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their first decrease at a depth 30 cm below that of N. pachyderma.. A.M.S. 14C dates demonstrate that this 30 cm lead of the G. bulloides record over the N. pachyderma record is an artifact of bioturbation. The inconsistancy between the two isotopic records roughly disappears when both records are deconvolved and plotted with respect to the '4 C ages measured on the same foraminiferal species (Fig. 2B). This time scale places the beginning of the deglaciation between 15,000 yr B.P. and 14,500 yr B.P. and basically fits that of core SU 81-18. The abundances of fourteen foraminiferal species have been deconvolved with the same bioturbation parameters which successfully explain the discrepancies between the isotopic records. The transfer function has been applied to these restored foraminiferal data in order to generate an unmixed S.S.T. record (Fig. 2C), which exhibits the same trends as the raw S.S.T. estimates but emphasizes the large amplitude of the temperature changes betwen 15,000 yr B.P. and 10,000 yr B.P.. The comparison of the paleoclimatic records of core CH 73-139 C and SU 81-18 may be used to estimate the rate of climatic change and the kinematics of the retreat of the cold water mass in the high latitude North Atlantic. The polar front is defined as the steep thermal gradient which separate the warm Gulf Stream waters from the cold polar water mass. The CLIMAP reconstruction (6) shows that core SU 81-18 is located in the southern part of the polar front during the last glacial maximum. Assuming that the passage of the polar front across the core will constitute the dominant thermal event within the general deglacial warming, A.M.S. 14C ages of G. bulloides indicate that its retreat from the latitude of Portugal is dated at about 12,500 yr B.P. (7). A.M.S. 14 C ages measured on G. bulloides between the levels 130 and 160 cm in core CH 73-139 C demonstrate that those levels correspond in fact to the dating of a single "pulse-like" abundance maximum for this species, of which shells have been bioturbated between 130 and 160 cm. The four radiocarbon ages are not statistically different and correspond to a mean value of 11,540 ± 170 yr B.P., which dates the invasion of warmer waters in the North Atlantic at 55°N. We therefore assume that this event marks the passage of the polar front at the location of core CH-73-139 C. We thus conclude that the North Atlantic polar front began its retreat at about 12,500 yr B.P. at the latitude of Portugal and reached the latitude of Ireland almost 1,000 years later, retreating with a mean velocity of about 2 km/year. Recently however, Broecker (pers. comm.) measured by A.M.S. an age of about 12,500 yr B.P. for the first warming phase in a core close to core CH 73-139 C. This discrepancy with our estimate may be due either to an artifact of bioturbation in Broecker's core or to a small sedimentation hiatus in core CH 73-139 C. The retreat velocity of the polar front that we determined is therefore a minimal value. The cooling associated with the Younger Dryas cold event and the warming which followed have been synchroneous (± 400 years) between 37°N and 55°N, since we do not observe any significant age difference for the temperature variations recorded in both cores. This favours theories explaining the Younger Dryas by instabilities within the climatic system itself, e.g. a catastrophic influx of polar ice into the North Atlantic. The high velocity of the advance and the retreat of cold waters suggests that these temperature changes had affected only the most surficial waters and not the whole hydrologic structure of the North Atlantic ocean. PENETRATION OF BOMB RADIOCARBON IN THE OCEAN Under natural conditions, the distribution of 14 C in the deep ocean is influenced by many processes. Bottom water formation in the Norwegian Sea and the Weddell
DOI link for their first decrease at a depth 30 cm below that of N. pachyderma.. A.M.S. 14C dates demonstrate that this 30 cm lead of the G. bulloides record over the N. pachyderma record is an artifact of bioturbation. The inconsistancy between the two isotopic records roughly disappears when both records are deconvolved and plotted with respect to the '4 C ages measured on the same foraminiferal species (Fig. 2B). This time scale places the beginning of the deglaciation between 15,000 yr B.P. and 14,500 yr B.P. and basically fits that of core SU 81-18. The abundances of fourteen foraminiferal species have been deconvolved with the same bioturbation parameters which successfully explain the discrepancies between the isotopic records. The transfer function has been applied to these restored foraminiferal data in order to generate an unmixed S.S.T. record (Fig. 2C), which exhibits the same trends as the raw S.S.T. estimates but emphasizes the large amplitude of the temperature changes betwen 15,000 yr B.P. and 10,000 yr B.P.. The comparison of the paleoclimatic records of core CH 73-139 C and SU 81-18 may be used to estimate the rate of climatic change and the kinematics of the retreat of the cold water mass in the high latitude North Atlantic. The polar front is defined as the steep thermal gradient which separate the warm Gulf Stream waters from the cold polar water mass. The CLIMAP reconstruction (6) shows that core SU 81-18 is located in the southern part of the polar front during the last glacial maximum. Assuming that the passage of the polar front across the core will constitute the dominant thermal event within the general deglacial warming, A.M.S. 14C ages of G. bulloides indicate that its retreat from the latitude of Portugal is dated at about 12,500 yr B.P. (7). A.M.S. 14 C ages measured on G. bulloides between the levels 130 and 160 cm in core CH 73-139 C demonstrate that those levels correspond