David Harning, Samuel Sacco, Jonathan H. Raberg, Nicolò Ardenghi, Julio Sepúlveda, Beth Shapiro, Gifford Miller, Áslaug Geirsdóttir

Ancient DNA and branched glycerol dialkyl glycerol tetraethers (brGDGTs) are new, powerful tools to reconstruct past ecosystems and climate in high-latitude lakes, but often require contrasting oxygen conditions for reliable interpretations. Here, we present a new approach using adjacent oxic and anoxic lakes to 1) better understand how redox potential impacts these proxies using isoprenoid GDGTs, and 2) reconstruct holistic changes in climate, human settlement, and plant history. We observe, first, that oxic lakes are likely to yield more reliable brGDGT-based paleotemperature records. Second, while anoxic lakes are optimal for ancient DNA preservation, commonly studied shallow lakes are more susceptible to past changes in oxygen availability that can lead to fragmentary DNA records. Finally, our dual lake approach highlights that climate, and not humans, has been the primary forcing behind Holocene vegetation changes in northeast Iceland, and therefore provides ideal constraints for earth system models.