Tracing the sources of carbon in clay minerals: An example from western Taiwan

Radiocarbon 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. This study examines carbon contained in clay minerals (kaolinite, illite and illite-smectite) extracted by temperature-stepped combustion 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.We applied the same procedure to sediments collected from terraces along the Kueichung River, in southwestern Taiwan, sieved to <64 μm. We obtained results similar to the pure 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 results on these various carbon fractions provide a range of dates enabling us to place constraints on the timing of terrace development in Taiwan.