Linking rock age and soil cover across four islands on the Galápagos archipelago

The Galápagos Islands have been a place of preeminent scientific discoveries. The archipelago has formed on a tectonic plate moving over a volcanic hotspot, which has generated a chain of volcanic islands of different ages. This tectonic setting has favored the evolution of a unique flora and fauna on the islands; however, little is known on how it has influenced the formation and evolution of soils and belowground ecosystems. In order to enable the investigation of soil and ecosystem evolution, the aim of this study was to establish a soil chronosequence covering the full age range from the younger islands in the west to the older islands in the east of the archipelago. Six sites in the humid zones of the islands as well as two drier comparison sites were selected for this study. We characterized the geochemistry and mineralogy of the soil parent materials and constrained their ages using ¹⁴C (<10 ka) and ⁴⁰Ar/³⁹Ar dating. The parent materials are scoriaceous with bulk densities ≤1.6 g/cm³. Their major element composition is basaltic, including both alkali basalts and tholeiites. The scorias show amorphous contents between 20 and 85%, and their mineralogical composition is dominated by plagioclase and pyroxene. The numerical ages of the soil parent materials at the humid sites were: 1.45 ± 0.06 ka and 4.29 ± 0.09 ka on Isabela Island, 26 ± 7 ka on Floreana Island, 165.5 ± 11.6 ka on Santa Cruz Island, 825.6 ± 11.2 ka and 1070 ± 10 ka on San Cristóbal Island. Hence, the ages are distributed logarithmically covering a time span of approximately one million years. Along this chronosequence, we observed a non-linear increase in solum thickness, rubification degree and clay content with rock age. Changes were rapid in the initial phase of pedogenesis and more gradual in the later stages. Compared to the humid sites of the chronosequence, the pedogenic development at the dry comparison sites was much less advanced. The established chronosequence shall provide a solid basis for future studies on soil and ecosystem evolution in the unique environment of the Galápagos Islands.