1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
Optically stimulated luminescence dating of glaciofluvial sediments on the
Canterbury Plains, South Island, New Zealand
Ann V. Rowana*, Helen M. Robertsb, Merren A. Jonesa, Geoff A.T. Dullerb, Steve J.
Covey-Crumpa and Simon H. Brocklehursta
a
School of Earth, Atmospheric and Environmental Sciences, University of
Manchester, Oxford Road, Manchester M13 9PL, UK
b
Aberystwyth Luminescence Research Laboratory, Institute of Geography and Earth
Sciences, Aberystwyth University, Aberystwyth, Ceredigion SY23 3DB, UK
*Corresponding author: ann.rowan@manchester.ac.uk
Tel: +44(0)161 306 9360
Fax: +44(0)161 306 9361
Keywords: OSL; coarse-grained quartz; SAR; Canterbury Plains; LGM; braided river
New Zealand is a key location for investigating the geomorphic response of fluvial
systems over glacial-interglacial timescales, and as such provides a potentially rich
archive of Quaternary climate change. Identification of the climatic response of fluvial
systems requires the application of a reliable geochronological method to place the
sedimentary record within the context of the regional climate history. Optically
stimulated luminescence (OSL) dating offers the opportunity to generate ages from
quartz in glaciofluvial sediments, and so has many possible applications in South
Island. However, in applying this method, previous studies have encountered
problems of low OSL signal intensities in quartz. This has limited the application of
quartz OSL in South Island; most geochronological studies have instead used
feldspar for luminescence dating, but have been affected by problems such as
weathering. In this study, we found that although the OSL signal levels from quartz
are low, a useable OSL signal can be observed from medium-sized aliquots
containing ~500 grains of quartz separated from samples from eastern South Island.
Mathematical component separation of the quartz OSL signal indicated that the
signal is dominated by a fast component. Ages produced using the central age model
range from 18.2 ± 1.3 to 36.7 ± 2.9 ka, are in stratigraphic order, and agree with
independent age control from two 14C ages. This study demonstrates the successful
application of quartz OSL to glaciofluvial sediments from Canterbury, and its potential
to provide a chronology for sedimentary records of climate change in this region.
1. Introduction
The three major braided river systems of the southern Canterbury Plains, South
Island, New Zealand (Fig. 1), the Rakaia, Ashburton and Rangitata, present an
excellent opportunity to investigate the impact of climate change in glaciofluvial
environments. The stratigraphic record resulting from these rivers is considered to
span the Last Glacial Maximum (LGM) (e.g. Ashworth et al., 1999), although the
precise timing of sediment deposition has not been correlated with the wellestablished late Quaternary climate chronology of New Zealand (Alloway et al.,
2007). To make this correlation between sediment deposition and climate variations
it is necessary to find a reliable geochronological method that can be applied to the
glaciofluvial sediments. The Canterbury Plains coastal stratigraphy is dominated by
gravel-bed braided river deposits in which organic material is rare, so a
geochronological method that does not rely on organic carbon is necessary;
1
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
luminescence dating is one such method.
2. Optically Stimulated Luminescence (OSL) Dating
2.1 OSL dating of quartz
Luminescence dating is one of the few geochronological methods that can be applied
in glaciofluvial sedimentary environments to the sediments themselves (e.g. Duller,
2006; Fuchs and Owen, 2008), whereas radiocarbon dating relies on the presence of
organic material. However, these environments pose challenges for optically
stimulated luminescence (OSL), because rivers typically have high suspended
sediment loads and the sediment transport paths are frequently short (Wallinga et al.,
2001; Rodnight et al., 2006; Thrasher et al., 2009). A key question that needs to be
considered is whether the OSL signal was completely reset during transport prior to
deposition, so that the age generated does not include an inherited component from
a previous depositional event.
Prior to burial, the OSL signal is reset by sunlight as the sediment is transported in
the river channel. Incomplete bleaching can be a problem in some fluvial applications
of OSL dating (Olley et al., 1998, 1999; Duller, 2004), leading to an over-estimation
of the age of deposition, as water and suspended sediment attenuate sunlight
through the water column. A review of the application of OSL to fluvial sediments by
Jain et al. (2004) showed that the inherited signal is likely to be small (<5 Gy) and so
less likely to affect older samples (>1 ka), but this survey did not consider
glaciofluvial sediments. If incomplete bleaching does occur then this may be detected
in the OSL measurement protocol by examination of the sample De distribution, as
seen by Olley et al. (1999). Furthermore, the OSL signal from quartz resets more
rapidly than that from feldspar (Thomsen et al., 2008) and so quartz is a more
suitable material to use for dating in high-energy environments such as proglacial
rivers (e.g. Spencer and Owen, 2004).
To reduce the effects of incomplete bleaching, careful selection of the appropriate
sedimentary facies from which the luminescence samples are taken greatly improves
the likelihood of success (Duller, 2006). The relevant criteria when dating glaciofluvial
sediments are outlined by Fuchs and Owen (2008) and Thrasher et al. (2009), and
include sampling from depositional environments known to have a high bleaching
potential, typically those deposited during waning flow stages.
2.2 OSL dating in New Zealand
There have been relatively few recent (<10 a) quartz OSL studies in New Zealand
(Nichol et al., 2003; Litchfield and Rieser, 2005). A notable success in early quartz
OSL dating from South Island, Holdaway et al. (2002) produced a luminescence
chronology for colluvial sediments (Richard Holdaway, pers. comm.) in Otago that
agreed well with 14C ages, using 10 mm aliquots containing ~800 grains. In contrast,
are results from a study by Preusser et al. (2006), who used quartz prepared by
crushing fluvial boulders and other rock samples from Westland, and one
sedimentary sample from the forefield of the Franz Josef Glacier, to investigate the
effect of bedrock geology, metamorphic grade and deformation history on quartz
luminescence. The Westland quartz displayed extremely low OSL signal intensities
and large changes in sensitivity. Preusser et al. (2006) noted that the OSL signal
appeared to originate from many dim grains rather than a few bright grains, and that
the signal was affected by thermal transfer (Rhodes, 2000; Rhodes and Bailey, 1997)
at low preheat temperatures. The poor luminescence behaviour of the Westland
quartz was not attributed to variations in the bedrock geology of the samples, but is
2
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
suggested to be related to the young sedimentary history of the quartz, and possibly
also due to the absence of suitable traps for OSL production in this quartz (Preusser
et al., 2006). Westland, on the west side of the Main Divide of the Southern Alps, has
short proglacial sediment transport distances (<25 km). The sedimentary sample
investigated by Preusser et al. (2006) was taken <10 km downstream from the ice
front of the Franz Josef Glacier. The east coast of South Island may be more viable
for quartz OSL because proglacial sediment transport distances are at least double
(>50 km) those on the western side of the Southern Alps, thereby generating the
possibility for increased luminescence sensitivity of quartz in the same depositional
environment (Pietsch et al., 2008).
Instead of working with the OSL signal from quartz, other recent luminescence
studies in South Island have used the infra-red stimulated luminescence (IRSL)
signal from coarse-grained potassium-rich feldspars from North Westland (Preusser
et al., 2005), or from polymineral fine-grained sediment samples (Hormes et al.,
2003; Vandergoes et al., 2005; Rother et al., 2009; Shulmeister et al., 2010) primarily
obtained from the abundant loess deposits (e.g. Berger et al., 2001a, 2002; Almond
et al., 2001, 2007; Litchfield and Lian, 2004; Preusser et al., 2005). While some
studies (e.g. Hormes et al., 2003) report good agreement between polymineral finegrain IRSL and 14C ages, others report significant stratigraphic inconsistencies in
IRSL ages (e.g. Berger et al., 2001b), which is attributed to the highly weathered
nature of feldspars from this region. Further complications are noted in southern
Westland where potassium-rich feldspars are less common in loess deposits (e.g.
Almond et al. 2001; Berger et al., 2001b). Some doubt therefore remains over
whether South Island sediments are suitable for any luminescence protocol. For
example, Almond et al. (2001, 2007) comment that on the basis of their studies using
IRSL for polymineral samples, neither feldspar (due to the highly weathered nature
and age underestimation) nor quartz (due to dim signal) is likely to be suitable for
routine luminescence dating, although they do conclude that further investigation of
quartz, or of the feldspathic inclusions within quartz, may still hold promise for the
determination of luminescence ages for sediments from New Zealand.
