Natural variations of δSi ratios during progressive basalt weathering, Hawaiian Islands

Silicon stable isotopes can be used to trace the biogeochemical pathways of Si as it moves from its continental sources to its sink in ocean sediments. Along the way, Si is incorporated into clay minerals, taken up by plants where it forms plant opal, and leached into rivers, the major land-to-ocean conduit. Compared to igneous rocks, the waters that drain continents are enriched in heavy Si isotopes, but the mechanisms that control fractionation have not been elucidated. We studied Si isotope fractionation along a 4 million yr basaltic soil chronosequence on the Hawaiian Islands. Using the natural context of these samples in combination with laboratory experiments, we demonstrate that the isotopic composition of dissolved Si in weathering systems is determined by the combined effects of rock disintegration, clay mineral neosynthesis, and Si biocycling. Weathering preferentially releases ²⁸Si into solution, whereas secondary mineral formation preferentially removes ²⁸Si from solution. In humid environments, leached soils have lost large amounts of this soluble Si, thus creating a net loss of ³⁰Si from the entire soil system. As soils develop and greater fractions of Si reside in neoformed clay minerals, δ³⁰Si_{bulk soil} values change progressively toward more negative values; basalt δ³⁰Si values are about −0.5‰, but older soils have δ³⁰Si values up to −2.5‰. The difference between the solid and solution δ³⁰Si values remains more or less constant with progressive weathering, and therefore, soil water from older soils has a more negative δ³⁰Si composition. In the upper horizons of the Hawaiian soils, this weathering-driven δ³⁰Si shift is modified by the addition of unweathered primary minerals via dust, carrying δ³⁰Si values of about −0.5‰, and by biocycling of Si via plants, producing negative δ³⁰Si values in phytoliths and positive δ³⁰Si values in soil solutions derived from upper horizons. Due to the high concentrations of dissolved Si in these near-surface layers, rivers have more positive δ³⁰Si values than predicted based on the weathering status of the lower horizons. When combined with published δ³⁰Si values from large rivers worldwide, we find that the results from Hawaii point to weathering control of Si isotopes delivered to the oceans, and thus, to an important continent-ocean linkage that warrants further investigation.