Cosmic-Ray Muography as a Novel Tool for Studying Warm and Humid Paleoclimates: An Example of Density Characterization of Sedimentary Bauxite Deposits

Bauxites are mature laterites develop under a long period of extreme chemical weathering in a humid, tropical to subtropical climate. Bauxites are, in fact, among the best climate-sensitive rock indicators for warm and humid paleoclimates. They also represent a useful proxy for past climate changes and are important for provenance studies and geodynamic and paleogeographic reconstructions. We propose a novel tool for investigating sedimentary bauxites for these purposes: cosmic-ray muography. Muography is based on the measurements of atmospheric muons after they have passed through the geological body of interest in straight trajectories and almost at the speed of light. The idea is to reconstruct a 2D or 3D density model of the volume of material between the open sky and muon detector from the raw data. This is possible since muons are lost in the rocks by attenuation according to average density similarly to the attenuation of X-rays (higher density stops more muons and X-rays alike). Muography is hence sensitive to density contrasts in the media. Consequently, it can be used in the studies of regoliths (laterites, saprolites) and paleosols, as well as economically interesting residual enrichments within. Here we focus to sedimentary bauxites, or allochthonous bauxites, which are one of the three genetic subtypes of the bauxite deposit family. In addition to climate studies, muography can be used for the exploration and resource delineation of (any type of) bauxite, which is the most important source of aluminum and a potential source of some Critical Raw Materials (as defined by EU). It can also be used for the studies of remnant paleosols trapped in scattered locations above ancient continental rocks. The sedimentary bauxite deposits are primarily formed when the more common in-situ lateritic bauxite deposits are eroded, transported, and proximally redeposited as reworked debris flow bauxites, until they are buried by further sedimentation. At present, many deposits are concealed (Fig. 1). The main geophysical methods used in their studies are gravity, magnetic, electrical and reflection seismic methods. Our concept offers a new way to study sedimentary bauxites by focusing on the density contrasts between laterite, bauxite, sedimentary interbeds and voids.