If varying orbital parameters were to result in ice growth on the Antarctic craton, this was most likely to have occurred when summer insolation is minimised either by the seasonal timing of aphelion (precession) during periods of relatively high eccentricity, periods of low eccentricity which minimised the effects of precession, even when perihelion occurs during austral summer, or during periods of reduced amplitude in obliquity. All of these orbital configurations result in relatively cool summers and hence reduce the potential for summer melt. Zachos et al. (2001b) measured oxygen isotope ratios from Ocean Drilling Programme Equatorial Atlantic (Ceara Rise) Site 929 for a ~5myr interval spanning the Oligocene-Miocene boundary. The Mil event was found to correlate with both a minima in the low frequency (400 ky) eccentricity cycle and a prolonged minima in the amplitude of obliquity, a co-occurrence with a reoccurrence interval of 2.4 myr or longer (Palike et al., 2004, 2006). This sustained period of unusually low seasonality (cold summers) was, however, relatively transient with warmer summers returning within a few hundred thousand years from the coincidence of increased eccentricity and high-amplitude variability in obliquity which would have resulted in warmer polar summers and increased summer melt (Zachos et al., 2001b).
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