Controls on 87Sr/86Sr of Plant- Available Ions and Soil Minerals across the Hawaiian Archipelago

Across the Hawaiian islands, continuous variation in substrate age and climate superimposed on near-constant parent material composition and limited species diversity provide an ideal natural experimental system to test hypotheses about soil development and nutrient sources. We present ⁸⁷Sr/⁸⁶Sr data on both bulk soil digests and NH₄OAc extractable cations from soil profiles covering a wide range of environments and substrate ages. Whole soils from dry climates and/or young substrate ages with > 100 µg/g Sr retain basalt-like Sr isotopic signatures (⁸⁷Sr/⁸⁶Sr ≤ 0.705), whereas those with Sr concentrations <100 µg/g can have isotope values that range from basalt-like values to more radiogenic signatures of >0.721. Variation in these signatures depend strongly on the amount of dust derived from continental source. Since both dust accumulation and lava weathering are time dependent the overall concentration of Sr drops with increasing time even as quartz and mica concentrations increase to >40%. At these elevated dust levels of quartz and mica, lava-derived Sr is low and dust-derived Sr is the dominant source, yielding a radiogenic “continental” signature. This relationship between strong weathering of lava and accumulation of dust can be further evaluated using δ³⁰Si which demonstrates that while the lava may be weathered much of the silicon isotope signal is derived from dust which is only minimally weathered compared with the host lava minerals. We conclude that whole soil Sr isotope values are controlled by an interplay between lava weathering and leaching and dust accumulation. Interestingly, this is not true for plant available Sr where we find that as the lava-derived Sr concentrations drop they are replaced by marine-derived Sr which is introduced in dilute solution in rainwater. Marine aerosol Sr becomes the main contributor to ecosystems even as the Sr isotopic composition in the bulk soil grows strongly radiogenic.