Strontium Isotopes Fingerprint Base Cation Sources During Early-Stage Soil Formation on Hawaiian Basalt Flows
Soil residence times for dynamic landscapes are often relatively short compared to geologic timescales. The nutrient status of these soils is controlled by the rates of chemical and physical rock weathering and soil development. Studies which constrain most external soil forming factors produce translatable results to more complicated systems, providing a baseline interpretation of incipient soil development. Therefore, I seek to understand soil formation over a relatively short time scale (0 to 7500 years) through the sourcing of plant available base cations by measuring the strontium (Sr) isotopic composition of the soil exchange complex and residual soil pools in a young, tropical chrono-climosequence. Our sampling strategy spans soils in three different precipitation ranges (950-1060 mm, 1180-1210 mm, and 1450-1500) and an array of soil ages from 500 to 7500 years in the Kona region on the island of Hawaii. In Hawaiian soils, 87Sr/86Sr values are determined by a mixture of three components: a mantle-derived component from the lava (0.7034, Kennedy et al., 1998), a rainfall component (0.7093, Kennedy et al., 1998) and a component from continental dust (0.720, Nakai et al., 1983; Kennedy et al., 1998). In contrast to the clear-cut differentiation in Sr isotopes with precipitation shifts observed in older soils, patterns on these young soils in Kona are complicated by low soil water residence times and small surface to volume ratios for the rock. This kinetic constraint to rock weathering is implied by the low proportion of bulk Sr compared to that of its rock substrate. The low concentrations of bulk Sr in the soil enables detection of dilute atmospheric Sr fluxes on the soil Sr isotope signatures. Precipitation is a driving factor controlling the proportion of atmospheric influence. In the driest sites, where leaching intensity is dramatically reduced, there is a buildup of rainfall-derived extractable Sr in the soil over time on top of the small supply of cations from rock weathering. Thus, extractable Sr isotope signatures reflect both the input of rainfall-derived cations and rock-derived cations with values that fall between rainfall and basaltic signatures. Sr isotopes and mixing model results point towards increased rock weathering in the wettest localities. However, elevational driven increased precipitation intensity leads to increased soil leaching at some sites which depletes the concentration of base cations supplied by basalt and a dilute resupply of cations by rainfall is apparent in the Sr isotope values. Soils in the intermediate precipitation range have Sr isotopic signatures consistent with both the wet and dry trends; suggesting that they lie close to the critical precipitation amount that marks a shift between these two processes. For the Kona region, this transition seems to occur at 1200 mm /yr. Finally, kinetic limitation to rock weathering allows dust accumulation, although small, to be detectable in the Sr signature for residual soils.