Chemical weathering along a one-million-year soil age gradient on the Galápagos Islands

Abstract

The Galápagos archipelago, a chain of islands formed by hotspot-induced volcanism on the Nazca tectonic plate, exhibits pronounced gradients of rock age and climate. Here, we investigate chemical weathering along a soil chronosequence (1.5 to 1070 ka) and under humid vs. dry conditions. Our results show considerable loss of base cations already in the early to intermediate phases of weathering under humid conditions (e.g. 95 % of Na and 78 % of Mg lost from the topsoil after 26 ka) and almost complete loss from the entire profile in soils older than 800 ka. Depletion of Si was less pronounced, with topsoil losses of 24 % and 63–68 % after 26 ka and >800 ka, respectively. Under dry conditions, weathering rates were much lower, e.g. 33 % of Na and 1.4 % of Mg lost from the topsoil after 26 ka. Indices of chemical weathering, e.g. the Chemical Index of Alteration and the Ruxton Ratio, correlated well with indicators of pedogenic development, such as solum thickness, soil pH, or the ratio of oxalate- to dithionite-extractable Fe. Total weathering flux and associated CO2 consumption rates estimated from profile-scale element losses in this study exceeded catchment-scale estimates reported for other volcanic islands or global averages during the early weathering phase, but were much lower in the intermediate and late phases. Nevertheless, total C drawdown was dominated by soil organic C sequestration (70–90 % share) rather than inorganic, weathering-induced CO2 consumption during early pedogenesis (≤4.3 ka), and the relative importance switched in the intermediate and late phases (90–95 % share of weathering-induced C drawdown at ≥166 ka). Dust deposition derived from a nearby ocean sediment core was <20 % of total basalt mass loss at the young and intermediate-aged sites, but reached 40–60 % at the older sites (>800 ka). Our results suggest that (1) young volcanic surfaces are very efficient (inorganic and organic) C sinks, (2) the development of thick soil covers at advanced pedogenic stages effectively shields the underlying rocks and decelerates weathering, and (3) dust inputs become an increasingly important biogeochemical factor in such highly weathered environments.