A refined paleotemperature calibration for New Zealand limnic environments using differentiation of branched glycerol dialkyl glycerol tetraether (brGDGT) sources

Abstract

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) were abundant in surface sediments of freshwater lakes and in catchment soils at altitudes from 10 to 2020 m in New Zealand. Significant differences in brGDGT compositions between lake sediments and soils indicate sources from separate microbial habitats. An expanded modern calibration dataset comprising 33 lakes has enabled a revised calibration function for determining past mean annual air temperature (MAT) from brGDGTs in New Zealand lake sediments: MAT (°C) = −31.664 × MBTm + 16.252 (n = 30). The function uses a modified methylation index of branched tetraethers (MBTm) that incorporates brGDGT III in the numerator to overcome the lower correlation found between our larger dataset and the unmodified MBT which had been used for previous calibrations. Calibrations combining the cyclization index of branched tetraethers (CBT) and the MBTm or using only certain brGDGTs are possible but have limitations. The revised function shows slightly higher correlation with MAT (R2 = 0.75) than previous calibrations, which were based on nine sites. The refined calibration function is applied to a ∼16 000‐year lake sediment sequence from northern South Island, New Zealand, and yields temperature reconstructions that are consistent with independently derived climate trends from the same sequence.

Reconstructing Holocene temperature and salinity variations in the western Baltic Sea region: a multi-proxy comparison from the Little Belt (IODP Expedition 347, Site M0059)

“…To analyse the GDGT content, aliquots of the polar fractions were dissolved in n-hexane:2-propanol (99 : 1; v/v) and passed through a 0.4 µm polytetrafluoroethylene filter. Analysis of GDGTs was performed using an Alliance 2695 (Waters, UK) high-performance liquid chromatograph coupled to a ZQ (Micromass, UK) single quadrupole mass spectrometer (MS) as detailed in Zink et al (2016) following the analytical protocols of Hopmans et al (2000) and Liu et al (2012). Isoprenoid and branched GDGTs were detected using single ion recording of their protonated molecules [M+H] + as outlined in Schouten et al (2013a).…”

Early Holocene Thermal Maximum recorded by branched tetraethers and pollen in Western Europe (Massif Central, France)

“…They defined new indices and established new calibrations. Lake sediment studies have involved the development of specific calibrations (Bendle et al, 2010;Blaga et al, 2010;Tierney et al, 2010;Sun et al, 2011;Zink et al, 2016;Russell et al, 2018). Most of these studies demonstrated that in situ production in the water column or in the sediment could occur, potentially biasing the results obtained (Tierney and Russell, 2009;Loomis et al, 2011;Tierney et al, 2012;Wang et al, 2012a;Buckles et al, 2014aBuckles et al, , 2014b.…”

Final author comments

“…To analyse the GDGT content, aliquots of the polar fractions were dissolved in n-hexane:2-propanol (99 : 1; v/v) and passed through a 0.4 µm polytetrafluoroethylene filter. Analysis of GDGTs was performed using an Alliance 2695 (Waters, UK) high-performance liquid chromatograph coupled to a ZQ (Micromass, UK) single quadrupole mass spectrometer (MS) as detailed in Zink et al (2016) following the analytical protocols of Hopmans et al (2000) and Liu et al (2012). Isoprenoid and branched GDGTs were detected using single ion recording of their protonated molecules [M+H] + as outlined in Schouten et al (2013a).…”