Climate variability over the last 2000 years inferred from glycerol dialkyl glycerol tetraethers (GDGTs) in alkaline Nalin Lake of Inner Mongolia, China

Nalin Lake from western Inner Mongolia of China is a shallow oxbow lake with a laminated sedimentary record. The distributions of glycerol dialkyl glycerol tetraethers in lacustrine sediment core collected in Nalin Lake in June 2011 were examined in combination with other paleoclimate proxies. The cyclization index of branched tetraethers (CBTs)-inferred pH values changed primarily between 7.67 and 8.63. These changes were synchronous with those from the actual measurement and previous studies. The newly proposed CBT/methylation index of branched tetraethers (MBTs) palaeothermometer was also applied to produce a mean annual air temperature record for Nalin Lake spanning circa the last 2000 years. Over much of the record temperature fluctuated from ~2.76 to 7.61 °C, with a mean value of ~4.76 °C. The proposed CBT/MBT-based temperatures revealed a strong similarity to the local records, both documenting the moderate Medieval Warm Period from 600 to 1200 A.D. (1400–800 cal year BP) and a cooling anomaly Little Ice Age around 16th century. The CBT/MBT temperature patterns seemed to follow variations in the historical and Northern Hemisphere records of the mean annual air temperature. These suggested that the signals were produced during this period. Identification of high alkalinity in association with the decreased fluvial input by multiple indices was consistent with the previous observations due to the cutoff of Yellow River channel. Therefore, CBT/MBT temperature records of lake sediment core can potentially be a useful tool in reconstructing past variations of alkaline lacustrine climate.