Variations of Mg isotope geochemistry in soils over a Hawaiian 4 Myr chronosequence

Magnesium (Mg) isotopes fractionate during rock/mineral weathering and leaching, secondary mineral neoformation, adsorption/desorption, and plant-related Mg recycling, but the mechanisms and extent of fractionation are not well under-stood. Here, we report the fate of Mg and its isotopes during basalt weathering and soil development in the Hawaiian Islands by sampling soils of varying age (0.3, 20, 150, 1400, and 4100 ka) in undisturbed humid rainforests. Magnesium concentrations in bulk soils are variable with depth and age, ranging from 0.07 to 8.79 wt.%, and significant Mg depletions (up to 99%) relative to parent basalts are visible after 20 ka. Bulk soils display a large age-dependent range of delta Mg-26 values ranging from -0.60 to +0.26 parts per thousand. A sequential leaching scheme showed that labile Mg is depleted whereas residual Mg is enriched in isotopically heavy Mg. The two youngest soils (0.3 ka) display delta Mg-26 value similar to basalt for both labile or residual Mg, indicating either that basalt weathering causes little Mg isotope fractionation or that delta Mg-26 value is overwhelmed by the primary minerals during 0.3 ka. However, in the older soils (>= 20 ka), the delta Mg-26 values of both labile and residual Mg vary non-linearly as a function of time, with an increase in the difference between them. These variations are explained by both plant-related Mg recycling and progressive mineral transformations, evolving from short-range-order (SRO) minerals (allophane and ferrihydrite) to more crystalline products (goethite, gibbsite and kaolin minerals). Indeed, plant-related Mg recycling causes the enrichment of light Mg isotopes in the labile Mg, while secondary phases incorporate more and more heavy Mg isotopes with time. These results reconcile experimental and field studies and highlight a weathering control of Mg isotopes delivered to the oceans. (c) 2020 Elsevier Ltd. All rights reserved.

Automated summary

An analysis of soil samples of varying age and depth from the Hawaiian Islands revealed complex fractionation of magnesium and its isotopes during weathering and soil development, with variations influenced by plant recycling and mineral transformations as time progresses.