Presented by: Cole Jensen “Thermoluminescence dating tests for lacustrine, glaciomarine, and floodplain sediments from western Washington and British Columbia,” - Berger and Easterbrook 1993 Presented by: Cole Jensen

What is TL Dating? Dates the last exposure to sunlight or heat. Within the crystal lattice of minerals there are light sensitive electron traps. Sunlight empties/bleaches these electron traps and effectively resets the clock. Once sediment is buried it will stop receiving sunlight and the electron traps are repopulated.

Purpose Few techniques for dating Quaternary sediments younger than 780 ka and older than 50 ka. TL dating along with optically stimulated techniques can date grains from Quaternary sediments that are within this age range. TL dating can also date deposits that are un-datable by other techniques. If a sediment lacks organics and is un-datable by C14, TL could be an option. Eolian sediments have been studied to determine accuracy of TL dating but water lain sediments have not been studied in depth. TL dates for waterlain sediments can be obtained but are they accurate?

Basics of the Study Looked at three different types water lain sediments lacustrine, glaciomarine, and floodplain. 2 samples were taken from a previous study and the other 15 were sampled specifically for this study. Looked at sediments in the Puget Sound, many of the samples were taken from Whidbey Island. Samples were taken from sediments that have already been accurately dated using other methods. These ”expected” dates were used to determine accuracy of TL.

Lab Procedure Samples are collected from each site and then heated for a number of days. The heat empties the electron traps and this produces luminescence that can be measured. This luminescence can then be related to an equivalent dose (de) from a calibrated source. After this point the effective dose rate can be obtained (Dose rate).

Issues with TL Many sediments are not effectively zeroed, this can especially be the case in water lain sediments. When dating using TL the assumption is that sediments are ”bleached” and if this is not the case then age overestimations occur. Signal stability in TL in sediments.

Lacustrine Sediments Zeroing of the TL sensitive feldspar was most effective in clays and strikingly ineffective in silts. Silts gave an over estimation of up to 5 times the expected age in some cases. Two reasons: Clay particles would remain in suspension for longer than silts in still water Clay particles would be transported further from the source This allows clays more time to be completely bleached SNOP-2s shows an example of over estimation

Glaciomarine Sediments Both of the proximal samples showed age over estimation, which is indicative of ineffective zeroing of ice proximal sediments. Estimation for the Double Bluff (DUBLF-1) sediment was consistent with expected age, effectively zeroed. Some glaciomarine sediments are datable by TL techniques but more guidelines for selection would be useful.

Floodplain Sediments Sample results are highly erratic, over and underestimations are present throughout. The two oldest floodplain sediments are more consistent with the expected age (close to within 1 standard deviation). Floodplain sediments with smaller grain sizes and that were deposited in quiet water show more promise. Refinement of sampling techniques, could make dating of floodplains more accurate

Overall Conclusion Lacustrine sediments can be dated accurately if using clay laminae instead of silts. Only distal glaciomarine sediments should be dated since ice proximal deposition has ineffective zeroing. Glaciomarine TL dating could benefit from more study on selection of samples. Floodplain sediments are highly erratic but show some promise in the older age samples, and especially in smaller grain lower energy regime sediments.

Sources Berger and Easterbrook, 1993

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