18th Swiss Geoscience Meeting, Zurich 2020

Formation calcified roots in terrestrial sediments and their implications for paleoenvironmental research - revisited

Guido L.B. Wiesenberg*, Martina I. Gocke**

*Department for Geography, University of Zurich, Winterthurerstrasse 190, 8057

Zurich, Switzerland (guido.wiesenberg@geo.uzh.ch)

** Institute of Crop Science and Resource Conservation, Division Soil Science, University of Bonn, Nussallee 13, D-53115 Bonn, Germany

Calcified roots, also known as rhizoliths, are widely distributed in Holocene soils

and sediments of many different climates, which enables their use in

environmental research. While one can frequently observe carbonate coatings

on living and sub-recent root surfaces, it remains questionable, how rhizoliths of

larger diameter (>10 mm) are formed and what their implications are for the

paleoenvironmental information of the surrounding soil and sediment. Often one

or a few dark colored channels can be observed within large rhizoliths, which

suggested that organic matter from former tree or shrub roots was preserved in

these areas. However, the internal structure of the rhizoliths commonly does not

look similar like the internal structure of living roots, which led to the questions:

1) Which was the source vegetation of rhizoliths?; 2) How can large rhizoliths

be formed?; 3) Where in the profile is the carbonate used for formation of

rhizoliths originating from?; 4) What are paleoenvironmental implications of

rhizoliths?

1) Stable carbon isotope and lipid molecular analyses analyses confirmed that

frequently rhizoliths are formed by C3-vegetation.

2) A combination of micro-computed tomorgaphy (CT) and scanning electron

microscopy (SEM) was applied to decipher the formation mechanisms of

rhizoliths. The CT measurements of intact soil and sediment cores clearly

confirmed that the calcification was related to former root growth. This was

underpinned by side roots, some of which being calcified, whereas others

remained visible in the CT scans as unfilled biopores. Surprisingly, SEM

analyses showed that all investigated rhizoliths had a very similar internal

structure, irrespective of their origin (Serbia, Hungary, or Germany). The large

rhizoliths (>10 mm diameter) consist of an agglomeration of many

microrhizoliths (<2 mm), which were formed by fine roots, showing the same

internal structure like living roots. The SEM analyses suggest that calcification

of roots starts at the surface, followed by coating of cortex cells of the roots and

subsequent coating of internal cells. Finally, carbonate can be precipitated even

within the root cells, the latter most likely after the death of the individual cell.

However, the question remains, why one can find so many microrhizoliths in

large rhizoliths? The answer is, that large biopores were formed by trees and

shrubs. But during degradation of the large roots, smaller roots colonized the

decaying root and used it as a source of nutrients. Such scenario is frequently

observed for living fine roots. With ongoing degradation, more and more fine

roots of different generations filled the pore space of the former root channel

and started to calcify. Finally, the organic matter and nutrients entrapped in the

primary large root are entirely consumed and one can find only the

microrhizoliths that have the potential to persist for millennia.

18th Swiss Geoscience Meeting, Zurich 2020

3) Stable calcium and strontium isotope as well as radiogenic Sr isotope

analyses suggest that Ca and Sr originate from a mixture of the leached fraction

of the loess from the same stratigraphic level of the respective rhizoliths and

from overlying soils or paleosols. However, we still lack of a detailed

understanding of related leaching and transport processes in very deep

subsoils, which requires further investigations.

4) The impact of rhizoliths on organic matter in soils and sediments depends on

rhizolith frequency and organic matter concentration in the archive. On the other

hand, rhizoliths themselves can provide valuable data on rooting plants and

thus can contribute useful paleoenvironmental information.

In our presentation, we will explain the latest findings on the nature of rhizoliths

and implications of these for their environmental interpretation.