Quaternary Geochronology

Radiocarbon Dating

Radiocarbon dating makes use of the carbon isotope, C14, to evaluate the ages of wood, charcoal, coral or any other carbon-included samples. £]-decay (rate of lose of electron in unit weight and unit time) is calculated to decide the age. This method has been widely used for more than 60 years, and is still considered as the most reliable dating method for samples younger than 50 ka.

Thermal-luminescence

Thermal-luminescence (TL) dating method is able to complement the deficiency of radiocarbon method when the sample age is older than 50 ka. When heat event occurred in the past, such as faulting, burning or illumination, most electrons existing within quartz lattice defects would be reset (a process called ¡§bleaching¡¨). TL method uses thermal stimulation to calculate the re-trapped electron since last heat event. The dating range is about 200 ka (in Taiwan ).

Optical Stimulated Luminescence

Optical Stimulated Luminescence (OSL) method, which has been rapidly developed during the 90's, has very similar principal with TL method. The only difference lies in the stimulation method: TL uses heat, while OSL uses laser beam. When sediments were illuminated by sun in the past, bleaching also took place, but on different kinds of lattice defects. We may then use laser beam to stimulate the re-trapped electrons within this kind of defects since last illumination event. OSL is especially applicable on terrace deposits. Dating ranges of both TL and OSL are almost the same, 200 ka (in Taiwan ).

Example I: Luminescence dating of neotectonic activity on the southwestern coastal plain, Taiwan

The southwestern coastal plain of Taiwan is heavily urbanised, with a population of more than 6 million. The area is known to be subsiding, and the resulting basin is filled with thick fluvial deposits, of at least 200m thickness. In the past century, three large earthquakes have occurred in the area; two caused surface ruptures, and resulted in large property loss and more than 2000 casualties. There is an urgent need for an understanding of the dynamics and recurrence intervals of this neotectonic activity, but little is known of the chronology of the late Pleistocene deposits. Recently more than 20 cores longer than 250m were taken from the coastal plain as part of a large hydrogeological investigation, and basic data on lithology, hydrogeology and palaeobiology were collected. The base of these cores is beyond the 14C age limit, and so the application of luminescence dating to these sediments has been investigated.

Optically stimulated luminescence methods have been applied to quartz sand-sized grains extracted from 29 samples. Dosimetry based on gamma spectrometry is also compared with ICP-MS and XRF analyses. In the age range up to B40 ka, radiocarbon ages are compared with the luminescence results, to give confidence that the initial bleaching of these sediments was sufficient. The luminescence ages are then discussed and differential rates of basin subsidence are deduced. It is clear from these data that the study area is tectonically active, and it may be that regions of similar subsidence rate correlate with identifiable geological structures.

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Full Text: Y.W. Chen, Y.G. Chen, A.S. Murray , T.K. Liu and T.C. Lai (2003) Quaternary Science Review 22, 1223-1229

Example II: Use of the optically stimulated luminescence method to date the sedimentary sequence in a trenching site on the source fault of the 1999 Chi-Chi earthquake, Taiwan

A trenching site was excavated for the purpose of paleoseismic exploration and slip-rate acquisition along the Chelungpu fault, the source fault for the 1999 Chi-Chi earthquake (Mw 7.3) in central Taiwan . In previous studies that used the conventional and AMS radiocarbon method, sample ages clustered in three unevenly spaced groups: <400, 1700-2200, and >30000 yrBP, respectively. Those prior studies also yielded questionable long-term slip-rates: the rate for the most recent 2 kyr rate was six times higher than the average for the past 40 kyrs. In order to reevaluate the time frame for this trench we applied an independent dating method, optical stimulated luminescence (OSL). The large- and small-aliquot techniques were compared by testing on modern fluvial samples collected nearby. Our results indicate that the 5 % minimum ages derived from the small-aliquot method is preferable for dating samples that may include unbleached sediments associated with the unbleached sediments associated with debris events which occur frequently in Taiwan. The final OSL dates indicate that the previously published radiocarbon ages include some incorrect ages resulting from contamination and reworking of deposits, and a revised time frame is presented. A long-term vertical slip-rate is also derived independently from trench log restoration and from depositional rate in the footwall: the two methods yield consistent results.

Trench log of Pineapple site at Wufeng (please see Fig. 1 for the location). The 1999 fault trace and older traces are colored as red and blue respectively. Twelve previously published radiocarbon ages (in both sides in red) can be grouped into three phases: <0.5 ka, 1.7-2.1 ka, and >35 ka, which yields dramatic rate change of the long-term slip on the Chelungpu fault. Newly worked out OSL ages (in green with measured aliquot number bracketed behind) allow us to reconstruct the time frame and sort out the unreliable radiocarbon ages (indicated by question marks).