Using Water Table Depths Inferred From Testate Amoebae to Estimate Holocene Methane Emissions From the Hudson Bay Lowlands, Canada

Abstract

AbstractWetlands are the largest natural source of methane, yet the roles of source region and paleoclimate in explaining the variability in Holocene atmospheric methane concentrations remain poorly constrained. The Hudson Bay Lowlands (HBL) is one of the world's largest continuous peatland regions and a significant source of methane. We present here, using a novel proxy‐based approach, Holocene methane fluxes for the HBL. Paleo‐methane fluxes were quantified based on water table depth (WTD), inferred from testate amoeba assemblages in nine peat records. WTDs were reconstructed using a North American transfer function and were used to estimate paleo‐methane flux through a linear regression model of contemporary growing season methane fluxes and WTDs from 88 sites across the region. Following HBL peatland initiation in the Middle Holocene, total methane flux is closely related to the increasing area of land emerging from below sea level, controlled by rapid rates of glacial isostatic adjustment. In the Late Holocene, rates of uplift slowed, but methane fluxes remained high due to lower evapotranspiration in a wetter and cooler climate. We estimate that 4.8 ± 1.6 Pg C has been released from HBL peatlands to the atmosphere as CH4 over the last 8,000 years, with an average annual methane emission of 1.1 Tg CH4 yr−1 in the Late Holocene. The values estimated here are broadly consistent with those calculated from other independent methods, on modern and Holocene timescales, demonstrating that testate amoeba records provide an effective approach for scaling local processes to regional paleo‐methane emissions.