Intense CO2 consumption by pulsed volcano weathering near interglacial peaks in the Azores Archipelago (North Atlantic Region)

The weathering of basaltic rocks, especially on volcanic islands, plays a crucial role in global carbon cycling. In these environments, intense precipitation and frequent exposure of fresh rocks accelerate weathering processes, thus favoring the uptake of atmospheric CO₂. While most estimates of weathering rates derive from river chemistry, soils and paleosols –the solid residue of protracted interaction between surface waters and the volcanic substrate– remain underexplored. Developed in contact with the atmosphere and incorporated into the geological record once sealed by volcanic deposits, paleosols record valuable environmental information, including the paleoclimatic conditions under which they were formed. In this study, we investigated the geochemistry of paleosols developed in the Azores Archipelago over the past 1 Myr. Precise geochronology of volcanic units bracketing paleosols revealed pulses of fast soil formation during interglacial peaks, and indicates high soil formation rates (3–180 mm kyr^-1), similar to modern soil formation rates in tropical volcanic islands. This suggests periods over which the Azores High-pressure system could have been weakened or centered farther to the south of its current position, allowing humid air masses to reach the Azores region. Geochronological evidence suggests high initial formation rates, rapidly decreasing to near zero after ~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 results highlight the criticality of precise geochronological control to estimate past weathering and soil formation rates, and that atmospheric CO₂ may have experienced short episodes of intense sequestration during interglacial stages, possibly contributing to subsequent cooling events over the past 1 Myr. A preliminary study of U-series geochronology on paleosols of the Azores provided promising results, consistent with our previous Ar geochronology. This is expected to provide a better understanding of the evolution of past weathering rates and consequent CO₂ consumption in the Azores and other volcanic settings.