Use of the OSL signal from quartz for dating sediments from South Island would
circumvent the potential problems of weathering and the paucity of potassium-rich
feldspars observed by Almond et al. (2001, 2007). Additionally, quartz is not affected
by anomalous fading, a phenomenon which some workers believe to be ubiquitous
(e.g. Huntley and Lamothe, 2001; Huntley and Lian, 2006), although observations of
fading in samples from New Zealand varies from one study to another; Preusser et
al. (2005) do not detect any fading, while Huntley and Lian (2006) observe g values
of 2-5% per decade. Use of a sensitivity-corrected Single Aliquot Regenerative dose
(SAR; Murray and Wintle, 2000) measurement protocol on quartz has been shown to
provide accurate and precise age determinations for sediments from a wide variety of
depositional settings around the globe (e.g. Murray and Olley, 2002; Rittenour, 2008;
Roberts, 2008). This paper seeks to apply quartz OSL to date glaciofluvial sediments
from the Canterbury Plains, South Island.
3. Canterbury Plains, New Zealand
New Zealand has undergone at least five major glaciations during the Quaternary, in
addition to the Ross and Porika glaciations at 2.4–2.6 and 2.1–2.2 Ma. The last
glacial, the Otiran (marine oxygen isotope stage (MIS) 2–4), was preceded by the
slightly more extensive Waimea (MIS 6) (Suggate, 1990; Newnham et al., 1999).
Multiple smaller, short-lived advances and retreats have been identified within the
3
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
Otiran (e.g. Soons, 1963; Soons and Gullentops, 1973; McGlone, 1995; Williams,
1996; Fitzsimmons, 1997; Suggate and Almond, 2005). During the LGM (18–24 ka,
Alloway et al., 2007), central South Island was extensively glaciated (Fig. 1), and ice
extended ~4 km beyond the range front in the Rakaia catchment (Shulmeister et al.,
2010). Several workers support the interpretation of an extended LGM, with the
onset of glaciation at ~29 ka, a warm period from 24–26 ka (Suggate and Almond,
2005; Newnham et al., 2007), and rapid deglaciation occurring from 14 ka (Suggate,
1990; Shulmeister et al., 2005). Evidence from terrestrial cosmogenic nuclide dating
in northern South Island suggests an additional Otiran glacial maxima occurred at
~32 ka (Thackray et al., 2009).
The Canterbury Plains are considered to have formed during cold stages as a major
region of glacial outwash deposition (Bal, 1996; Ashworth et al., 1999). Termination 1
was marked by a regional change from aggradation to incision (Alloway et al., 2007)
followed by rapid environmental change, as rivers reworked and removed glacial
sediments (Mabin, 1987). Postglacially, a series of smaller glacial advances and
periods of Holocene warming have been identified from various climate proxy
records (Suggate, 1990; Newnham et al., 1999; Alloway et al., 2007; Schaefer et al.,
2009; Putnam et al., 2010; Kaplan et al., 2010) although some controversy exists as
to their timing (e.g. Denton and Hendy, 1994, and replies; Barrows et al., 2007a, and
replies).
3.1 Geological setting
The southern Canterbury Plains, on the eastern side of central South Island, New
Zealand (Fig. 1), comprise a series of coalesced alluvial floodplains of late
Quaternary age that presently cover an area of ~5000 km2. Glacial outwash and
gravel-bed braided river sediments up to 650 m thick have been deposited over the
last 400 ka (Bal, 1996), on top of the Carboniferous-Triassic Rakaia terrane
greywackes that form the Torlesse Supergroup basement of the region (Cox and
Barrell, 2007). The sediment supplied to the braided river systems is predominantly
derived from the greywackes. The modern Rakaia, Ashburton and Rangitata Rivers
occupying the Canterbury Plains flow perpendicular to the coast. Palaeo-drainage
patterns are considered to have also had this orientation (Leckie, 2003), although it is
thought that the braidplains were at times more mobile than at present (Browne and
Naish, 2003). The rivers have incised into the Plains since the end of the Otiran
glacial, despite glacioeustatic sea level rise, and now the uppermost 6–25 m of the
late Quaternary deposits form a coastal cliff which extends in width 70 km from the
mouth of the Rangitata to the Rakaia (Fig. 2c). The modern coastal cliff section is
located ~50 km from the range front of the Southern Alps. The shallow slope of the
Canterbury shelf means that during glacial periods when ice reached the range front,
the coastline moved ~60 km offshore from its modern location following sea level fall
of 120 m (Fig. 1; Newnham et al., 1999), and so at such times the modern coastal
section would have been in the medial part of the braided river systems.
The chronology of the sediments in the exposed cliff sections is poorly known, but
can be constrained towards the base by two radiocarbon ages of >35.5 and >35.4
14
C ka BP (NZ5290A and NZ5291A; Brown et al., 1988) for samples taken from 5
and 11 m below current sea-level in a core drilled within a few km of the current
mouth of the Ashburton River. At the top, the overlying loess sheet is generally
considered to be Holocene in age (Tonkin et al., 1974; Berger et al., 1996);
consequently the braided river stratigraphy includes the LGM. The climate history of
New Zealand through the period represented by the Canterbury coastal section is
4
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
well constrained, and has been collated from a wide variety of sources by the NZINTIMATE project (Alloway et al., 2007).
3.2 Stratigraphy
The gravel-bed braided river stratigraphy exposed at the coast is divided by subhorizontal surfaces that appear to extend over tens to hundreds of metres, and which
perhaps represent periods of little or no deposition (Ashworth et al., 1999). The
majority of the gravel accumulation is usually considered to have occurred in rapid
pulses during glacial periods (Ashworth et al., 1999; Leckie, 2003; Browne and
Naish, 2003), but more detailed analysis of the timing of sediment aggradation is
required to confirm such suggestions. Between the surfaces, stacked sets of gravelbed braided river deposits can be clearly seen, representing either one or two
channel flow depths in height. The gravel-bed braided river sediments are overlain by
a fluvial sand sheet 1–2 m thick, which is in turn covered by the 0.25–2.5 m thick
loess sheet that makes up the top of the cliff (Fig. 2a).
Finer-grained units occur within the fluvial gravel stratigraphy, and the OSL samples
were taken from these sand bodies, i.e. from channel fills, bar margin and bar top
sediments (Fig. 2b). Sedimentary facies were defined using the classification scheme
of Moreton et al. (2002). The facies selected for sampling are those that are most
likely to have had their OSL signal reset, as they have been deposited during the
waning stages of flow (Thrasher et al., 2009). Samples were taken for OSL dating at
four sites within the Canterbury coastal section (Fig. 1), and include material from
each of the three river systems. From north to south the sampling sites are: K04
(43.9442°S, 172.0673°E), C06 (44.0169°S, 171.8890°E), A25 (44.0575°S,
171.7937°E) and B08 (44.1179°S, 171.6472°E).
4. Methods
4.1 Sampling and sample characteristics
Nine unconsolidated coarse-grained sand samples were collected for OSL dating
from the Canterbury coastal cliff section using aluminium cylinders (250 mm long x
55 mm diameter) hammered into a cleaned section face (Table 1). The sampling
cylinders were removed and sealed with light- and water-tight plastic material for
transport to the UK. Before shipping, several centimetres of sediment was removed
from the light exposed end of each sample under darkroom conditions to prevent this
sediment mixing with the non-exposed sample material.
4.2 Sample preparation
Sample preparation and luminescence measurements were carried out at
Aberystwyth Luminescence Research Laboratory. The samples were opened in
subdued red light laboratory conditions. A standard laboratory preparation procedure
for coarse-grained quartz was used: ~200 g of each sample was treated with a 10%
volume for volume dilution of concentrated (37%) hydrochloric acid followed by 20
vols. hydrogen peroxide to remove any possible carbonates and organic material
respectively. The samples were then dry sieved to separate the 180–211 μm
diameter fraction, prior to density separation using solutions of sodium polytungstate
to isolate the 2.62–2.70 g cm-3 fraction. This quartz-rich fraction was etched with 40%
hydrofluoric acid for 45 min to etch any remaining feldspars and the outer alphairradiated portion of grains, followed by concentrated (37%) hydrochloric acid for 45
minutes to dissolve any insoluble fluorides that may have formed. The samples were
then sieved again as a further purification step to remove any grains reduced in
5
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
volume by this etching procedure. This resulted in a 180–211 μm fraction of pure
quartz grains that was used for OSL measurements.
The complete removal of feldspar from the samples is established by internal checks
within the SAR measurement protocol (using the OSL-IR depletion ratio of Duller,
2003), and was verified using scanning electron microscopy (SEM) and X-ray
diffraction (XRD) analyses at the University of Manchester. In one sample (C06-A3),
XRD revealed the presence of aluminium fluoride (AlF3) formed during sample
preparation, but this appears not to have had an adverse impact on the
luminescence characteristics of the prepared material.
