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


The investigation of laterites and lateritic soils is multifaceted and has become increasingly important as the role of the critical zone (CZ) is recognized. The CZ encompasses a near-surface region where complex chemical, biological, and mechanical interactions take place and, as such, is vital to human, plant, and animal life. In particular, the CZ in the tropical regions of southeast Asia, India, Africa, and South America is lateritic (Kellogg and Orvedal, 1969). These areas of the world are heavily populated with developing economies and a strong dependence on agriculture. As such, understanding the CZ, and the processes that operate in it, is vital to sustainable development and food security.

Laterites play a number of important roles in the tropics in terms of economic deposits, engineering, long-term regulation of the climate, and solute fluxes. Laterites have been investigated as ore deposits for Al, Fe, Ni, Au, Nb, and P (e.g., Meyer et al., 2002; Gleeson et al., 2004; Freyssinet et al., 2005). Tropical saprolites typically have low shear wave velocities (VS) of ~300 m/s (Yaede et al., 2015), corresponding to seismic design site class D (stiff soil; NEHRP, 2003). This is important in areas where there is a seismic hazard, like the Big Island of Hawaii (USA) (e.g., Wong et al., 2011). Chemical weathering of laterite substrates is an important long-term sink for atmospheric CO2 (e.g., Dessert et al., 2003; Navarre-Sitchler and Brantley, 2007; Beaulieu et al., 2012; Nelson et al., 2013), thereby playing an important role in regulating the climate. GEOSPHERE GEOSPHERE; v. 14, no. 3 11 figures; 1 set of supplemental files CORRESPONDENCE: CITATION: Sowards, K.F., Nelson, S.T., McBride, J.H., Bickmore, B.R., Heizler, M.T., Tingey, D.D., Rey, K.A., and Yaede, J.R., 2018, A conceptual model for the rapid weathering of tropical ocean islands: A synthesis of geochemistry and geophysics, Kohala Peninsula, Hawaii, USA: Geosphere, v. 14, no. 3, p. 1–3, Science Editor: Shanaka de Silva Received 30 November 2017 Revision received 5 February 2018 Accepted 9 April 2018 OPEN ACCESS GOLD This paper is published under the terms of the CC‑BY-NC license. © 2018 The Authors Research Paper GEOSPHERE | Volume 14 | Number 3 Sowards et al. | Rapid weathering of basalt 2 In fact, basalts play an oversized role. Because basalt generally lacks quartz, more carbonic acid in the vadose zone is expended on a mole-per-mole basis than in felsic rocks.

Laterite formation controls weathering and denudation rates (e.g., Hilley et al., 2010; West, 2012), and its study can reveal the processes and mass balances underlying denudation, including the leaching of alkaline and alkaline earth elements accompanied by the release of Si as a function of climate, topographic slope, and water-rock contact time (e.g., Nelson et al., 2013). In particular, the leaching of rock-derived, base-cation nutrients (Na+, K+, Ca2+, and Mg2+) is a fundamental consequence of laterite development. Other nutrients may be derived from the atmosphere and fixed by soil microbes (N), removed from rocks and cycled through plants (Si4+), or derived from the parent rock and returned to the soil by biological processes (P, K+) (e.g., Vitousek and Sanford, 1986; Derry et al., 2005). Other cations such as Ca2+ are highly leached in strongly weathered material and effectively removed, leaving atmospheric deposition as their dominant source (e.g., Kennedy et al., 1998).

The importance of understanding laterite development ranges from economic to ecological to engineering to biogeochemical considerations. In this context, the purposes of this study are twofold. First, we investigate weathering processes and rates of soil and saprolite development on a relatively young basaltic substrate. In this case, we consider the development of a ~15 m thick soil-saprolite system on ca. 300 ka lavas versus saprolites developed on ca. 2 Ma lavas on the island of Oahu, Hawaii (Nelson et al., 2013; Yaede et al., 2015). We examine the effects of primary lava flow textures by holding fixed both bedrock composition (tholeiites to quartz tholeiites) and climate. The second purpose is to synthesize geochemical, mineralogical, and geophysical data into a conceptual model for the development of a laterite weathering profile over the relatively young substrate. This is accomplished by selecting a single, well-exposed study site at the northern tip of the Kohala Peninsula on the Big Island of Hawaii (Fig. 1).

