Anne E. Taunton

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.

Geomicrobiological controls on light rare earth element, Y and Ba distributions during granite weathering and soil formation

In this study we examine the redistribution of trivalent and tetravalent light rare earth elements (LREEs), Ba and Y during chemical weathering of granites from southern New South Wales, Australia. In essentially abiotic zones in the lower weathering profile, primary allanite is dissolved, and apatite is extensively replaced by secondary LREE lanthanide phosphates such as rhabdophane and florencite. This association is attributed to the relatively high concentrations of phosphorus at dissolving apatite surfaces and low solubility products for lanthanide phosphates. Bulk chemical data from the lower profile indicate considerable enrichment of Y, La and Nd. In contrast, in granites weathered in proximity to the soil zone, secondary lanthanide phosphates are rare and phosphate surfaces are often colonized by bacteria and fungal hyphae. Bulk chemical data show that Y, La and Nd decrease in abundance with increasing weathering. Low dissolved phosphate concentration due to microbial uptake of phosphorus suppresses secondary phosphate precipitation and also leads to dissolution of secondary lanthanide phosphates formed prior to colonization. In the most highly weathered rocks and soils, secondary phosphates are extremely rare, and only Ce oxides remain. Bulk chemical data show Ce concentrations in excess of 1200 ppm in some soils. Retention of Ce as Ce-oxides reflects the low mobility of (tetravalent) Ce under oxidizing conditions. These observations explain heterogeneities in LREE abundances in weathering profiles, development of extreme Ce anomalies, and greatly elevated concentrations of trivalent LREEs in some regions.

Bacterial mineralization patterns in basaltic aquifers: implications for possible life in martian meteorite ALH84001

To explore the formation and preservation of biogenic features in igneous rocks, we have examined the organisms in experimental basaltic microcosms using scanning and transmission electron microscopy. Four types of microorganisms were recognized on the basis of size, morphology, and chemical composition. Some of the organisms mineralized rapidly, whereas others show no evidence of mineralization. Many mineralized cells are hollow and do not contain evidence of microstructure. Filaments, either attached or no longer attached to organisms, are common. Unattached filaments are mineralized and are most likely bacterial appendages (e.g., prosthecae). Features similar in size and morphology to unattached, mineralized filaments are recognized in martian meteorite ALH84001.