A Pervasive 1470-Year Climate Cycle in North Atlantic Glacials and
Interglacials: A Product of Internal or External Forcing?
Gerard C. Bond (Lamont-Doherty Earth Observatory, Rt. 9w, Palisades, N.Y., 10964; ph. 914-365-8478; fax 915-365-8154; Internet: firstname.lastname@example.org. columbia. edu)
New evidence from deep sea piston cores in the eastern and western subpolar North Atlantic suggests that regional climate underwent rapid sub-Milankovitch variability, not only during the last glaciation, as has been previously documented on a global scale, but also during the present interglacial (Holocene) and the previous interglacial (stage 5e). The evidence consists of recurring shifts in lithic grain concentrations, lithic grain petrology and percentages of foraminiferal species. Amplitudes of this cycle during interglacials are much smaller than during glacials, typically by a factor of 2 to 3 in temperature and by more than one order of magnitude in amounts of ice rafted debris.
Three features are especially noteworthy in our records. First, we find a persistent quasi-periodic cycle with a mean pacing of 1470 years in both glacials and interglacials, demonstrating that climate on that time scale oscillated independently of ice volumes. During the last glacial, especially stage 3, Dansgaard-Oeschger shifts were locked into the quasiperiodic cycle, suggesting that cycle and the D/O shifts had a common origin. Second, during the last glaciation, climatically sensitive portions of glaciers, such as the St. Lawrence ice stream and the Icelandic ice cap, underwent increases in calving rates in response to each cooling phase of the 1470-year cycle. Finally, all but Heinrich events 3 and 6 are immediately preceded by the cold phase of a 1470-year event, implying that during most Heinrich events, massive collapses of Laurentide ice in Hudson Strait were in some way triggered by the cooling phases of the pervasive climate cycle. Clearl,y, the 1470-year cycle appears to have exerted a major controlling influence on the sub-Milankovitch climate variability which has been revealed in the North Atlantic's glacial and interglacial records.
The origin of the 1470-year cycle is far from clear. Its persistence across glacial- interglacial boundaries is evidence that it cannot have been produced by any internal process involving ice-sheet instabilities. On the other hand, the cycle pacing is close to the overturning time of the ocean, raising the possibility that it arises from an internal oscillation within the ocean's circulation. External processes, such as solar forcing and harmonics of the orbital periodicities cannot be ruled out, but are, at least presently, difficult to test.