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

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 (similar to 37 degrees 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 (similar to 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 mu m kaolinite separates showed delta O-18 values ranging from 17.3 parts per thousand to 20.0 parts per thousand, and delta D values between -71 parts per thousand and -60 parts per thousand. The delta O-18 and delta D mean values indicated a crystallization temperature of about 24 degrees C, which is higher than the local present-day annual mean air temperature (similar to 18 degrees C). For the calculated temperature, the oxygen isotope fractionation factor between kaolinite and water (alpha(k-w) = 1.0248) implies that kaolinite formed in equilibrium with meteoric waters (delta O-18 = -5.4 parts per thousand and delta D = -33 parts per thousand) 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.