OSL measurements were carried out using a Risø TL/OSL-DA-20 reader [30
], with optical stimulation of quartz provided by an array of blue light (470 nm, FWHM 20 nm) diodes, optical stimulation of feldspar with infrared (870 nm, FWHM 40 nm) diodes, a calibrated 90
Y beta source (~0.16 Gys−1
) to administer laboratory radiation doses, and a heating stage for thermal stabilization. All luminescence signals were detected in the ultraviolet (peak transmission ~340 nm) using 7.5 mm of Hoya U-340 filter with an EMI 9235QB photomultiplier tube.
Determination of the equivalent dose (De
) was carried out using a single-aliquot regenerative-dose (SAR) protocol [31
] with modifications. Continuous power or continuous wave OSL (CW-OSL) was conducted in all measurements. We routinely utilize post-infrared optically stimulated luminescence (post-IR OSL) measurement approaches [29
], which in certain instances has been shown to improve dose recovery results even if infrared signals are negligible or absent [29
]. Post-IR OSL was used to measure the luminescence from the quartz grains in this study. This procedure removes charge sensitive to infrared stimulation, commonly associated with remnant feldspathic minerals, before measuring OSL from the quartz grains. The post-IR OSL measurement comprised 40 s infrared-stimulated luminescence (IRSL) at ~117.9 mWcm−2
(22 Vishay TSFF5200 IR led’s at 90% power) at a sample temperature of 50 °C, followed by 40 s OSL at 38.7 mWcm−2
(28 Nichia NSPB500S blue led’s at 90% power) at a sample temperature of 125 °C. After measurement of the natural OSL in the first SAR cycle, regenerative doses in subsequent cycles were approximately 0.8, 1.6, 2.4, 0, and 0.8 Gy. The test dose administered for sensitivity correction was typically ~1.6 Gy, exceeding typical De
values by a factor of ~10 to 400 (consistent with data from [7
]). Test doses were heated to 160 °C prior to measurement. A hot bleach measurement of 40 s OSL at 280 °C was incorporated at the end of each SAR cycle [32
All natural IRSL signals appeared negligible but natural OSL signals (Figure 3
) were also very dim. There was little observable scaling in size of IRSL at the regenerative dose level suggesting IR contamination may not be a problem. However, for such small OSL signals, negligible IRSL may still be a source of overestimation and requires careful assessment if a post-IR protocol is not used.
For all measurements we compared two approaches to define the net OSL signal: (1) late background subtraction where the signal was defined as the initial 0.8 s integral with subtraction of the final 8 s integral [31
], and (2) early background subtraction with the same initial 0.8 s integral with subtraction of the following 0.8 to 2.72 s integral. The latter method has been assessed to optimize the contribution from the fast component [41
]. The De
value was estimated by interpolation of the natural OSL with a best-fit linear or saturating exponential curve fitted to regenerative OSL data. Uncertainty in De
was estimated by combining error from counting statistics for the natural OSL, curve fitting, and instrumental systematic uncertainty [42
Dose-rate measurements were conducted using the core portions described in Section 3.1
. High-resolution gamma spectrometry was performed using a small-sample 2 g well geometry for assessment of U and Th. With the use of such a small sample for gamma spectrometry, a homogenous medium is assumed for accurate assessment of the radioactivity within a 30 cm radius sphere; the fluvial sand samples studied here are of uniform composition with well-sorted grain sizes, and thus a homogeneous medium is a good approximation. Li-metaborate fusion ICP-OES and ICP-MS were performed for K and Rb, respectively. These data were converted to annual dose rate using conversion factors [43
]. Calculated beta dose was corrected using attenuation factors for grain size and HF etching (described in detail in [44
] and references therein). In the absence of detailed imagery or documented evidence of the nature of subaerial-to-subaqueous cyclicity at the Tapuama and Cupari sites, attenuation of dose rate via moisture conditions over the burial time of the samples was calculated by using present day field moisture content with a maximum absolute error of 5% to allow for past changes. The dose rate from the ionizing cosmic ray component was calculated following [45
]. For the purposes of this feasibility study, a constant overburden depth was assumed; we deliberately chose the deepest part of the cores for our sample selection in an attempt to minimize shallow gamma and hard cosmic corrections [46
]. Finally, an estimate of an internal dose rate of 0.01 ± 0.002 mGya−1
] was incorporated into total dose-rate assessment.
