Imbrie, J., A. Berger, E. A. Boyle, S. C. Clemens, A. Duffy, W. R. Howard, G. Kukla, J. Kutzbach, D. G. Martinson, A. Mcintyre, A. C. Mix, B. Molfino, J. J. Morley, L. C. Peterson, N. G. Pisias, W. L. Prell, M. E. Raymo, N. J. Shackleton and J. R. Toggweiler, 1993: On the Structure and Origin of Major Glaciation Cycles .2. The 100,000-Year Cycle. Paleoceanography, 8(6): 699-735.
Climate over the past million years has been dominated by glaciation cycles with periods near 23,000, 41,000, and 100,000 years. In a linear version of the Milankovitch theory, the two shorter cycles can be explained as responses to insolation cycles driven by precession and obliquity. But the 100,000-year radiation cycle (arising from eccentricity variation) is much too small in amplitude and too late in phase to produce the corresponding climate cycle by direct forcing. We present phase observations showing that the geographic progression of local responses over the 100,000-year cycle is similar to the progression in the other two cycles, implying that a similar set of internal climatic mechanisms operates in all three. But the phase sequence in the 100,000-year cycle requires a source of climatic inertia having a time constant (similar to 15,000 years) much larger than the other cycles (similar to 5,000 years). Our conceptual model identifies massive northern hemisphere ice sheets as this larger inertial source. When these ice sheets, forced by precession and obliquity, exceed a critical size, they cease responding as linear Milankovitch slaves and drive atmospheric and oceanic responses that mimic the externally forced responses. In our model, the coupled system acts as a nonlinear amplifier that is particularly sensitive to eccentricity-driven modulations in the 23,000-year sea level cycle. During an interval when sea level is forced upward from a major low stand by a Milankovitch response acting either alone or in combination with an internally driven, higher-frequency process, ice sheets grounded on continental shelves become unstable, mass wasting accelerates, and the resulting deglaciation sets the phase of one wave in the train of 100,000-year oscillations.
Imbrie, J., A. C. Mix and D. G. Martinson, 1993: Milankovitch Theory Viewed from Devils Hole. Nature, 363(6429): 531-533.
VARIATIONS in the oxygen isotope content (deltaO-18) of late Quaternary deep-sea sediments mainly reflect changes in continental ice mass1, and hence provide important information about the timing of past ice ages. Because these sediments cannot yet be dated directly beyond the range of radiocarbon dating (40-50 kyr), ages for the deltaO-18 record have been generated2,3 by matching the phase of the changes in deltaO-18 to that of variations in the Earth's precession and obliquity. Adopting this timescale yields a close correspondence between the time-varying amplitudes of these orbital variations and those of a wide range of climate proxies4, lending support to the Milankovitch theory that the Earth's glacial-interglacial cycles are driven by orbital variations. Recently Winograd et al.5 reported a record of deltaO-18 variations in a fresh-water carbonate sequence from Devils Hole, Nevada, dated by U-Th disequilibrium6. They concluded that the timing of several of the features in the record, which reflects changes in the temperature of precipitation over Nevada as well as changes in the isotopic composition of the moisture source5,7, showed significant deviations from that predicted by Milankovitch theory. Here we demonstrate that applying the Devils Hole chronology to ocean cores requires physically implausible changes in sedimentation rate. Moreover, spectral analysis of the Devils Hole record shows clear evidence of orbital influence. We therefore conclude that transfer of the Devils Hole chronology to the marine record is inappropriate, and that the evidence in favour of Milankovitch theory remains strong.
Wamser, C. and D. G. Martinson, 1993: Drag Coefficients for Winter Antarctic Pack Ice. Journal of Geophysical Research-Oceans, 98(C7): 12431-12437.
This paper presents air-ice and ice-water drag coefficients referenced to 10-m-height winds for winter Antarctic pack ice based on measurements made from R/V Polarstern during the Winter Weddell Sea Project, 1986 (WWSP-86), and from R/V Akademik Fedorov during the Winter Weddell Gyre Study, 1989 (WWGS-89). The optimal values of the air-ice drag coefficients, made from turbulent flux measurements, are C-10 = (1.79 +/- 0.06) x 10(-3) for WWSP-86 and (1.45 +/- 0.09) x 10(-3) for WWGS-89. Neutral drag coefficient values are C(N10) = 1.68 x 10(-3) for WWSP-86 and 1.44 x 10(-3) for WWGS-89. The slightly lower values for WWGS-89 reflect a smaller surface roughness (z0) attributed to the thicker snow cover present in the 1989 study region (median z0BAR = 0.47 mm for WWSP-86 and 0.27 mm for WWGS-89). These values are consistent with Arctic measurements for 80-100% concentration of sea ice and with those of Andreas et al. (this issue) for the Antarctic. A single (average) ice-water drag coefficient for both WWSP-86 and WWGS-89, estimated from periods of ice drift thought to represent free-drift conditions (air-ice stress balanced by ice-water drag and Coriolis force), is (1.13 +/- 0.26) x 10(-3), and the ice-water turning angle betaBAR = 18 +/- 18-degrees. This drag value is significantly lower than Arctic values for thick multiyear ice, but it is similar to the values obtained by Langleben (1982) for first-year Arctic ice. Consistent with previous findings for WWSP-86, the free-drift form of the momentum balance can be used to describe the observed WWGS-89 ice drift observations by using an ''effective'' drag coefficient and turning angle that subsume the influence of ice-ice interaction. For a typical Antarctic winter pack ice cover, it appears that the ice cover reduces the momentum flux from the atmosphere to the ocean by approximately 33%.
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