Oliver Chadwick, Univ of California, Santa Barbara, CA 93106-4060
Hawaii provides an exquisite natural laboratory for studying soil development. The islands are formed over a stationary mantle plume and then are carried to the northwest on the Pacific Plate. Thus the islands get older with distance from the hotspot; Kauai has remnant shield surfaces whose lavas date to about 4,000 ky. It is possible to sample soils that are developing on different age flows ranging from a few hundred years to a few million years. Additionally, individual volcanoes are impacted by differing amounts of rainfall depending on location with respect to the northeasterly trade winds. Whereas rainfall over the open ocean near Hawaii is about 700 mm, rainfall over the Islands ranges from 150 to 11,000 mm. Hawaii is minimally impacted by mineral aerosol additions compared to continental areas and this has a significant impact on soil development. More than 100 pedons have been sampled along the Hawaii time-climate matrix with some surprising results. For example, in arid soils developing in Hawaii, soils might be expected to develop a preponderance of 2:1 clays, but they do not. They are rich in halloysite and allophane. Importantly, these same soils show a trend from high-Mg calcite to dolomite as carbonates accumulate within the profiles – this is one of the first documented occurrences of pedogenic dolomite that is not associated with high levels of salts. It appears that lack of smectite formation lowers the incorporation of Mg into silicate clays and increases its incorporation into carbonates. This is an unusual pedogenic process that seems to be enhanced by the lack of substantial amounts of mica in the basalt derived soils. The only mica is in surface horizons that receive dust derived from distant continents. Without mica there is no template to allow 2:1 clay formation under the rapid wetting and drying regimes encountered in the arid soils. At the same time that halloysite is forming, iron and aluminum oxides tend to move rapidly from poorly crystalline to crystalline forms, which in turn leads to formation of Oxisols under an arid climate regimes – Torrox formation without substantial climate change. By contrast, soils forming in humid environments along the same time trajectory take much longer to go through the same transformations (allophane => halloysite; poorly crystalline goethite => well crystallized goethite; poorly crystalline gibbsite => well crystallized gibbsite). The reasons for the longer period of gestation appear to be continually wet rather than wet-dry cycles and interference by organic carbon in the transformation process. Thus whereas it takes about 400,000 years to form a Torrox, it takes more than three times that long to form a humid zone Oxisol. The continuous view soil formation afforded by the Hawaii time-climate matrix suggests several important thresholds in soil properties associated with external and internal process: 1) the shift from udic to perudic soil moisture regime is accompanied by reduction related changes in soil properties particularly accumulation of organic matter and loss of iron-bound phosphorus; 2) shift from ustic to udic moisture leads to rapid loss of nutrients with far reaching implications for soil exchange properties and prehistoric land use, 3) the shift from from aridic to ustic soil conditions leads to lower losses of plant nutrients (bases, P, Si) due to less wind erosion. Many of these features are found in continental environments but in Hawaii they are easier to tease apart and less confounded by the pervasive dust input that affects continental soils.