Spatial variations of soil weathering processes in a tropical mountain environment: The Baturité massif and its piedmont (Ceará, NE Brazil)

Abstract

Strong spatial variations in soil weathering processes were identified in the uplands and piedmont zones of the Baturité massif (Ceará, NE Brazil), a medium-elevation tropical mountain characterized by a steep ecocline between humid climate of the summit zone and semi-arid conditions of the erosional piedmont. With the combined help of field surveys and laboratory analyses, involving micromorphological observations as well as geochemical and mineralogical investigations, this study demonstrates a close spatial correlation between soil weathering processes and present-day water balance (WBI) variations: (i) in the humid massif (WBI > 500 mm·y− 1), monosiallitization is exclusive in soil and upper saprolite horizons and leads to the neoformation of 1:1 clay minerals (well-crystallized kaolinite) with minor amounts of gibbsite; (ii) in the subhumid peripheral area of the massif (50 < WBI < 500 mm·y− 1), monosiallitization coexists with bisiallitization in the soil profiles, producing a mixture of 1:1 (kaolinite) and 2:1 (illite) clay minerals along with lower weathering intensity; (iii) in the semiarid piedmont (0 < WBI < 50 mm·y− 1), bisiallitization becomes the dominant weathering pathway conducive to grus formation and fersiallitic pedogenesis, with the most vulnerable primary minerals weathered into high-charge, 2:1 clay minerals (smectite and illite). Given that soils and weathering mantles are integrators of “average” bioclimatic conditions that prevail at timescales of 1 to 10 Myr, the distribution of soil weathering signatures in the study area is probably reflective of inevitable, long-term (Quaternary to Neogene) climatic fluctuations around median values close to the present ones. Paleoclimatic legacies inherited from the Paleogene lateritization event are rare, because of the denudation pulse occurring in the Late Tertiary and favoring soil stripping to the paleoweathering front on the erosional piedmont.