HEINRICH EVENTS IN THE NORTH-ATLANTIC - RADIOCHEMICAL EVIDENCE

Heinrich events are a series of apparently synchronous sediment horizons in the North Atlantic with unusually high ratios of lithic fragments to foraminifera in the coarse fraction. They occur more prominently during cold climatic periods and could have been produced either by dramatic decreases in surface water productivity, by brief increases in carbonate dissolution on the seafloor, or by rapid accumulation of ice-rafted debris. The excess Th-230 activity profile in core CHN82 31 11PC (42-degrees 23'N, 31-degrees 48'W) provides evidence in support of the last of these explanations for the two most recent events (H1 and H2). Sediment flux reconstruction based on the constant Th-230 flux model indicates that sediment rain rates increased from an average of ca 1.6 g cm-2 ka-1 to at least 19 g cm-2 ka-1 during Heinrich event H1 and 11 g cm-2 ka-1 during Heinrich event H2. These estimates are lower limits, as bioturbation likely smoothed the Th-230 profile. Our data also suggest a maximum of 600 y for the duration of H1 and 800 y for H2. Two independent approaches provide similar estimates of sediment mass accumulation (g cm-2) at the site during the two events; ca 10 g cm-2 and ca 9 g cm-2 were deposited from the overlying water column during event H1 and H2. An additional ca 4 g cm-2 were brought to the site by sediment focusing during event H1. These methods provide a means of mapping and quantifying IRD mass accumulation over the North Atlantic for studying the processes that controlled the distribution of the Heinrich deposits.