Impacts of Climate and Vegetation Change on Carbon Accumulation in a South-Central Alaskan Peatland Assessed with Novel Organic Geochemical Techniques

To constrain the effect of climate and peatland type on carbon accumulation, we reconstructed these parameters from a Holocene-length core of a Sphagnum-dominated peatland near Cordova, AK, USA. We determined peat type using a combination of peat texture and density, macrofossils, distributions of leaf-wax biomarkers, and soil pH reconstructions based on distributions of branched glycerol dialkyl glycerol tetraether lipids (brGDGTs). We produced an independent record of hydroclimate and temperature change using hydrogen isotope ratios of leaf-wax biomarkers and distributions of brGDGTs. Carbon accumulation rates were constrained with 14 AMS 14C dates from identified macrofossils and ash-free bulk density. In the early Holocene, the site was a shallow pond with evidence for emergent macrophytes, Sphagnum, and algae growing in a warm, moist climate. At 9.2 kyr (1 kyr = 1000 cal. yr BP), the site became a Sphagnum-dominated bog. Under mid-Holocene warm, evaporative climate conditions, the site became sedge- dominated. As climate cooled and effective precipitation increased, Sphagnum was able to gain dominance abruptly at ~3.5 kyr. Large changes in the vegetation assemblage and hydrology and climate are contemporaneous with significant changes in the rate of carbon accumulation. Carbon accumulated most rapidly when Sphagnum dominated and effective moisture was high and most slowly when sedges were dominant and conditions were warmer and drier. Estimates of future climate change indicate warmer, more evaporative conditions that, in the past, favored a sedge-dominated environment, suggesting that this peatland and those similar can contribute to a positive feedback to warming by transitioning to less efficient carbon sinks.