4.3 Equipment
Luminescence measurements were made on two Risø TL/OSL readers (BøtterJensen et al., 2003) equipped with 1.48 GBq 90Sr/90Y beta sources. Grains were
mounted in a monolayer on 9.7 mm diameter aluminium discs using silicone oil, to
make medium aliquots containing ~500 grains covering a diameter of 5 mm (Duller,
2008). Quartz OSL was stimulated using blue (470 Δ 20 nm) light emitting diodes
(LED), and signals were detected using 7.5 mm of Hoya U340 filter in front of the
photomultiplier tube. The two OSL units have very different optical stimulation
powers; one is equipped with a prototype diode system and delivers 2.28 mW cm-2,
while the other delivers 19.7 mW cm-2. However, the stimulation spectrum of both
systems, and their performance, is otherwise identical. To accommodate these
different stimulation powers, OSL was measured for either 40 s or 100 s, and the
initial 0.8 s and the final 8 s or the initial 2 s and the final 20 s respectively of each
OSL decay curve were used to define the signal and background.
4.4 Dose rate assessment
Environmental dose rates (Table 1) were calculated from concentrations of
radioisotopes obtained using mass and emission spectrometry on unseparated, dried
sediment taken from each sample tube. Potassium oxide was measured by ICPOES, and uranium and thorium by ICP-MS at Royal Holloway University, London.
The concentrations of U, Th and K were then used to calculate the dose rates using
the conversion factors of Adamiec and Aitken (1998). Cosmic dose rates were
determined using a mean sediment density (including pore spaces) of 1.8 g cm-3, the
sample site latitude, elevation relative to sea level and an overburden thickness
based on modern values for each sample (Prescott and Hutton, 1994).
Gravimetric field water content (WCgrav, expressed as mass of water/mass of dry
sediment x 100%) was measured in the laboratory using the innermost part of each
sample, and gave field water contents that ranged from 1.4–20.1% (Table 1). A value
of 15 ± 5% was chosen as representative of mean water content throughout the
depositional history of all the samples. This value is higher than modern WCgrav
values (7 ± 3%) measured for sand facies in Canterbury coastal deposits (Dann et
al., 2009), but has been chosen to represent the value over the last 40 ka, on the
basis that the water content is likely to have been higher in the past when these sites
were buried, whereas in the present day the sites form a cliff. The effective beta and
gamma dose rates to the samples were calculated taking account of grain size and
water content (Table 1).
The thickness of the units sampled varied from 15–200 cm. In each case, care was
taken to sample from the centre of these units, and where possible at least 30 cm
from a stratigraphic boundary. The gamma contribution to the dose rate originates
6
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
from a sphere with a radius of ~30 cm, and so for the sand samples collected from
units <60 cm in thickness, a proportion of the gamma dose rate arises from the
surrounding gravels. However, calculations based on the thinnest unit (15 cm) show
that the contribution from the surrounding gravels accounts for 34% of the gamma
dose rate, which in turn is only 13% of the total dose rate. Additionally the gravels
and sands have a homogeneous source lithology, and so large variations in the
concentration of radionuclides would not be expected.
To confirm that the assessment of gamma dose rate based upon geochemical
measurements of the sand unit sampled is appropriate, in situ dosimeters made of
aluminium oxide treated with carbon (Al2O3:C; Burbidge and Duller, 2003) were
deployed to directly measure the field gamma dose rate, using a plastic housing to
shield the dosimeters from any beta dose contribution. Three sample locations were
selected; the thickest unit (B08-B2, 200 cm) and the two thinnest units (A25-A1, 15
cm; K04-A1, 16 cm). The ratio of these field and laboratory gamma dose rates was
0.99 ± 0.10, indicating that the doses obtained from laboratory geochemical analyses
were representative of bulk sediment dosimetry. Since geochemical values were
available for all of the samples, these were used for determination of all dose rates
shown in Table 1.
4.5 Determining measurement conditions
The Single Aliquot Regenerative (SAR) dose protocol (Murray and Wintle, 2000) has
been widely adopted for OSL dating of coarse-grained quartz. To optimise the SAR
measurement protocol, a preheat plateau test is typically used to determine suitable
preheat conditions. However, where sediments are likely to be incompletely
bleached, such tests are not appropriate because scatter in the natural De values can
obscure any trends with preheat temperature. Instead, suitable preheat conditions
can be determined using the ability to recover a given laboratory dose for different
preheat temperatures.
In this study, 34 aliquots of sample C06-A2 had their natural signal removed by
exposure to blue LED stimulation for 1 ks at room temperature, with a 10 ks (2.8 hr)
pause between bleaches to allow charge in the 110°C trap to empty (Murray and
Wintle, 2006), followed by another 1 ks blue LED stimulation. The aliquots were then
given a beta dose of 27.8 Gy. Measurements were made using a range of OSL
preheat temperatures (160–280°C) using a minimum of three aliquots for each
temperature. Initially a preheat of 160°C with immediate cooling was used prior to
measurement of the test dose response (Tx). Dose recovery results are shown in Fig.
3a (open symbols) and a plateau is identified between temperatures of 180–220°C,
but a large amount of scatter is observed between aliquots. A test dose preheat of
220°C for 10 s was also investigated (Fig. 3a) and this reduced the scatter. Optimal
dose recovery was found by making both preheats 220°C for 10 s. Fig. 3b shows the
mean dose recovery for three aliquots of each sample in this study using these
preheats. Low signal intensities lead to uncertainties on the De of typically 8% on
individual aliquots, but statistical analysis of the doses recovered shows a mean and
standard deviation of 26.7 ± 2.6 Gy, that is, 96 ± 9% of the given laboratory dose
(27.8 Gy). For the remainder of this study, the measurement protocol given in Fig. 4
is used, which employs regenerative and test dose preheats of 220°C for 10 s.
4.6 Aliquot size
The glaciofluvial origin of these sediments means that incomplete bleaching is a
potential consideration. Duller (2008) highlights the importance of aliquot size in OSL
7
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
dating in such environments. A balance must be struck between creating an aliquot
containing sufficient grains to give a viable OSL signal for dating, versus creating an
aliquot small enough so as to avoid the averaging effects that arise when aliquots
contain many grains which may have experienced different degrees of bleaching and
hence have varying De values. To assess the appropriate aliquot size to use for the
Canterbury samples, single grain measurements were made to determine the
proportion of grains giving signal and the amount of signal given by each grain.
Single grain measurements were made using a focused 10 mW Nd:YVO4 green laser
emitting at 532 nm (Duller, 2003). Single grain analyses were carried out on 1000
grains each from two samples (C06-A4 and K04-A2). Grains had their natural OSL
signal removed by optical stimulation at room temperature, were given a 93 Gy dose
and a preheat of 220°C for 10 s, and the OSL signal arising from this dose was then
measured.
402
403
404
405
406
407
408
409
410
411
412
413
414
where I is the total signal, t is measurement time in s, and n0i and bi are respectively
the number of trapped electrons at t=0 and a constant describing the decay of the
luminescence curve that is proportional to the detrapping probability, each for the two
exponentials, and a constant c represents the signal background (Bailey et al.,
1997). Eq. 1 was fitted to the OSL and test dose responses using a least squares
fitting method employing the Levenberg-Marquardt algorithm (implemented in
MATLAB®) following the method used by Choi et al. (2006). Data were also
transformed to pseudo linearly modulated OSL curves (Bulur, 2000) to confirm that
an adequate number of components had been fitted, but the component coefficients
used were generated using the CW-OSL data. Component fitting of CW-OSL data is
challenging, and when signals are weak, noise in these data can create unrealistic
fitting results. Therefore a 5-step strategy was adopted to obtain the best estimates
for the values of b1 and b2 in Eq. 1 for each sample:
The grains were ranked in order of their net OSL brightness giving the cumulative
light sum shown in Fig. 5a; for both samples, 90% of the OSL signal is derived from
~3% of the grains. The small proportion of grains contributing to the total light sum is
typical of observations for detrital quartz (Duller, 2008), but this does not give any
information about the absolute intensity of the signal. Fig. 5b shows the net OSL
signal expressed in counts per 0.17 s per Gy and shows that even the brightest
grains do not exceed 20 counts per 0.17 s per Gy, which is at the bottom of the
range seen by Duller (2008, Fig. 5), making single grain work impracticable. This
observation concurs with that of Preusser et al. (2006) for quartz from Westland, who
concluded that Westland quartz is unlikely to be suitable for dating using small
aliquots or single grains due to the dim OSL signal. Medium-sized aliquots containing
~500 grains are therefore used throughout the remainder of this study, as a
compromise between achieving detectable signal levels whilst providing the
opportunity to detect variability in De.
