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.