PDFArticle Abstract
Over the last decade considerable attention has been given to evidence for the iceberg rafting of sediments into the North Atlantic, especially during Heinrich (H) events (presumed to be associated with short-lived, abrupt intervals of increased iceberg and meltwater flux). However, our knowledge of the present sediment distribution within icebergs and the fate of this sediment during iceberg drift is rudimentary. Debris is usually concentrated in a 1-3 m layer at the base of a glacier; if present in calved icebergs, this layer is rapidly melted within a few 10s to 100s of km from an ice margin. Ice rafted debris (IRD) is frequently characterized by the weight percentage or counts of mineral grains within certain size fractions (i.e. > 63, > 250 µm, etc.) with an implicit assumption that the selection of the size limits does not bias the data nor their interpretation. However, studies of glacial and glacial marine sediments indicate there is a poor correlation between the weight percentage in different sand-size classes, moreover, the bulk of glacially derived sediments are typically silts and clays (< 63 µm). In areas close to the probable sources of iceberg discharge during Heinrich events, the abrupt changes are poorly defined by the sand-size IRD fraction but are dramatically highlighted by the input of fine-grained (< 63 µm) detrital carbonate. During the last glaciation, abrupt changes in IRD concentrations (IRD spikes) appear to be an important feature of the mid-latitude belt of the North Atlantic, whereas at higher latitudes, such as areas of the Labrador and Nordic seas, IRD input was more continuous and individual H-events cannot be identified solely by their IRD signal.