The origin of Heinrich layers: evidence from H2 for European precursor events

Recent well-dated isotopic (Sr-Nd) fingerprinting of Heinrich layer ice-rafted detritus (IRD) on the European margin indicates supply from European ice sheets as precursors to Laurentide Ice Sheet (LIS) supply [F.E. Grousset et al., Geology 28 (2000) 123-126, H. Snoeckx et al., Mar. Geol. 158 (1999) 197-208]. These precursor events lead LIS input by up to 1.5 ka [F.E. Grousset et al., Geology 28 (2000) 123-126] and have been interpreted to indicate LIS collapse during Heinrich events stimulated by events originating on the European side of the Atlantic [F.E. Grousset et al., Geology 28 (2000) 123-126]. Such phasing of IRD supply from different sources within Heinrich layers therefore has implications for the origin and mechanics of Heinrich events. We present evidence here that the IRD comprising Heinrich layer 2 (H2; similar to 20-21 C-14 ka BP) on the European continental margin contains detrital Campanian Upper Chalk deriving from bedrock sources eroded on the Celtic shelf by the British Ice Sheet (BIS) in addition to lithic material sourced from the LIS. High-resolution radiocarbon chronology indicates chalk grain deposition as discrete pulses both before and coincident with supply of LIS-sourced detritus. The specificity of the chalk fingerprint to the BIS enables a 700-1000 yr lag between the BIS and LIS events to be identified. This phasing indicates a more rapid response of the outlet lobes draining the smaller BIS than those draining the LIS and implicates external climatic forcing of Heinrich events. It is unlikely that this precursor event represents IRD event is, the recently identified 1-2 ka IRD cycle event which immediately precedes H2, because the lag between precursor and main event is here less than 1.5 ka and because such pervasive periodicity is not apparent in European continental margin IRD records. The later synchroneity between the BIS and LIS input in H2 identifies glacio-eustatic sea-level rise associated with LIS discharges as a possible feedback mechanism causing destabilisation of ice streams elsewhere during Heinrich events. (C) 2000 Elsevier Science B.V. All rights reserved.