The Fe/Na ratio, a framework for modelling trace element distributions in Jamaican soils

摘要

The trace element geochemistry of Jamaican soils (<150 m) is extremely variable due to their development on diverse parent materials, varying maturity (ages range from recent to >5 Ma) and the variety of soil-forming processes. It is demonstrated that the Fe/Na ratio of the soils forms a useful quantitative framework for studying trace element distributions. The ratio varies 2.5 orders of magnitude from <2 in newly developed inceptisols to >500 in oxisols and terra rossas that have been developing for >5 Ma. The form of the trace element distributions with respect to Fe/Na ratio may be interpreted in the context of mineralogy of the parent material, removal of labile trace element fractions during soil development, sequestration in stable secondary forms, and ultimate concentration of stable primary resistate and secondary minerals by terra rossa soil formation processes that lead to elevated Fe and Al levels and the depletion of silica and base cations, Ca~(2+), Mg~(2+), Na~+ and K~+. As examples, the distributions of U and As are modelled as functions of the Fe/Na ratio. Use of robust regression procedures identifies individual soil samples that exhibit divergent patterns of trace element concentration. Some of these can be linked to local bedrock sources, while others are more likely related to exotic volcanic ash-fall material from Central America deposited on proto-Jamaica in the late Miocene.