Holocene variations in North Atlantic export productivity as reflected in bathyal benthic foraminifera

Abstract

The subpolar NE Atlantic Ocean experiences seasonal fluxes of labile organic matter (phytodetritus) which are expected to strongly influence the composition of benthic foraminiferal assemblages and benthic foraminiferal accumulation rates. We studied export production over the last 12 kyr at a sampling resolution of approximately 250–300 years through an investigation of bathyal benthic foraminiferal assemblages (>63 μm) at Ocean Drilling Program (ODP) Site 980 on the Feni Drift (55°29′N, 14°42′W, 2179 m water depth). During the last 12 kyr, faunas at Site 980 were dominated (~75%) by Cassidulina obtusa, Nonionella iridea, Bolivina difformis, Trifarina pauperata, Alabaminella weddellensis, Stainforthia fusiformis, Cassidulina laevigata and Eilohedra vitrea. The absolute and relative abundances and diversity of these and other species varied significantly. In the interval ~12–10 ka, A. weddellensis, S. fusiformis and T. pauperata had higher % abundance (named here ‘H10 species’), but this is not reflected in a higher accumulation rate, suggesting that surface productivity was low, at highly variable conditions. Species at lower % abundance during this time include B. difformis, C. laevigata, C. obtusa, E. vitrea and N. iridea (so-called ‘L10 species’). The ‘8.2 ka cold event’ was characterized by increased carbonate dissolution (reflected in decreases in the absolute abundance, benthic foraminifera accumulation rate, weight % coarse fraction, and presence of poorly preserved/fragmented benthic foraminifera). Peaks in the relative abundance of species which, in our opinion, exploited phytodetritus (‘phytodetritus species’: N. iridea, A. weddellensis, C. obtusa, and rare Epistominella exigua) occurred at 8.0 ka, 7.0 ka, 6.3–5.6 ka, 4.7 ka, 4.3–3.4 ka and 2.4 ka. These peaks generally correspond to peaks in absolute abundance (number of specimens per gram, accumulation rate), indicating increases in the seasonality of export productivity. However, the ‘phytodetritus species’ do not covary in absolute and relative abundance over the studied interval, suggesting that they have somewhat different ecological requirements. There appears to be no simple relationship between changes in the degree of seasonality of export productivity (i.e., abundance of ‘phytodetritus species’) and records of palaeoclimatic/palaeoceanographic proxies, suggesting that bentho-pelagic coupling (arrival of food on the seafloor with local surface productivity) might not have been straightforward in this region. Site 980 is located in the hydrodynamically active area of Feni Drift, and during the Holocene, currents might have winnowed and removed fine-grained organic matter, making it unavailable to benthic organisms. Alternatively, there may have been changes in remineralization and/or mid-water competition for food, so that the fraction of the organic flux that reached the seafloor may have varied. Holocene benthic foraminiferal assemblages thus reflect highly dynamic conditions in export productivity and arrival of organic matter at the seafloor.