Benthic foraminiferal responses to mesoscale environmental heterogeneity at the Porcupine Abyssal Plain, NE Atlantic

Abstract

Although our knowledge on the vast deep-sea biome has increased in recent decades, we still have a poor understanding of the processes regulating deep-sea diversity and assemblage composition, as well as their underlying natural variability in space and time. In the face of unprecedented anthropogenic impact on this environment, addressing this knowledge gap remains of paramount importance. In this thesis I focus on the effect of mesoscale (10s of kilometres) spatial heterogeneity, in the form of abyssal hills and surrounding abyssal plains, on benthic communities and specifically on foraminiferal faunas living at abyssal depths in the northeast Atlantic. ‘Live’ (Rose-Bengal-stained) and dead benthic foraminiferal assemblages, including rarely-studied soft-walled monothalamous species, were analysed based on a total of 16 Megacorer samples (0.25 cm2 surface area, 0-1 sediment horizon, >150 ?m sieve fraction) from five sites within the area of the Porcupine Abyssal Plain Sustained Observatory (PAP-SO, NE Atlantic, ~4850 m water depth). Three sites were located on the tops of small abyssal hills (~200-500 m elevation) and two on the adjacent abyssal plain. The main results of this analysis include the following. (1) Description of new morphotypes of poorly known primitive benthic foraminifera associated with (i.e. sessile on) planktonic foraminiferal shells and mineral grains. Some of these forms were more common on the hills, while others were more common on the plain. (2) Agglutinated foraminifera selected particles of different sizes on the hills compared to the plain, which affected their test morphometry and visual appearance. Distinct hydrodynamic conditions, and consequently distinct sediment granulometric characteristics between the two settings (hills, plain) resulted in foraminifera on the hills having more coarsely agglutinating tests. This information could be useful in palaeoecological interpretations of the fossil record. (3) Live benthic foraminiferal assemblages were significantly influenced by seafloor topography. Abyssal hills had a higher species density compared to the plain, supported a distinct fauna, and therefore tended to increase regional diversity. Enhanced bottom-water flow on hills, which affects organic matter supply and local sedimentology, were proposed to be responsible for these differences. (4) During the transition from live to dead benthic foraminiferal faunas there was a significant loss of delicate agglutinated and organic-walled forms. Unlike ‘live’ assemblages, the composition of the dead assemblages was very similar in hill and plain settings, suggesting that it would not be possible for paleoceanographers to differentiate between fossil foraminiferal faunas originating from these topographically contrasting settings. In conclusion, this study highlighted the significant effect of hills on agglutination patterns, assemblage composition and regional diversity of living benthic foraminifera. Since abyssal hills are one of the most common landforms on Earth, their presence may substantially enhance abyssal biodiversity, with important implications of deep-sea ecosystem functioning.

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