BASALT SURFACE ALTERATION DURING INCIPIENT SOIL FORMATION

Monitoring incipient soil formation is used to understand the initial stages and processes of rock transformation to soil. Basalt weathering is one of the most important contributors in controlling atmospheric carbon dioxide concentrations in geologic time, and to observe this the world’s largest controlled experiment, Biosphere 2, was built in Tucson, AZ. This experiment was designed to investigate basalt alteration, hydrology of zero-order basins, and chemical fluxes of various elements. The Landscape Evolution Observatory (LEO) provides an opportunity to monitor basalt transformation in relatively simple zero-order basins that are composed of ground rock that mimic a loamy sand-soil texture under controlled environmental conditions. Three replicated slopes, currently lacking vegetation, are instrumented with a dense network of sensors and samplers that are capable of capturing water, carbon, and energy dynamics at a high spatiotemporal resolution to establish landscape evolution patterns. The goals of our study were a) to investigate the surfaces of the basalt grains for micro- and nano-scale weathering patterns, and b) to quantify mineral transformation at high resolution. To reach these goals a combination of scanning electron microscopy (SEM), electron microprobe (EMP) equipped with wavelength dispersive spectroscopy (WDS), and synchrotron transmission x-ray diffraction (STXRD) were used on samples from one of the locations of each slope and compared to the original material. The SEM results show various degrees of organo-mineral layer formation on surfaces of the basalt grains and dehydrated amorphous oxide/hydroxide phases in all samples. The original material also exhibited some of these features, but at a lower percentage. These layers could potentially slow the weathering rates. We expected to see elemental depletion zones to about 30-50 nm depth into the minerals and an increase in the proportion of amorphous materials in the samples. However, there were no observable depletion zones with EMP, and the STXRD did not show secondary mineral peaks with one wt% resolution. Further investigations using higher resolution techniques are needed to understand rock alterations during incipient soil formation prior to plant colonization of the landscapes.