4.7 Signal characterisation
The presence in the OSL signal of a fast component is essential to the success of
the SAR protocol (Watanuki et al., 2005; Wintle and Murray, 2006). To verify that a
fast component is dominant in the OSL signals measured in this study, the OSL
responses were mathematically separated into their component parts (Fig. 6a and b).
This was done by fitting a summed exponential function to the continuous wave OSL
(CW-OSL) data;
I t   n01b1eb1t  n02b2eb2t  c
Eq. 1
8
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
1. Eq. 1 was fitted to every SAR OSL decay curve for a single aliquot.
2. The same data were then refitted, holding b1 at its mean value derived from
step one.
3. The data were refitted again using the mean b1 value from step one and the
mean b2 value from step two.
4. Steps one to three were then repeated for every aliquot of the sample.
5. The mean values of b1 and b2 for the sample were calculated from step four
(Table 2) and were used for a final fitting to determine n0 for each component
for each aliquot.
This process has been automated to enable component fitting for every OSL and test
dose response for all aliquots (n = 384 aliquots) of each sample. Fig. 6a and b show
examples of CW-OSL data and the results of component fitting. The lack of structure
in the residuals shows the quality of the fit obtained.
The b values for each component were used to calculate the photoionisation crosssection of that component. The stimulating powers of the two LED units used were
19.7 mW cm-2 for samples C06-A3, K04-A1 and K04-A2, and 2.28 mW cm-2 for the
remaining samples. Mean photoionisation cross-section values for the Canterbury
samples are 5.4 x 10-17 cm2 for the first component and 7.4 x 10-18 cm2 for the second
component. These values are the same order of magnitude as those for the fast and
medium component as defined by Jain et al. (2003).
After fitting Eq. 1 as described above, the value of n0 for the fast component for each
OSL response (Lx and Tx) was used to construct a dose response curve for each
aliquot. Fig. 6c and 6d compare the responses of two aliquots based solely on the
fitted fast component with those obtained from integration of the OSL signal from the
first 2 s (stimulated using the weaker of the two LED units) of the OSL decay curve.
The Lx/Tx ratios and resulting dose response curves produced using either the
integrated OSL response, or a separated fast component, are indistinguishable; the
resulting De values are also consistent within uncertainty.
The fast component De and the integrated OSL De values are compared (Fig. 7). On
average, for all samples, the ratio of the De values derived from the fast component
divided by those from the integrated OSL signal is 1.11 ± 0.30 (Table 2). The large
scatter in the data shown in Fig. 8 makes it difficult to assess whether there is any
systematic difference between the two sets of De values. The component separation
results show that, for aliquots that pass the SAR screening criteria, the fast
component dominates the luminescence signal (Fig. 6). Given the difficulties involved
in fitting dim OSL signals such as those observed in this study, due to low signal to
noise ratios, the integrated OSL signal is used to determine De in this study.
4.8 Equivalent dose determination
The SAR protocol (Murray and Wintle, 2000; Wintle and Murray, 2006) as described
in Fig. 4 was applied to determine De values for each aliquot. All OSL measurements
were collected at a temperature of 125°C using a minimum of four regenerative dose
points selected to bracket the expected De value. One of these regenerative points
was repeated to allow the calculation of a recycling ratio (Murray and Wintle, 2000).
The suitability of the data was assessed based on several criteria: (1) a recycling
ratio of 1.0 ± 0.1, (2) an OSL-IR depletion ratio of 1.0 ± 0.1 (Duller, 2003), (3) a
detectable OSL signal (i.e. >3σ above background), (4) whether the sensitivity
corrected natural signal (Ln/Tn) intersects the dose response curve, and (5) whether
signal recuperation following a zero dose was detected. The most common cause for
9
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
rejecting an aliquot was criterion 3, lack of a detectable OSL signal, and 22% of the
measured aliquots failed on this basis. This is not surprising given that the single
grain data presented in section 4.6 implied that the signal from each aliquot
originates from a small number of grains. A smaller number of aliquots failed the
recycling ratio test (4%), giving values that were not within 1.0 ± 0.1.
The OSL-IR depletion ratio (Duller, 2003) was assessed within the SAR sequence to
identify feldspar contamination in the samples; values of within 10% of unity were
required for aliquots to be accepted, with unity indicating no luminescence response
from feldspar. At low signal levels, the OSL-IR depletion ratio may give a non-unity
result when a very small IR response is present; this is due to the relatively large
variation between two similarly low signals. The IR response of each aliquot was
therefore also inspected visually, and if a decay curve was observed then the aliquot
was rejected; this resulted in 10% of aliquots being rejected.
72 aliquots were measured for each sample, with the exception of B08-B2 for which
48 aliquots were measured. The mean pass rate of aliquots for all samples was 62%.
Data were analysed using Analyst® software (v3.24, Duller, 2007) by fitting an
exponential or exponential plus linear term (depending on the degree of saturation)
to the dose-response curve. The instrumental uncertainty of the readers as described
in Duller (2007) is about 1% and this value is included in the calculation of
uncertainty of individual De values. Between 28 and 60 aliquots were accepted for
the samples (Table 3).
5. Results and discussion
5.1 Equivalent dose distributions
Equivalent dose distributions for each of the Canterbury samples are shown in Fig. 9
as radial plots. The overdispersion (σOD, a measure of the dispersion of the De
distribution beyond that expected based on measurement uncertainty) of these
distributions was calculated using the approach of Galbraith et al. (1999), and varied
from 10–31% (Table 3).
Values of overdispersion for well-bleached aeolian sediments reported in the
literature range from 0–18% for multiple grain aliquots, and 9–22% for single grains
(Galbraith et al., 2005), with a threshold σOD value of 20% (Olley et al., 2004) being
commonly used to define whether aeolian and fluvial samples are incompletely
bleached or not. Deciding whether overdispersion is due to incomplete bleaching or
other factors is important since it informs the method used to analyse the distribution
of De values obtained (e.g. central age model, CAM, versus the minimum age model,
MAM, Galbraith et al., 1999).
To investigate the sources of overdispersion, the dose recovery data (Fig. 3b) from
all samples were combined, analysed, and yielded an σOD of 4%. Ideally the value for
such an experiment would be expected to be zero, but it is not uncommon for a nonzero σOD value to be observed in dose recovery experiments. Jacobs et al. (2006)
observed values of 4.4 and 6.0% while Roberts et al. (2000) had values of 9 and
14%. The σOD measured on naturally irradiated samples will combine both intrinsic
sources of uncertainty (e.g. counting statistics) and extrinsic sources (e.g. beta
microdosimetry, incomplete bleaching) (Duller, 2008).
The De distributions of our samples have a mean σOD of 25% (Table 3), slightly larger
than the threshold of 20% commonly used to differentiate between well-bleached and
10
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
incompletely bleached samples. However, our dose recovery experiment showed
that there is 4% σOD associated with intrinsic sources of uncertainty that we are not
able to account for (cf. Thomsen et al., 2005), and so it is not necessary to invoke
incomplete bleaching to explain the σOD values seen in our natural samples.
Furthermore, where incomplete bleaching has been observed (Olley et al., 1999) the
distributions are characterised by a sharp ‘leading edge’; the data in this study (Fig.
9) do not have this form. A similar lack of asymmetry led Alexanderson and Murray
(2007) to conclude that their samples were not incompletely bleached, and the same
conclusion is drawn in the present study. Therefore the central age model (CAM,
Galbraith et al., 1999) has been applied to the De data for all samples, giving the De
values and ages shown in Table 3 and Fig. 10.
5.2 OSL ages and comparison with independent age control
The coarse-grain quartz OSL ages produced in this study range from 18.2 ± 1.3 to
36.7 ± 2.9 ka and so occur within the last glacial period identified by Suggate (1990).
The validity of these OSL ages can be assessed by a number of methods.
Firstly, one can assess the internal consistency by looking at samples C06-A4 and
C06-B4 which were taken from the same sand body spaced five metres apart
laterally (Fig. 2a). These have ages (31.7 ± 2.6 and 36.7 ± 2.9 ka) that agree within
one standard deviation. Secondly, the OSL ages generated (Table 3, Fig. 10) are in
stratigraphic order (within 2σ uncertainties) and all are older than the Holocene,
which is consistent with the assumed age of the overlying loess sheet.
