The denudation of ocean islands by ground and surface waters: The effects of climate, soil thickness, and water contact times on Oahu, Hawaii

Abstract

Ease of access, size, and basalt as the dominant bedrock make Oahu an ideal locality for investigating chemical weathering-driven denudation rates as a function of climate (rainfall varies by an order of magnitude), water–rock contact time, and soil thickness. New and compiled surface and groundwater solute data permit calculation of mass balances for solute fluxes from Oahu, revealing that groundwater dominates surface water solute fluxes by a factor of 3–12.Weathering reactions were written consistent with the mineralogy of Oahu soils, permitting denudation rates to be partitioned between dissolved and suspended loads. Total denudation rates, indexed to the leaching of SiO2, vary from 0.016 to 0.063m/ka, with about 70% of Si transport due to dissolved loads. Drier regions of Oahu have distinctly lower denudation rates, and areas with thick weathering profiles have suppressed surface-water solute loads.Indexing denudation in basaltic terranes to dissolved SiO2 rather than other solutes leads to improved estimates of weathering rates. Other approaches require correction for the atmospheric depositions of sea salts based on Cl− abundances in waters that are assumed to derive solely from the ocean via atmospheric deposition.Recent work indicates that Oahu is tectonically emerging at 0.060m/ka. As long as this uplift continues, the net size of the island will slowly increase and the Koolau Range should persist as an orographic trap to precipitation, maintaining relative aridity in the Waianae Range. Comparing emergence and denudation rates suggests that growth of the island will be non-uniform, with arid regions experiencing the greatest emergence with wet regions in balance with denudation. More importantly, however, this work offers an increased appreciation of the controls on the rates and mechanisms of denudation in basaltic and intermediate composition terranes in the tropics.