Lanthanides in granulometric fractions of Mediterranean soils. Can they be used as fingerprints of provenance?

Abstract

There is geochemical interest in the lanthanides because they behave like a group that is closely related to the parent materials during surface processes, although they also undergo fractionation as a result of supergene dynamics. We analysed lanthanide concentrations (ICPms) in the granulometric fractions fine sand, clay and free forms of clay (FFclay‐CDB and FFclay‐Ox: extracted with citrate‐dithionite‐sodium bicarbonate and with ammonium oxalate, respectively) from a soil chronosequence of Mediterranean soils. There was a relative enrichment of heavy rare earth elements (HREE) in the clay fraction and its free forms with respect to fine sand. The clay free forms behaved as scavengers of lanthanides, and oxidative scavenging of cerium (Ce) in FFclay‐CDB was also detected. Lanthanide concentrations (lanthanum to gadolinium in fine sand; terbium to lutetium in clay) varied with soil age, and chronofunctions were established. There was a strong positive collinearity between most of the lanthanide concentrations. Furthermore, the value of the correlation index (Pearson's r) of the concentrations between couples of lanthanides (rCLC) decreased significantly with increasing separation between the elements in the periodic table; this has never been described in soils. Several geochemical properties and indices in the fine sand and clay soil fractions and in the geological materials of the Guadalquivir catchment showed, on the one hand, a genetic relation between them all, enabling the lanthanides to be used as fingerprints of provenance; on the other hand, fractionation between fine sand and clay showed these are actively involved in soil lanthanide dynamics. Highlights Are lanthanides from fine sand and clay genetically related to the geological materials? Lanthanide concentrations of fine sand and clay fit chronofunctions Pearson's r of lanthanide couples decreases when separation increases in the periodic table Free forms of clay are scavengers of lanthanides and concentrate HREE and cerium

Pedogenic information from fine sand: A study in Mediterranean soils

“…The chemical element composition (mean values per profile) of the whole fine sand (Tables 3 and 4) differed from that of the clay (Martín-García et al, 2019) in that the former contained lower quantities of major elements Al, Ca, Fe and K, and also lower quantities of most of the minor elements. This may be related, first, to the lower contents of phyllosilicates and, especially, of the (hydr)oxides goethite and haematite, in the fine sand, and, second, to the role of the clay fraction as a scavenger of heavy elements (Martín-García et al, 2016).…”

“…the existence of an authentic soil chronosequence, previously demonstrated by other compositional and morphological properties of the soils of the chronosequence (Calero et al, 2008(Calero et al, , 2009(Calero et al, , 2013Martín-García et al, 2016, 2019. No previous studies of chronofunctions established using the soil fine sand have been found in the bibliography.…”

“…Following a lithium metaborate fusion and nitric acid digestion, the following were determined: (a) Major oxides, by ICP-AES: SiO2, Al2O3, CaO, Fe2O3, K2O, MgO, Na2O, P2O5, TiO2; (b) trace elements by ICPms: Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Mo, Sn, Cs, Ba, Hf, Ta, W, Tl, Pb, Th, U. The sum of the lanthanide elements (ΣREE) was taken from Martín-García et al (2019). Several weathering indices were used to estimate the extent of chemical alteration: SiO2/CaO, Chemical Index of Weathering (Harnois, 1988) Weathering Ratio (Chittleborough, 1991): WR = (CaO+MgO+Na2O)/ZrO2, and Sr/Zr (Muhs et al, 2001).…”