Rare-earth element and stable isotope signatures of kaolin from a Pliocene lateritic weathering profile at mid-latitude region (Andalusia, Spain): Implications for paleoweathering and paleoclimatic reconstructions

Abstract

This study reports the impact of weathering on behavior of rare earth elements (REE) in a coastal lateritic profile developed on Pliocene sediments at mid-latitude location (~37° N), in the Guadalquivir Basin (Spain). It also explores the stable isotope signature of kaolinite providing constraints on paleotemperature and isotopic composition of the ancient meteoric water. The paleoweathering profile (~20 m thick) comprises a white sandy clayey saprolite overlain by a red-mottled clay zone, which in turn is capped by a pisolitic ferricrete. The kaolinitic regolith was practically reduced to a mixture of kaolinite and quartz by intense chemical weathering under warm and seasonally humid climate.

The kaolinized material is markedly depleted in total REE (mean value of 77.30 mg kg−1 in the white saprolite and 72.70 mg kg−1 in the mottled zone) relative to the parent rock (168.96 mg kg−1). Development of acidic and oxidizing conditions promoted a suitable soil environment for REE release and leaching with percolating rainwater, leading to an apparent loss of REE from the profile (up to 87%). The parent rock-normalized REE patterns display concave-upward shapes tracing a remarkable depletion in middle REE (MREE), with no significant anomalies. The REE concentrations normalized against the North American Shale Composite (NASC) showed consistently similar fractionation patterns. The MREE-depleted signature (NASC-normalized ratios of La/Sm up to 3.32 and Gd/Lu as low as 0.45) probably arises from reductive dissolution of iron oxyhydroxide phases due to seasonal water saturation, as indicated from redoximorphic features.

The <2 μm kaolinite separates showed δ18O values ranging from 17.3‰ to 20.0‰, and δD values between −71‰ and −60‰. The δ18O and δD mean values indicated a crystallization temperature of about 24 °C, which is higher than the local present-day annual mean air temperature (~18 °C). For the calculated temperature, the oxygen isotope fractionation factor between kaolinite and water (αk-w = 1.0248) implies that kaolinite formed in equilibrium with meteoric waters (δ18O = −5.4‰ and δD = −33‰) that closely reflect the regional meteoric precipitation.

The lateritic weathering profiles evolved during the mid-Pliocene warm period in the southwestern Iberian margin could be useful for predicting potential future environmental effects of increased atmospheric CO2 on the Earth's critical zone.