A micromorphological evaluation of pedogenesis on Isla Santa Cruz (Galápagos) .

Abstract

Thin sections of about 200 horizons or layers, representing 60 soil profiles in the coastal area and on the windward slopes of Isla Santa Cruz (Galápagos Islands) were analysed. Based on the fabric and the composition of the groundmass and the presence of pedofeatures in individual layers or horizons, clusters of similar material are made and 7 units and 3 subunits distinguished. Plotting these units on a survey map gives a good insight in the spatial distribution of soil materials, expressing different combinations of parent material and precipitation. In the coastal area and the lowest slopes, with a summer dry climate, respectively reddish and greyish and brown materials with a porphyric c/f related distribution pattern and striated b-fabrics, and often with fragmented illuvial clay coatings are observed. The coarse material contains mainly holocrystalline basalt fragments, unweathered in the coastal area, or basalt derived individual minerals. The micromass has a halloysitic-smectitic composition. On the higher slopes, with a permanent moist climate, materials have also a porphyric or fine monic c/f related distribution pattern, but the b-fabric is undifferentiated and no illuvial features are present, gibbsitic features sometimes occur and the micromass consists mainly of halloysite and gibbsite. Subunits are distinguished here according to the microstructure, and the quantity and type of coarse material. Mesocrystalline basalt, often vesicular, dominates over holocrystalline, pointing to the influence of scoria. Soils on the higher slopes are more strongly weathered than those on the drier lower slopes and the coastal area. In depressions in the higher areas, materials with a yellowish or brownish grey micromass with a striated b-fabric, and well developed limpid illuvial clay coatings and impregnative iron oxide nodules occur. The micromass has a halloysite-smectite composition. Contrary to existing hypotheses the reddish soils in the coastal area are not considered as palaeosoils (roots of tropical soils), but as modern soils developed in colluvium on the lower slopes, which was deposited on totally eroded surfaces (marine terraces?). On the slopes the distribution of units is not only determined by hypsometric zones, as suggested in literature, but is rather clustered according to types of parent material. The soils on Santa Cruz are supposed to be formed after the last interglacial period, different from the red soils of San Cristóbal which are older.

Pedogenic bayerite in andic material of Isla Santa Cruz, Galápagos archipelago

“…In Dystrandepts from an area north of Bellavista (Eswaran et al, 1973), and in a Hapludoll sampled near that village (Morrás, 1974(Morrás, , 1976, gibbsite occurs in part as coatings and infillings of vesicles in volcanic rocks, and as derived spherical aggregates, interpreted as having formed by transformation of a zeolite precursor (Eswaran et al, 1973) or of an amorphous gel phase (Morrás, 1974(Morrás, , 1976. In the Hapludoll profile, gibbsite also occurs as alteration product of plagioclase, again with amorphous gels as presumed intermediate phase (Morrás, 1974(Morrás, , 1976 see also Stoops, 2013). Based on the nature of these gibbsite features, there is no reason to assume that their development involved a bayerite stage.…”

“…This is also the case for gibbsite that, together with kaolinite, forms the main constituent of the clay fraction of a Dystrandept studied by Adelinet et al (2008), despite its proximity to the profile identified as bayeritebearing by the present study. In other soils of Santa Cruz, gibbsite is only a minor component of the clay fraction, in association with amorphous material (Stoops, 1972), with halloysite and allophane (Morrás, 1977), or with kaolinite (Candra et al, 2021), and the mineral has also been detected by XRD analysis of bulk samples (Gerbazek et al, 2023). However, these various gibbsite mentions mainly indicate that conditions for gibbsite formation by strong alteration of volcanic parent materials are met in parts of Santa Cruz, in settings that are unrelated to that of the bayerite-bearing shallow profile of the Santa Rosa area.…”

“…The site of the studied shallow profile is located about 2 km south-east of Santa Rosa, on Isla Santa Cruz of the Galápagos archipelago, at an altitude of about 380 m. The profile was sampled, as profile n° 42 (XLII), in august 1962 by Georges Stoops, in the course of an extensive pedological and geological field campaign, organized by Ghent University, that covered several islands. Data for soil samples that were collected on Santa Cruz during that expedition have been presented in various early studies (e.g., Laruelle, 1966;Laruelle and Stoops, 1967;Stoops, 1972;Eswaran et al, 1973;Morràs, 1974Morràs, , 1976Morràs, , 1977 and also in some more recent publications (e.g., Taboada et al, 2016Taboada et al, , 2019Stoops et al 2022), all without including results for profile n° 42.…”

Water Quality from Natural Sources for Sustainable Agricultural Development Strategies: Galapagos, Ecuador

“…Topography is another influential factor in climate modulation, which gives rise to three main microclimates or climatic zones: the dry lowlands (arid zone), the transition zones, and the humid highlands [62]. Precipitation ranges from an annual average of 100 to 300 mm of rain in the arid zones [63] and from 1500 to more than 2000 mm of rain in the humid zones, according to records from Santa Cruz and San Cristobal [64,65].…”

Soil Fertility Changes With Climate and Island Age in Galápagos: New Baseline Data for Sustainable Agricultural Management

“…Systematic information on (top)soil properties in Galápagos is still scarce, and most of the available baseline information is drawn from a geo-pedological mission in 1962 (Stoops, 2013;Taboada et al, 2016;Rial et al, 2017) with very few additional later studies (e.g., Adelinet et al, 2008). In 2016 and 2017, three soil scientific expeditions laid the foundation for further, more detailed investigations, and the present study forms part of these ongoing research efforts carried out by the University of Natural Resources and Life Sciences Vienna in cooperation with the Galápagos National Park Directorate and other partner institutions.…”