Organic-Matter Accumulation and Degradation in Holocene Permafrost Deposits Along a Central Alaska Hillslope

The quantity and preservation of carbon-rich organic matter (OM) underlying permafrost uplands, and the evolution of carbon accumulation with millennial climate change, are large sources of uncertainty in carbon cycle feedbacks on climate change. We investigated permafrost OM accumulation and degradation over the Holocene using a transect of sediment cores dating back to at least c. 6 ka, from a hillslope in the Eight Mile Lake watershed, central Alaska. We find decimeter-scale organic-rich (111 +/- 45 kg C m(-3)) and organic-poor (49 +/- 30 kg C m(-3)) layers below an upper peat, which store 35% +/- 11% and 41% +/- 20% of the carbon in the upper 1 m, respectively. In organic-poor layers, scattered 14C ages of plant macrofossils and higher percentages of degraded Alnus and Betula pollen indicate reworking by cryoturbation and hillslope processes. Whereas organic carbon to nitrogen ratios generally indicate OM freshening up-core, amino acid bacterial biomarkers, including d-enantiomers and gamma-aminobutyric acid, suggest enhanced degradation prior to 5 ka. Carbon accumulation rates increased from similar to 4 to 14 g C m(-2) year(-1) from c. 8 to 0.2 ka, coinciding with decreasing temperatures and increasing moisture regionally, which may have promoted OM accumulation. Carbon stocks within the upper 1 m average 66 +/- 13 kg C m(-2), varying from 77 kg C m(-2) in a buried depression on the upper slope to 48 kg C m(-2) downslope. We conclude that heterogeneity in preserved OM reflects a combination of hillslope geomorphic processes, cryoturbation, and climatic variations over the Holocene. Plain Language Summary Permafrost, ground that remains frozen for two or more years, stores enormous amounts of carbon at risk of degrading in a warming climate. Less is known about carbonrich organic matter (OM) stored in permafrost in upland areas compared with lowlands, so we analyzed four sediment cores from a hillslope in central Alaska to develop a carbon inventory and history of carbon preservation. The sediment cores include layers of OM-rich and OM-poor hillslope deposits within the top 1 m, both of which hold significant portions of the total carbon inventory. The overall buildup of sediment during the last 8,000-6,000 years, together with freeze-thaw mixing, helped preserve carbon deeper in sediments. But in the early stages of accumulation, higher temperatures drove enhanced OM degradation. There is significant, climatically sensitive carbon stored across the hillslope and variability in preserved OM is due to a combination of hillslope processes, freeze thaw mixing, and climatic variations over the past 10,000 years.