來的碳來源的年代意義。
第一個研究地點在苗栗縣南庄鄉南富村，階地選在離獅潭和斗煥坪左右兩斷層之間，中間段的最低位河階，目的是檢視本研究方法，在構造影響最小的情狀之下，沉積物年代是否可以紀錄氣候變遷的可行性。南富階地沉積物經過篩之後分成粗、細兩組分樣 (150-1410 μm and <64 μm)，此兩組分樣分別經過酸-鹼-酸的化學前處理，將溶解在鹼液的腐植酸 (humic acid)，利用再沈澱的方式析出，再以單一高溫燃燒氧化，而化學前處理的產物-腐植質 (humin)，則是採用階段式加溫的方式燃燒氧化，得到多個數據。將標本中選一(1E)的細粒分樣 (<64 μm) 析出的腐植質，經由階段式加溫燃燒的結果中發現，有低溫(LT)年輕和高溫(HT)年老的兩個端成分，以攝氏400度為分界燃燒其他樣本，同樣也有高、低溫兩個端成分的組成。採用合併過篩和階段式加溫的實驗步驟得到的結果，腐質酸呈現最年輕，低溫腐植質次年輕，高溫腐植質最年老的規律性分布。由於過篩將沉積物顆粒分離，將碳來源變異度侷限在小於64 μm粒徑範圍內的有機碳-礦物包裹體，再用階段式加溫方式有效分離其中碳成分，此兩種方法的組合，可成功分離出數個不同年代的碳來源。由此不同年代的分樣可估算出，紀錄在南富階地沉積物中土壤發育的綜合年代，而此一發育年代與台灣過去兩千年到四千年之間的暖、濕氣候時間相當吻合。
第二個研究地點在台南六甲斷層區域，沿著龜重溪開挖了五個槽溝，分別位於不同高度的河階上，由於此河流穿越六甲斷層的上盤區域，因此河階抬升速率應可反應該斷層的活動速率。來自五個槽溝的沉積物，如同上述先過篩，但只採用小於64 μm粒徑的分樣，經過化學前處理之後的腐植質，皆採用階段式加溫方式燃燒氧化，與腐質酸共同比對。根據前人研究，有機碳主要吸附和鍵結在黏土礦物上，所以本研究選取高純度、結晶良好的黏土礦物，同步進行分析；研究結果顯示有機碳以鍵結方式與黏土礦物連結，因此釋放溫度也與礦物結晶結構變形溫度同步，野外沉積物雖然組成複雜，但也有類似的趨勢，證明有相當量的有機碳建結在黏土礦物上。本研究將龜重溪的五個河階沉積物分離出數個不同的分樣分別來定年，將之轉換成抬升速率，並探討不同河階土壤發育方式不同，對抬升速率（沉積物年代）的影響。
本研究修改過去前人針對沉積物碳十四定年的研究方法，利用階段式加溫燃燒小於63um粒徑的有機碳礦物包裹體，成功將碳來源簡化成兩到三個端成分；由於沉積物為開放系統，腐質酸以及低溫腐植質年代，並不表示階地形成年代，而是從階地形成之後，持續混合現生碳，直到封閉不再碳交換的平均年代，因此會比階地形成年代年輕；階地沉積物中，有一部分黏土礦物是從上游沖刷下來沈積，而高溫腐植質則是代表黏土礦物在上游形成時鍵結的有機碳，因此會比階地形成年代老；由此兩方年代的控制，可以估計出階地抬升速度。此一方法在找不到碳屑或是樹枝的沉積層的條件之下，可以提供年代的參考範圍。
本研究方法成功分離河階沉積物中多個不同年代的碳來源，證明此簡單的物理方法加上過去長期使用的化學前處理，即可將碳來源的變異度降低，進而討論碳來源的年代意義。此方法亦可應用在其他沈積環境的沉積物中，尤其在找不到其他定年物質的情形之下，沉積物中數個分樣的碳來源年代亦可當作重要參考年代指標。

關鍵字

黏土礦物 ； 碳十四定年 ； 沉積物 ； 粒徑分選 ； 階段式加溫燃燒

並列摘要

Radiocarbon (14C) has been widely used in determining the age of various geological materials and events. The validity of ages, particularly for sediments, has often been questioned due to the incorporation of carbon-bearing components with different abundances of 13C and 14C. For the purpose of isolating various carbon sources in sediment, a procedure is adopted in this study including 1) size fractionation before regular sample pretreatment and 2) temperature stepped-combustion in place of conventional high temperature oxidation. Then, this procedure was applied on terrace sediments from two study areas at western Taiwan.

