isi

A new sediment tracer uses the interactions between radiation, charge, and the Sun to uncover the hidden transport histories of sand grains.

Schematic representation of Guyez et al.’s model of the luminescence of sand grains traveling through a river system. Grains in transport see sunlight which removes or “bleaches” their luminescence, whereas when a sand grain is buried, luminescence regenerates. This mechanism potentially allows one to quantify the sand grain’s rate of transport through the river system. D_e represents the dose, that is, the intensity of the luminescence signal (units are “Grays” – Gy). L_T is transport length between resting periods (e.g., during a flood), and R_t is resting time; the longer R_t, the greater the intensity of the luminescence signal. Credit: Guyez et al. [2023], Figure 4a

Source: Journal of Geophysical Research: Earth Surface

The Earth’s surface is continually reshaped by the processes that create and move sediment. For surface dwellers, cohabitating with this sprightly dance of matter is a fact of life that is sometimes fruitful, and sometimes fraught. One may be surprised to learn that despite our experience, the simple question of how fast sediment travels from point A to point B is surprisingly hard to answer.

Over the past century, methods attempting to answer this question have ranged from the classical image of a geologist observing minerals under a microscope, to the more dubious radioactive tagging of sand released into a river. In a new study, Guyez et al. [2023] present an environmentally friendly method towards answering this long-standing question and apply it to the Rakaia and Waimakariri Rivers of New Zealand, a region where ancient climate change led to vast changes in the Earth’s surface. Their new method takes the physics behind an established geologic dating method and reimagines it as an innovative sediment tracer.

This new tracer method uses the luminescence of individual sand grains of feldspar. Luminescence is a property of minerals wherein energy absorbed from natural background radiation is stored as charge trapped in defects in the mineral’s molecular structure over time. When exposed to sunlight, this trapped charge is free to escape, leading to a dynamic growth and decay of luminescence as sand grains travel through sunlight-variable river systems. Guyez et al. use this framework to quantify the range of sunlight-exposing transport and burial histories of sand grains to answer how fast said grains get from point A to point B.

While further work is needed to refine the details of the method, the use of this method on geologically ubiquitous feldspar sand offers much potential for future studies and offers a new tool towards answering large-scale questions of how Earth’s surface continues to change.

Citation: Guyez, A., Bonnet, S., Reimann, T., Carretier, S., & Wallinga, J. (2023). A Novel Approach to Quantify Sediment Transfer and Storage in Rivers – Testing Feldspar Single-Grain pIRIR Analysis and Numerical Simulations. Journal of Geophysical Research: Earth Surface, 128, e2022JF006727. https://doi.org/10.1029/2022JF006727

—Harrison Gray, Associate Editor, JGR: Earth Surface

Source: https://eos.org/editor-highlights/what-electrons-can-tell-us-about-the-speed-of-sand