“Thermoluminescence dating tests for lacustrine, glaciomarine, and floodplain sediments from western Washington and British Columbia,” - Berger and Easterbrook 1993 Presented by: Cody M Mack ESS 433

Purpose of the study:  Use TL dating in a reconnaissance fashion on sediments with known ages to determine how effective TL dating is  With new constraints on the accuracy of TL, apply it to other sediments for dating

Why Thermoluminescence(TL)?  Few geochronometric techniques exist to capture the time frame that TL can  ~ 40ka to 780ka  Useful for deposits such as volcanic ash and most unheated sediments (the Quaternary sediment record is very diverse in terms depositional environment)  Lacustrine  Glaciomarine  Floodplain

Issues with TL  Incomplete zeroing of signal in some subaqueous depositional environments (Floodplain)  TL clock ‘resets’ when deposited under water, although there has been some issue with the clock resetting in some subaqueous environments when exposed to sunlight  May have issues with long-term-signal stability of TL in sediments from this region  Age underestimation on sediments older than 100ka

Previous Relevant TL Dating  Lacustrine-  Most effective zeroing of light-sensitive TL occurs during slow deposition by rainout – clay- rich lamina are preferred (Berger et al. 1987a)  Glaciomarine-  Zeroing of light-sensitive TL is ineffective withing ~1km of icefront (Jennings and Forman, 1992)  Floodplain  Slow deposition allows for effective zeroing of the light sensitive TL (Berger et. Al. 1990)

TL Dating Procedures  “Daylight empties light-sensitive electron traps (e.g., lattice-defect sites), and after burial natural ionizing radiations repopulate emptied traps.” – Berger and Easterbrook, 1993  In lab: traps are emptied by heating with electron- hole recombination, producing TL

TL Dating Procedures cont..  Used thick-source alpha-particle counting to measure U and Th concentrations in finely powdered samples  K concentrations were measured using commercial atomic absorption spectrophotometry  Data from these two methods were combined with estimates of average cosmic-ray dose rate (Prescott and Hutton, 1988) and estimates of past water content in the sample to get an effective dose rate, and from that sample ages

Conclusions - Lacustrine  Confirmation that clayey laminae are likely the only sediments with effectively zeroed light-sensitive TL  Tests of older lamina indicates no age underestimation of sediments older than 100ka  Promising for older lacustrine sediments being accurately dated through TL

Conclusions - Glaciomarine  Tests on two out of the three glaciomarine sediments produced large overestimates of TL age  These two samples were deposited close to the ice fronts  Future TL dating studies of glaciomarine deposited sediments will have to choose samples further from ice front so that they have effective zeroed light-sensitive TL

Conclusion - Floodplain  Confirmation of earlier observations; deposits laid down near fluvial channels unsuitable for TL dating procedures  Fine-grained floodplain sediments deposited in low energy environments might still be suitable  E.g. oxbow lakes, marshes, swamps, abandoned channels

Sources  Berger and Easterbrook, 1993  Berger et al. 1987a  Jennings and Forman, 1992  Berger et. Al  Prescott and Hutton, 1988