in fact to the dating of a single "pulse-like" abundance maximum for this species, of which shells have been bioturbated between 130 and 160 cm. The four radiocarbon ages are not statistically different and correspond to a mean value of 11,540 ± 170 yr B.P., which dates the invasion of warmer waters in the North Atlantic at 55°N. We therefore assume that this event marks the passage of the polar front at the location of core CH-73-139 C. We thus conclude that the North Atlantic polar front began its retreat at about 12,500 yr B.P. at the latitude of Portugal and reached the latitude of Ireland almost 1,000 years later, retreating with a mean velocity of about 2 km/year. Recently however, Broecker (pers. comm.) measured by A.M.S. an age of about 12,500 yr B.P. for the first warming phase in a core close to core CH 73-139 C. This discrepancy with our estimate may be due either to an artifact of bioturbation in Broecker's core or to a small sedimentation hiatus in core CH 73-139 C. The retreat velocity of the polar front that we determined is therefore a minimal value. The cooling associated with the Younger Dryas cold event and the warming which followed have been synchroneous (± 400 years) between 37°N and 55°N, since we do not observe any significant age difference for the temperature variations recorded in both cores. This favours theories explaining the Younger Dryas by instabilities within the climatic system itself, e.g. a catastrophic influx of polar ice into the North Atlantic. The high velocity of the advance and the retreat of cold waters suggests that these temperature changes had affected only the most surficial waters and not the whole hydrologic structure of the North Atlantic ocean. PENETRATION OF BOMB RADIOCARBON IN THE OCEAN Under natural conditions, the distribution of 14 C in the deep ocean is influenced by many processes. Bottom water formation in the Norwegian Sea and the Weddell
their first decrease at a depth 30 cm below that of N. pachyderma.. A.M.S. 14C dates demonstrate that this 30 cm lead of the G. bulloides record over the N. pachyderma record is an artifact of bioturbation. The inconsistancy between the two isotopic records roughly disappears when both records are deconvolved and plotted with respect to the '4 C ages measured on the same foraminiferal species (Fig. 2B). This time scale places the beginning of the deglaciation between 15,000 yr B.P. and 14,500 yr B.P. and basically fits that of core SU 81-18. The abundances of fourteen foraminiferal species have been deconvolved with the same bioturbation parameters which successfully explain the discrepancies between the isotopic records. The transfer function has been applied to these restored foraminiferal data in order to generate an unmixed S.S.T. record (Fig. 2C), which exhibits the same trends as the raw S.S.T. estimates but emphasizes the large amplitude of the temperature changes betwen 15,000 yr B.P. and 10,000 yr B.P.. The comparison of the paleoclimatic records of core CH 73-139 C and SU 81-18 may be used to estimate the rate of climatic change and the kinematics of the retreat of the cold water mass in the high latitude North Atlantic. The polar front is defined as the steep thermal gradient which separate the warm Gulf Stream waters from the cold polar water mass. The CLIMAP reconstruction (6) shows that core SU 81-18 is located in the southern part of the polar front during the last glacial maximum. Assuming that the passage of the polar front across the core will constitute the dominant thermal event within the general deglacial warming, A.M.S. 14C ages of G. bulloides indicate that its retreat from the latitude of Portugal is dated at about 12,500 yr B.P. (7). A.M.S. 14 C ages measured on G. bulloides between the levels 130 and 160 cm in core CH 73-139 C demonstrate that those levels correspond in fact to the dating of a single "pulse-like" abundance maximum for this species, of which shells have been bioturbated between 130 and 160 cm. The four radiocarbon ages are not statistically different and correspond to a mean value of 11,540 ± 170 yr B.P., which dates the invasion of warmer waters in the North Atlantic at 55°N. We therefore assume that this event marks the passage of the polar front at the location of core CH-73-139 C. We thus conclude that the North Atlantic polar front began its retreat at about 12,500 yr B.P. at the latitude of Portugal and reached the latitude of Ireland almost 1,000 years later, retreating with a mean velocity of about 2 km/year. Recently however, Broecker (pers. comm.) measured by A.M.S. an age of about 12,500 yr B.P. for the first warming phase in a core close to core CH 73-139 C. This discrepancy with our estimate may be due either to an artifact of bioturbation in Broecker's core or to a small sedimentation hiatus in core CH 73-139 C. The retreat velocity of the polar front that we determined is therefore a minimal value. The cooling associated with the Younger Dryas cold event and the warming which followed have been synchroneous (± 400 years) between 37°N and 55°N, since we do not observe any significant age difference for the temperature variations recorded in both cores. This favours theories explaining the Younger Dryas by instabilities within the climatic system itself, e.g. a catastrophic influx of polar ice into the North Atlantic. The high velocity of the advance and the retreat of cold waters suggests that these temperature changes had affected only the most surficial waters and not the whole hydrologic structure of the North Atlantic ocean. PENETRATION OF BOMB RADIOCARBON IN THE OCEAN Under natural conditions, the distribution of 14 C in the deep ocean is influenced by many processes. Bottom water formation in the Norwegian Sea and the Weddell
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