Megarhizoliths in Pleistocene aeolian deposits from Gran Canaria (Spain): Ichnological and palaeoenvironmental significance

Abstract

The Pleistocene dune field of Tufia, located on the east of Gran Canaria (Spain), contains different stratigraphic levels of indurated pillar-like structures that are interpreted as megarhizoliths. The megarhizoliths occur at the top of different aeolian sets and reach 31.5 cm in diameter and over 1 m in height. These scattered, free-standing, vertical, cylindrical-to-slightly conical columns usually appear as hollow cylinders, displaying elliptical cross-sections aligned with the prevailing wind. On the leeward side of some specimens the external wall shows a tail of rock matrix resembling a sort of "wind shadow". These tails and other remains of the associated rock matrix show a texture composed of long, horizontal, parallel cylinders orientated with the wind. Internally the most complete structures show five concentrically arranged zones: Zone (a), is a central pore corresponding to the cavity originally occupied by the root; Zones (b) and (c), which include alveolar and laminated carbonate textures indicating that carbonate precipitation was mostly induced by the roots and their associated microorganisms; and Zones (d) and (e), consisting mostly of aeolian sands. In (d) the sand grains show thin micritic coatings whereas in (e) vadose aragonite cements can be seen on the grain surface suggesting a less biogenic influence in their formation. The degree of cementation and the time of the precipitation of carbonate around the roots controlled the preservation of these zones. Thus, in some cases, Zones b, c and/or d are not preserved. Cylinders are up to 30× the diameter of the root that nucleated them. The presence of the megarhizoliths at the top of the aeolianite beds indicates that aeolian sedimentation halted several times, allowing soil formation and plant colonisation during slightly more humid periods. The occurrence of megarhizoliths is further proof of the alternation of arid and slightly more humid climates in the north Atlantic during the last glacial period. It is also noted that they may be misinterpreted as animal trace fossils or tree trunk casts, resulting in incorrect ichnological or palaeoenvironmental interpretations.

Polygenic Red Calcic Soils in Coastal Middle Palaeolithic Environments, Israel: Taxonomy and Pedosedimentary Reconstructions

“…The 1-10 palaeosols in AJU contains (1) small-sized, poorly consolidated carbonate; (2) macrorhizoliths similar to those described by Khadkikar et al (2000) and Alonso-Zarza et al (2008) either in vertical or inclined configuration ( Fig. 7.2d; and (3) large crust-like concretions at the base of the soil profile.…”

Pinnacle Syngenetic Karst in Nambung National Park, Western Australia

“…e same process was suggested by Alonso -Zarza et al (2008), who interpreted certain vertical tube-like structures with cemented rims on Canary Island as "megarhizoliths", because they appear to be the remains of roots. In addition to plantroots, focused water ow could also be concentrated by the upper parts of the plants (stem ow, see Lundberg & Taggart 1995), or it could just be focused beneath surface hollows in an exposed calcrete (Lundberg & Taggart 1995) or through porous patches in a developing calcrete (Grimes 2004, in prep.).…”

“…Later on the ll in the vertical tube was also cemented by carbonate derived from an overlying dune layer. In this ll smaller solution pipes are found, which could, according to Alonso -Zarza et al (2008), also be interpreted as rhizoliths or megarhizoliths.…”

Taxonomic and stratigraphic revision of Lower Cretaceous cicatricose spores from Patagonia. 2. Genera Cicatricosisporites Potonié & Gelletich and Ruffordiaspora Dettmann & Clifford

“…16c). Estructuras similares fueron descriptas y atribuidas por Bordy et al (2004) a termiteros fósiles, mientras Alonso-Zarza et al (2008) interpreta otras estructuras semejantes como megarizolitos.…”