Sowards figure 1.PNG
Figure 1. (A) Location of the study area on the Kohala Peninsula, Big Island of Hawaii, including the locations of sampled regional soils. Pololu Volcanics lavas are in pink, Hawi Volcanics lavas in dark red. Pie diagrams summarize regional soil and saprolite mineralogy (gray—plagioclase; black—Fe-Ti oxides; blue—halloysite; red—gibbsite; yellow—quartz; dark green—pyroxene; light green—olivine). Magenta isohyets (in mm/yr) are from Frazier et al. (2016). Black lines are roads. (B) Google Earth historical image from 2013 illustrating the location of rappel points on the cliff and well as the location of geophysical profiles BIH-2 and BIH-3. Note the wave-cut platform in fresh basalt immediately north of the rappel points. UTM coordinates, NAD83, Zone 5Q.

Geologic and Hydrologic Setting

The Hawaiian Islands are an excellent natural laboratory for the investigation of chemical weathering. They are overwhelmingly comprised of basalt, in particular tholeiitic lavas, with minor alkaline basalt and more evolved lithologies (e.g., Sherrod et al., 2007). This minimizes variability in the composition of fresh bedrock, thereby simplifying the study. Rainfall, by contrast, varies enormously from >6 m/yr on the windward side of the large islands to <0.25 m/yr on leeward or rain shadow areas (Frazier et al., 2016). The islands also vary in age from ca. 5 Ma for Kauai to 0 Ma for the Big Island of Hawaii (Frey et al., 1991; Sherrod et al., 2007). These factors permit examination of weathering processes, rates, and products as a function of time and climate by the careful choice of field localities. In this study, we have chosen one site for intense investigation where the bedrock substrate varies little in age and has weathered under the same climate.

Our study area lies on the flank of Kohala, the oldest of five shield volcanoes that comprise the Big Island of Hawaii (Fig. 1). Sherrod et al. (2007) provided an excellent summary of its geologic history. The peninsula is underlain by tholeiitic basalts of the Pololu Volcanics, which range in age from ca. 0.46 to ca. 0.26 Ma. The Pololu Volcanics are overlain by alkaline post-shield Hawi volcanic rocks that range in age from ca. 0.3 to ca. 0.1 Ma (Chadwick et al., 2003; Sherrod et al., 2007). Chemically weathered Pololu lavas compose the sea cliff exposure examined in this study (Fig. 2).

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The study area is mesic, receiving 1500–2000 mm/yr of rain (Fig. 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 CO2 concentrations are undoubtedly high and help drive weathering reactions through the formation of carbonic acid in soil waters. Cliff exposures are from 14.6 m to 12.7 m thick in deeply weathered material (Fig. 2). The ground surface slopes toward the northeast at ~3°, reflecting the average topographic surface of the distal portions of the Kohala shield volcano. Outside the Kohala Peninsula, most of the Big Island is either too dry or too young to form thick laterite profiles.

Mineralogy of Hawaiian Laterites

The mineralogy of soil-saprolite systems in the Hawaiian Islands represents the early to intermediate to end stages of chemical weathering, depending on the degree of leaching, where the minerals present in any one horizon are indicators of the intensity of leaching. Rainfall obviously influences the thickness of laterite zones as well as the degree of leaching. For example, Yaede et al. (2015) noted a positive correlation between the thickness of laterites and precipitation on Oahu. 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.

e much younger Kohala lavas. The abundance and species of clays forming in the laterite zones also depend on variables such as elapsed time, composition, texture, and porosity (Johnsson and Stallard, 1989; Bates, 1962; Barshad, 1957; Jackson, 1957). Bates (1962) observed that a mineral present as a transition phase in areas of high rainfall may be an end product of weathering in dry regions.

In Hawaii, the weathering of primary igneous minerals produces an assemblage of allophane, halloysite or kaolinite, and Fe-Ti oxides (Jones et al., 2000; Chadwick et al., 2003; Nelson et al., 2013). Locally, smectite may occur (Nelson et al., 2013; Dessert et al., 2003; Chadwick et al., 2003; Johnsson et al., 1993), and may be an intermediate phase of weathering before recrystallization to kaolinite or halloysite (Glassman and Simonson, 1985; Eggleton et al., 1987). Joussein (2016) and Vitousek et al. (2007) suggested that soil mineralogy ripens through a series of steps from allophane or imogolite to halloysite and finally kaolinite on a time frame of ~1 m.y. Clearly, the weathering of parental basalt to an assemblage of halloysite-kaolinite and Fe-Ti oxides requires the extensive leaching of Si4+, Ca2+, Mg2+, Na+, and K+.

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