Host: Abstracts of Annual Meeting of the Geochemical Society of Japan
Pages 29-
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.
