School of Earth & Environmental Sciences
Hannan-Joyner, Alex, The effect of time on soil development: study of a soil chronosequence from Reunion Island, Indian Ocean, BSci Hons, School of Earth & Environmental Sciences, University of Wollongong, 2016.
Here, presented for the first time, are the results of a geochemical analysis on a soil chronosequence from Reunion Island, Indian Ocean. The soils are defined by three profiles of ages 20 ka, 70 ka and 2 Ma, but all have evolved from the same basaltic parent material, climate, biota and topography, and so form a chronosequence. Using X-Ray Diffraction (XRD), we observe the rapid depletion of the major rock forming minerals – olivine, pyroxene, plagioclase – during the first 70 ka of soil formation. Conversely, there is rapid formation of secondary minerals over the same period, here defined as gibbsite, halloysite and goethite. Interestingly, halloysite, rather than gibbsite, is the dominant phase in the 2 Ma profile, and suggests Si is being conserved at the expense of Al. Additionally, Quadrupole Inductively Coupled Plasma Mass-Spectrometry (Q-ICP-MS) has enabled the concentrations of 19 elements to be measured with remarkable precision. By computing mass-transfer coefficients, identifying elements that are depleted and enriched throughout the profile can be achieved. At Reunion Island, niobium (Nb) is chosen as the index element for these mass-transfer calculations because it shows the greatest immobility. The alkali and alkaline earths are rapidly lost at the surface during the first 20 ka, mainly as a result of plagioclase dissolution. By 70 ka, however, the elements are gained at the surface, and suggests that marine aerosol and dust are able to contribute to the soil nutrient budget. The extent of these gains increases with increasing ionic radius (or equivalently, with decreasing hydration energy), and hypothesize that the fixation ability of clays, which increases with decreasing hydration energy, is responsible for the progressive enrichment of Mg (least) to Cs (most). After 2 Ma, the elements approach 100% depletion, except for Cs and Ba, and is assumed that these elements become immobile once adsorbed onto the surfaces of clays. It is our understanding this is the first time this has been observed in-situ. Meanwhile, the rare earth elements (REE), Mn, and Cu become progressively depleted with soil age, and hypothesise this is due to declining pH and Eh, both with decreasing depth in the soil profiles and over time across the chronosequence. Uranium and Th remain immobile, but are slightly enriched in the 70 ka soil, further supporting that the addition of dust has measurable effects on soil development. The balance between element gain from atmospheric dust and loss via mineral dissolution may be the main control on the soil nutrient budget at Reunion Island, but further research is needed to quantify the composition of the dust in order to make meaningful estimates of long-term soil sustainability.