Organic-matter rich soils hidden inside alpine periglacial blockfields: paleosols or aeolian?

Permafrost warming due to rising global temperatures has the potential to alter carbon (C) cycling in high-mountain environments, yet alpine permafrost regions remain poorly studied compared to Arctic systems. In particular, coarse blockfields on alpine summits (>2900 m a.s.l.), often vegetation-free and traditionally assumed to be devoid of soil organic carbon (SOC), may conceal previously unrecognized carbon stocks. To address this gap, we investigated SOC occurrence, stocks, age, and composition in alpine blockfields across the Swiss and Italian Alps. On nine mountain tops (2900–3350 m a.s.l.) within the periglacial zone (with the addition of a low-elevation fossil Pleistocene blockstream at ca. 1200 m a.s.l.), we removed a layer up to more than one meter thick of coarse blocks and described 18 soil profiles hidden below. Some of the sites were completely barren, while in others a few plants grew between the stones or in the central parts of sorted patterned ground morphologies. Weakly decomposed plant fragments of aeolian origin were found at the base of the stone layer in six soil profiles. Below, the observed soils showed dark brown to black A horizons with thickness between few and 62 cm, overlying Bw, Bs or C horizons. The texture was usually sandy loamy to loamy sand, with very few stones, and the pH was usually acidic to strongly acidic. In different sites, soils could be classified as Akroskeletic Cambic Umbrisols, Dystric Akroskeletic Cambisols, Akroskeletic Regosols, and one as an Akroskeletic Entic Podzol. In the A horizons, the C content was between ca. 3.2 and 37.2 g kg-1, usually with C/N ratios between 8 and 15, indicating well-decomposed organic matter. The C stocks were significant, considering the barren landscape of blockfields, and they were between 0.8 and 10 kg C m⁻². The soils showing the strongest weathering degree like the Umbrisols, Cambisols and the Podzol (likely predating the Holocene) had the largest C stock. Radiocarbon dating revealed that plant fragments were mostly recent, while the humic materials in the dark A horizons ranged between ~4,000 years near the surface to ~12,900 years at depth, with some extreme values (17,000 and 22,000 years) in a Monte Rosa massif blockfield. Between 65% than 90% of organic carbon was stored in a stable mineral-associated organic matter pool, highly decomposed and dominated by lipids, waxes, cellulose, and hemicellulose. Thin section observations under optical microscope and SEM-EDS images verify the strong decomposition degree of such humified material. Although the aeolian origin of the soil was well visible in one site (Venerocolo summit, Adamello massif), the organic matter composition did not allow us to distinguish an aeolian pool, and one originated from ancient alpine tundra vegetation that developed during warmer climatic phases. Together, these findings demonstrate that alpine blockfields and periglacial features can host substantial, millennia-old, and largely stable SOC pools, preserved for long times by cold soil temperatures and metal-organic stabilization. These “hidden” carbon reservoirs likely formed through a combination of Holocene vegetation inputs and atmospheric deposition and may become vulnerable to mobilization under future climatic warming, with implications for alpine carbon–climate feedbacks