A new benthic foraminiferal proxy for near-bottom current velocities in the Gulf of Cadiz, northeastern Atlantic Ocean

Abstract

Recent benthic foraminiferal assemblages were analyzed in the Gulf of Cadiz, northeastern Atlantic, to study the impact of the Mediterranean Outflow Water (MOW) undercurrent on the benthic environment. Foraminiferal counts and the analysis of specimens attached to hard substrates from 26 surface samples reveal a relationship of epibenthic assemblages with sedimentary and hydrodynamic environment. Epibenthic species make up as much as 60% of the living assemblage at proximal sites with high current velocities and 3–18% in distal areas or near the margins of the MOW flow paths at low velocities. These foraminifers inhabit elevated substrates only within the MOW, which evidently provides an ecological niche for opportunistic suspension feeders. They adapt their settling elevation dynamically and occur at greater heights above the ambient sediment surface under stronger currents. Mobility, fixation strength, suspension feeding, and reproduction efficiency emerge as individual capabilities promoting the occupation of elevated substrates by certain epibenthic species. The active microhabitat selection is pursued as basic strategy of these foraminifers to optimize their food acquisition. A better access to food sources stimulates reproduction and leads to a greater contribution of foraminiferal tests to the surface sediments. Elevated epibenthos percentages from the dead assemblage and current velocities prevailing at the sampling sites are closely correlated. A compilation including other data from southern Portugal, Florida Straits, and the English Channel infers an exponential relationship between epibenthic abundances and flow strength implying that endobenthic species prevail even under high current velocities. A linear model provides a significantly better fit for the Gulf of Cadiz data however. This relation is used for a case study in order to estimate near-bottom current strengths for the late Holocene Peak III contourite in core M39008-3. Trends and absolute values of current velocities, inferred from the benthic foraminiferal proxy, are the same scale as estimates obtained from sediment grain-size distribution and hydrodynamic models. Epibenthic foraminifera thus bear a high potential as proxy for palaeocurrent studies that even may overcome objections by predetermined grain-size distributions in deep-sea cores.