Root Trace Fossils are new Tools for Paleoecological Studies

Biopores and root features as new tools for improving paleoecological understanding of terrestrial sediment-paleosol sequences

Authors:

Gocke et al

Abstract:

The important role of roots and rhizosphere processes is accepted nowadays for the top soil, but is still under debate for the deep subsoil including soil parent material. Especially for terrestrial sediments like loess and dune sands, roots and root traces are mostly recognized in profile description, but not interpreted in the paleoenvironmental context. Further, synchrony of sediment deposition and root trace formation is commonly assumed within a respective stratigraphic unit (sediment or paleosol). This is challenged by partially large maximum rooting depth of plants, exceeding the soil depth, and by frequent occurrence of secondary carbonates and biopores of potential root origin below recent soil and paleosols. To improve understanding of paleoenvironmental records in terrestrial sediment-paleosol sequences, recent roots and root traces, including calcified roots and root-derived biopores, were investigated in six soil, loess and dune sand profiles across Middle and Southeastern Europe. Visualization of small carbonate accumulations (diameter ≤ 1 mm), frequently called ‘pseudomycelia‘, by X-ray microtomographic scanning, and morphologic comparison with rhizoliths (calcified roots; diameter mostly 3–20 mm, up to 100 mm possible) indicate root origin of the former, therefore requiring renaming to so-called microrhizoliths. Quantification of roots, biopores, rhizoliths and microrhizoliths on horizontal levels yielded maximum frequencies of 2,100 m- 2, 4,100 m- 2, 196 m- 2 and 12,800 m- 2, respectively. Considering the pore volume remaining from former root growththis indicates their significant contribution to structural properties of the sediments and paleosols. Depth distribution of roots and root traces was frequently related to soil and paleosols, respectively, and mostly showed maximum frequencies within or immediately below these units. Root traces are therefore not necessarily directly associated to the surrounding sediment, but are typically of younger age. The time lag between root traces and the surrounding stratigraphic unit can vary between small time periods (likely decades to centuries) in case of microrhizoliths and several millenia in case of larger rhizoliths penetrating several stratigraphic units. With assumed radii of former rhizosphere extention of 5 mm for microrhizoliths, a frequency of 12,500 m- 2 corresponded to 100% rhizosphere area in the respective depth interval. These findings emphasize the meaning of root traces in sediment-paleosol sequences. Potential temporal and spatial inhomogeneity of root growth on the one hand, especially for shrub and tree vegetation, and occurrence of root remains of different age and origin in identical depth intervals on the other hand, hamper the assessment of the chronologic context of these with the surrounding sediment or paleosol. Nevertheless, root traces in terrestrial archives provide valuable information with respect to paleovegetation and paleoenvironmental conditions, if their chronological context is known.