Paleoecological reconstruction and modern analogues from foraminiferal proxies in the Middle Branch of Bue Marino Cave (Sardinia, Italy). Work in progress

Abstract

Being at the boundary between marine and continental realms, marine caves are affected by wide environmental variability for the interaction of marine and continental waters; for this, they are natural laboratories for studying the effects of seasonal and long-term environmental variability also in the perspective of global changes. The first studies of benthic foraminifera from marine caves of the Mediterranean Sea have been carried out in the Gulf of Orosei in the context of a scientific project which started in 2014 and has continued to this day. They demonstrated that benthic foraminifera are very sensitive environmental proxies in these habitats, due to their high diversity and adaptability. The Bue Marino system is constituted by North, Middle and South Branch which join together close to the coast. The study of benthic foraminifera and sediment texture from surface samples, associated with parameters of bottom water, indicated that the Middle Branch was characterized by warmer, saltier and less oxygenated water than the rest of the cave, and by finer sediment with abundant vegetal debris, probably supplied from the mainland during strongly rainy periods (Romano et al., 2021). Then, it was reasonable hypothesizing that cave sediments, although deposited in enclosed environment, are reliable archives of past climatic changes. A 18 cm long sediment core, collected in 2018 in the Middle Branch, was studied for grainsize and benthic foraminifera in order to highlight evidence of temporal changes of environmental conditions and their causes. In the whole core, sediment texture was homogeneous, with prevailing sand and silt in similar proportions, while clay was less abundant and gravel nearly absent. Two hyaline (Ammonia inflata and A. tepida) and one agglutinated species (Eggerelloides advenus), dominated the assemblage, accounting for a total of 70-92%. A main change occurred along the core, at 11 cm depth, for the shift from an assemblage in the lower section with prevailing A. inflata, to another one where the highly opportunistic E. advenus and A. tepida dominated. Of these two assemblages, the latter was more similar to that recorded in the surface sample (0-2 cm), collected at the same sampling station, for the dominance of E. advenus, despite A. inflata slightly prevailed on A. tepida. It was interpreted as the result of a highly organically-enriched environment where opportunistic taxa benefited of abundant degraded organic material under low-oxygen conditions. The prevalence of the “indifferent” A. inflata in the lower core suggests less stressed conditions with minor supply of continental vegetal debris, probably due to less frequent and/or intense rainy episodes in past times.