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内容記述 |
Marine sediments contain important archives of past ocean and climate changes,<br />but at high latitudes, such as the polar regions, the absence of carbonate has prevented<br />the construction of accurate chronological models. To get an age model, a method<br />which does not rely on carbonate is needed. In this thesis we have investigated the<br />potential of optically stimulated luminescence (OSL) dating to establish a chronology<br />for our deep sea sediment cores from the Northern Pacific area. OSL dating makes use<br />of the omnipresent quartz and feldspar grains in the sediment so there is no limitation<br />in the presence of the dosimeter. Optical dating is a widely accepted dating method for<br />terrestrial sediments, but only few studies have tested its reliability in the marine<br />environment.<br /> A luminescence age reflects the time that has elapsed since the sediment grains were<br />last exposed to sunlight. The luminescence age equation contains two equally<br />important factors: the equivalent dose and dose rate. The equivalent dose is the total<br />radiation dose that crystals (quartz, feldspar) have absorbed during burial, and the dose<br />rate is the rate at which the sample was exposed to ionising radiation in the<br />environment. Dividing the equivalent dose by the dose rate gives the luminescence age<br />of the sample.<br /> This study is focused on testing the reliability of the optical dating method when<br />applied to marine sediments from the Northern Pacific ocean, more specifically in<br />regions that are known for seasonal-sea ice. The aims of this thesis are (1) determine<br />the accuracy of luminescence ages, e.g. by comparison with AMS <sup>14</sup>C dating and<br />marine oxygen isotope stratigraphy where possible, (2) produce a high resolution<br />sequence of absolute ages which can describe changes in sedimentation rate through<br />time, and (3) test whether it is possible to date back to marine isotope stage 5e (MIS<br />5e).<br /> In this thesis we have made use of fine (4- 11 μm) grains of quartz extracted from the<br />marine sediment cores taken in the south-western Sea of Okhotsk, the Bering Sea and<br />the Central Sea of Okhotsk. In the first study, a high resolution optical age dataset (64<br />samples) from the south-western Sea of.Okhotsk showed continuous ages up to 24 ka<br />and indicated clear sedimentation rate differences during glacial and interglacial<br />periods. Also for the most northern located core in the Bering Sea (sub-arctic area), the<br />OSL ages (12 samples) could date to 64ka and the ages are in good agreement with the<br /><sup>14</sup>Cage ～25ka. The OSL ages in this study are in good agreement with the <sup>14</sup>C ages.<br />In the third study in the central Sea of Okhotsk, a test of accuracy of the OSL ages<br />back to MIS 5e was carried out. The OSL ages (40 samples) are in good agreement<br />with the well-established oxygen isotope stratigraphy at this site back to the MIS 5e<br />(130ka). In all these studies, the luminescence characteristics of the dosimeter are<br />investigated in detail and extensive tests of the performance in the chosen<br />measurement protocol are presented.<br /> Despite the good agreement of the optical ages with the independent age control, it<br />must be noted that the OSL ages are largely dependent on the water content. The<br />evaluation of the appropriate water content model is discussed. It appears that the<br />observed water content values, measured immediately after core extraction, seem to<br />give the most accurate ages.<br /> This thesis shows that OSL dating is a useful method for dating marine sediment in<br />the North Pacific area. The data confirm that the OSL dating using fine-grain quartz<br />that is distributed all over the ocean has very great potential in the establishment of an<br />absolute chronology for deep sea sediments; because luminescence does not depend on the presence of carbonate for AMS <sup>14</sup>C dating, it is now likely that we can establish a<br />chronology is regions of the ocean that were previously undatable. |
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