A further means of assessing the OSL ages is by comparison with independent age
control. Fossil wood samples are not uncommon in the gravels of the Canterbury
Plains, and Brown et al. (1988) give details of a number of radiocarbon ages on
wood collected from bore holes. As described in Section 3.1, two samples collected 5
and 11 m below current sea level near the mouth of the Ashburton River yielded
infinite radiocarbon ages.
A large wood fragment (20-30 cm in diameter) was collected at the base of the
coastal cliff 4 km to the south of A25-A1 (Phil Ashworth, pers. comm.) (Fig. 10).
Although the OSL and radiocarbon samples are not immediately adjacent to one
another, the limited topography on the Canterbury Plains means that one can be
confident that the radiocarbon sample is ~2m lower in the stratigraphy than OSL
sample A25-A1. The sediment surrounding the organic material was a mottled, light
grey silt bed, interpreted to represent floodplain deposits within the braided river
system. The occurrence of organic material in the braided river sediments, whilst not
common, is expected, as the floodplain would have been stable enough for the
development of vegetation, including trees, as can be seen in the modern systems.
Furthermore, trees persisted in South Island through the last glacial period. Two
separate sub-samples of the wood were submitted for AMS radiocarbon dating. In
both laboratories the sample underwent a standard acid alkali acid pretreatment.
Radiocarbon ages of 35630 ± 500 BP (Beta-141092) and 30490 ± 320 BP (NZA11860) were obtained (δ13C values of -29.2 and -28.3%0 respectively) and yield
calibrated ages of 38.7 ± 1.1 cal. ka BP and 33.5 ± 0.8 cal. ka BP with a 95%
confidence interval (OxCal IntCal09, Reimer et al., 2009). The reason for the
discrepancy in the two radiocarbon ages is unclear, but is probably related to the
difficulties in measuring samples of such low radiocarbon activity. Although close to
the limit of radiocarbon, both analyses returned finite ages, bolstering confidence in
their veracity. Sample A25-A1 gives an OSL age of 33.0 ± 2.5 ka, entirely consistent
with both of the 14C determinations for the tree fragment found below it.
11
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
All De measurements were made on medium-sized aliquots containing ~500 grains;
this was a compromise between providing sufficient grains per aliquot to obtain an
OSL signal detectable above background, whilst keeping the number of grains
sufficiently low to avoid averaging that might obscure inter-aliquot variability in De.
Single grain measurements (section 4.6) showed that ~3% of grains contribute 90%
of the OSL signal. Thus, for a 500 grain aliquot on average ~15 grains dominate the
signal, but this will vary due to statistical fluctuations. This variability is confirmed by
the observation that 22% of the aliquots measured for De determination are rejected
due to the lack of a detectable signal (section 4.8). The agreement between duplicate
samples, stratigraphic relationships, and independent age control, all support the
selection of medium aliquots for analysis of these samples.
5.3 Stratigraphic context
The ages show that for the four sites in this study, the majority of the exposed cliff
section was deposited during the latter part of the Otiran (last) glacial, with only
limited vertical aggradation during the postglacial. Previous workers (e.g. Suggate,
1990; Bal, 1996; Ashworth et al., 1999; Browne and Naish, 2003) consider the
Canterbury gravels to have accumulated during cold stages, but their actual
chronology remained unknown, despite efforts to date these sediments (e.g. Brown
et al., 1988; Ashworth et al., 1999). As no evidence for gravel deposition was found
in well cores taken from the Canterbury shelf ~40 km offshore (Fulthorpe et al.,
2008), and as the modern braided rivers are incisional, it can be inferred that gravel
deposits found at the coast are associated with glaciation. Moreover, the OSL ages
presented here confirm that for at least two of our study sites all of the gravel units
were deposited between ~37 and 18 ka.
It is interesting to note that our OSL samples appear to fall into two age ranges (see
inset to Fig. 10), the upper five being between 18–24 ka, and the lower four being
between 31–37 ka. Although the number of OSL ages is limited, one possible
interpretation of the OSL ages is that, for the studied stratigraphy, sediment
accumulation appears to have occurred in two periods, with little or no sediment
apparently preserved within the period from 30–24 ka corresponding to the LGM
warming phase identified by Newnham et al. (2007). The 18–24 ka period of
sediment accumulation identified by the OSL ages coincides with the timing of ice
maxima at the LGM (Fig. 11 of Alloway et al., 2007). The evidence for gravel
accumulation between 31–37 ka is consistent with a period of glacial advance at this
time, similar to that seen by Thackray et al. (2009) in the Cobb Valley, Nelson,
northern South Island, and in Westland by Almond et al. (2001) and Suggate (1990).
Sea surface temperature data (inset to Fig. 10) indicate that the climate remained
mostly cool between 30–40 ka (Pahnke et al., 2003), shown by Barrows et al.
(2007b) to be a regional trend, supporting this hypothesis. Previous identification of a
pre-LGM glacial signal in the Canterbury Plains deposits could have been limited by
the coastal gravel sedimentology from this period being visually indistinguishable
from that deposited during the LGM.
6. Conclusions
OSL dating was applied to nine samples of glaciofluvial quartz from the Canterbury
Plains, New Zealand. Although glaciofluvial settings can result in low OSL signal
intensities, and this has been found by other workers elsewhere in South Island, we
found that the coarse-grained quartz from eastern South Island was suitable for a
SAR protocol. Dose recovery tests were used to determine an appropriate set of
12
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
measurement conditions, and on this basis preheats of 220°C for 10 s were selected
for luminescence measurements. Mathematical deconvolution of the OSL signal for
each step of the SAR protocol verified that, for all the aliquots that passed the SAR
screening criteria, the fast component was dominant. The CAM was applied to all of
the Canterbury samples to generate ages ranging from 18.2 ± 1.3 to 36.7 ± 2.9 ka,
all of which are in stratigraphic order within uncertainty, and are consistent with
independent age control. The low OSL signal intensity forced the use of medium
sized aliquots (~500 grains) that potentially might mask evidence for incomplete
bleaching. However the chrono-stratigraphic consistency achieved and agreement
with independent age control show that incomplete bleaching has not had a
significant impact on the OSL ages produced in this study. Future studies working in
glaciofluvial environments and with similarly dim quartz will also need to incorporate
similar checks.
This suite of OSL ages allows us to place the deposition of the Canterbury coastal
sediments at our study sites in the context of climate change that occurred within the
last ~40 ka. The OSL ages suggest that the majority of the studied stratigraphy was
deposited during the last glacial period, possibly as two distinct glacial advances,
with limited sediment accumulation taking place between these two periods and
since the end of the LGM (~18 ka).
Acknowledgements
This work was undertaken whilst AVR was in receipt of NERC studentship
NE/F008295/1. Additional funding for fieldwork was gratefully received from the
American Association of Petroleum Geologists Grants-In-Aid program. Profs. Phil
Ashworth and Jim Best are thanked for providing the two unpublished 14C dates
included in this paper (obtained under NERC grant GR3/11330). AVR would like to
thank everyone at ALRL for their assistance and hospitality, particularly Julie Durcan,
Hollie Wynne, Rachel Smedley and Lorraine Morrison. ALRL benefits from support
from the Climate Change Consortium of Wales (C3W). Thanks also to Kate Brodie
and John Waters at the University of Manchester for assistance with the SEM and
XRD analyses, and to Peter Bain for field assistance in New Zealand. Thorough
reviews by Frank Preusser and Olav Lian, and comments from the editor, Bert
Roberts, greatly improved this manuscript.
References
Adamiec, G., Aitken, M.J., 1998. Dose-rate conversion factors: update. Ancient TL,
16, 37–50.
Alexanderson, H., Murray, A.S., 2007. Was southern Sweden ice free at 19–25 ka, or
were the post LGM glaciofluvial sediments incompletely bleached? Quaternary
Geochronology, 2, 229–236.
Alloway, B.V., Lowe, D.J., Barrell, D.J.A., Newnham, R.M., Almond, P.C.,
Augustinus, P.C., Bertler, N.A.N., Carter, L., Litchfield, N.J., McGlone, M.S.,
Shulmeister, J., Vandergoes, M., Williams, P., NZ-INTIMATE members, 2007.
Towards a climate event stratigraphy for New Zealand over the past 30 000 years
(NZ-INTIMATE project). Journal of Quaternary Science, 22, 9–35.
Almond, P.C., Moar, N.T., Lian, O.B., 2001. Reinterpretation of the glacial chronology
of South Westland, New Zealand. New Zealand Journal of Geology and
Geophysics, 44, 1–15.
