Radiocarbon dating reveals the timing of formation and development of pedogenic calcium carbonate concretions in Central Sudan during the Holocene
Articolo
Data di Pubblicazione:
2018
Citazione:
Radiocarbon dating reveals the timing of formation and development of pedogenic calcium carbonate concretions in Central Sudan during the Holocene / G. Dal Sasso, A. Zerboni, L. Maritan, I. Angelini, C. Compostella, D. Usai, G. Artioli. - In: GEOCHIMICA ET COSMOCHIMICA ACTA. - ISSN 1872-9533. - 238(2018), pp. 16-35. [10.1016/j.gca.2018.06.037]
Abstract:
Calcic soil horizons are common in arid and semi-arid lands and represent the result of the progressive accumulation of
calcium carbonate in the soil profile over time. This process leads to the occurrence of several pedogenic phases of calcium
carbonate dissolution/precipitation. For this reason, timing the formation and development of calcic horizons through radiometric
dating is not straightforward as time-averaging effects, due to the superimposition of the same process over time, occur.
On this basis, this study aims to define the timing and dynamics of formation and development of pedogenic calcium carbonate
concretions from semi-arid Central Sudan, highlighting the relevance of a multi-disciplinary approach and the effectiveness
of radiocarbon dating (coupled to an accurate sampling strategy) applied to pedogenic carbonates. Calcic soil horizons
(Bk) were sampled during the archaeological excavation of site 16-D-4 at Al Khiday (Central Sudan) and studied by optical,
cathodoluminescence and scanning electron microscopy, as well as by chemical-physical and stable isotopes (C and O) analyses.
Micromorphological analyses reveal the occurrence of distinctive calcitic pedofeatures, here described as calciticcemented
nodules and calcitic-rich matrix. Radiocarbon ages obtained for these pedofeatures mostly refer to the Early Holocene
for calcitic-cemented nodules (11.5, 9.9 and 9.6 cal. ka) and to the Middle Holocene for powdery calcitic-rich matrix (7.9,
7.7, 6.3 and 6.1 cal. ka) samples. Additionally, d13C and d18O values indicate major shifts of environmental conditions from
the Early to the Middle Holocene, in agreement with the available information from detailed palaeoenvironmental studies
carried out in the region. In particular, Early Holocene pedogenic calcitic features can be related to short arid phases during
general wetter climate conditions, whereas those formed in the Middle Holocene can be related to short humid phases. Results
show that Bk horizons were characterized by alternate periods of calcite accumulation (precipitation) and dissolution, and
periods of quiescence or extremely slow growth rates. Thus, the formation and development of Bk horizons have been a
long-lasting process significantly influenced by climatic fluctuations. This study represents a further step in the comprehension
of the development of calcium carbonate concretions and (at a wider perspective) calcrete; the paper also contributes to the
definition of a reliable method to radiometrically date the formation of calcic pedofeatures. On a broader perspective, this work may offer a significant tool for archaeometric studies, where the interaction between secondary calcite and archaeological
material is significant (e.g., radiocarbon dating of bioapatite in bones), and palaeoenvironmental studies in arid lands,
where Bk horizon development is a function of past rainfall.
Tipologia IRIS:
01 - Articolo su periodico
Keywords:
Holocene; Calcic soil horizon; Calcium carbonate concretions; Calcite; North Africa; Cathodoluminescence microscopy; Radiocarbon dating; Pedogenesis
Elenco autori:
G. Dal Sasso, A. Zerboni, L. Maritan, I. Angelini, C. Compostella, D. Usai, G. Artioli
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