A GEOCHEMICAL CRITERION FOR THE RECOGNITION OF HEINRICH EVENTS AND ESTIMATION OF THEIR DEPOSITIONAL FLUXES BY THE (TH-230(EXCESS))(0) PROFILING METHOD

A core from the Armorican Seamount(46 degrees 23.8'N, 12 degrees 32.8' W; 3849 m), in the deep NE Atlantic southwest of the British Isles, exhibits unusual Mg/Al ratios in the sediments deposited during the last glacial period. High Mg/Al ratio values are restricted within layers of less than or equal to 14 cm thick, and mark the presence of ice-rafted detrital material deposited by Heinrich events H1, H2 and H4. These peaks in the ratio are caused by local increases in the dolomite, quartz and feldspar content and concomitant dilutions in the phyllosilicate content, which overall increase Mg and decrease Al. The known times of the Heinrich events provide a chronology for the core, and indicate a considerably slower mean sediment accumulation rate during and after the glacial/interglacial transition (2.2 cm ky(-1)) than in earlier glacial times (mean 5.6 cm ky(-1)). Within the glacial, however, (Th-230(excess))(0) profiling demonstrates that accumulation fluxes were at least 7 X higher than normal during the deposition of Heinrich events (up to 25 g cm(-2) ky(-1) versus a background glacial level of 3.7 g cm(-2) ky(-1)). Because of post-depositional bioturbation, peak fluxes are likely to have been even higher. The (Th-230(excess))(0) method also reveals the presence of event H3, which bears minor dolomite and is the least well developed of the last four Heinrich events. Despite the marked compositional and flux variations, the total mass deposited at this site is estimated at only 2.4, 4.9 and > 7.9 g cm(-2) for events H1, H2 and H4 respectively, These inventories were deposited over maximum periods of 2.0, 1.1 and > 0.7 ky respectively, but delivery could have been much more rapid (of the order of a few hundred years).