Abstract
Abyssal hills, small topographic features rising above the abyssal seafloor (< 1000 m altitude), have distinct environmental characteristics compared to abyssal plains, notably the presence of coarser-grained sediments. As a result, they are a major source of habitat heterogeneity in the deep sea. The aim of this study was to investigate whether there is a link between abyssal hills and the test characteristics of selected agglutinated benthic foraminiferal species. We analysed 1) the overall morphometry, and 2) the granulometric and chemical (elemental) characteristics of the agglutinated tests of ten common foraminiferal species (Adercotryma glomerata, Ammobaculites agglutinans, Cribrostomoides subglobosus, Lagenammina sp.1, Nodulina dentaliniformis, Portatrochammina murrayi, three Reophax sp. and Recurvoides sp. 9) at four sites (two on top of abyssal hills and two on the adjacent plain) in the area of the Porcupine Abyssal Plain Sustained Observatory, northeast Atlantic. The foraminiferal test data were compared with the particle size distribution and elemental composition of sediments from the study sites in order to explore possible grain size and mineral selectivity. We found differences in the visual appearance of the tests (i.e. the degree of irregularity in their shape), which was confirmed by morphometric analyses, related to seafloor topography. The agglutinated foraminifera selected different sized particles on hills and plains, reflecting the distinct granulometric characteristics of these settings. These characteristics (incorporation of coarse particles, test morphometry) could provide evidence for the recognition of ancient abyssal hill environments, as well as other palaeoceanographic settings that were characterised by enhanced current flow. Furthermore, analyses of sediment samples from the hill and plain sites using wavelength dispersive X-ray fluorescence (WD-XRF) yielded different elemental profiles from the plains, probably a result of winnowing on the hills, although all samples were carbonate-rich. In contrast, the majority of the agglutinated tests were rich in silica, suggesting a preferential selection for quartz.
Highlights
Abyssal plains are vast areas of the ocean floor situated at water depths between 3500 and 6500 m
During the analysis of foraminiferal samples collected in the area of the Porcupine Abyssal Plain Sustained Observatory (PAP-SO; Hartman et al, 2012) in the northeast Atlantic (4800 m water depth), we found variation in the community composition of benthic foraminiferal assemblages obtained at sites on the tops of abyssal hills and on the adjacent abyssal plain (Stefanoudis et al, 2015)
As our samples were fixed in formalin, we could not obtain molecular data to support our contention that the same foraminiferal species occur at the hill and plain sites
Summary
Abyssal plains are vast areas of the ocean floor situated at water depths between 3500 and 6500 m. Often regarded as topographically homogeneous, abyssal plains are populated by abyssal hills, typically up to a few hundred meters in height and a few kilometres in width. These represent one of the most important geomorphic features in the oceans (Heezen et al, 1959; Heezen and Holcombe, 1965; Goff and Arbic, 2010). Very few studies (e.g., Durden et al, 2015) have explored the impacts of abyssal hills on deep-sea communities and none has dealt with meiofaunal groups such as the foraminifera
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