Depth and character of rock weathering across a basaltic‐hosted climosequence on Hawai‘i

Abstract

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. The transport of organic carbon (C) to deep mineral horizons in soils can lead to long-term C stabilization. In basaltic soils, C associations with short-range-ordered (SRO) minerals often lead to colloid-sized aggregates that can be dispersed and mobilized by changes in soil solution chemistry. In the montane forest region of Hawaii, basaltic soils are exposed to high rainfall and anoxic conditions that facilitate ferric (Fe III ) (oxyhydr)oxide reduction. We explored the potential of iron (Fe)-reducing conditions to mobilize C by exposing the surface mineral horizons of three soils from the Island of Hawai'i (aged 0.3, 20, and 350 ky) to 21 days of anoxic incubation in 1:10 soil slurries. Mobilized C was quantified by fractionating the slurries into three particle-size classes (b430 nm,b 60 nm,b2.3 nm ≈ 10 kDa). In all three soils, we found Fe reduction (maximum Fe 2+ (aq) concentration ≈ 17.7 ± 1.9 mmol kg −1 soil) resulted iñ 500% and~700% increase of C in the 2.3-430 nm, and b 2.3 nm size fractions, respectively. In addition, Fe reduction increased solution ionic strength by 127 μS cm −1 and generated hydroxyl ions sufficient to increase the slurry pH by one unit. We compared this to C mobilized from the slurries during a 2-h oxic incubation across a similar range of pH and ionic strength and found smaller amounts of dissolved (b 2.3 nm) and colloidal (2.3-430 nm) C were mobilized relative to the Fe reduction treatments (p b 0.05). In particular, C associated with the largest particles (60-430 nm) was dispersed almost exclusively during the Fe reduction experiments, suggesting that it had been bound to Feoxide phases. Our experiments suggest that colloidal dispersion during Fe-reducing conditions mobilizes high concentrations of C, which may explain how C migrates to deep mineral horizons in redox dynamic soils.

A conceptual model for the rapid weathering of tropical ocean islands: A synthesis of geochemistry and geophysics, Kohala Peninsula, Hawaii, USA

“…However, with between 700 and 1200 mm/yr precipitation, the thickness of the weathering zone varied between 6 and 40 m. It was only for annual precipitation >1300 mm that weathering zones exceeded 45 m thickness. Chadwick et al (2003) and Goodfellow et al (2014) noted similar precipitation thresholds at 1400 mm/yr and 1200 mm/yr, respectively, albeit on the much younger Kohala lavas.…”

“…1), wetter than the ~1400 mm/yr threshold for thick and thorough leaching of bedrock on the Kohala Peninsula (Chadwick et al, 2003;Goodfellow et al, 2014). The area is heavily vegetated, such that soil CO 2 concentrations are undoubtedly high and help drive weathering reactions through the formation of carbonic acid in soil waters.…”

Mineralogical Transformations and Soil Development in Shale across a Latitudinal Climosequence

“…Elevation gradients encompass a range of precipitation and temperature across short distances and record climate effects on soil development and weathering (e.g., Koch et al, 1995;Dahlgren et al, 1997;Bockheim et al, 2000;Egli et al, 2003;Stiles et al, 2003;Goodfellow et al, 2014). Similarly, soil profiles observed across latitudinal gradients (Muhs et al, 2001;Williams et al, 2010) or at a variety of latitudes (Rasmussen et al, 2011) show correlations of climate to weathering rates and mineral transformations.…”

“…In landscapes where physical erosion is low, studies at multiple spatial scales have demonstrated that precipitation and temperature strongly control chemical weathering rates and soil development when all other state factors are held constant (Strakhov, 1967;Ruhe, 1984;Chadwick et al, 2003;Dahlgren et al, 1997;Rasmussen et al, 2007;Williams et al, 2010;Goodfellow et al, 2014). The degree of soil development varies considerably with changes in climate, with thresholds observed in weathering processes and soil formation between water-limited systems and energy-limited systems (Dahlgren et al, 1997;Rasmussen et al, 2011;Goodfellow et al, 2014).…”

“…The degree of soil development varies considerably with changes in climate, with thresholds observed in weathering processes and soil formation between water-limited systems and energy-limited systems (Dahlgren et al, 1997;Rasmussen et al, 2011;Goodfellow et al, 2014). Soil development indices correlated with climate include increased clay content (Dahlgren et al, 1997;Bockheim et al, 2000;Lybrand and Rasmussen, 2015), elemental losses and redistributions during pedogenesis (Muhs et al, 2001;Egli et al, 2003;Stiles et al, 2003;Chadwick et al, 2003) and mineral transformations from parent material to secondary clay minerals in soils (Dahlgren et al, 1997;Mirabella and Egli, 2003).…”