Both redox potential and climate control molecular proxies in Icelandic Holocene lake sediments

Ancient DNA and branched glycerol dialkyl glycerol tetraethers are powerful tools to reconstruct past ecosystems and climate in high-latitude lakes, but often require contrasting redox conditions for reliable interpretations. Here, we present a new approach using adjacent oxic and seasonally suboxic lakes from Iceland to 1) better understand the impacts of redox potential on these molecular proxies, and 2) reconstruct Holocene climate, human settlement, and plant history. We observe that oxic lakes provide more reliable branched glycerol dialkyl glycerol tetraethers-based paleotemperature records, possibly due to the diminished presence of bacterial communities associated with reducing environments. On the other hand, oxic conditions in shallow lakes, which are often targeted for ancient DNA, can lead to poor preservation. By combining oxic and suboxic lake sediment records, we show that natural climate, and not humans, has controlled Holocene vegetation changes in northeast Iceland, and therefore provides ideal constraints for Earth system models. Recovery of complementary materials from well-ventilated and seasonally stratified lakes can allow reliable reconstruction of local temperature and vegetation changes in Iceland, according to an analysis of ancient branched dialkyl glycerol tetraethers and sedimentary DNA.