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

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.