Organic acids and high soil CO2 drive intense chemical weathering of Hawaiian basalts: Insights from reactive transport models

Abstract

We have investigated how biota contributes to rapid chemical weathering of Hawaiian basalts using a reactive transport model and chemical data from a soil chronosequence. These Hawaiian soils have developed under a tropical forest with rainfall >200 cm/yr and exhibit extensive weathering on timescales of 104 years. We developed a series of multicomponent reactive transport models to examine the role of soil respiration and low molecular weight organic acids in generating these intense weathering patterns. The base model starts with a 1-m basaltic porous media reacting with a fluid of rainwater composition in equilibrium with atmospheric CO2. Subsequent simulations incorporate soil respiration modeled as a constant flux of CO2 at 10× atmospheric and continuous input of organic ligands – oxalate and citrate – at 10−4 molar. After 20 kyr of weathering, the base model shows limited elemental losses, high soil pH and is overall CO2(acid)-limited. Soil respiration lowers soil pH to circumneutral values, leaches all Mg and Ca from the basalt and allows precipitation of Fe(III)-oxyhydroxides, while Al stays immobile as secondary clays accumulate. After adding organic ligands, soil pH is reduced to values similar to the Hawaiian soils and Si, Al and Fe are exported from the system by dissolution of secondary phases, resulting in mass depletion patterns similar to the ones observed in Hawai’i. Dissolution of secondary minerals is generated by low pH and relatively low free activities of Al3+ and Fe3+ when organic ligands are added. These results suggest that organic acids in basalt weathering in tropical environments can sustain far-from-equilibrium conditions that drive fast elemental losses and that biologic activity contributes to weathering processes both by generating high soil PCO2 and organic acids.