『Abstract
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-1. Mass loss rates
were highest for the 0-10 ka interval under the wettest climatic
conditions (54 t km-2 yr-1), 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-2 yr-1 from 0-10 ka to
<2 t km-2 yr-1 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 Hawai‘i 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.
Keywords: weathering rate; mass loss; Hawai‘i; dust; chemical
depletion; climate; age』
1. Introduction
2. Site description
3. Methods
3.1. Mass loss calculations
3.2. Quantifying dust inputs
4. Results
5. Discussion
5.1. Mass loss
5.2. Dust
5.3. Uncertainties
6. Conclusion
Appendix A. Supplementary data
References