A Novel Proxy-Based Estimate of Holocene Methane Fluxes for the Hudson Bay Lowlands, Canada

Abstract:

Wetlands are the largest natural sources of methane to the atmosphere, but the impacts of paleoclimate on regional sources remains poorly constrained. Further, the Hudson Bay Lowlands (HBL) region of Canada is one of the largest peatland complexes in the world and a significant methane source, yet relatively little work has been done to constrain methane emissions from this region across the Holocene. We utilized a novel proxy-based approach to quantify methane emissions from the HBL by combining water table depth reconstructions from a testate amoeba-based transfer function (N= 9 sites) and a linear regression model of contemporary growing season methane fluxes and water table depth (N=93 sites). We calculated the average growing season flux in 500-year bins since peatland initiation in the Middle Holocene across the HBL, and scaled growing season fluxes to total regional emissions for the HBL using growing season length, available land area, and an average non-growing season methane flux from the literature. We estimate that 4.8 ± 1.6 Pg C has been released from HBL peatlands as methane over the Holocene. The Late Holocene accounts for 76% of the total Holocene methane emission from the HBL, with an average of 1.1 Tg CH₄ per year. Total methane emissions from the HBL increase over the Middle Holocene coincident with the increasing available land area, which is controlled by rapid rates of glacial isostatic adjustment (GIA) and associated marine regression. The Late Holocene is characterized by cooler and wetter climate and higher methane fluxes per unit area, as the rate of GIA slowed. The testate amoeba-inferred paleohydrological reconstructions show relatively drier conditions in the Middle Holocene, coincident with lower lake levels and higher temperatures reconstructed from lake sediment proxies in nearby regions. Therefore, evapotranspiration rates are likely an important factor in both Holocene and future carbon budgets for the HBL. Overall, we provide an approach to calculating paleo-methane fluxes for sites with paleoecological records and demonstrate that these fluxes can be scaled to regional estimates. Further, we estimate paleo-methane emissions of the HBL at a 500-year resolution based on land availability, providing data needed to constrain trends in atmospheric methane across the Holocene.