Almond, P.C., Shanhun, F.L., Rieser, U., Shulmeister, J., 2007. An OSL, radiocarbon
and tephra isochron-based chronology for Birdlings Flat loess at Ahuriri Quarry,
Banks Peninsula, Canterbury, New Zealand. Quaternary Geochronology, 2, 4–8.
13
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
Ashworth, P.J., Best, J.L., Peakall, J., Lorsong, J.A., 1999. The influence of
aggradation rate on braided alluvial behaviour: field study and physical scale
modelling of the Ashburton River gravels, Canterbury Plains, New Zealand. Fluvial
Sedimentology VI. Special Publication No. 28 of the International Association of
Sedimentologists, 333–346.
Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay form
characteristics of quartz OSL. Radiation Measurements, 27, 123–136.
Bal, A.A., 1996. Valley fills and coastal cliffs buried beneath an alluvial plain:
evidence from variation of permeabilities in gravel aquifers, Canterbury Plains,
New Zealand. Journal of Hydrology (New Zealand), 35, 1–27.
Barrows, T.T., Lehman, S.J., Fifield, L.K., de Deckker, P., 2007a, Absence of Cooling
in New Zealand and the Adjacent Ocean During the Younger Dryas Chronozone:
Science, 318, 5847, 86–89.
Barrows, T.T., Juggins, S., de Deckker, P., Calvo, E., Pelejero, C., 2007b. Long-term
sea surface temperature and climate change in the Australian-New Zealand
region. Paleoceanography, 22, PA2215.
Berger, G.W., Tonkin, P.J, Pillans, B.J., 1996. Thermoluminescence ages of postglacial loess, Rakaia River, South Island, New Zealand. Quaternary International,
34-36, 177–181.
Berger, G.W., Pillans, B.J., Tonkin, P.J., 2001a. Luminescence chronology of loesspaleosol sequences from Canterbury, South Island, New Zealand. New Zealand
Journal of Geology and Geophysics, 44, 501–516.
Berger, G.W., Almond, P.C., Pillans, B.J., 2001b. Luminescence dating and glacial
stratigraphy in Westland, New Zealand. New Zealand Journal of Geology and
Geophysics, 44, 25–35.
Berger, G.W., Pillans, B.J., Bruce, J.G., McIntosh, P.D., 2002. Luminescence
chronology of loess-paleosol sequences from southern South Island, New
Zealand. Quaternary Science Reviews, 21, 1899–1913.
Bøtter-Jensen, L., Andersen, C.E., Duller, G.A.T., Murray, A.S., 2003. Developments
in radiation, stimulation and observation facilities in luminescence measurements.
Radiation Measurements, 37, 535–541.
Brown, L.J., Wilson, D.D., Maor, N.T., Mildenhall, D.C., 1988. Stratigraphy of the
later Quaternary deposits of the northern Canterbury Plains. New Zealand Journal
of Geology and Geophysics, 31, 305–330.
Browne, G.H., Naish, T.R., 2003. Facies development and sequence architecture of
a late Quaternary fluvial-marine transition, Canterbury Plains and shelf, New
Zealand: implications for forced regressive deposits. Sedimentary Geology, 158,
57–86.
Bulur, E., 2000. A simple transformation for converting CW-OSL curves to LM-OSL
curves. Radiation Measurements, 32, 141–145.
Burbidge, C.I., Duller, G.A.T., 2003. Combined gamma and beta dosimetry, using
Al2O3:C, for in situ measurements on a sequence of archaeological deposits.
Radiation Measurements, 37, 285–291.
Choi, J.H., Duller, G.A.T., Wintle, A.G., 2006. Analysis of quartz LM-OSL curves.
Ancient TL, 24, 9–20.
Cox, S., Barrell, D.J.A., 2007. Geology of the Aoraki area. GNS Science. Institute of
Geological and Nuclear Sciences, Lower Hutt. 1:250,000 geological map 15.
Dann, R., Close, M., Flintoft, M., Hector, R., Barlow, H., Thomas, S., Francis, G.,
2009. Characterization and Estimation of Hydraulic Properties in an Alluvial
Gravel Vadose Zone. Vadose Zone Journal, 8, 651–663.
Denton, G.H., Hendy, C.H., 1994, Younger Dryas Age Advance of Franz Josef
Glacier in the Southern Alps of New Zealand: Science, 264, 5164, 1434–1437.
14
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence
measurements. Radiation Measurements, 37, 161–165.
Duller, G.A.T., 2004. Luminescence dating of Quaternary sediments: recent
advances. Journal of Quaternary Science, 19, 183–192.
Duller, G.A.T., 2006. Single grain optical dating of glacigenic deposits. Quaternary
Geochronology, 1, 296–304.
Duller, G.A.T., 2007. Assessing the error on equivalent dose estimates derived from
single aliquot regenerative dose measurements. Ancient TL, 25, 15–24.
Duller, G.A.T., 2008. Single-grain optical dating of Quaternary sediments: why aliquot
size matters in luminescence dating. Boreas, 37, 589–612.
Fitzsimmons, S.J., 1997. Late-glacial and early Holocene glacier activity in the
Southern Alps, New Zealand. Quaternary International, 38, 69–76.
Fuchs, M., Owen, L.A., 2008. Luminescence dating of glacial and associated
sediments: review, recommendations and future directions. Boreas, 37, 636–659.
Fulthorpe, C.S., Hoyanagi, K., Geldmacher, J., 2008. Integrated Ocean Drilling
Program Expedition 317 Scientific Prospectus Global and Local Controls on
Continental Margin Stratigraphy: Canterbury Basin, Eastern South Island, New
Zealand (Preliminary report).
Galbraith, R.F., Roberts, R.G., Laslett, G.M., Yoshida, H., Olley, J.M., 1999. Optical
dating of single and multiple grains of quartz from Jinmium rock shelter, northern
Australia: Part 1, experimental design and statistical models. Archaeometry, 41,
339–364.
Galbraith, R.F., Roberts, R.G., Yoshida, H., 2005, Error variation in OSL palaeodose
estimates from single aliquots of quartz: a factorial experiment. Radiation
Measurements, 39, 289–307.
Holdaway, R.N., Roberts, R.G., Beavan-Athfield, N.R., Olley, J.M., Worthy, T.H.,
2002. Optical dating of quartz sediments and accelerator mass spectrometry 14C
dating of bone gelatin and moa eggshell: A comparison of age estimates for nonarchaeological deposits in New Zealand. Journal of the Royal Society of New
Zealand, 32, 463–505.
Hormes, A., Preusser, F., Denton, G., Hajdas, I., Weiss, D., Stocker, T.F.,
Schlüchter, C., 2003. Radiocarbon and luminescence dating of overbank deposits
in outwash sediments of the Last Glacial Maximum in North Westland, New
Zealand. New Zealand Journal of Geology and Geophysics, 46, 95–106.
Huntley, D. J., Lamothe, M., 2001. Ubiquity of anomalous fading in K-feldspars and
the measurement and correction for it in optical dating. Canadian Journal of Earth
Sciences, 38, 1093–1106.
Huntley, D. J., Lian, L. B., 2006. Some observations on tunnelling of trapped
electrons in feldspars and their implications for optical dating. Quaternary Science
Reviews, 25, 2503–2512.
Jacobs, Z., Duller, G.A.T., Wintle, A.G., Henshilwood, C.S., 2006. Extending the
chronology at Blombos Cave, South Africa, back to 140 ka using optical dating of
single and multiple grains of quartz. Journal of Human Evolution, 51, 255–273.
Jain, M., Murray, A.S., Bøtter-Jensen, L., 2003. Characterisation of blue-light
stimulated luminescence components in different quartz samples: implications for
dose measurement. Radiation Measurements, 37, 441–449.
Jain, M., Murray, A.S., Bøtter-Jensen, L., 2004. Optically stimulated luminescence
dating: how significant is incomplete light exposure in fluvial environments?
Quaternaire, 15, 143–157.
Kaplan, M.R., Schaefer, J.M., Denton, G.H., Barrell, D.J.A., Chinn, T.J.H., Putnam,
A.E., Andersen, B.G., Finkel, R.C., Schwartz, R., Doughty, A.M., 2010, Glacier
retreat in New Zealand during the Younger Dryas stadial: Nature, 467, 194–197.
15
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
Leckie, D., 2003. Modern environments of the Canterbury Plains and adjacent
offshore areas, New Zealand: an analog for ancient conglomeratic depositional
systems in nonmarine and coastal zone settings. Bulletin of Canadian Petroleum
Geology, 51, 389–425.
Litchfield, N.J., Lian, O.B., 2004. Luminescence age estimates of Pleistocene marine
terrace and alluvial fan sediments associated with tectonic activity along coastal
Otago, New Zealand. New Zealand Journal of Geology and Geophysics, 47, 29–
37.
