Changes in the sedimentation and oxygen depletion during the Heinrich stadials 4 and 5 in a distal region of the NW Iberian Margin

This work aims to study changes in sedimentation during the Heinrich Stadials (cold periods contemporary with Heinrich Events, HE) HS5 and HS4 in a distal region of the NW Iberian Continental Margin, which has been poorly documented. Physicochemical, geochemical and mineralogical data and radiocarbon dates for the model age were analyzed. During the HS5 (recorded between ∼46.9 and 43.9 ka; duration ∼3.0 ka), the sediment accumulation rate was higher (∼20 cm/ka) than HS4 (recorded between ∼40.7 and 36.8 ka; duration ∼3.9 ka; ∼3.4 cm/ka). Both events were marked by the darkening of the sediment and the increase in lithic grains >350 μm in size, magnetic susceptibility values, sediment resistivity and density, Fe/Ca and Ti/Ca ratios, rare earths (REEs) contents, among other lithogenic chemical elements, minerals such as quartz, feldspars, phyllosilicates, Neogloboquadrina pachyderma, Globigerina bulloides δ¹⁸O. These events were also marked by reduced fractional porosity (due to the accumulation of fine detrital material), calcite, pyrite, ferrihydrite contents, sea surface temperatures, and exported productivity (Pexp). The greatest mineralogical and geochemical changes are associated with the deposition of ice-rafted debris (IRD) due to the melting of icebergs. The Pexp was higher between the HE5 and HE4. However, the values of the V/(V + Ni) ratio indicate that during the analyzed period (∼47.0–36.2 ka) the oxygenation of the sediments was poor, but improved considerably from the end of the HS5 (between ∼43.9 and 43.5 ka) associated with a sudden reduction in δ¹⁸O values, due to an improvement in climatic conditions related to the reduction of the ice caps. The reduction in Pexp values and the decrease in pyrite and ferrihydrite allow us to deduce that during HE5 and HE4, the oxygen shortage was not caused by a higher amount of organic matter degradation; it should have been a consequence of the weakening in the North Atlantic Deep Water (NADW) production, as suggested by several studies. During HS5, several pulses of heating and cooling (indicated by the δ¹⁸O values), with the highest density of IRD discharges, occurred in a relatively warmer phase.