Holocene Terrestrial Permafrost Contributes More Highly Reactive Organic Matter to the Laptev Sea Shelf Than Pleistocene Permafrost

Warming can lead to mobilization of organic matter (OM) initially stored in circumarctic permafrost and subsequent greenhouse gas release to the atmosphere. Our understanding remains limited regarding how the extent of carbon release, that is, OM reactivity, varies across terrestrial permafrost types and how it changes during transport from land to marine shelves. In this study, we measured bulk organic (TOC, C/N), isotopic ( δ 13 C, Δ 14 C), and thermogravimetric properties (TGA) as proxies of OM reactivity on bulk and water‐soluble fractions (leachates) from terrestrial Holocene and Pleistocene permafrost, bulk surface sediments from the Laptev Sea, and sediment cores from the western Laptev Sea. Bulk OM from terrestrial Pleistocene permafrost exhibited lower reactivity compared to Holocene permafrost, as indicated by its lower thermoreactivity and more advanced degradation state, reflected in higher δ 13 C values and lower C/N ratios. Marine surface sediments showed relatively old radiocarbon ages and reduced OM thermoreactivity in the eastern Laptev Sea shelf compared to the central and western Laptev Sea shelf. This likely resulted from a higher contribution of Pleistocene permafrost‐derived OM. In the central and western Laptev Sea, a rapid decrease in OM thermoreactivity was observed near the coast, followed by a more gradual decline offshore. Downcore analyses revealed that the reduction in OM thermoreactivity primarily reflected degradation during cross‐shelf transport rather than after burial. Our results advance the understanding of OM reactivity differences between Pleistocene and Holocene permafrost, as well as changes in terrestrial permafrost OM thermoreactivity during transport and post‐burial.