Application of bacterial glycerol dialkyl glycerol tetraethers (GDGTs) to develop modern and past temperature estimates from New Zealand lakes

Abstract

Branched isoalkyl glycerol dialkyl glycerol tetraethers (GDGTs) of bacterial origin have been found in high abundance in both modern and glacial sediments of New Zealand South Island freshwater lakes covering a wide range of altitude (101–2000 m). Like isoprenoid GDGTs of archaeal origin, they provide excellent potential for temperature assessment. For this study, their distribution patterns (MBT, methylation ratio and CBT, cyclisation ratio of branched GDGTs) have been successfully used to develop an initial temperature calibration for the study area and to provide preliminary (palaeo)environmental interpretations. MBT data from modern lake sediments correlate well with measured annual air temperature ( R 2 0.74), enabling a regional calibration for reconstructing palaeotemperatures for fossil samples. MBT-derived palaeotemperatures for Alpine Lake, calibrated against mean annual temperature, were determined for the Last Glacial during an early cold phase (between 29,000 and 26,000 years BP) and for later less cold phases (between 23,000 and 18,000 years BP). Compared with the modern temperature regime, the MBT data indicate a decrease of ca. 5.6 and 2.8 °C respectively, during this time. Modern and past MBT-derived temperatures correlate with chironomid-based temperature reconstructions in the area. Archaeal GDGTs, commonly used for the TEX 86 index, are abundant in fossil sediments (Alpine Lake) but scarce in modern sediments, hindering a new local calibration for this palaeotemperature proxy.