Litchfield, N.J., Rieser, U., 2005. Optically stimulated luminescence age constraints
for fluvial aggradation terraces and loess in the eastern North Island, New
Zealand. New Zealand Journal of Geology and Geophysics, 48, 581–589.
Mabin, M., 1987, Early Aranuian sedimentation in the Rangitata Valley, mid
Canterbury: New Zealand Journal of Geology and Geophysics, 30, 87–90.
McGlone, M., 1995, Lateglacial landscape and vegetation change and the Younger
Dryas climatic oscillation in New Zealand. Quaternary Science Reviews, 14, 867–
881.
Moreton, D.J., Ashworth, P.J., Best, J.L., 2002. The physical scale modelling of
braided alluvial architecture and estimation of subsurafce permeability. Basin
Research, 14, 265–285.
Murray, A. S., Olley, J. M., 2002. Precision and accuracy in the optically stimulated
luminescence dating of sedimentary quartz: a status review. Geochronometria,
21, 1–16.
Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved
single-aliquot regenerative-dose protocol. Radiation Measurements, 32, 57–73.
Newnham, R.M., Lowe, D.L., Williams, P.W., 1999. Quaternary environmental
change in New Zealand: a review. Progress in Physical Geography, 23, 567–610.
Newnham, R.M., Lowe, D.J., Giles, T., Alloway, B.V., 2007. Vegetation and climate
of Auckland, New Zealand, since ca. 32000 cal. yr ago: support for an extended
LGM. Journal of Quaternary Science, 22, 517–534.
Nichol, S.L., Lian, O.B., Carter, C.H., 2003. Sheet-gravel evidence for a late
Holocene tsunami run-up on beach dunes, Great Barrier Island, New Zealand.
Sedimentary Geology, 155, 129–145.
Olley, J.M., Caitcheon, G.G., Murray, A.S., 1998. The distribution of apparent dose
as determined by Optically Stimulated Luminescence in small aliquots of fluvial
quartz Implications for dating young sediments. Quaternary Science Reviews, 17,
1033–1040.
Olley, J.M., Caitcheon, G.G., Roberts, R.G., 1999. The origin of dose distributions in
fluvial sediments, and the prospect of dating single grains from fluvial deposits
using optically stimulated luminescence. Radiation Measurements, 30, 207–217.
Olley, J.M., Pietsch, T., Roberts, R.G., 2004. Optical dating of Holocene sediments
from a variety of geomorphic settings using single grains of quartz.
Geomorphology, 60, 337–358.
Pahnke, K., Zahn, R., Elderfield, H., Schulz, M., 2003. 340,000-year centennial-scale
marine record of Southern Hemisphere climatic oscillation. Science, 301, 948–
952.
Pietsch, T., Olley, J.M., Nanson, G.C., 2008. Fluvial transport as a natural
luminescence sensitiser of quartz. Quaternary Geochronology, 3, 365–376.
Prescott, J. R., Hutton, J. T., 1994. Cosmic ray contributions to dose rates for
luminescence and ESR dating: large depths and long-term time variations.
Radiation Measurements, 23, 497–500.
Preusser, F., Andersen, B.G., Denton, G.H., Schlüchter, C., 2005. Luminescence
chronology of Late Pleistocene glacial deposits in north Westland, New Zealand.
Quaternary Science Reviews, 24, 2207–2227.
16
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
Preusser, F., Ramseyer, K., Schlüchter, C., 2006. Characterisation of low OSL
intensity quartz from the New Zealand Alps. Radiation Measurements, 41, 871–
877.
Putnam, A.E., Denton, G.H., Schaefer, J.M., Barrell, D.J.A., Andersen, B.G., Finkel,
R.C., Schwartz, R., Doughty, A.M., Kaplan, M.R., Schlüchter, C., 2010, Glacier
advance in southern middle-latitudes during the Antarctic Cold Reversal: Nature
Geoscience, 3, 10, 700–704.
Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, J.W., Blackwell, P.G., Bronk
Ramsey, C., Buck, C.E., Burr, G.S., Edwards, R.L., Friedrich, M., Grootes, P.M.,
Guilderson, T.P., Hajdas, I., Headton, T.J., Hogg, A.G., Hughen, K.A., Kaiser,
K.F., Kromer, B., McCormac, F.G., Manning, S.W., Reimer, R.W., Richards, D.A.,
Southon, J.R., Talamo, S., Turney, C.S.M., van der Plicht, J., Weyhenmeyer,
C.E., 2009. IntCal09 and Marine09 radiocarbon age calibration curves, 0–50,000
years cal BP. Radiocarbon, 51, 1111–1150.
Rhodes, E. 2000. Observations of thermal transfer OSL signals in glacigenic quartz.
Radiation Measurements, 32, 595–602.
Rhodes, E., Bailey, R. 1997. The effect of thermal transfer on the zeroing of the luminescence
of quartz from recent glaciofluvial sediments. Quaternary Science Reviews, 16, 291–298.
Rittenour, T. M., 2008. Luminescence dating of fluvial deposits: applications to
geomorphic, palaeoseismic and archaeologic work. Boreas, 37, 613–635.
Roberts, H.M., 2008. The development and application of luminescence to loess
deposits: a perspective on the past, present and future. Boreas, 37, 483–507.
Roberts, R.G., Galbraith, R.F., Yoshida, H., Laslett, G.M., Olley, J.M., 2000.
Distinguishing dose population in sediment mixtures: a test of single-grain optical
dating procedures using mixtures of laboratory-dosed quartz. Radiation
Measurements, 32, 459–465.
Rodnight, H., Duller, G.A.T., Wintle, A.S., Tooth, S., 2006. Assessing the
reproducibility and accuracy of optical dating of fluvial deposits. Quaternary
Geochronology, 1, 109–120.
Rother, H., Shulmeister, J., Rieser, U., 2009. Stratigraphy, optical dating chronology
(IRSL) and depositional model of pre-LGM glacial deposits in the Hope Valley,
New Zealand. Quaternary Science Reviews, 1–17.
Schaefer, J.M., Denton, G.H., Kaplan, M., Putnam, A., Finkel, R.C., Barrell, D.J.A.,
Andersen, B.G., Schwartz, R., Mackintosh, A., Chinn, T., Schluchter, C., 2009,
High-Frequency Holocene Glacier Fluctuations in New Zealand Differ from the
Northern Signature: Science, 324, 5927, 622–625.
Shulmeister, J., Fink, D., Augustinus, P., 2005, A cosmogenic nuclide chronology of
the last glacial transition in North-West Nelson, New Zealand—new insights in
Southern Hemisphere climate forcing during the last deglaciation: Earth and
Planetary Science Letters, 233, 455–466.
Shulmeister, J., Thackray, G.D., Rieser, U., Hyatt, O.M., Rother, H., Smart, C.,
Evans, D., 2010. The stratigraphy, timing and climatic implications of
glaciolacustrine deposits in the middle Rakaia Valley, South Island, New Zealand.
Quaternary Science Reviews, 29, 2362–2381.
Soons, J., 1963. The Glacial Sequence in Part of the Rakaia Valley. New Zealand
Journal of Geology and Geophysics, 6, 735–756.
Soons, J., Gullentops, F., 1973. Glacial advances in the Rakaia valley, New Zealand.
New Zealand Journal of Geology and Geophysics, 16, 425–438.
Spencer, J.Q., Owen, L.A., 2004. Optically stimulated luminescence dating of Late
Quaternary glaciogenic sediments in the upper Hunza valley: validating the timing
of glaciation and assessing dating methods. Quaternary Science Reviews, 23,
175–191.
17
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
Suggate R.P., 1990. Late Pliocene and Quaternary glaciations of New Zealand.
Quaternary Science Reviews, 9, 175–197.
Suggate, R., Almond, P., 2005, The last glacial maximum (LGM) in western South
Island, New Zealand: implications for the global LGM and MIS 2: Quaternary
Science Reviews, 24, 16-17, 1923–1940.
Thackray, G.D., Shulmeister, J., Fink, D., 2009. Evidence for expanded Middle and
Late Pleistocene glacier extent in northwest Nelson, New Zealand. Geografiska
Annaler, 91A, 291–311.
Thomsen, K.J., Murray, A.S., Bøtter-Jensen, L., 2005. Sources of variability in OSL
dose measurements using single grains of quartz. Radiation Measurements, 39,
47–61.
Thomsen, K.J., Murray, A.S., Jain, M., Bøtter-Jensen, L., 2008. Laboratory fading
rates of various luminescence signals from feldspar-rich sediment extracts.
