Chemical weathering, mass loss, and dust inputs across a climate by time matrix in the Hawaiian Islands

We determined the total mass loss and rate of chemical weathering from three minimally eroded, Hawaiian lava flows that are ∼10, 170, and 350 ka old. Using a backhoe, we sampled the entire weathering zone at 28 sites and measured the depletion or enrichment of each major element in each soil horizon relative to parent material. We were able to assess the influence of both climate and substrate age on chemical weathering because each flow crosses a precipitation gradient from ∼600 to ∼2500 mm yr. Mass loss rates were highest for the 0–10 ka interval under the wettest climatic conditions (54 t km yr), and decreased to near zero in the wet sites during the 10–170 and 170–350 ka intervals. Not surprisingly, weathering rates were lower in drier sites; ∼24 t km yr from 0–10 ka to b2 t km yr thereafter. However the effects of precipitation were non-linear. There was a precipitation threshold below which mass loss was relatively small, and above which mass loss was substantial but insensitive to increased rainfall. Chemical weathering rates depend on tectonic uplift, erosion, climate, rock type or some combination thereof. By working on stable, uneroded surfaces of a single rock type across a well-constrained precipitation gradient, we were able to identify another potential driver: the rate of dust deposition. Although Hawaiʻi is one of the least dusty places in the northern hemisphere, dust inputs reached 82% of the total mass loss from the weathering zone at some sites, and averaged 30% on the 170 ka flow. This highlights the potential importance of dust as a component of observed weathering fluxes from catchments worldwide. © 2007 Elsevier B.V. All rights reserved.