The first step in this study was to carry out a radiocarbon study of sediment samples collected from Nanfu terrace in western Taiwan. Humic acids (HA) and humins were extracted from the very fine and coarse grain-size fractions of sediment using a standard acid-base-acid (ABA) pretreatment. The humin extracts were combusted at 400 and 1100 °C by stepped-combustion, to yield a low-temperature (LT) carbon component and a high-temperature (HT) carbon component. A consistent relationship can be observed by comparing the ages of the LT and HT humin fractions to the HA fractions of samples collected at 2 depths within the Nanfu terrace profile. The HA ages are the youngest on average, and overlap with the LT ages, and the carbon contained in the HT fraction is always distinctly older than both the LT and HA ages. To better understand the relationship between 14C age and combustion temperature, incremental stepped-combustion experiments were conducted with one of the samples (1E) using 50 °C steps that ranged from 300°C to 1100 °C. The 14C results of the stepped-combustion products show a clear division between 2 isotopically identifiable carbon constituents, from carbon released below 400 °C and carbon released above 550 °C. By comparing the δ13C and 14C results, a third carbon isotopic component is identified in the humin that is released when combusted at ~500 °C.

Since organic matter is mainly bound to clay minerals in fine sediments, the next phase of this study was to focus on carbon contained in clays. The same temperature stepped-combustion procedure was applied to well-crystalized clay minerals (kaolinite, illite and illite-smectite) in a series of experiments ranging from 350°C to 1100°C. Distinct carbon components were identified in the samples using both their 13C and 14C contents. Two carbon end-members were observed for all of the clay minerals: a low temperature component (<400°C), relatively depleted in 13C and enriched in 14C; and a high temperature component (>550°C), relatively enriched in 13C and depleted in 14C. The high temperature carbon component is relatively old and not released until the clay is completely oxidized. This observation suggests that this carbon was incorporated into the clay mineral when it first formed by weathering.

The same procedure was applied to field sediments collected from terraces along the Kueichung River, in southwestern Taiwan, sieved to <64 μm. The results from sediments are similar to the well-crystallized clay minerals, with distinct low temperature (relatively young) and high temperature (relatively old) end members. One of the sediments also contained a third carbon component, possibly detrital carbon. The radiocarbon dates on these various carbon fractions provide a range of dates enabling us to place constraints on the timing of terrace development in Taiwan.

The procedure used in this study successfully separates carbon sources in terrace sediments. Organic carbon bound on minerals is well accepted in previous researches, but this study is the first to provide 14C ages of distinct carbon sources from organo-mineral complex. It implies by this procedure radiocarbon dating technique can be applied on any kind of sediment, and its age interpretation will be varied based on the binding mechanism.

並列關鍵字

clay minerals ； radiocarbon dating ； sediments ； size fractionation ； temperature stepped-combustion

參考文獻

Anderson DW and Paul EA (1984) Organo-mineral complexes and their study by radiocarbon dating. Soil Science Society of America Journal 48 (2), 298-301.

Baldock JA and Skjemstad JO (2000) Role of the matrix and minerals in protecting natural organic matter against biological attack. Organic Geochemistry 31, 697-710.

Balesdent J (1987) The turnover of soil organic fractions estimated by radiocarbon dating. The Science of the Total Environment 62, 405-408.

Bennett CL, Beukens RP, Clover MR, Gove HE, Liebert RB, Litherland AE, Purser KH and Sondheim WE (1977) Radiocarbon dating using electrostatic accelerators: negative ions provide the key. Science 198, 508-510.

Bird MI, Turney CSM, Fifield LK, Jones R, Ayliffe LK, Palmer A, Cresswell R and Robertson S (2002) Radiocarbon analysis of the early archaeological site of Nauwalwabila I, Arnhem Land, Australia: implications for sample suitability and stratigraphic integrity. Quaternary Science Reviews 21, 1061-1075.