Radiation Measurements, 43, 147–41486.
Thrasher, I.M., Mauz, B., Chiverrell, R.C., Lang, A., 2009. Luminescence dating of
glaciofluvial deposits: A review. Earth Science Reviews, 97, 133–146.
Tonkin, P.J., Runge, E.C.A., Ives, D.W., 1974. A study of Late Pleistocene loess
deposits, South Canterbury, New Zealand. Quaternary Research, 4, 217–231.
Vandergoes, M.J., Newnham, R.M., Preusser, F., Hendy, C.H., Lowell, T.V.,
Fitzsimmons, S.J., Hogg, A.G., Kasper, H.U., Schlüchter, C., 2005. Regional
insolation forcing of late Quaternary climate change in the Southern Hemisphere.
Nature, 436, 242–245.
Wallinga, J., Murray, A.S., Duller, G.A.T., Tornqvist, T.E., 2001. Testing optically
stimulated luminescence dating of sand-sized quartz and feldspar from fluvial
deposits. Earth and Planetary Science Letters, 193, 617–630.
Watanuki, T., Murray, A.S., Tsukamoto, S., 2005. Quartz and polymineral
luminescence dating of Japanese loess over the last 0.6 Ma: Comparison with an
independent chronology. Earth and Planetary Science Letters, 240, 774–789.
Williams, P., 1996. A 230 ka record of glacial and interglacial events from Aurora
Cave, Fiordland, New Zealand: New Zealand Journal of Geology and Geophysics,
39, 225–241.
Wintle, A.G., Murray, A.S., 2006. A review of quartz optically stimulated
luminescence characteristics and their relevance in single-aliquot regeneration
dating protocols. Radiation Measurements, 41, 369–391.
18
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
Figure Captions
Figure 1. Location map showing the four sampling sites and their relationship
to the major braided rivers. The three-digit site name that is used
as the ID code for all samples taken from it is shown in bold. The
sample ID suffix indicates the stratigraphic order from the cliff top.
(Inset) Map of South Island, New Zealand showing location of the
Canterbury Plains coastal section under investigation in this study.
The position of the LGM coastline is indicated by the dashed line
(Newnham et al., 1999).
Figure 2. (A) Photograph of site C06 showing the location of the OSL samples
(white circles) within the stratigraphy. Sand units are highlighted in
yellow, dashed lines show subhorizontal erosional surfaces, blue
lines show cross-bedding identified in the gravel sediments, red
lines show cross-bedding identified in the sand units. (B) A closeup photograph of the C06-B4 sample site, showing the fine
resolution cross-bedding in this bar margin sand body. (C) View of
the Canterbury coastline looking north from site C06.
Figure 3. Dose recovery test results, following laboratory bleaching and a beta
dose of 27.8 Gy, for (A) 34 aliquots of sample C06-A2 tested at a
range of OSL preheat temperatures from 160–280˚C for 10 s using
test dose preheats of either 160˚C (open circles) or 220˚C for 10 s
(closed circles); (B) dose recovery for the mean of three aliquots of
each of the nine Canterbury samples after preheats of 220˚C for 10
s. In each plot, the solid line indicates unity for dose
recovered/dose delivered, with dashed lines indicating ±10%.
Points in grey indicate aliquots that failed the recycling ratio
criterion.
Figure 4. Outline SAR protocol (Murray and Wintle, 2000) used for OSL
dating. A typical SAR measurement sequence was N, 0, R1, R2, R3,
R4, 0, RR, OSL-IR where each Rx refers to one cycle of
measurement, as shown here; RR indicates measurement of a
recycled dose (typically a repeat of R1), and OSL-IR refers to
determination of the OSL-IR depletion ratio of Duller (2003). At
least four regenerative points were measured for every aliquot.
Figure 5. Results from single grain analyses, the total number of grains
measured for each sample was 1000: (A) Cumulative percentage
of the total light sum (following a 93 Gy dose, preheat temperature
220°C for 10 s) as a function of the cumulative percentage of
grains measured for C06-A4 and K04-A2. 90% of the luminescence
emitted is derived from 2.7% and 3.4% of the grains respectively.
(B) Absolute grain brightness plot showing the distribution of OSL
signal from grains of C06-A4 and K04-A2, for which less than 1%
grains gave more than 10 counts per 0.17 s per Gy, compared with
19
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
quartz samples described by Duller (2008) from Tasmania
(TNE9517) and Chile (LCF2).
Figure 6. Deconvolution of the natural OSL signal for two different samples
stimulated using an LED unit delivering 2.28 mW cm-2: (A) A25-A1
aliquot 1-15 and (B) C06-A4 aliquot 1-4. The summed exponential
function (solid black line), fast component (dashed line) and
medium component (dotted line) fitted to the OSL response data
(grey dots) are shown for the first 10 s of measurement. The total
OSL response collected over 100 s, with the first 2 s (5 channels)
defining the integrated signal and the last 20 s (50 channels)
defining the signal background are shown by the grey shading, and
the residuals to the component fit are shown in the insets. R
indicates the degree of correlation between the data and the fitted
summed exponential function. (C, D) Dose response curves
generated for these two aliquots (C is A25-A1 and D is C06-A4)
showing the integrated OSL signal (black) and the fast component
signal (grey). The y-axis (Lx/Tx) shows the sensitivity-corrected
OSL responses, with the circles on this axis representing the
Natural OSL signal. The De values are shown by the vertical dotted
lines.
Figure 7. Comparison of De generated using the separated fast component
and the De obtained using the integral OSL signal (first 2 s of
excitation for B08-B2 and C06-A1; 0.4 s for K04-A1). Only aliquots
that passed the SAR screening criteria are shown. The dashed line
represents the 1:1 relationship between the different methods of De
calculation.
Figure 8. Mean fast component De/integral OSL De each sample. For each
sample, the number of aliquots that passed the SAR criteria is
given in bold.
Figure 9. Radial plots showing equivalent dose distributions for all of the
Canterbury samples. In each case, the grey bar is centred on the
De determined using CAM. The number of aliquots that passed
SAR criteria and make up each distribution is given by n, and the
overdispersion by σOD.
Figure 10. Stratigraphic cross-section through the Canterbury Plains coastal
section showing our quartz OSL chronology, two 14C ages (Phil
Ashworth, pers. comm.), and the location of the mapped
stratigraphy. Field observations were made at the mapped
locations. The base line of this section is sea level datum NZ1949.
Inset shows the OSL ages compared to the regional climate
chronology; the black line is the sea surface temperature data from
marine core MD97-2120 taken from the Chatham Rise, ~300 km
east of South Island (Pahnke et al., 2003) and the climate phases
20
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
defined by Alloway et al., (2007); LGIT is Last Glacial Interglacial
Transition, LGCP is Last Glacial Cold Period. The dashed line
shows the LGM peak at 21 ka.
Table Captions
Table 1: Sample information and dosimetry. Depth from which samples were
taken is given in metres below the cliff top. Sedimentary facies were
interpreted using the classification scheme of Moreton et al. (2002).
Gravimetric field water content (Field WCgrav) is calculated as mass
of water/mass of dry sediment x 100%; for all samples, a water
content of 15 ± 5% was used to represent the mean water content
over the depositional history of the sediments (see text for
discussion). The grain size used for luminescence measurements
was 180–211 μm diameter quartz. Dose rates were calculated
following geochemical analysis of the uranium, thorium, and
potassium concentrations for each sample (as described in section
4.4), using the conversion factors of Adamiec and Aitken (1998), and
include a cosmic dose rate contribution assessed according to
Prescott and Hutton (1994). The uncertainties associated with
measurement of radioisotope concentrations were 5% in each case.
Total dose-rates were calculated using values prior to rounding.
Table 2. Component separation results showing mean values for the
photoionisation cross-section (σ) and detrapping probability (b) for
both of the components identified in the OSL responses of the
Canterbury samples. Two different Risø readers were used for
measurements of the suite of samples. Consequently samples C06A3, K04-A1 and K04-A2 have higher b values, as they were
measured using a reader with a greater LED intensity. This is
corrected for in the calculation of photoionisation cross-section
values. The De ratio is calculated as De generated from the fast
component/De generated using the integrated OSL signal and is
plotted in Fig. 8.
Table 3. Equivalent doses and quartz OSL ages calculated for each sample
using the central age model. The number of aliquots that passed
the SAR screening criteria is given by n, and the overdispersion of
the De distribution by σOD. OSL ages are expressed as thousands of
years before 2010 AD, and rounded to the nearest 100 years.
21