High-resolution optical dating for marine sediments from North Pacific Ocean

Marine sediments contain important archives of past ocean and climate changes,but at high latitudes, such as the polar regions, the absence of carbonate has preventedthe construction of accurate chronological models. To get an age model, a methodwhich does not rely on carbonate is needed. In this thesis we have investigated thepotential of optically stimulated luminescence (OSL) dating to establish a chronologyfor our deep sea sediment cores from the Northern Pacific area. OSL dating makes useof the omnipresent quartz and feldspar grains in the sediment so there is no limitationin the presence of the dosimeter. Optical dating is a widely accepted dating method forterrestrial sediments, but only few studies have tested its reliability in the marineenvironment. A luminescence age reflects the time that has elapsed since the sediment grains werelast exposed to sunlight. The luminescence age equation contains two equallyimportant factors: the equivalent dose and dose rate. The equivalent dose is the totalradiation dose that crystals (quartz, feldspar) have absorbed during burial, and the doserate is the rate at which the sample was exposed to ionising radiation in theenvironment. Dividing the equivalent dose by the dose rate gives the luminescence ageof the sample. This study is focused on testing the reliability of the optical dating method whenapplied to marine sediments from the Northern Pacific ocean, more specifically inregions that are known for seasonal-sea ice. The aims of this thesis are (1) determinethe accuracy of luminescence ages, e.g. by comparison with AMS 14C dating andmarine oxygen isotope stratigraphy where possible, (2) produce a high resolutionsequence of absolute ages which can describe changes in sedimentation rate throughtime, and (3) test whether it is possible to date back to marine isotope stage 5e (MIS5e). In this thesis we have made use of fine (4- 11 μm) grains of quartz extracted from themarine sediment cores taken in the south-western Sea of Okhotsk, the Bering Sea andthe Central Sea of Okhotsk. In the first study, a high resolution optical age dataset (64samples) from the south-western Sea of.Okhotsk showed continuous ages up to 24 kaand indicated clear sedimentation rate differences during glacial and interglacialperiods. Also for the most northern located core in the Bering Sea (sub-arctic area), theOSL ages (12 samples) could date to 64ka and the ages are in good agreement with the14Cage ～25ka. The OSL ages in this study are in good agreement with the 14C ages.In the third study in the central Sea of Okhotsk, a test of accuracy of the OSL agesback to MIS 5e was carried out. The OSL ages (40 samples) are in good agreementwith the well-established oxygen isotope stratigraphy at this site back to the MIS 5e(130ka). In all these studies, the luminescence characteristics of the dosimeter areinvestigated in detail and extensive tests of the performance in the chosenmeasurement protocol are presented. Despite the good agreement of the optical ages with the independent age control, itmust be noted that the OSL ages are largely dependent on the water content. Theevaluation of the appropriate water content model is discussed. It appears that theobserved water content values, measured immediately after core extraction, seem togive the most accurate ages. This thesis shows that OSL dating is a useful method for dating marine sediment inthe North Pacific area. The data confirm that the OSL dating using fine-grain quartzthat is distributed all over the ocean has very great potential in the establishment of anabsolute chronology for deep sea sediments; because luminescence does not depend on the presence of carbonate for AMS 14C dating, it is now likely that we can establish achronology is regions of the ocean that were previously undatable