<P> Based upon solid geological evidence, consisting largely of the analysis of numerous deep cores from coral reefs, variations in the rates of sea level rise have been reconstructed for the postglacial period . For the early part of the sea level rise that is associated with deglaciation, three major periods of accelerated sea level rise, called meltwater pulses, occurred . They are commonly called meltwater pulse 1A0 for the one between 19,000 and 19,500 calendar years ago; meltwater pulse 1A for the one between 14,600 and 14,300 calendar years ago and meltwater pulse 1B for the one 11,400 and 11,100 calendar years ago . The Younger Dryas occurred after meltwater pulse 1A, a 13.5 m rise over about 290 years, centered at about 14,200 calendar years ago, and before meltwater pulse 1B, a 7.5 m rise over about 160 years, centered at about 11,000 calendar years ago . Finally, not only did the Younger Dryas postdate both all of meltwater pulse 1A and predate all of meltwater pulse 1B, it was a period of significantly - reduced rate of sea level rise relative to the periods of time immediately before and after it . </P> <P> Possible evidence of short - term sea level changes has been reported for the beginning of the Younger Dryas . First, the plotting of data by Bard and others suggests a small step, less than 6 m, in sea level near the onset of the Younger Dryas . There is a possible corresponding change in the rate of change of sea level rise seen in the data from both Barbados and Tahiti . Given that this change is "within the overall uncertainty of the approach," it was concluded that a relatively smooth sea - level rise, with no significant accelerations, occurred then . Finally, research by Lohe and others in western Norway has reported a sea - level low - stand at 13,640 calendar years ago and a subsequent Younger Dryas transgression starting at 13,080 calendar years ago . They concluded that the timing of the Allerød low - stand and the subsequent transgression were the result of increased regional loading of the crust, and geoid changes were caused by an expanding ice sheet, which started growing and advancing in the early Allerød about 13,600 calendar years ago, well before the start of the Younger Dryas . </P> <P> The prevailing theory is that the Younger Dryas was caused by significant reduction or shutdown of the North Atlantic "Conveyor", which circulates warm tropical waters northward, in response to a sudden influx of fresh water from Lake Agassiz and deglaciation in North America . Geological evidence for such an event is so far lacking . The global climate would then have become locked into the new state until freezing removed the fresh water "lid" from the North Atlantic . An alternative theory suggests instead that the jet stream shifted northward in response to the changing topographic forcing of the melting North American ice sheet, which brought more rain to the North Atlantic, which freshened the ocean surface enough to slow the thermohaline circulation . </P> <P> There is also some evidence that a solar flare may have been responsible for the megafaunal extinction, but that cannot explain the apparent variability in the extinction across all continents . </P>

Which of the following is not a presumed cause of ice ages
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