| Resumo: |
Primary productivity and water mass reconstructions based on planktic Foraminifera reveal distinct interglacial/glacial variations for the past 208 ka in a mid-latitude Northeast Atlantic piston core. Average total planktic foraminiferal absolute frequencies and accumulation rates, which are interpreted to reflect primary productivity, are higher in interglacial than in glacial sediments. Low total planktic foraminiferal absolute frequencies and accumulation rates in 'Heinrich layers' are likewise interpreted to show low production of Foraminifera due to low surface ocean fertility. 'Heinrich layers' are enriched in ice-rafted debris, recording periods of massive iceberg production and rapid melting in the Northeast Atlantic. The dominance of Neogloboquadrina pachyderma (sinistral) in these layers reflects an extension of cold low salinity polar waters. The fresh water along with turbidity caused by melting icebergs may account for the low productivity during these events. In contrast, the dominance of Globigerina bulloides, Neogloboquadrina incompta, Globorotalia scitula, Globigerinita glutinata, and Globorotalia inflata group in interglacial sediments is interpreted to reflect conditions comparable with the present day North Atlantic Current (NAC) waters in the area. In the modern ocean, the Gulf Stream and its extension, the NAC, are driven by seasonally strong westerly winds which induce mixing, supplying nutrients from deep to surface waters. Enough food and sufficient light combine to provide for pulses of algal blooms which support large populations of Foraminifera. The important planktic foraminiferal test contributors in glacial sediments are not only Turborotalita quinqueloba, a subpolar species, but also polar and NAC indicator species. Primary productivity is inferred to be extremely seasonal, with low productivity during winter when there was little sunlight and partial ice cover. Planktic Foraminifera are good primary productivity indicators in this carbonate-dominated, openocean Northeast Atlantic site. In contrast, organic carbon, which is directly linked to primary productivity and is extensively used as a proxy in upwelling areas, is unsuitable in this site because of the very low organic carbon content of the sediments and the contamination by terrigenous organic matter.
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