Electronic Supplement to
'Slip rate on the Wellington Fault, New Zealand, during the late Quaternary: Evidence for variable slip during the Holocene'
by Dee Ninis, Timothy A. Little, Russ J. Van Dissen, Nicola J. Litchfield, Euan G. C. Smith, Ningsheng Wang, Uwe Rieser and C. Mark Henderson
Part 1: Optically Stimulated Luminescence (OSL) Dating: Technical Report and Results
Preparation for Measurement of Equivalent Dose (De)
Samples had their outer surfaces removed. Of this removed outer scrapings, 100 g was weighed and dried in an oven in preparation for gamma spectrometer analysis. A plastic cube was then filled with remaining scrapings in preparation for water content measurment. About 150 g of unexposed light sample material was treated in 10% HCl. This was carried out overnight until all carbonate was removed by the reaction. Following this treatment the sample was further reacted overnight with 10% H2O2 in order to remove organic matter. The next step involved 200 ml CBD solution (71 g sodium citrate, 8.5 g sodium bicarbonate, and 2 g sodium dithionate per litre of distilled water) being added to the sample for 12 hours to remove iron oxide coatings. After every chemical treatment procedure distilled water was used to wash the sample several times .
Fine Grain Technique (4-11 μm)
After chemical treatment, calgon solution (1 g sodium hexametaphosphate per litre distilled water) was added to make a thick slurry. This slurry was placed into an ultrasonic bath and mechanically agitated for 1 hour. The sample was then placed into a 1 L measuring cylinder, filled with a certain amount of distilled water to separate out the 4-11 μm grains according to Stokes' Law. The 4-11 μm grains were then rinsed with ethanol and acetone and a suspension of these grains were then deposited evenly onto 70 aluminium disks.
Preparation of Measurement of Dose Rate
The dry, ground and homogenised sample material were weighed and sealed in air tight perspex containers and stored for at least four weeks. This storage time minimizes the loss of the short lived noble gas 222Rn and allows 226Ra to reach equilibrium with its daughters 214Pb and 214Bi.
Luminescence age was determined by two factors: the equivalent dose (De) (obtained from the lab equivalents to the paleodose absorbed by samples during the burial time in the natural environment since their last exposure to the light) and the dose rate (amount dose received by the sample each year).
Determination of Equivalent Dose (De) using MAAD and SAR
Multiple Aliquot Additive Dose Method (MAAD)
The test dose obtained from an initial test measurement was used for the MAAD. As luminescence vary between disks, all disks for MAAD need to be normalised before β irradiation. Six groups (30 disks divided by five) were β irradiated up to five times of the test dose. Three groups (three disks per group) were ∝ irradiated up to three times of the test dose. ∝ irradiation was carried out by the ELSEC Littlemore irradiator. These 39 disks together with nine non-irradiated disks (total of 48 disks) were stored for four weeks to relax the crystal lattice after irradiation.
After storage, the 48 disks were preheated for five minutes at 230oC, then were measured using a Riso TL-DA-15 reader with infrared diodes at 880 nm used to deliver a stimulated beam at the room temperature for 100 s. Blue luminescence centred about 410 nm emission from feldspar was then detected by an EMI 9235QA photomultiplier fixed behind two filters consisting of a Schott BG-39 and Kopp 5-58. Luminescence growth curve (β induced luminescence intensity versus added dose) was constructed by using the first a few seconds of the shine down curves and subtracting the average of the last 20 s, along with the so called late light which was thought to be a mixture of background and hardly bleachable components. Extrapolation of this growth curve to the dose axis was obtained using the equivalent dose De as a paleodose. The shine plateau was checked to be flat after this manipulation.
Measurement of a-value
A similar plot for the alpha irradiated disks allows for an estimation of ∝ efficiency and a-value (a-value is measured by comparing the luminescence induced by alpha irradiation with that induced by beta or gamma irradiation). The a-value was for dose rate calculation.
Single Aliquot Regenerative Method (SAR)
The Single Aliquot Regenerative Method (SAR) was used to determine the equivalent doses. This technique is described by Murray and Wintle (2000). For the SAR method, a number of aliquots (disks) were subjected to a repetitive cycle of irradiation, preheating and measurement. Firstly, natural shining down curves was measured after preheating. Then shining down curves were measured for the next four or five cycles for different beta doses. From the variety of shining down curves, a luminescence growth curve (β induced luminescence versus added dose) was established. This was used to determine the equivalent dose (equivalent to the palaeodose). The measurement for the aliquots resulted in a variety of equivalent doses, spread over the arithmetic mean of the data. In order to correct potential sensitivity changes from cycle to cycle, the luminescence response to a test dose was measured after preheat between cycles. Preheating temperature and time was 260oC for 20 s; and measurement time was 100 s at room temperature.
Determination of Dose Rate
Dose rate consisted of two parts (i) Dose rate from the sample's burial environment; and (ii) Dose rate from cosmic rays. Dose rate from the sample's burial environment was determined by radionuclide contents of 238U, 232Th and 40K, a-value and water content. Gamma rays produced from sample material was counted for a minimum time of 24 hrs by a high resolution and broad energy gamma spectrometer. The spectra were then analysed using GENIE2000 software. The contents of U, Th and K were obtained by comparison with standard samples. The dose rate calculation was based on the activity concentration of the nuclides 40K, 208Tl, 212Pb, 228Ac, 214Bi, 214Pb and 226Ra using dose rate conversion factors published by Adamiec and Aitken (1998). Water content was measured as weight of water divided by dry weight of the sample taking into account a 10% uncertainty. Dose rate from cosmic rays were determined by the depth of sample below the surface along with its longitude, latitude and altitude.
Table S1. Cosmic Dose Rates
|Laboratory Code||Depth Below Surface (m)||Cosmic Dose Rate (Gy/ka)||Field Code|
Table S2. Water Contents, Radionuclide Contents
|Laboratory Code||Water content %||U (ppm) from 234Th||U (ppm) from 226Ra, 214Pb, 214Bi||U (ppm) from 210Pb||Th (ppm) from 208Tl, 212Pb, 228Ac||K %||Field Code|
Table S3. a-Values, Dose Rates, Equivalent Doses and Luminescence Dating Ages
|Laboratory Code||a-value||De (Gy)||Dose rate (Gy/ka)||OSL age (ka) (1s)||Field Code|
|*WLL482||0.071±0.013||323.7±13.9||9.90±0.29 (3.34±0.29)||95.4±9.4 (97.0±9.4)||WLL482|
|WLL855||0.08±0.01||351.0±3.6, #243.0±22.0||4.72±0.35||74.3±5.5, #51.4±6.0||DOG684|
|WLL856||0.018±0.003||198.8±28.4, #248.0±15.6||3.29±0.10||60.3±8.8, #75.3±5.3||DOG552|
|WLL857||0.040±0.004||304.3±44.75, #176.5±15.21||3.50±0.10||87.0±13.0, #50.5±4.6||DOG484|
Adamiec, G. and M. Aitken (1998). Dose- rate conversion factors: update. Ancient TL, 16, no.2, 37-50.
Murry, A.S. and A.G. Wintle (2000). Luminescence dating of quartz using an improved single aliquot regenerative dose protocol. Radiat. Meas. 32 57-73.
Prescott, J.R. and J.T. Hutton (1994). Cosmic ray contributions to dose rates for luminescence and ESR dating: Large depths and long-term time variations. Radiat. Meas. 23, nos. 2/3, 497-500.