Landscape modification due to agricultural irrigation: carbonate tufa formation on Tenerife, Canary Islands, Spain

Abstract The use of irrigation water on Tenerife has modified the hydrogeological system resulting in the formation of an anthropogenic carbonate tufa. The processes involved in its formation are: 1) infiltration of rainwater and mixing with groundwater rich in volcanic carbon dioxide (CO2), 2) weathering of volcanic rocks increasing the ion content of the water, 3) extraction of this water from subterranean galleries and channels and spilling it onto the Lomo Morin slope, 4) mechanical CO2 degassing and photosynthetic activity driving calcite precipitation. The tufa consists of boundstones of algae and coated stems of reeds, along with laminated microbial crusts. Textures range from microspar to coarse mosaics to fibrous-radial calcite, all containing biogenic features. The presence of the green macroalga Blidingia is noticeable. Carbon stable isotope ratio (δ13C) varies between -3.58 and +1.03 ‰ VPDB (Vienna Pee Dee Belemnite) and oxygen stable isotope ratio (δ18O) between -6.77 and -5.00 ‰ VPDB, highlighting the largely meteoric origin of the water with some addition of deep CO2. A strontium isotope ratio (87Sr/86Sr) of 0.7030 confirms the weathering of volcanic rocks as the source of calcium. The Lomo Morin tufa provides a scale model of how water use contributes to rapid landscape change through alterations in the hydrogeological system. It also provides a good example of a CO2 sink in a continental setting, and is comparable to the global carbon cycle, which mostly involves marine limestones. Although anthropogenic processes are commonly viewed negatively, the human modification of the hydrogeological system reported in this study has produced changes to geological and ecological conditions that have increased the geo - and biodiversity of the island.