Soil potassium isotope composition during four million years of ecosystem development in Hawai‘i

WashU Affiliated Authors: Kun Wang (Dept. of Earth and Planetary Sciences and McDonnell Center for the Space Sciences)

Abstract:We combine spectroscopic and geochemical approaches to interpret the fate of potassium (K) during forest soil development along a 4-million-year chronosequence sampled from relatively undisturbed rainforests in Hawai‘i. Potassium derived from weathering of lava is dominant in the youngest site (0.3 ky), but its contribution to the soil K budget declines as weathering progresses. Sites older than 0.3 ky are characterized by substantial K depletion (τK,Nb ∼−1), with soil isotopic composition (41K/39K, δ41K) varying from −1.91 ± 0.08‰ to −0.09 ± 0.08‰, relative to the homogeneous basaltic substrate (−0.48‰). Exchangeable and interlayered K show δ41K values ranging from −1.32 ± 0.06‰ to 0.06 ± 0.08‰, higher than their corresponding bulk δ41K values. The δ41K patterns of soils and exchangeable components are vertically similar, implying similar environmental controls. The variability in K phase and isotope composition reflects the accumulative effect of different processes. Chemical weathering and plant cycling retain isotopically light K in soils, in particular for 20–150 ky sites. In contrast, atmospheric inputs of marine aerosols (0.14‰) and mineral aerosols (−0.44‰) add heavier K (than native basalts) and crust-like K (similar to basaltic δ41K) in soils, respectively. In sites older than 150 ky, nearly complete depletion of basaltic K and reduced plant K imprints result in the dominance of atmospheric inputs in soils. In sum, this study emphasizes the sensitivity of δ41K to terrestrial biotic and abiotic K cycles.

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