Origin of mounds in the Pantanal wetlands: An integrated approach between geomorphology, pedogenesis, ecology and soil micromorphology

<div><p>Vegetated mounds are an important geomorphological feature of the Pantanal, where the influence of floods dictates not only hydropedological processes, but also the distribution and ecology of the flora and fauna. This work aimed to identify factors and processes that influence the formation and spatial distribution of the mounds, which are commonly associated with termite activity. In order to characterize pedological processes, macro and micro morphological descriptions, satellite image interpretation, dating of the sandy sedimentary material using OSL and carbon dating using ¹⁴C AMS were carried out. This dating of the materials indicates that the sediments in which the soils were formed were deposited during the Pleistocene, while the carbonates are from the Holocene. The basin-like format of the laminar structures suggests that part of the more clayey material was deposited in lacustrine environments. The more humid climate in the Holocene intensified argilluviation, which at an advanced stage, led to a more pronounced textural gradient, reducing drainage and leading to ferrolysis and thickening of the E horizon. Besides pedogenic processes, more erosive flooding during the Holocene began reducing and rounding the landscape’s more elevated structures (paleolevees). In the final stage, these structures were occupied by termites to shelter from flooding. Thereafter, the bio-cementation action of the termite nests has increased the resistance of the vegetated mounds to processes of erosion.</p></div

Soil classification and most important attributes for each site.

<p>Soil classification and most important attributes for each site.</p

Morphological features from site 4.

<p>(A) Powdered white material. (B) Fe depletion in root channels, an incipient stage of ferrolysis process.</p

Study area and its boundary.

<p>Study area bordered to the west by the river Cuiabá and to the east by the São Lourenço river, study sites are indicated by stars (1 to 5). Coordinates in UTM projection system 21K zone (south). Printed under a CC BY license, with permission from “Secretaria de Estado de Planejamento do Mato Grosso” (SEPLAN-MT), original copyright 2008.</p

Soil profiles corresponding to five study sites.

<p>Profiles from study sites 1 (A), 2 (B), 3 (C), 4 (D) and 5 (E). Undisturbed sample sites for the thin sections are indicated by black rectangles.</p

Soil classification and morphological attributes for each site.

<p>Soil classification and morphological attributes for each site.</p

Model for mound, carbonate and natric character formation.

<p>(A) Flooding process raising higher parts of the landscape. (B) Erosive processes acting on the mound, with thick sediment burying clay and carbonates which becomes a subsurface layer. (C) Narrowing of the structure through erosion. (D) Thickening of the E horizon and formation of preferential drainage paths and remnants of the Bt horizon. At the most advanced stage of alteration and erosion, mound fragments will be narrowed.</p

Micromorphological description from thin sections, according to Bullock <i>et al</i>. [30] and Stoops [31].

<p>Micromorphological description from thin sections, according to Bullock <i>et al</i>. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179197#pone.0179197.ref030" target="_blank">30</a>] and Stoops [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0179197#pone.0179197.ref031" target="_blank">31</a>].</p

Satellite images from studied sites showing their geomorphological context.

<p>Close on site 1 (A), 2 (B), 3 (C), 4 (D) and 5 (E). Printed under a CC BY license, with permission from “Secretaria de Estado de Planejamento do Mato Grosso” (SEPLAN-MT), original copyright 2008.</p

Satellite image showing mound alignment.

<p>Largest paleolevees (dotted lines) and some aligned mounds (continuous circles). Printed under a CC BY license, with permission from “Secretaria de Estado de Planejamento do Mato Grosso” (SEPLAN-MT), original copyright 2008.</p