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

Recent well-dated isotopic (Sr-Nd) fingerprinting of Heinrich layer ice-raf ted detritus (IRD) on the European margin indicates supply from European ic e 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. Grou sset et al., Geology 28 (2000) 123-126] and have been interpreted to indica te 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 contai ns detrital Campanian Upper Chalk deriving from bedrock sources eroded on t he Celtic shelf by the British Ice Sheet (BIS) in addition to lithic materi al sourced from the LIS. High-resolution radiocarbon chronology indicates c halk grain deposition as discrete pulses both before and coincident with su pply of LIS-sourced detritus. The specificity of the chalk fingerprint to t he BIS enables a 700-1000 yr lag between the BIS and LIS events to be ident ified. This phasing indicates a more rapid response of the outlet lobes dra ining the smaller BIS than those draining the LIS and implicates external c limatic forcing of Heinrich events. It is unlikely that this precursor even t represents IRD event is, the recently identified 1-2 ka IRD cycle event w hich immediately precedes H2, because the lag between precursor and main ev ent 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 r ise 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.