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

Abstract

Much of the world's population lives in developing countries in regions with deeply weathered soils and thick, subjacent saprolites. These areas are widespread in the tropics and compose an important component of the critical zone (CZ). The Hawaiian Islands (USA) make an excellent natural laboratory for examining the tropical CZ, where the bedrock composition (basalt) is nearly uniform but of variable age and where climate (rainfall) varies greatly. The purpose of this study is to develop a model for rapid weathering to deep regolith profiles in tropical ocean islands.In the Kohala Peninsula, Hawaii, a variably weathered, thick soil and saprolite profile is exposed along sea cliffs in a mesic climate setting. Laterite development includes the entire vadose zone, with a mineralogy that chiefly includes halloysite ± gibbsite and Fe-oxides and mixed Fe-oxides and hydroxide species developed on a 303 ka substrate. Gibbsite-rich horizons occur in enhanced zones of weathering. At the base of the weathering front and on the rinds of core stones, transient smectite clays are developed, but rapidly break down to halloysite. Excess Al and Fe found in soil and saprolite likely originated from the decomposition of volcanic ash deposited on the ground surface from later effusive volcanism.Shear-wave velocity data derived from multichannel analysis of surface waves (MASW) and common depth point (CDP) seismic reflection profiles reveal important internal details of the weathering profiles that complement information derived from nearby rock outcrop along a sea cliff. In addition to identifying the depth of the weathering front, stiff horizons within the laterite correlate to high gibbsite abundances in the MASW profile. Parallel to the paleo-lava flow direction, relict igneous stratigraphy is expressed as seaward-dipping reflectors on CDP profiles, whereas perpendicular to flow, reflector geometry suggests lenticular bodies within laterite.A synthesis of geochemistry and geophysical studies leads to the development of a conceptual model to explain variable weathering within the CZ. First, although there is an overall trend for the downward migration of the weathering front with time, zones of high initial permeability (rubble above or below a'a flows; tephra or scoria deposits) are influenced by both downward and lateral fluxes of water, leading to enhanced weathering in what are now gibbsite-rich horizons. Second, the dense cores of a'a flows with widely spaced joints preserve large core stones that weather inward where halloysite-rich saprolite gives way to smectite-rich rims. Thus, differential weathering is a natural consequence of textural variations in primary igneous stratigraphy with superimposed additions of Al and Fe from the dissolution of basaltic glass. This study shows how weathering features and processes may be synthesized from outcrop, geochemical observations, and geophysical profiles into a multi-stage conceptual model of weathering.

Application of HVSR to estimating thickness of laterite weathering profiles in basalt

“…As noted above, d and V s are well constrained for our study sites by outcrop, well logs, and prior MASW results on Oahu and the Kohala Peninsula (Yaede et al, 2015;Sowards et al, 2018). Since these sites have been well characterized, the H/V forward modeling was employed to estimate vertical V s profiles using the previously obtained geophysical and geological data.…”

“…The Big Island of Hawaii was constructed by five shield volcanoes, the oldest of which is the 0.44 to 0.10 Ma Kohala Volcano. The LWP of the Kohala Peninsula cliff locality on Hawaii (Figure 1) developed over the Pololu series lavas that have a minimum age of 0.30 Ma (Sherrod et al, 2007;Sowards et al, 2018). The geologic and hydrologic settings at Kohala are described in detail by Sowards et al (2018).…”

“…They also exert influence on atmospheric CO 2 abundances (Dessert et al, 2003;Navarre-Sitchler and Brantley, 2007;Beaulieu et al, 2012) and ground stability, especially where there is a seismic hazard (Wong et al, 2011). Laterites also figure into estimates of weathering and denudation rates (Hilley et al, 2010;West, 2012) and the processes underlying denudation (Nelson et al, 2013;Sowards et al, 2018).…”

“…Targets of opportunity such as stream banks and road cuts may not universally expose the base of the LWP and outcrop may be obscured by dense tropical vegetation. Yaede et al (2015) and Sowards et al (2018) have employed the MASW (multichannel analysis of surface waves; Park et al, 1999;Schuster et al, 2017) method in modified form to image the base of the LWP and underlying basalt in the Hawaiian Islands to depths of up to 70 m. Although standard P-wave reflection surveys are able to image relict igneous textures within LWPs (Sowards et al, 2018) it could be difficult to detect the gradational transition from saprolite to bedrock. Yaede et al (2015) concluded that MASW works well and generally detects the base of the weathering front to within a few meters accuracy, where depths to bedrock were known from drillers logs, geologic logs, and measured section from outcrop.…”

“…> 1 Hz). The method was tested by re-occupying three Hawaiian localities investigated by Yaede et al (2015) and Sowards et al (2018) in central Oahu and the northern tip of Hawaii (Figure 1), respectively. The reader should note that the localities reoccupied have geologic well log or outcrop control for subsurface LWP thicknesses.…”