A Journey of Permafrost Organic Carbon
Authors:   , Jong Dirk Jacob 
Published: June 2023

Recent climate warming is disproportionately impacting the Arctic. Over the last decades, the Arctic is warming nearly four times faster than the global average. This has a significant local and global impact, for example through thaw and destabilisation of permafrost, the ‘permanently’ frozen ground below the Arctic. Thaw of permafrost reintroduces the soil organic carbon (OC) it holds into the short-term carbon cycle. This OC, when mobilised by thaw and subject to degradation, can be a significant source of greenhouse gases. However, redeposition and storage of permafrost OC in river- and marine sediments can attenuate this ‘permafrost carbon feedback’. It is still poorly understood whether lateral transport of permafrost OC through aquatic systems will strengthen this feedback through degradation of OC along the way, or weaken it through burial of OC in sediments.

The objective of this thesis is to assess the fate of permafrost OC in aquatic systems. We will start from the source, and move along the path the permafrost OC takes to its destination in an integrated approach, and assess what is left and what is lost along its journey. We will combine sedimentological (grain size, mineral surface area) and geochemical analyses (elemental C and N, molecular biomarkers, and carbon isotopes δ13C and radiocarbon) on samples from the source (permafrost thaw sites), transitional environments (rivers, thaw slumps, nearshore zone), the marine shelf and marine basins to assess the degradation, deposition, and transport potential of permafrost OC along the land-to-ocean continuum, on both a small scale (5 km, Herschel Island – Herschel Basin, Yukon, Canada) and a large scale transect (1500 km, Kolyma River – East Siberian Sea, North East Siberia, Russia).

This thesis highlights that every pool of OC needs to be included when studying the pathway and destination of permafrost OC on a land-ocean transect: dissolved in water (DOC), as particulate matter (POC), and within the soil and sediments (SOC). This type of integrated carbon cycle-approach is necessary for both river and marine studies, where we should consider that each pool of permafrost OC is ‘processed’ at a different stage along its journey, and acknowledge the complex and dynamic interactions between different carbon pools along the way. We find that the transitional zones play a crucial role for processing permafrost OC: particularly the zone between ‘undisturbed’ permafrost and ‘thaw-mobilised’ permafrost material (e.g. the thaw slumps, thaw streams, mudpools), and the zone on the border of land and ocean, (e.g. the nearshore zone for coastal erosion or the delta for larger rivers). Furthermore, we find that the combination of biogeochemical and sedimentological methods is especially necessary when comparing samples from different sedimentary environments, and recommend further integration of these methods. Sorting of OC during transport through deposition and resuspension, and the affinity of certain types of OC to mineral particles through adsorption and desorption can affect biomarker proxies for degradation just as well as actual degradation.