B21D-0477
Strontium isotopes provide clues for a process shift in base cation dynamics in young volcanic soils
Tuesday, 15 December 2015
Poster Hall (Moscone South)
Nina Bingham1, Matthew G Jackson2, Bodo Bookhagen3, Katharine Maher4 and Oliver Chadwick1, (1)University of California Santa Barbara, Santa Barbara, CA, United States, (2)University of California Santa Barbara, Department of Earth Sciences, Santa Barbara, CA, United States, (3)University of Potsdam, Potsdam, Germany, (4)Stanford University, Stanford, CA, United States
Abstract:
Despite advances in soil development theory based on studies of old soils or over long timescales, little is known about soil thresholds (dramatic changes in soil properties associated with only small shifts in external forcing factors) that might be expressed in young soils (less than 10 kyr). Therefore, we seek to understand infant soil development in a tropical environment through the sourcing of plant available base cations by measuring the strontium (Sr) isotopic composition of the soil exchange complex. 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), a rainfall component (0.7093) and a component from continental dust (0.720). Elevation-controlled leaching intensity in the wettest localities produces a decline in the concentration of base cations supplied by basalt and a dilute resupply by rainfall. In the driest sites, where leaching intensity is dramatically reduced, there is a buildup of rainfall-derived extractable Sr in the soil over time. Slow rock weathering rates produce a small rock-derived cation input to the soil. Thus, Sr isotope signatures reflect both the input of rainfall-derived cations and rock-derived cations with values that fall between rainfall and basaltic signatures. 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. In contrast to the clear-cut differentiation in strontium 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. These factors lead to a kinetic limitation to the release of rock-derived Sr and other nutrient ions.
