Heterocysts of Rivularia Type for Interpreting a Palaeoenvironmental Context of the Late Quaternary in Northern Italy

Abstract

This paper presents new results on a quali-quantitative analysis of the heterocysts of the Rivularia type as a key bioindicator informative on local eutrophic conditions. The Rivularia type is usually reported in palynological analyses due to the thick, multilayered envelope that ensures the preservation of heterocysts in sediments. Samples come from two continuous terrestrial cores (N-S3: 77 samples, C-S1: 20 samples) drilled in the area surrounding the Bronze Age site of the Terramara S. Rosa di Poviglio (Po Plain; N Italy) and spanning at least over the last 15 ka years. Concentrations and percentages of the Rivularia type, combined with pollen curves of wet environments, describe local variability and rapid changes in ecological conditions over the millennia of deposition. Given the abundance and diversity of heterocysts of the Rivularia type in the studied samples, this paper attempts to group these cells based on morphology (ellipsoidal or elongated) and the state of preservation of the sheaths (presence or absence). Actually, it is difficult to confirm a relationship between heterocyst morphologies and the presence of different cyanobacteria species. Increasingly accurate identification of heterocysts from biostratigraphical archives may improve the data available on these bioindicators for achieving more detailed decoding of wetland (and terrestrial) transformations. Since the Bronze Age and at the most recent levels, the Rivularia type may be a good indicator of the local presence of agriculture and livestock, which lead to trophic and water changes in the soil.

Pollen and Molecular Biomarkers from Sedimentary Archives in the Central Po Plain (N Italy): Assessing Their Potential to Deepen Changes in Natural and Agricultural Systems

“…Marsh and alluvial plain deposits from almost the whole Late Pleniglacial/Lateglacial have also been identified at other sites in the lower central Po Plain [68]. Pollen records indicate that vegetation changes in the early Holocene were mainly related to climate oscillations, and fairly open riparian communities (dominated by hygrophilous herbs: Cyperaceae, Sparganium emersum type, and Typha latifolia type) characterized the study area [69]. The open landscape is also indicated in the upper part of the cores and is associated with the increasing pressure of human activities in the area, as evidenced by the rapid and significant increase in Anthropogenic Pollen Indicators (API group [58] in Figure 6).…”

“…According to Cremaschi et al [37], broomcorn millet spread during the Bronze Age in the Po Plain, and it was probably introduced by the population of the Terramara S. Rosa di Poviglio during drier phases (Bronze Age Warm Period, or BAWP; [70]) or sown at the end of the season in the same fields where other cereal crops had already been harvested. Widespread arid conditions and poor nutrient supply are also indicated by the decrease in cyanobacteria colonies and riparian vegetation [37,69] and by the increase in Cichorieae and other Asteraceae [37], suggesting an expansion of dry meadows and pasturelands in the area. The biomolecular analyses also indicated the presence of triterpenyl acetates specific for Asteraceae-Picris hieracioides (such as bauerenyl and pichierenyl acetates) in the Bronze Age samples.…”