GDGTs in Canadian Arctic lakes: insight into the methane cycle and climatic changes

Abstract:

GDGTs are ubiquitous biomolecules whose structural diversity or isotopic composition is increasingly used to reconstruct past environmental changes such as air temperatures or pCO₂. Isoprenoid GDGTs (iGDGT), and in particular GDTG-0, are biosynthetized by a large range of Archaea, from planktonic auto- and mixotrophic to methanogens Archaea (Elling et al., 2017; Koga et al., 1993; Oger and Cario, 2013; Schouten et al., 2013; Sollai et al., 2019). To test the potential of GDGT-0 to record the methane cycle, three lacustrine sediment cores from the Mackenzie River delta (Northwest Territories, Canada) have been studied for iGDGT and diploptene concentration, distribution and stable carbon signature. The near absence of crenarchaeol, and high GDGT-0 vs crenarchaeol ratio, as well as ¹³C-enriched carbon signature of GDGT-0 (Weber et al., 2015, 2018) hint towards a production by lacustrine acetoclastic methanogens feeding on carbon originating from macrophyte as well as heterotrophic Archaea. The oxidation of methane seems to be dominated by bacteria as indicated by the high abundance and ¹³C-depleted diploptene. Branched GDGTs, thought to be produced by heterotrophic bacteria (Sinninghe Damsté et al., 2014), are dominated by hexa- and penta-methylated 5-methyl compounds and their 6-methyl isomers. The presence of 5,6-methyl isomer IIIa’’ points towards in situ production of brGDGTs in the Mackenzie delta lakes (Weber et al., 2015, 2018), with only a minor input from soil branched GDGT brought by the Mackenzie River. Carbon isotope values are in agreement with a heterotrophic producer, likely living during summer time. The reconstructed temperature using a global lake calibration reflect recorded summer air temperature (± 2.14 °C) during the past 60 years, and further highlight the absence of warming in summer in this region in the last 200 years. Oxygen availability and connection time to the Mackenzie River also seems to control the distribution of branched GDGT with an increase in 6-methyl and 5,6-methyl isomers with increased period of anoxia.