Speleothems: a Quaternary archive of hydroclimate change in Iran

Studying the paleoclimate of Iran fills an important gap in the paleoclimate history of the Middle East.Previous research has focused on lake, alluvial and desert sediments, as well as soils and glacial deposits.An archive that has received little attention so far in Iran are caves and their chemical sediments(speleothems). In addition, speleothems are known to be sensitive recorders of the hydroclimate. Here, wepresent a concise summary of the state of the art of speleothem-based palaeoenvironmental andpalaeoclimate studies in this country covering the Middle and Upper Pleistocene as well as the Holocene.Kaufmann (۲۰۰۲) analysed calcite shelfstones in Alisadr cave, NW Iran, which record a significantly higherkarst groundwater table in the past. Dating of these deposits using radiocarbon proved challenging butsuggest a possible early Holocene age for some of the high palaeo-watertable marks, calling for a wetterclimate. Mehterian et al. (۲۰۱۷) studied stalagmites from Qal'e Kord cave in central NW Iran and foundevidence of a strong atmospheric teleconnection between the North Atlantic and Iran during the timeperiod ۱۲۷-۷۳ ka and the early Holocene. Oxygen isotope data suggest wetter conditions during times ofhigh summer insolation. This was confirmed by a study of Carolin et al (۲۰۱۹), which suggested dry periodsat ۸۸, ۷۷ and ۷۳ ka BP during severe cold events in the North Atlantic, based on oxygen isotopes, carbonisotopes and trace elements in speleothems from Pir Ghar cave in NE Iran. Moreover, Carolin et al. (۲۰۱۸)reported evidence of a prevailingly dry period between ۵.۲ ka and ۳.۷ ka BP based on speleothems fromGol-e-Zard cave on the southern slopes of the Alborz mountains. Mehterian et al. (۲۰۱۷) and Andrews etal. (۲۰۲۰) demonstrate the effect of earth’s orbital configuration on rainfall amount in northwest Iran usingstalagmite oxygen isotope records, which suggest wetter conditions when the northern hemisphereexperienced hotter summers and colder winters due to earth’s orbital precession (i.e. early Holocene). Inongoing studies by the authors, speleothems from several caves in Central and Southwestern Iran areexamined in order to provide well-dated proxy records covering parts of the last half million years.Methods and Materials: The stalagmites were cut along their growth axis and their petrography wasstudied to identify possible growth interruptions. Samples for carbon and oxygen isotopic analysis weredrilled at low (۱-۱۰ mm) and high (۰.۲ mm) resolution traces along the main growth axis. Results andDiscussions: The presence of speleothems documents vadose groundwater flow and soil coverage in thecatchment of the cave for given time intervals in the Quaternary. As such, dated speleothem growth is astrong qualitative indicator of non-arid conditions in the past. Our current dataset from central Irancomprises growth periods from very old phases (more than ۵۰۰ ka) to young generations (late Holocene).Our current proxy time series are in agreement with the hypotheses of Mehterian et al. (۲۰۱۷) and Carolinet al. (۲۰۱۹), i.e. Iran experienced drier condition during glacials and more humid conditions duringinterglacials.

کلیدواژه ها:

Climate Proxy ، Carbon ، Oxygen ، Palaeoclimate ، speleothem ، Iran