Organic-matter Accumulation and Degradation in Holocene Permafrost Deposits Along a Central Alaskan Hillslope
Abstract
Permafrost in high-latitude regions stores a substantial quantity of carbon (C) that is susceptible to microbial decomposition and subsequent greenhouse gas release in a warming climate. However, the quantity and quality of C-rich organic matter (OM) that underlies hilly uplands, and the evolution of C accumulation with changing climate over millennia, are large sources of uncertainty in C cycle feedbacks on climate change. To investigate OM accumulation and degradation over the Holocene, we analyzed a transect of four sediment cores (110 to 128 cm long), each extending back about 6 to 8 kyr, from a hillslope in the Eight Mile Lake watershed, near Healy, Alaska. First, we estimated organic C stocks and analyzed radiocarbon (14C) ages of macrofossils (n = 74). We subdivided the sediment below the upper peat into distinct decimeter-scale organic-rich (111 ± 46 kgC m-3) and organic-poor (51 ± 31 kgC m-3) layers. These store 35 ± 11 % and 41 ± 21 % of the C, respectively, while making up approximately equal thicknesses in the upper 1 m. 14C ages of plant macrofossils are more scattered in organic-poor than organic-rich layers, which may suggest reworking by cryoturbation and hillslope processes. C accumulation rates increased from an average of 2 ± 2 gC m-2 yr-1 prior to around 5.5 ka to 12 ± 5 gC m-2 yr-1 thereafter, with the highest rates in the undecomposed peat that accumulated during recent centuries. The higher rates following 5.5 ka coincide with decreasing temperatures and moisture, which may have promoted OM accumulation over degradation. At the top of the hillslope, we found in situ peat accumulation and higher C stocks (77 kg C m-2) in a buried depression. In contrast, we found low C stocks (48 kg C m-2) at the base of the slope. These downslope trends challenge the conventional model of gravity driven hillslope sediment transport. Second, we measured indicators of microbial influence and the extent to which OM had been degraded. Whereas the ratio of carbon to nitrogen generally indicates a shift to fresher OM up-core, amino acid bacterial biomarkers, including D-enantiomers and gamma-aminobutyric acid, indicate periods of enhanced degradation prior to 5 ka. We conclude that heterogeneity in preserved OM reflects a combination of hillslope geomorphic processes, cryoturbation and climatic variations over the Holocene.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMPP15F0724M