Methane (CH4) emissions from Arctic lakes are of global concern in a warming world. Past Holocene warming provides an opportunity to examine carbon-climate feedbacks that develop over hundreds to thousands of years, but thus far records of past long-term changes in lake CH4 dynamics are rare. Here, we demonstrate for the first time that the hydrogen stable isotopic composition of aquatic plant wax records incorporation of CH4 in plant (aquatic moss) biomass. Trends in δ2H and δ13C values of aquatic plant derived leaf waxes point to widespread and sustained middle Holocene shifts in CH4 cycling at climatically diverse sites across Greenland during millennia of elevated summer temperatures. Independent proxies indicate concurrent increases in local primary productivity and decreases in hypolimnetic oxygen. These data portend that ongoing warming may promote an enduring shift towards conditions that enhance methanogenesis in many Arctic lakes, including in lakes where these conditions do not exist today. This work highlights a previously unrecognized factor influencing δ2H values of aquatic leaf waxes in some high-latitude lakes, and also draws attention to the role of common aquatic mosses as a potentially important sink of lake CH4 across the Arctic that has yet to be quantified.

The Ediacaran-aged Shuram excursion was the last and largest of the Neoproterozoic negative carbon isotope anomalies. Recognized in stratigraphic successions around the globe, it precedes diverse evidence for macroscopic, multicellular life, and follows the Cryogenian global glaciations and Ediacaran Gaskiers glaciation. Hypotheses for the cause of the Shuram excursion can be broadly grouped into those that argue for post-depositional diagenetic alteration of the carbon isotope record and those that argue the extremely low δ13C values reflect a primary perturbation to the carbon cycle. Given the timing and magnitude of this event, distinguishing between these disparate hypotheses, or combining them, is critical for reconstructing the environmental conditions under which complex life evolved on Earth. We test specific predictions of each model using a range of stratigraphic observations and micro- and macro-analytical techniques. We find that the type sections in Oman where the Shuram excursion was first described are well preserved and contain a range of features difficult to reconcile with a post-depositional origin. However, many salient features are consistent with an extreme warming event coupled to a carbon cycle perturbation, analogous to the Paleocene-Eocene Thermal Maximum (PETM), and increased middle Ediacaran volcanism. We propose that cooling associated with the recovery was critical for origination rates of macroscopic soft-bodied organisms.