Evaporite mineralogy and major element geochemistry as tools for palaeoclimatic investigations in arid regions: A synthesis

摘要

This paper presents a synthesis of the applications of evaporite mineralogy and the relationship between major elements for the palaeoclimatological research of arid regions, with examples from Playa Oum el Krialate in Tunisia, Wadi Natron in Egypt, East African Rift Valley, etc. The numerous evaporite minerals serving as indicators of palaeo-drylands (salinity and evaporation) include carbonates, sulfates, and Na, K, Ca, and Mg chlorides. The occurrence of double salts, such as glauberite, carnallite, kainite, gaylussite, pirssonite, burkeite, etc., suggests disequilibrium conditions. Apart from that, the presence of very rare Fe-sulfates, such as rozenite and szomolnokite, indicates anoxic conditions with higher salinity. The formation of Na-silicates, such as magadiite and kenyaite, implies a decrease in pH of a highly alkaline Na concentrated brine. The Mg-silicates (palygorskite, Mg-montmorillonite and talc) form quickly and then re-dissolve when conditions change. Identification of fulgurites in the Sahara has been related to palaeo-lightning. We have also discussed a simple geochemical approach of using the ratios of soluble/insoluble elements to identify palaeo-arid events with examples from loess-soil sequences from Feiran Oasis in the Sinai Desert (Egypt) and salty silt lacustrine sequences from Thar Desert (India).