Two methods, two views: Integrating phytoliths in thin sections and bulk samples on the urban Dark Earths from the DIVA-site (Antwerp, Belgium)

Abstract

Traditionally, phytolith analyses are carried out by extraction from bulk (sediment) samples. This technique provides valuable information, not only on the morphological and/or taxonomic assignment of phytoliths, but also on their concentration (quantitative analysis). However, extraction leads to the loss of the (micro-)context in which they are embedded. Over the past 20 years, the study of phytoliths in soil thin sections has proven to be a consistent method. As phytoliths are neither removed from their sedimentary matrix nor artificially concentrated, their analysis provides information on their taphonomical history, but their morphological identification is sometimes limited. Therefore, it seems obvious that the next step to improve phytolith analysis is to combine the two approaches. The aim of this paper is to explore the potential of this integration. For this purpose, we focus on the urban Dark Earth of the DIVA-site (Antwerp, Belgium), with a chronology between the end of the Gallo-Roman Empire and the 11th century AD. Three different stratigraphic units, micromorphologically recognized within the Dark Earth, have been studied. They correspond to an agricultural field, unconsolidated walking surfaces and a floor. Our results confirm the added value of combining the two methods. The possibility of observing the phytoliths in their (micro-)context allowed us to characterize each stratigraphic unit with a particular phytolith assemblage. At the same time, the information derived from the bulk samples overcomes the difficulties in the morphological identification of phytoliths in soil thin sections.

Fig 1. Profile 62 of the archaeological site of DIVA (Antwerp).

The studied thin sections are shown on the left, and their provenience in the profile is marked in orange.

Fig 2. Plots showing the percentage of distribution patterns per sample and the percentage of phytoliths present in each pattern.

Fig 3. Plots showing the percentages of each of the categories of the VPC index for each of the stratigraphic units analyzed in this work.

Fig 4. Microphotographs taken at 500x under PPL (a) and blue light (b) of an articulated system formed by Elongate dentate from FL.

Note that organic matter is partially masking the articulated system, making the observations difficult. Under blue light, organic matter is auto-fluorescent.

Fig 5. Microphotograph taken at 500 magnifications of an isolated melted phytolith (AEB) under PPL.

Fig 6. Microphotographs taken at different magnifications under PPL: (a) Isolated GSSCP Rondel (AAA) taken at 80x from STS 196.5; (b) Isolated Acute bulbosus (ACA) at 80x from STS 196.5, note that the phytolith is fragmented into three pieces and slightly moved as there is space between the pieces; (c) Isolated Elongate dentate (ACA) taken at 80x from STS 196.4; d) Isolated spicule (BCA) taken at 50x from STS 196.5.

Fig 7. Microphotograph from phytolith extractions of FL sample taken at 500 magnifications.

The arrows point to the Elongate dendritic/dentate while the circle shows a Papillate base.

Fig 8. Plots showing the principal morphotypes identified in the STS and the BS.

Fig 9. Microphotograph taken at 200 magnifications of an articulated system from sample 196.1.

Arrows indicates the location of the articulated system. Note that there is a continuation of the plant tissue on the left side of the picture, revealing this was broken by trampling.