Ongoing warming of Arctic lakes is predicted to drive physical and biologic changes, including longer ice-free seasons, higher summer water temperatures, and heightened primary productivity. In lakes with geometries prone to thermal stratification, these changes could drive increasing methanogenesis and methane storage in the water column. However, evidence for changes in lake redox conditions and methane storage during prior warm periods is limited, inhibiting observations of long-term changes to carbon cycling in lakes that result from Arctic warming.

Although methane is a known influence on C isotopes of lacustrine organic materials, the influence of methane on H isotopes of plant biomarkers has not been documented. H isotopes of mid-chain plant waxes in lacustrine settings are conventionally interpreted as recording lake water H isotopes. Here, we present the first evidence, to our knowledge, that methane cycling can act as an influence on the H isotopes of these materials. More specifically, we present evidence from three Greenland lakes that insolation-driven warming during the early-middle Holocene led to summer stratification, low oxygen, and higher seasonal methane production. We posit that these conditions promoted a symbiotic relationship between aquatic mosses and methanotrophic bacteria and that this symbiosis contributed exceptionally depleted H to aquatic mosses, resulting in depleted H isotopes of sedimentary mid-chain waxes. In contrast, the isotopic composition of lake water and precipitation, inferred using the isotopic composition of chironomid and terrestrial plant waxes respectively, track multi-millennial cooling through the Holocene at all sites. This data supports that Arctic lakes were a potentially major source of atmospheric methane in the early-middle Holocene, although the role of aquatic mosses as a compensating C-sink has yet to be quantified.