Elemental trends and weathering indices to assess volcanic ash deposition on soil weathering status along Maui climosequences

Abstract

The western slopes of Haleakalā, Maui, are greatly affected by an orographic climate effect, creating a massive precipitation gradient and rain shadow with significant impact on pedogenic weathering and differentiation of soil formed in basalt. However, the presence of volcanic vents along East Maui's northwest rift have created irregularity in the deposition of volcanic ash across the landscape, making it difficult to determine a soil's relative age and degree of ash influence on its development. The objectives of this study were to 1) characterize and classify the basalt substrate of pedons sampled across a wide climatic gradient; 2) determine how elemental composition and soil chemical properties vary with precipitation and elevation; 3) identify pedogenic thresholds of western Haleakalā climosequences; and 4) determine if elemental trends and weathering indices can distinguish pedons and soil horizons that have been influenced by volcanic ash. We sampled 16 soils across elevational and precipitation gradients formed in Kula Volcanics on northwest-facing slopes <5 %. Soils were sampled and described to 1-m depth. Total elemental analysis was determined on rock samples and soil horizons by x-ray fluorescence, and various soil physical and chemical properties were measured in the laboratory. We calculated a total of 22 weathering indices across coastal and elevational climosequences. Soil pedon rock samples were alkalic basalt and predominantly classified as tephrite basanite with four samples classifying as foidite. We identified pedogenic thresholds for SiO₂/TiO₂ at 500 mm and 1300 mm mean annual precipitation (MAP) for the coastal climosequence, showing that although the pedon in Kahului (AIR) had lower MAP than the two pedons in Paia (CEM, PAIA), it is more weathered. Crystalline Fe (CD-AO) was variable in pedons below 1500 mm MAP, increased sharply with increased MAP, then decreased in pedons OL and HAI-E likely due to the reduction and loss of iron. Crystalline Fe (CD-AO) was generally higher at lower elevation pedons likely due to the relatively older age of lowland soils. Values of SiO₂/TiO₂ generally decreased with soil depth, but whole pedon values of SiO₂/TiO₂ were highly variable with increasing precipitation due to the influence of volcanic ash before also dropping substantially in high rainfall pedons. Soil SiO₂/TiO₂ and MnO/TiO₂ normalized to the pedon rock fragments may be viable indicators for distinguishing ash-rejuvenated soil pedons and genetic horizons from those that are not.