3.4. Towards OSL Dating of Multi-Grain Quartz Aliquots
A summary of the OSL analysis is given in Table 1
data, calculated using late background subtraction, were indistinguishable from those data analyzed using early background subtraction. An important observation is that recuperation measured during the De
SAR cycle is significant for the quartz from Arapiuns (Figure 7
a,b) and Tapuama. For the Arapiuns sample (ARA-040808-05), we calculate a similar De
result when the growth curves are forced through the origin (Figure 7
c; all aliquots accepted) compared to when the acceptance threshold for recuperation was set at 35% to achieve a satisfactory De
dataset (Figure 7
d; 18 of 22 aliquots accepted); for Tapuama (TAP-030808-03 and TAP-030808-04), recuperation was much more significant and as a consequence all growth curves were forced through the origin to obtain De
values. High recuperation is somewhat surprising given the minimal thermal transfer for preheats <~220 °C (Figure 5
), and the likelihood of numerous bleaching events occurring in these shoreline environments which have been linked to substantially reduced recuperation effect [50
]. Given that the values measured are unusually high (e.g., for Tapuama aliquots, recuperation exceeds Ln
by a factor ranging from ~1 to ~60), we suspect that the majority of the recuperation signal recorded could be a measurement artefact due to the comparatively large regenerative doses (lowest beta dose was ~800 mGy) used compared to the measured De
values (Table 1
). Furthermore, if for example the De
values were ~400 mGy, then the majority would be accepted below the 5% threshold level. Future work will investigate whether there is a systematic dose-dependent effect on the size of the recuperation signal.
]) values of 0% for the Arapiuns and Tapuama samples indicate the OSL signals were completely reset, although the true extent of resetting may not be revealed due to the medium-sized aliquots measured. For these samples the final De
was calculated using the central age model (CAM) [52
]. The second of the Cupari samples, CUP-030808-02, had a moderate over-dispersion value of 13.3%, but was also well suited to a CAM analysis. CUP-030808-01 was the only sample with a lower quartz yield and, subsequent to De
plateau, thermal transfer and dose recovery tests, only 10 aliquots were available for De
analysis. Over-dispersion for this sample was higher at 20.1% but, despite the low number of aliquots a minimum age model (MAM; [52
]), analysis returned a result with a reasonable number of significant aliquots contributing to the MAM result (p
-value = 0.333).
The OSL age for sample ARA-040808-05, from the shoreline dune feature at Arapiuns, is 24 ± 3 a from 2009. For Tapuama, the sample from the unvegetated distal end of the spit, TAP-030808-03, is 13 ± 5 a, and from the sands within the vegetated proximal end of the spit, TAP-030808-04, is 34 ± 8 a. Although we lack direct independent dating evidence, these are plausible ages for the Tapuama samples, with the sands from the unvegetated distal end of the spit of younger depositional age than the sands in the vegetated proximal end of the spit. Vegetation adds stability to sediments via root networks increasing cohesive strength, and by grasses, shrubs, and trees increasing surface roughness and dissipating some energy of wind or water; together, these lower the effectiveness of erosion by wind or water. The result for TAP-030808-04 is supportive of apparent decadal stability of other vegetated landforms (cf. Figure 2
), whereas TAP-030808-03 suggests continual reworking and redeposition occurs in more active zones of the spit. The duplicated samples from the densely vegetated bar at Cupari have significantly older OSL ages of 324 ± 29 a and 557 ± 35 a; these ages in the 100s-of-years range are more consistent with youngest ages from other OSL studies of Tapajós sand bars [15
]. Although the Cupari bar seems likely to be an older feature, we suspect that the discordance in the two ages may in part be related to poor resetting that is not apparent because of the large aliquots measured. For these samples the natural OSL is sufficiently large that smaller aliquots, or potentially single grains, could be measured to investigate this age discordance. 210
Pb data from a series of channel bottom cores from the Tapajós indicate sedimentation rates of 0.2–0.7 cmyr−1
in the upper stretch of the ria (consistent with the sampling localities in the work described here), and 0.2–1.9 cmyr−1
sampled across the entire ria [25
]. If we make the assumption that these values represent sedimentation rates not only for clays, silts, and sands in the channel bottom but also for sands in shoreline features, we derive an age range of ~30–300 years (for 60 cm depth assuming uniform linear deposition) similar to the age range indicated from this OSL study.
This study demonstrates the potential of OSL to determine depositional age of very young fluvial landforms in the Rio Tapajós. Importantly it has revealed how future experimental approaches should be modified in the following ways: (1) use of ultra-low-dose beta source (and assessment of possible dose-dependency of recuperation); (2) optimization of aliquot size or single grain analyzes (including assessment of F-statistic [53
] and un-logged age model approaches [6
]); and (3) considering the seasonal river-level oscillation and extremely low external dose rates, careful assessment of water content fluctuation, accurate measurement of internal dose rates, and modelling of gamma and cosmic dose rates [46
]. Given the nature of this study as one of feasibility of OSL approaches on a small selection of samples, it follows that geomorphological interpretation is somewhat speculative and should be limited. We propose future work with detailed stratigraphic and lateral sampling strategies which, in combination with differential remotely sensed imagery and hydrological data, will provide a sensitive monitor of how fluvial landforms change in response to land cover and land use change (including planned dam projects) and modern analogue data for depositional models of ancient tidal rhythmites.
The landforms investigated in this study were all quartz rich, but quartz yield was dependent on grain size distribution. For the ~5 cm core samples from Cupari, the majority of the sample was >310 μm, and quartz yields for sieve fractions <310 μm were correspondingly low. Conversely, ~5 cm core samples from Arapiuns and Tapuama had high quartz yields in the 250–310 μm and 212–250 μm sieve fractions, respectively. This indicates that in certain localities, a 1–2 cm sampling resolution may be possible.