An exploration of panarctic lake formation and methane emission since the Last Glacial Maximum

Abstract

Polar ice cores show that atmospheric methane concentrations nearly doubled in response to rapid climate warming over the last deglacial transition. Since concentrations of this potent greenhouse gas are tightly coupled to the Earth’s climate system, understanding the climate­ ecosystem interactions that precipitated this event may help predict feedbacks to current and future warming. This work reconstructs panarctic lake areas and methane emissions to assess their contribution to global atmospheric methane budgets since the Last Glacial Maximum. In the first two chapters of my dissertation, I show that climate warming and deglaciation caused widespread lake formation across land surfaces poised toward this trajectory by glaciation. Thermokarst (thaw) lake formation that accelerated in response to climate warming released methane from a mixture of radiocarbon-depleted permafrost soils and contemporary carbon sources, creating a positive climate feedback that helped sustain early Holocene temperature increases. Younger, albeit ultimately larger sources of methane from more extensive glacial lakes, lagged those from thermokarst lakes but were more than twice their magnitude throughout most of the Holocene. These findings are consistent with top-down polar ice core ¹⁴CH₄ constraints. Not included in my initial analysis were exceptionally large proglacial lakes dammed by continental ice sheets. These proglacial lakes, which had never been explored as a methane source in the literature, were the focus of my third dissertation chapter. I found that within a single large proglacial lake, Lake Agassiz, lake lowering and subsequent re-expansion into shallow aquatic and subaerial environments provided the most significant opportunity for methane production, which was otherwise limited by substantial water depth. Since it is unlikely that projected warming will cause extensive lake formation on the order of that observed during last deglacial, much of the 21st century permafrost carbon feedback will ultimately depend on how many new lakes the landscape can support. Over geologic timescales, the function of northern lakes as a carbon source could be unique to early interglacial stages due to the inevitability that lake drainage and terrestrialization will transform these features into climate stabilizing carbon sinks.