Abrupt climatic changes induced by ice-rafting events in the eastern North Atlantic during the late Quaternary: stable isotope and X-ray mineralogical analyses

Abstract

Stable oxygen- and carbon-isotope and X-ray mineralogical analyses are used to obtain information on ice-rafting (Heinrich) events in the eastern North Atlantic during the last glacial period. A sediment core collected from the continental slope off Portugal reveals that the X-ray mineralogy of some ice-rafted debris (IRD) layers is similar to that of the Heinrich layers H1, H2, and H4 in the North Atlantic, although the quantities of various detrital fragments are relatively low. The IRD layers are marked by a maximum abundance of planktonic foraminiferaNeogloboquadrina pachyderma (sin.), minima or lowered values of planktonic δ18O (Globigerina bulloides), and negative δ13C anomalies of benthic foraminiferaCibicidoides wuellerstorfi. Significant decreases in seasurface temperature and salinity are also characteristic. These data suggest that the icebergs passed along the western Iberian margin were of the same origin as those which led to the formation of Heinrich layers in the North Atlantic. In particular, a decrease in benthic δ13C of the IRD events implies a decreased ventilation of the North Atlantic that may have enhanced the advection of a12C-enriched intermediate water from the south. The absence of an IRD layer corresponding to the Heinrich layer H3 in the studied cores suggests that the reduction of convection in the North Atlantic prevented the iceberg drifting to the study area off Portugal during the period H3.