Unexpected intense weathering during glacial periods in the Central North Atlantic as recorded by paleosols from Flores Island (Azores)

Global climate variations (e.g., during glacial-interglacial transitions) induce local climatic effects such as temperature and precipitation changes, significantly impacting the chemical and physical degradation of volcanic islands. Conversely, the weathering of volcanic rock, especially on volcanic islands, consumes CO₂, thus impacting its concentration in the atmosphere and consequently the global climate. The Azores Archipelago (Central North Atlantic) is particularly sensitive to climate changes due to its position influenced by regional climatic drivers such as the North Atlantic Oscillation atmospheric system and the oceanic North Atlantic Gyre. Paleosols are key targets to reconstruct paleo-environmental conditions, as they constitute a valuable archive of both paleoclimatic conditions and weathering processes. Recent work on paleosols spanning the past 1 Myr in the Central and Eastern Azores showed pulses of fast soil formation during wet and warm interglacial stages locally promoting intense atmospheric CO₂ consumption through weathering. Flores Island, in the Western Azores, is the perfect target to further study rates of weathering and paleosol formation, and document paleoclimate at the regional scale (~600 km separation between Western and Eastern Azores). In this work, K-Ar geochronology of volcanic units under and overlying paleosols was used to precisely constrain their mean ages and formation times. This was complemented with paleoclimatic proxies based on paleosol whole-rock geochemistry, which allowed us to reconstruct Mean Annual Precipitation (MAP) and Mean Annual Air Temperature (MAAT) at the time the paleosols were formed. Our results show two groups of paleosols formed mainly during glacial periods (~ 550 ka, 630-670 ka), in contrast with the Central and Eastern Azores, where paleosols were formed near interglacial peaks. Our MAAT and MAP reconstructions show that mild and wet conditions prevailed in Flores, reaching 21.5°C and 1340 mm yr^-1, respectively. These conditions are hotter and drier than current mean annual conditions (17°C and 1716 mm yr^-1). However, they show wetter/warmer conditions than those reached around interglacial peaks in the Central and Eastern Azores, consistent with modern climatic differences (wetter/hotter conditions to the west). As paleosol ages between Flores and other Azores islands do not overlap, our data could indicate (1) persistent wet/warm local paleo-conditions in Flores due to its position farther to the north-west compared to the Central and Eastern Azores, closer to the westerlies’ main trend; or (2) a regional warm and wet climate around 550 ka and 650 ka that remains to be investigated in the other parts of the Archipelago and the Atlantic region at a broader scale (e.g., the Canary volcanic archipelago). In any case, our data evidence periods of fast soil formation during glacial stages (10 to 367 mm kyr^-1), supposed to be too dry and cold to allow the efficient weathering of the volcanic substrate, according to recent reconstructions in the Central and Eastern Azores. Such intense and fast weathering likely resulted in significant atmospheric CO₂ consumption, at least at local scale. Further investigations of paleosols could improve our temporal and spatial resolutions, and consequently our understanding of the feedback between volcanic islands weathering and global climate.