Deep sea sediment records from North Atlantic cores (40°N-55°N) provide evidence of several massive iceberg discharges, known as Heinrich events, during the last glacial period. High resolution benthic [delta]18O and [delta]13C records from North Atlantic sediment cores were used to monitor the impact of Heinrich events on thermohaline circulation and to estimate the sensitivity of deep oceanic circulation to changes in freshwater input to the North Atlantic surface waters. Our data indicate that major rearrangements of deep-water masses were directly associated with these massive iceberg discharges. To trace in detail the deep water conditions in the North Atlantic, benthic [delta]13C values in several cores were used to generate time slices before, during and after Heinrich event 4 dated at ~ 35 ka BP. Although North Atlantic Deep Water continued to form during the oxygen isotope stage 3 at 37 ka BP, deep circulation was characterized by an increased incursion of deep waters of southern origin, which reduced the [delta]13C composition of North Atlantic deep waters, particularly in the eastern Atlantic basin. North Atlantic Deep Water production was reduced during Heinrich Layer 4 (HL4 at ~ 35 ka BP) synchronously with the changes in the surface water hydrology. Deep convection processes may have occurred in areas not affected by the salinity decrease. Soon after HL4 ( ~ 33 ka BP) the [delta]13C distribution was similar to that before the event. Similarly, a rapid return to initial [delta]13C values was observed at the end of the most clearly defined Heinrich events (HL5, HL4 and HL1). Comparison between deep circulation patterns corresponding to HL4, the Last Glacial Maximum and Heinrich event 1 indicates that each of these periods was characterized by a different circulation state associated with changes in convection sites.
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