Environmental Variations in the North Atlantic in Response to Quaternary Glaciations

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In this book, the sea surface and deep water conditions in the mid-latitude North Atlantic during the Quaternary are reconstructed based on the analysis of both planktonic and benthic microfaunal assemblages from the SW Iberian margin. The key location of the study site, and the comparison with other sites in the North Atlantic, allow for the drawing of basin-wide conclusions.

Millennial-scale variability in Sea Surface Temperature (SST) occurred during interglacial and glacial periods, but with a wider amplitude (> 5 ºC) at glacial onsets and terminations. Pronounced stadial events were recorded at all deglaciations during the Quaternary. This prominent cooling of surface waters is the result of major reorganizations of North Atlantic circulation in response to freshwater release to the North Atlantic when ice sheets collapse at the onset of deglaciations. The slowdown of North Atlantic deep-water formation associated to such freshening events reduced the northward flow of the warm subtropical North Atlantic Drift and enhanced the southward advection of cold subpolar waters. Following each episode of severe cooling at the onset of deglaciations, surface water experienced abrupt warming that initiated the climatic optimum during the early phase of interglacials. Abrupt warming was recorded by a sudden increase of the subtropical assemblage that indicates enhanced northward transport of heat through the North Atlantic Drift. At the onset of glaciations, SST in middle latitudes remained relatively warm while the surface waters of the North Atlantic experienced cooling, producing a wide latitudinal SST gradient.

The Southwest Iberian margin is highly sensitive to changes in the distribution of North Atlantic currents and both surface and deep water masses, as well as to changes in the position of the Arctic and subtropical fronts. Integrative research including deep and surface analysis has been conducted in this area to evaluate the incidence that surface changes had on deep-sea environments. Results reveal the predominance of well-oxygenated bottom and pore-waters, as well as an abundance of food in the sediment for benthic communities. Nevertheless, significant changes in surface productivity and food availability in the sea floor are recorded between glacials and interglacials. Interglacial surface productivity is much lower than during glacial activity, which, combined with the presence of the highly oxygenated North Atlantic deep water, results in reduced organic matter availability for benthic communities. On the other hand, during glacials when the bottom-water mass is the oxygen-depleted Antarctic bottom water and the organic matter supply from the surface is higher, the combination results in better environmental conditions for benthic communities.