METHANE CYCLING IN CLARKIA PALEOLAKE DURING MIDDLE MIOCENE CLIMATE OPTIMUM (MMCO)

Methane is the second most significant greenhouse gas after carbon dioxide. Although it remains in the atmosphere for a much shorter timescale, its heat-trapping capacity is much greater. Lakes and wetlands are one of the most significant sources of natural methane emissions and exhibit active methane cycles involving methanogenic and methanotrophic archaea and bacteria. Glycerol dialkyl glycerol tetraether (GDGT) lipids are ubiquitous microbial biomarkers used to reconstruct air and water temperature, pH; they are also effective indicators for methane cycling in both modern and ancient environments.

The middle Miocene Clarkia lake was formed by the damming of the proto-Saint Maries River by lava flows from the Columbia River Basalt Group during a thousand years at the peak of the middle Miocene Climate Optimum (MMCO) around 15.78 Ma. The deposit yields not only world renowned exceptionally preserved plant and animal fossils but also in situ biomolecules recording a millennium of environmental changes with high resolution of decadal scale. Previous reports of changes in water depth of the Clarkia paleolake revealed a history from a deep, stratified lake to a shallower swamp, as is also reflected in the change in fossil floral assemblages. Here, we report a high resolution GDGT analysis from nearly 90 samples of the whole stratigraphic column of Clarkia P-33 outcrop.

We applied both marine-based Methane Index (MI) and ratio of GDGT-0 over crenarchaeal and found their values increase upward along the lake sequence over the history of the Clarkia paleolake, corresponding with the transition from a deeper lake environment to a shallower swamp environment. This suggests a more active methane cycle in the swamp environment and is consistent with observations of modern wetlands as an even greater source of natural methane emissions than lakes. This dataset is significant as it represents timeframe during the MMCO, an approximately one-million-year greenhouse period which is one of the closest analogues in the geological record for near future climate change, including higher temperatures and moderately higher atmospheric carbon dioxide concentrations than today, making this high-resolution record with well constrained time and physical parameters valuable for climate mitigation and adaption.