Fast CO2 uptake by intense weathering of volcanic islands during interglacial stages

Abstract

The weathering of basaltic rocks, especially on volcanic islands, plays a crucial role in global carbon cycling. Intense precipitation and warm atmospheric conditions accelerate weathering processes in these environments. While most estimates of weathering rates derive from river chemistry, soils and paleosols remain underexplored. In this study, we investigated the geochemistry of paleosols developed from volcanic rocks in the Azores Archipelago over the past 1 Myr. Precise geochronology of volcanic units bracketing paleosols indicates high soil formation rates (3–180 mm kyr⁻¹), similar to modern soil formation rates in tropical volcanic islands. Geochronological evidence suggests a logarithmic decrease of soil formation rates over time, with high initial values reaching near zero in less than 35 kyr. This might be attributed to a combination of cation depletion and precipitation of stable minerals. Paleosols have generally developed faster on pyroclastic deposits than on lava flows. However, those formed on lava flows required less vertical development to sustain high cation exports due to their higher density. Based on the geochemistry of paleosols and their parental materials, we estimated cation exports (0–2600 t km⁻² yr⁻¹) and associated CO₂ uptake (0–35 × 10⁶ Mol km⁻² yr⁻¹). These estimates generally exceed previous estimates based on the geochemistry of modern rivers in the Eastern Azores, by a factor of up to tenfold. Our data highlight the transient character of weathering processes and the criticality of precise geochronological control to constrain past weathering and soil formation rates. They further imply that atmospheric CO₂ may have experienced short episodes of intense consumption during interglacial stages, possibly contributing to subsequent cooling events over the past 1 Myr.