Geochemical features of a soil chronosequence developed on basalt in Hainan Island, China

Abstract

Hainan Island is located in the South China Sea. Basalts of different geological ages are widely distributed in the north of the island, and soils developed from them form a well dated soil chronosequence. The absolute ages of basalt from which soils have developed varied from 1.0×104 a to 1.81 Ma. A few indicative soil features, especially typical geochemical features, were studied and related to the stages of soil development. Relative depletion/enrichment ratios of macroelements and microelements, as well as of rare earth elements (REE), were calculated and several indices were proposed to illustrate the development of the soil chronosequence.

The study found that Fe and Al were relatively enriched, while Ca, Mg, K and Na as well as Si were strongly lost during rock weathering and soil formation. Indices based on soil macroelements, such as the chemical index of alteration (CIA), the chemical index of weathering (CIW), the silicon-aluminum ratio (Sa), and the Silicon-aluminum-iron ratio (Saf) could not adequately express the sequential weathering degree of the chronosequence. The weathering index (WI) gave a good relationship with soil age and can be taken as an index of soil development. Ba/Nb correlated significantly with soil age and can be used as an indicator of soil development. REE content increased linearly with soil age so it can express the degree of soil development. The relative depletion rates of major elements showed that Si was lost up to 80% of the original content before 1 Ma and remained in constant concentrations afterwards. Easily mobilized elements were lost quickly during the initial stage of weathering and more than 90% of them were depleted within the first 1.0×104 a.