Differential bleaching of quartz and feldspar luminescence signals under high turbidity conditions
Abstract. Sediment burial dating using optically stimulated luminescence (OSL) is a well-established tool in geochronology. An important but often inapplicable requirement for its successful use is that the OSL signal is sufficiently reset prior to deposition. However, subaqueous bleaching conditions during fluvial transport are vastly understudied, for example the effect of turbidity and sediment mixing on luminescence bleaching rates is only poorly established. The possibility that slow bleaching rates may dominate in certain transport conditions led to the concept that OSL could be used to derive sediment transport histories. The feasibility of this concept is still to be demonstrated and experimental setups to be tested. Our contribution to this scientific challenge involves subaquatic bleaching experiments, in which we suspend saturated coastal sand of Miocene age in a circular flume and illuminate it for discrete time intervals with natural light. We record the in-situ energy flux density received by the suspended grains in the UV-NIR frequency range by using a broadband spectrometer with a submersible probe. Our analysis includes pre-profiling of each sample following a polymineral multiple signal (PMS) protocol. Using the PMS, the quartz dominated blue stimulated luminescence signal at 125 °C (BSL-125) decays slower than the K-feldspar dominated infrared stimulated luminescence signal at 25 °C (IR-25) even under subaerial conditions. The BSL-125 from purified quartz shows the opposite behaviour, which renders the PMS unreliable in our case. We find a negative correlation between suspended sediment concentration and bleaching rate for all the measured signals. For outdoor bleaching experiments we propose to relate the measured luminescence dose to the cumulative received irradiance rather than to the bleaching time. Increases in the sediment concentration lead to a stronger attenuation of the UV/blue compared to the red/NIR wavelength. This attenuation thereby follows an exponential decay that is controlled by the sediment concentration and a wavelength-dependent decay constant, λ. As such λ could potentially be used in numerical models of luminescence signal resetting in turbid suspensions.
Jürgen Mey et al.
Status: final response (author comments only)
RC1: 'Comment on gchron-2023-4', Harrison Gray, 04 May 2023
- AC1: 'Reply on RC1', Jürgen Mey, 16 May 2023
RC2: 'Comment on gchron-2023-4', Anonymous Referee #2, 06 May 2023
- AC2: 'Reply on RC2', Jürgen Mey, 16 May 2023
Jürgen Mey et al.
Jürgen Mey et al.
Viewed (geographical distribution)
Review of Differential bleaching of quartz and feldspar luminescence signals under high turbidity conditions by Mey et al.
Review by H. Gray
In Differential bleaching of quartz and feldspar luminescence signals under high turbidity conditions by Mey et al., the authors describe an experiment to determine the effect of variable subaqueous suspended sediment concentration on sunlight irradiance and the bleaching (resetting) rates of quartz and feldspar luminescence. The authors constructed an experimental set up consisting of a chamber to control sunlight exposure surrounding a filled beaker with magnetic stir bar and submerged light spectrometer. The authors then performed experiments in which they measured the resulting irradiance and luminescence bleaching as a function of sediment concentration. They discovered that increasing suspended sediment concentration leads to larger residual doses and that differential attenuation of light by wavelength did not appear to have a notable effect. The authors then concluded that turbulent upwelling leading to grains reaching the water’s surface probably resulted in most of the overall bleaching.
Overall, I think this work is a useful contribution. The effect of differential attenuation of light by wavelength is a notable step towards answering the broader problem of where and how bleaching occurs in natural fluvial environments. The latter question being an important topic of study within the development of luminescence as a sediment tracer. I also like the author's point that cumulative irradiance is more important towards bleaching than overall exposure time.
I have minor comments about the experimental set up, but nothing so large as to require performing the experiment over. I agree with the basic mechanics of this experiment (to be fair, I did a very similar experiment during a past study (Gray et al. 2018, Supplemental material: Figure S7)). I would have liked the authors to have gone further with this data, perhaps coupling it with a numerical model simulating particle trajectories in fluvial turbulence or similar, but time is limited and this contribution is worth entering into the public domain.
In the experiment, the authors use suspended sediment concentrations of 33, 66, and 100 grams per liter. These seem like very very high sediment concentrations. My experience has been that suspended sediment concentrations in rivers are typically 1-2 orders of magnitude lower than used in this experiment (for example John Gray and Francisco Simões 2008’s manual). Certainly, the authors have identified the concentrations of fairly extreme flood events, but I think the authors should state that these may not be reflective of the general conditions of sediment transport in rivers.
For example, in some river systems, the majority of the sediment is transported in Wolman and Miller (1960) style 'effective discharge' conditions, which are a balance between the frequency and magnitude of flooding events. Very large floods with high sediment concentrations, such as those simulated here, move a large amount of sediment, but their effects are offset by their infrequency. In contrast, medium-sized floods that optimize the frequency and magnitude of sediment transport ('effective discharge events') end up doing the most work in the system. I think it is worth considering that these effective discharge events represent the conditions which need to be simulated in order for these experiments to have the most relevance for luminescence sediment tracing.
It would be helpful to have a Table of the PMS protocol
Line 41: "anisotropies" doesn’t seem exactly right. How about “heterogeneities” ?
Line 60: Stokes et al., 2001 (quartz) and Gray et al, 2018 (feldspar) have downstream bleaching trends as well
Line 111: "tab water" -> "tap water"
Line 109: This experimental setup is similar to that of Gray et al. 2018 (see, supplemental material). It may be worth mentioning that study to provide support for the conception of the experimental apparatus shown here.
Line 124: Does the sunlight beam setup (non reflecting sides) result in an overall lower radiance? I wonder because light scattering in natural environments might change the flux from various wavelengths if different wavelengths are scattered differently.
Line 127: See main comment above.
Line 270: "tab water" -> "tap water"
Line 297-300: I have also had this problem when I tried the PMS approach on a project a few years ago.
Also general observations with PMS:
One thing to consider is the possibility that the heating during the IR steps are bleaching/modifying the BSL signal prior to measurement. Potentially moving the fast component electrons into slower components. This happens to us here in the western USA because the quartz is often thermally unstable. ALSO with the polymineral approach, the relative qtz/feldspar ratio may matter as the IR-insensitive quartz grains may somewhat shield the feldspar from the radiation source in the Risoe. I had this happen on a quartz/feldspar mixture and purifying the feldspar fixed the problem.
Figure 3c: "wavelenght" -> "wavelength"
Gray, H. J., Tucker, G. E., & Mahan, S. A. (2018). Application of a luminescence‐based sediment transport model. Geophysical Research Letters, 45(12), 6071-6080.
Gray, J. R., & Simões, F. J. (2008). Estimating sediment discharge. Sedimentation engineering–processes, measurements, modeling, and practice, manual, 110, 1067-1088.
Stokes, S., Bray, H. E., & Blum, M. D. (2001). Optical resetting in large drainage basins: tests of zeroing assumptions using single-aliquot procedures. Quaternary Science Reviews, 20(5-9), 879-885.
Wolman, M. G., & Miller, J. P. (1960). Magnitude and frequency of forces in geomorphic processes. The Journal of Geology, 68(1), 54-74.