Description: The strength of the North Atlantic Meridional Overturning Circulation during climatically highly variable Marine Isotope Stage (MIS) 3 has attracted much attention in recent years. Here we present highresolution Nd isotope compositions of past seawater derived from authigenic Fe-Mn oxyhydroxides recovered from drift sediments on the Blake Ridge in the deep western North Atlantic (ODP Leg 172, Site 1060, 3481 m water depth). The data cover the period from 45 to 35 ka BP, tracing circulation changes during major Heinrich iceberg discharge event 4 (H4, ~40-39kaBP), The Nd isotope record suggests that there was no northern source water (NSW) mass like modern NADW at the deeper part of Blake Ridge at any time between 45 and 35 ka. This is fundamentally different from the hydrographic situation during the Holocene where NADW extends below 4500 m at this location. The εNd of past deep water recorded in the Blake Ridge sediments was least radiogenic during Dansgaard/Oeschger (D/O) Interstadial (IS) 8 (εNd = -11.3)and most radiogenic immediately preceding IS 9 (εNd = -9.8). More radiogenic compositions were also recorded during H4 (-10.2 ≤ εNd ≤ 9.9). The Nd isotope variability in MIS 3 matches that of a physical bottom current strength reconstruction from the same location. Neither record follows the pattern of Northern Hemisphere D/O climatic cycles. In our record, reduced mixing with northern source waters started in stadial 12 and lasted until after H4 in stadial 9, followed by a rapid increase in NSW contribution thereafter. This major change in the Nd isotope record predates the iceberg discharge event Heinrich 4 by more than 3 ka indicating a shallowing of the water mass boundary between Glacial North Atlantic Intermediate Water and Southern Source Water beneath. This early change in bottom water properties at the deep Blake Ridge suggests that North Atlantic deep water advection may already have decreased several thousand years before the actual iceberg discharge event and associated freshening of the surface waters in the North Atlantic. The change can thus not be attributed to climatic events in the North Atlantic but may be related to changes in flux of deep water from the South.

Description: It has been suggested that salinity increases in the subtropical gyre system may have pre-conditioned the North Atlantic Ocean for a rapid return to stronger overturning circulation and high-latitude warming following meltwater events during the Last Glacial period (Schmidt et al., 2006). Planktonic foraminifera oxygen isotopes (δ18O) measured on the surface dwelling Globigerinoides ruber show a positive gradient going from the Gulf Stream into the subtropical gyre during the Holocene as well as the Last Glacial Maximum (Keigwin, 2004). This gradient is due to decreasing temperatures, increasing salinity and a change from a summer to year round occurrence of G. ruber(Keigwin, 2004). During rapid climate oscillations between 54 and 46 ka of Marine Isotope Stage 3, including Heinrich ice-rafting event 5, this gradient may have been largely absent; G. ruber δ18O of ODP Site 1060 (subtropical gyre location) and ODP Site 1056 (Gulf Stream location) are virtually identical. Lower G. ruber δ18O that characterize the major warm DO interstadials 14 and 12 suggest a mainly summer occurrence of this species and indicate a more zonal and wider Gulf Stream influencing both ODP Sites. A large vertical δ18O gradient between shallow dwelling G. ruber and the deep dwelling species Globorotalia inflataat site 1056 is associated with strong summer stratification, supporting a Gulf Stream presence during these major interstadials. From about 51 ka until the end of Heinrich event 5 G. ruber δ18O is increased and the δ18O gradient between G. ruber and G. inflata is greatly reduced, suggesting a more year round occurrence of G. ruber and absence of summer stratification at site 1056, associated with subtropical gyre conditions. From this we infer that the Gulf Stream may have taken up a position nearer to the continental shelf during this cold period, where it may have been narrower with reduced transport.

Description: The strength of the North Atlantic Meridional Overturning Circulation during climatically highly variable Marine Isotope Stage (MIS) 3 has attracted much attention in recent years. Here we present high-resolution Nd isotope compositions of past seawater derived from authigenic Fe-Mn oxyhydroxides recovered from drift sediments on the Blake Ridge in the deep western North Atlantic (ODP Leg 172, Site 1060, 3481 m water depth). The data cover the period from 45 to 35 ka BP, tracing circulation changes during major Heinrich iceberg discharge event 4 (H4, ~40-39 ka BP). The Nd isotope record suggests that there was no northern-source water (NSW) mass like modern NADW at the deeper part of Blake Ridge at any time between 45 and 35 ka. This is fundamentally different from the hydrographic situation during the Holocene where NADW extends below 4500 m at this location. The epsilon-Nd of past deep water recorded in the Blake Ridge sediments was least radiogenic during Dansgaard/Oeschger (D/O) Interstadial (IS) 8 (epsilon-Nd = -11.3) and most radiogenic immediately preceding IS 9 (epsilon-Nd = -9.8). More radiogenic compositions were also recorded during H4 (-10.2 〈= epsilon-Nd 〈= -9.9). The Nd isotope variability in MIS 3 matches that of a physical bottom current strength reconstruction from the same location. Neither record follows the pattern of Northern Hemisphere D/O climatic cycles. In our record, reduced mixing with northern source waters started in stadial 12 and lasted until after H4 in stadial 9, followed by a rapid increase in NSW contribution thereafter. This major change in the Nd isotope record predates the iceberg discharge event Heinrich 4 by more than 3 ka indicating a shallowing of the water mass boundary between Glacial North Atlantic Intermediate Water and Southern Source Water beneath. This early change in bottom water properties at the deep Blake Ridge suggests that North Atlantic deep water advection may already have decreased several thousand years before the actual iceberg discharge event and associated freshening of the surface waters in the North Atlantic. The change can thus not be attributed to climatic events in the North Atlantic but may be related to changes in flux of deep water from the South.