The late Pleistocene palaeoclimatic record of North Atlantic deep-sea sediments revealed by mineral-magnetic measurements

Eight cores of North Atlantic deep-sea sediment are correlated using simple, rapid, and non-destructive measurements of magnetic susceptibility and isothermal remanence. This stratigraphy is demonstrably related to late Pleistocene palaeoclimatic variations by comparison with the oxygen-isotope and microfossil records of these cores. Fluctuations in the concentration-dependent magnetic properties of the cores largely reflect their lithological composition, consisting of climatically modulated cycles of marl (= glacial horizons) and nannofossil chalk (= interglacial horizons). High magnetic concentration characterises glacial horizons, which correspond to periods when carbonate productivity was severely restricted and ice-rafted detrital inputs were prolific, coincident with the migration south of the North Atlantic polar front, to within 3° of latitude north of the coring site. In contrast, interglacial horizons are characterised by low magnetic concentration and high carbonate content of the sediment, consequent on the retreat of the polar front and the re-establishment of coccolith floras in its wake, during these periods. Ratios formed between individual magnetic parameters are independent of magnetic concentration and are thus free of any climatic influences wrought by carbonate oscillations. In this study, however, such ratios exhibit a definite response to palaeoclimatic variations, attributable to differences in the composition and grain size of the magnetic mineral assemblage of glacial vis-a-vis interglacial horizons of the cores. These differences in mineral-magnetic composition are substantially confirmed by Mössbauer-effect studies of selected samples representing peak glacial and interglacial horizons of one core. Such glacial/interglacial contrasts in the concentration, mineralogy and grain size of the magnetic assemblage, most probably result from variations in the source and flux-density of detritral inputs, induced by changes in climate. X-ray diffraction studies of the same core indicate that variations in the mineralogy of the detrital fraction of the sediment, are consistent with those of the magnetic assemblage. Together, these data point to the conclusion that a subsidiary low latitude, possibly aeolian detrital flux supplements high-latitude ice-rafted input, in glacial horizons of the cores. The low-latitude flux, however, is also identifiable in interglacial horizons. Independent evidence suggests that this low-latitude aeolian flux may be derived from the Sahel region of North Africa and transported to the coring site by Harmattan winds associated with Saharan dust outbreaks.