An intrinsic sediment tracing method using natural quartz- and feldspar luminescence

Abstract

The goal of a sediment tracer is to give quantitative or relative feedback on (coastal) morphological changes or track either sediment or other particles (e.g. contaminations) in sediment dynamic environments. A sediment tracer is a visible or invisible marked (sediment) grain that is artificially injected or is present in-situ. This Master Thesis main objective is a feasibility study and the development of a new type of sediment tracer. This sediment tracer focuses on the intrinsic luminescence properties of the quartz and feldspar minerals as a measure for sediment transport. Utilizing an intrinsic property as tracer has a number of advantages over the use of artificial or natural tracers. First the tracer is a guaranteed match to its environment in terms of transport and interaction within the sediment system. The use of artificial tracers is especially problematic in large study areas, because the deploy scope depends on the quantity of injected material to the environment it is placed in. Natural tracers (partly) deal with this problem, since they make use of in-situ material. An example is the utilization of naturally occurring radionuclides. However, they don’t account for internal dynamics that can provide insight in the transport history from the original source to the position of deposition. Hence natural tracers are mostly used for provenance studies. Luminescence is the latent light that is emitted by a mineral after it is stimulated by a source of heat or light. It is a well described phenomenon in sediment grains and the mechanics have a simple analogy to a battery that is charged and uncharged (i.e. luminescence signal is accumulate and depleted). The accumulation of a luminescence signal is triggered by ionizing radiation, which is the product of naturally decaying radioactive isotopes during prolonged burial. The depletion occurs when the grains are exposed to heat or daylight. The rate at which the signal depletes depends on the daylight conditions as well as the depth of immersion in water. This makes every sediment grain a possible data recorder for its own (day)light exposure history or deposition- /erosion history, put in terms of morphology. We hypothesized that the degree of daylight exposure, hence the depletion of luminescence signal during transport, could represent the distance- and/or mode (aeolian, water lain etc.) of transport. Luminescence is measured in an equivalent dose in Grays. There are a multitude of stimulation methods to recover an equivalent dose from sediment, moreover are each of these methods are coupled to unique luminescence properties. Usually these stimulation methods are tested separately on either a quartz or a feldspar sample. In this thesis we have developed a new protocol to simultaneously retrieve a range of signals (blue light-, (post infrared) infrared light- and thermal stimulation) from a polymineral sample of mixed quartz and feldspar grains. This Polymineral Multi Signal Single Aliquot Regenerative protocol, PMS-SAR for short, proved to be representative for separate measurements of quartz and feldspar. Next, we have conducted controlled daylight exposure- or bleaching experiments to investigate the bleaching behaviour per signal. We found that each signal has a unique dynamic bleaching rate and range that differs approximately one order of magnitude. To test the feasibility as a sediment tracer we have been applying the PMS-SAR luminescence protocol and the results of the bleaching experiments in a test case at the Sand Engine, a mega-nourishment at the Dutch coast. An intensive monitoring campaign at the Sand Engine pilot project has provided bathymetry- and hydrological data. With this data a model is made to investigate the historical evolution of a sand spit that arose at the North-eastern tip of the Sand Engine after construction. Next luminescence measurements were conducted on selected sediment samples taken on the Sand Engine to see whether the results are in accordance with the historical evolution. The luminescence measurement results from the Sand Engine samples were then reflected upon the bleaching curves to give us the Equivalent Exposure Time (EET). The measurement results reveal an average increase in EET value for the different signals with distance from the first core position, which is consistent with our expectations. In addition to this we could also make a clear distinction between the presumably water lain transported samples and the aeolian sample. These distinct observations reveal the true potential of a multi signal luminescence technique and the use of it in morphological studies. However, these observation were made with very straight forward assumptions, hence to get a more accurate picture that can by appointed to morphological evolution and processes in mega-nourishment additional experiments are recommended.