Abstract
Temporal changes in soil development were assessed on fluvial terraces of the Little River in the upper Coastal Plain of North Carolina. We examined five profiles from each of six surfaces spanning about 100,000 years. Soil-age relationships were evaluated with inter-surface clay mineral comparisons and regression of chemical properties versus previously reported optically-stimulated luminescence ages using the most developed subsoil horizon per profile. Bases to alumina (Bases/Al2O3) ratios have negative correlations with age, whereas dithionite-Fe (FeD) concentrations are positively correlated with time and differentiate floodplain (<200 yr BP) from terrace (≥10 ± 2 ka) soils and T4 pedons (75 ± 10 ka) from younger (T1-T3b, 10 ± 2–55 ± 15 ka) and older (T5b, 94 ± 16 ka) profiles. Entisols develop into Ultisols with exponentially decreasing Bases/Al2O3 ratios, reflecting rapid weatherable mineral depletion and alumina enrichment during argillic horizon development in the first 13–21 kyr of pedogenesis. Increasing FeD represents transformation and illuviation of free Fe inherited from parent sediments. Within ~80–110 kyr, a mixed clay mineral assemblage becomes dominated by kaolinite and gibbsite. Argillic horizons form by illuviation, secondary mineral transformations, and potentially, a bioturbation-translocation mechanism, in which clays distributed within generally sandy deposits are transported to surface horizons by ants and termites and later illuviated to subsoils. T5b profiles have FeD concentrations similar to, and gibbsite abundances greater than, those of pedons on 0.6–1.6 Ma terraces along Coastal Plain rivers that also drain the Appalachian Piedmont. This is likely because the greater permeability and lower weatherable mineral contents of sandy, Coastal Plain-sourced Little River alluvium favor more rapid weathering, gibbsite formation, and Fe translocation than the finer-grained, mineralogically mixed sediments of Piedmont-draining rivers. Therefore, recognizing provenance-related textural and mineralogical distinctions is crucial for evaluating regional chronosequences.
Highlights
Soil chronosequences are assemblages of soils of differing ages that have formed on similar parent materials and experienced comparable topographic, vegetative, and climatic conditions through time [1,2]
A general working hypothesis for soil chronosequences is that certain mineral, chemical, and morphological components are non-randomly enhanced or depleted as a function of time
Chronosequences are the chief source of data used to evaluate rival theories of pedogenesis and ascertain rates of soil development [3]
Summary
Soil chronosequences are assemblages of soils of differing ages that have formed on similar parent materials and experienced comparable topographic, vegetative, and climatic conditions through time [1,2]. When the time required to develop certain pedogenic features is known, soil chemical and mineralogical properties are useful for establishing relative-age relationships among landforms [5,10], mapping and correlating Quaternary deposits [11], and aiding chronological interpretations in archaeological investigations [12,13]. Given these attributes, soil chronosequences serve important functions in pedology, geomorphology, archaeology, and Quaternary geology [3]
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