The origin of the magnetic record in Eocene-Miocene coarse-grained sediments deposited in hyper-arid/arid conditions: Examples from the Atacama Desert

Abstract

Magnetic proxies for paleoclimate are tools widely used to understand climate variability, yet current proxies focus on loess-derived soils in humid to temperate climate zones, whereas coarse-grained sediments in arid-hyperarid climate zones remain poorly investigated. To test the potential paleoclimatic-environmental significance of the magnetic record of coarse-grained sediments deposited in a region with a mean annual precipitation (MAP) <200 mm yr−1, we selected three previously studied sedimentary sequences from the Atacama Desert (Centinela area) and explored their magnetic properties, paedogenic features, salt chemistry and mineralogy. These sequences were deposited under different climate-environmental conditions from the Eocene to the Miocene, and correspond, from oldest to youngest, to the Atravesados II gravels (AtII), the Arrieros gravels (Arr) and the Ratones sediments (Rat). The magnetic susceptibility values (k) obtained in the stratigraphic record of the gravels are mainly controlled by the concentration of detrital (titano)magnetite, which is concentrated in the finest fraction (<0.5 mm) of sediments. The values decrease in the coarse sediments from AtII to Arr following a climatic transition from arid to hyperarid conditions, an interpretation that is supported by the transition from carbonate-rich (MAP ~ 40 mm yr−1) to sulfate-rich paleosols (MAP < 10 mm yr−1), and changes in the tectonic conditions and/or sedimentary source. In contrast, the Rat fine-grained sediments record changes in paleo-wetland dynamics. The high frequency-dependent magnetic susceptibility (kfd%) values obtained from these layers are mainly controlled by the concentration of authigenic magnetite-maghemite and/or hematite crystals of superparamagnetic/single magnetic size. The increase in kfd% is linked to an increase in the authigenic degree, which is related to variations in the depth of the local water table. These results demonstrate the potential of magnetic proxies to reveal climatic/environmental signals in coarse-grained sediments deposited under desert climate conditions.