Soils of the coastal and lower belt of Galápagos Islands – The case of Isla Santa Fé and Isla Santa Cruz

Abstract

This paper compares the development of soils in low-altitude zones of two eastern Galápagos islands having a similar composition of the volcanic rock substrate but contrasting topographical features, i.e. the small and relatively flat island of Santa Fé and the larger, mountainous, younger island of Santa Cruz. The soils of the lower, dry zone of Santa Cruz are less weathered compared to those of the higher slopes on the same island. They contain basalt-derived mineral grains, swelling 2:1 phyllosilicates, and poorly crystalline kaolinite, and they are rich in calcite features. The soils on Santa Fé, in a similar altimetric position, are less weathered, containing more weatherable basalt-derived mineral grains and a clay fraction composed of open 2:1 phyllosilicates and small amounts of mica-like components, together with likely sepiolite. Sepiolite seems to occur in part in the form of small yellowish nodules. Some soils on both islands contain significant amounts of phosphate, probably linked to populations of sea birds. Compared to the parent rock, an increase in Al, Fe, Ti and K, and a loss of Mg, Ca and Na, are observed for both islands, but this is less pronounced for Santa Fé. The observed micromorphological, mineralogical and chemical differences are explained by geomorphology and related climatological factors, since the present altitudinal setting of the soils is similar. The soils of Santa Fé receive less precipitation because the island has no central mountain chain. On both islands, the heterogeneous nature of the low-elevation soils indicates major colluvial input. Biological factors, such as nesting of seabirds and the development of phytogenic calcite, may play an important role in local soil development. Remnants of illuvial clay coatings, sometimes entrapped in calcitic features, indicate wetter conditions in the past.