Pedogenic carbonate nodules as soil time archives: Challenges and investigations related to OSL dating

Abstract

Pedogenic carbonate nodules have been observed in the Far North region of Cameroon. They have been interpreted as Vertisol relic components inherited from the African Humid Period (AHP), however their processes of formation remain poorly constrained. Here we explore the possibility of targeting the minerals trapped within such nodules, which represent a residual soil fraction, to date the deposition time of the soil parent material using optically stimulated luminescence (OSL). As the trapped minerals have a complex history, related to pedogenesis and sedimentary processes since their deposition, it was necessary to identify a suitable mineral population. To achieve this, a nodule collected at the surface was investigated in order to compare the OSL signals from feldspar minerals from within its external and internal parts. The resultant luminescence data were then compared with minerals trapped in a buried nodule, with the scatter in OSL signals between the different populations indicating potential partial bleaching of grains and/or pedoturbation processes. Our results show that the minerals from the internal part of the nodule are appropriate for OSL dating. Furthermore, the environmental dose rate has changed through time following carbonate precipitation and the radionuclide decay series may be in disequilibrium. Because of its solubility, the 238 U-series is particularly affected, and when a complete absence of 234 U daughters is assumed, our calculations show a maximum and significant decrease of 20% in the environmental dose rate compared to secular equilibrium. Finally, changing the time of carbonate nodule precipitation results in a maximum OSL age range of 12 ka (from 15 to 27 ka) for the deposition of the soil parent material. Despite this relatively high uncertainty, these ages suggest that the deposition occurred before the AHP. Consequently, if their formation time is well constrained from independent methods, the pedogenic carbonate nodules can be exploited as valuable time archives, providing temporal information on soil parent material deposition and thus a maximum age for the onset of pedogenesis.

Age determination using feldspar: Evaluating fading-correction model performance

“…A function for age correction using the Huntley and Lamothe (2001;H+L) approach has been implemented in the R package 'Luminescence' (Kreutzer et al, 2012; and is commonly used to correct feldspar luminescence ages (e.g. Bickel et al, 2015;Diaz et al, 2016).…”

Medieval demise of a Himalayan giant summit induced by mega-landslide

“…Sample preparation followed the procedure described in ref. 62 to isolate K-feldspar fraction. Bulk samples were sent to Actlabs or SARM-CRPG for major and trace element analysis, and used to estimate (see Supp.…”

Calcium transfer and mass balance associated with soil carbonate in a semi‐arid silicate watershed (North Cameroon): An overlooked geochemical cascade?

“…Diaz et al (2018) concluded that these dates correspond to the time period when organic matter was integrated into the Vertisols. The age of the CRPM deposition has been assessed using OSL dating (applied to K‐rich feldspar grains trapped in nodules; Diaz et al, 2016b) and provided ages ranging from 18.0 to 12.0 ka bp (Diaz et al, 2018). Finally, Dietrich et al (2017) demonstrated that the CRPM is a 1:1 mixture of the weathered granitic bedrock and Saharan dust using a multi‐isotopic approach.…”