Li Peiji
A continuous 500 kyr climate record from vein calcite in Devils Hole,Ncvada shows that the duration of the last four glacial cycles increased from 80 kyr.to 130 kyr.This variationindicates that major climate changes were aperiodic. Broecker(1992)questioned the validityof the Milankovitch theory.He suggested that althouth the date of the end of the penultimateice age(termination 11)was only 17 kyr.earlier than the previously accepted date of 128 kyr.this change is enough to bring the Milankovitch mechanism into serious deubt. Shackleton (1993) refuted the argument.He commented that the discrepancy arises dat-ing of the Devils Hole calcite vein by measuring the growth of 230Th from its parent 234U,because 230Th is produced by decay ofdissolved 234U in the water. Additionally, the value of 140±3 ka was deteimined by interpolation between the closest directly dated subsamples(132 ±3 and 150±3ka)。Imbrie(1993)demonstrated that applying the Devils Hole chronology to ocean cores requires physically implausible changes in sedimentation rate,therefor transfer it to the marinerecord is inappropriate,And the evidence in favour ofmilankovith theory remains strong. Emiliani(1993)suggested that terminations,transitional episodes which extend throughtime,are poor time markers for the correlation of pleistocene section, The maxima(hypsithermals)and especially the minima(bathytherinals)in the isotope curves are muchsharuer and thus afford a more precise correlation. The astronomical parameters can triggeronly the initiation of an ice age,further development of glaciation and deglaciation is drivenby the internal dynamics of the ice-ocean-atmosphere system.The timing of high sea levels over the past 200 kyr.(Gallup et al.,1994)concluded thatfor the last three interglacial and two intervening interstadial rteriods,sea level peaked at orafter peaks in summer insolation in the Northern Hemisphere. This overall patternsupportsthe idea that glacial-interglacial cycles are caused by changes in Earth’s orbital geometry.Crowley suggests that both the Devils Hole and deep-sea chronologies may be correct,with differences between 130 ka and 150 ka years ago reflecting a regionally varying to cli-mate shifts associated with a very extensive glaciation. And extending the vostok ice-core record back to the penultimate glacial period(～140-200 ka ago)yields a new glaciologicaltimescale which is consistent with ocean records.
