Microbial controls on phosphate and lanthanide distributions during granite weathering and soil formation

Abstract

Both microbial and geochemical factors control the form, distribution, and abundance of lanthanides during weathering and soil formation in a profile in the Bemboka Granodiorite from southern New South Wales, Australia. During the initial stages of weathering, hydrous Ce-bearing lanthanide and lanthanide-aluminum phosphates (including rhabdophane and florencite) crystallize on etched apatite surfaces throughout the profile. However, their fate depends greatly on their location within the profile. In rocks weathered 5–6 m below the soil zone (the lower profile), secondary lanthanide phosphates persist long after all apatite has been dissolved. However, in rocks weathered within 2 m of the soil zone (the upper profile), secondary phosphates are dissolved and lanthanides other than Ce are removed. Bacteria and fungal hyphae are localized on secondary phosphate surfaces, suggesting that rhabdophane and florencite are solubilized in the upper profile due to organic complexation of dissolved lanthanides and/or uptake of phosphate by cells. In the soils (defined by loss of granitic texture), secondary phosphates are replaced by Ce-oxides. Lanthanides (other than Ce) removed in solution from the upper profile precipitate as Ce-poor phosphates in the lower profile when the solubility product is exceeded due to high dissolved phosphate concentrations in proximity to apatite. Thus, the upper profile is the source of lanthanides added to the lower profile. In both the lower and upper weathered granite, the degree to which weight-based Ce abundances increase with increasing weathering is consistent with Ce immobility on the centimeter scale. We attribute the very high weight-based Ce abundances (up to 12× concentrations in fresh rock) to extensive leaching and compaction during transformation of weathered rock to soil.