Rock magnetic detection of distal ice-rafted debries: clue for the identification of Heinrich layers on the Portuguese margin

摘要

High resolution measurements of concentration and grain size dependent magnetic parameters along a sedimentary sequence deposited on the Portuguese margin (38°N) during the last 120 ka allowed the detection of multiple thin layers slightly enriched by a coarse lithogenic fraction. Sizes, mineralogy, shapes and mode of deposition identify the material as ice-rafted detritus (IRD). The four most recent events dated by calibrated radiocarbon ages are contemporaneous with Heinrich events HE1-HE4. Older events can be identified as HE5, HE6 or with minor IRD that occurred during stage 5. The resolution and sensitivity obtained with rock magnetic methods enable the detection of multiple secondary events showing that iceberg delivery was a frequent phenomenon directly linked to temperature variations over ice caps; Heinrich events representing the extreme cases.