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We deployed a full-day bleaching field experiment with irradiated feldspar samples tethered to a pole at various positions within and above the water column to quantify: 1) the bleaching potential, i.e. the light intensity and spectrum as a function of time, depth, and tidal stage; and 2) the bleaching efficiency, i.e. the degree of bleaching of infrared stimulated luminescence (IRSL) and post-infrared IRSL (pIRIR) signals measured at 150, 225 and 290°C after a full day of light exposure above and below water. + Luminescence measurements All measurements were performed on an automated Risø TL/OSL-DA-15 reader with a single-grain attachment with IR laser. A LOT I-410 interference filter was used to target the 410 nm peak emission of K-feldspar (Huntley et al., 1991). We used a multiple-elevated-temperature (MET) pIRIR measurement protocol after (Li & Li, 2011). Sixty datapoints were read-out in 2s (0.033s per datapoint), of which 50 datapoints were collected during stimulation. For each sample, 3 full single-grain discs were measured. + Light spectra measurements Two TriOS RAMSES SAMIP ACC-2 VIS spectrometers, with a spectral wavelength range between 320 and 950 nm, were used to monitor the light spectra at the surface and below water. The subaqueous spectrometer was attached to a sturdy frame and moved up and down the water column with the help of a pulley to measure the subaqueous light penetration. Inundation depth was monitored using a pressure sensor. The other spectrometer was attached to the jetty and measured the subaerial light conditions. Both spectrometers faced upward and were read out simultaneously. + Measurement protocols and data formats The measurement protocols were written in Riso's sequence editor software. All Riso software is downloadable at: https://www.fysik.dtu.dk/english/research/radphys/research/radiation-instruments/tl_osl_reader/software. + Processing and analysis scripts in R All processing of data (modifications, calculations, etc) and

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 under 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 set-ups are still 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

+ One-grain experiment We measured 20 single-grain discs with one grain per disc from sample NCL-1821112, which has been bleached for 48 hours in a solar simulator prior to measurements. Please visit the Netherlands Centre for Luminescence dating (NCL) database for more information on the sample background at: https://www.lumid.nl/. On the website go to the 'projects' tab and search for NCL-code 1821. The measurement protocol, as reported in Table 1 of the manuscript, was repeated 8 times for incremental aperture steps. + Reader comparison experiment We measured 20 single-grain discs per reader all originating from NCL-1821112, which has been bleached for 48 hours in a solar simulator prior to measurements, to compare the performance of both devices. + Measurement protocols and data formats The measurement protocols were written in Riso's sequence editor software. All Riso software is downloadable at: https://www.fysik.dtu.dk/english/research/radphys/research/radiation-instruments/tl_osl_reader/software. The raw data is stored in a Riso .binx file which can be opened in Riso's analyst software environment. EMCCD generated luminescence images has been analyzed in Riso's Viewer+ software before further analyses. + Processing and analysis scripts in R All processing of data (modifications, calculations, etx) and analysis of data was performed in R. All scripts are commented. Per experiment an R script was written. + Figures and tables All figures and tables were created through R. Refer to the R scripts for detailed information within the script.