Planktonic foraminifera assemblage composition and flux dynamics inferred from an annual sediment trap record in the central Mediterranean Sea

Abstract

The Sicily Channel, located in the central Mediterranean Sea, represents a key point for the regional oceanographic circulation, as it is regarded as the sill that separates the western and eastern basins. Therefore, it is regarded as a unique zone in the well-documented west-to-east Mediterranean productivity gradient. Here we present a time series of settling planktonic foraminifera assemblages from November 2013 to October 2014. Altogether, 19 samples from the sediment trap C01 deployed at a water depth of around 400 m have been used. More than 3700 individuals and 15 different species have been identified. Globorotalia inflata, Globorotalia truncatulinoides, Globigerina bulloides, Globigerinoides ruber, and Globigerinoides ruber (pink) were the five main species identified, accounting for more than 85 % of the total foraminifera. The total planktonic foraminifera flux mean value was 630 shells m−2 d−1, with a minimum value of 45 shells m−2 d−1 displayed during late autumn 2013 and a maximum of 1890 shells m−2 d−1 reached during spring 2014. This is likely due to the regional oceanographic configuration and the marked seasonality in the surface circulation. During spring and winter, Atlantic Water (AW) dominates the surface circulation, bringing cool and nutrient-enriched waters. This results in a planktonic foraminifera flux increase and a dominance of western basin taxa. During summer and autumn, the circulation is dominated by the eastern warm and oligotrophic Levantine water, which leads to a planktonic foraminifera flux decrease and the dominance of eastern basin species. Our comparison with satellite-derived sea surface temperatures (SSTs) and chlorophyll a data showed that G. inflata was associated with cool and nutrient-rich conditions, while both G. ruber morphotypes were associated with warm and oligotrophic conditions. However, no trends were identified for G. truncatulinoides or G. bulloides. As the latter species flux increased coincidently with that of benthic foraminifera, we considered that this species might have a resuspended origin. The comparison of the Sicily Channel data with other Mediterranean time series indicates that the annualized planktonic foraminifera flux was lower than in the westernmost Alboran Sea but higher than in the easternmost Levantine Basin. The Sicily Channel species diversity was the highest among the compared zones, highlighting the influence of the different basins and its transitional aspect from a planktonic foraminifera population perspective. Finally, we compared the settling planktonic foraminifera assemblage with the assemblages from seabed sediment located in the vicinity of the Sicily Channel. The differences in the seabed populations varied according to the sites studied. The deep-dwelling species dominated the settling assemblage samples, while eutrophic and oligotrophic species were more abundant in the sediment. Finally, a high-resolution chronology comparison allowed us to show that this planktonic foraminifera population shift likely developed during the late Holocene prior to the industrial period; however, its causes remain uncertain.