CH4 and δ18O of O2 records from Antarctic and Greenland ice: A clue for stratigraphic disturbance in the bottom part of the Greenland Ice Core Project and the Greenland Ice Sheet Project 2 ice cores

Jérôme Chappellaz,Bruno Malaizé,Thomas Blunier,Ed Brook

doi:10.1029/97jc00164

Abstract

The suggestion of climatic instability during the last interglacial period (Eem), based on the bottom 10% of the Greenland Ice core Project (GRIP) isotopic profile, has been questioned because the bottom record from the neighboring Greenland Ice Sheet Project 2 (GISP2) core (28 km away) is strikingly different over the same interval and because records of the δ18O of atmospheric O2 from both cores showed unexpected rapid fluctuations. Here we present detailed methane records from the Vostok (Antarctica), GRIP, and GISP2 cores over the relevant intervals. The GRIP and GISP2 data show rapid and large changes in methane concentration, which are correlative with variations of the δ18O of the ice, while the Vostok record shows no such variations. This discrepancy reinforces the suggestion that the bottom sections of the Greenland records are disturbed. By combining the methane data with measurements of δ18O of O2 in the same samples, we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples. Our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the GRIP core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation.

Highlights

Bycombinintghemethandeatawithmeasuremeonft/s51Oof 02 in thesamesamplesw,e attempto constrainthenatureof thestratigraphidcisturbancaendtheageof the analyzed ice samplesO. ur resultssuggestthatice layersfrompartof thelastinterglaciapl eriod existin the lower sectionof bothice coresandthat someof the apparentclimateinstabilities in the GreenlandIce Core Project (GRIP) core would be the result of a mixture of ice from the last interglacial with ice from the beginningof the lastglaciationor from the penultimateglaciation
First,asin GRIP,datapairscorresponditnogtheveryend warmingsandcoolingsin themethane-producirneggionsI.n of MIS-6 and to the beginningof marine isotopestage5e (MIS-5e) are not found contrastl,arge and fast CH4 changesare observedin the anywherein the GISP2 record.This strengthentshe GRIP andGISP2 recordsbelow2750 m of depth,whichare suggestioonf Benderet al. [1994b]thatGISP2missetshe correlativwe iththelargeandrapidvariationosf 158O. iIcne sectionbetween-130 and 140 kyr B.P. in theVostokEGT the absenceof anyknownartifactcausinga distortionof the chronology
Second,there are a number of data points, conclusionresachefdrom18Omartecordsw, ebelievethatthe from the > 3000 m section of GISP2, with methane stratigraphicsequenceof the GRIP and GISP2 coresis concentrationbsetween520 and 680 ppbv and 18Omat altered

Summary

Introduction

Bycombinintghemethandeatawithmeasuremeonft/s51Oof 02 in thesamesamplesw,e attempto constrainthenatureof thestratigraphidcisturbancaendtheageof the analyzed ice samplesO. ur resultssuggestthatice layersfrompartof thelastinterglaciapl eriod existin the lower sectionof bothice coresandthat someof the apparentclimateinstabilities in the GRIP core would be the result of a mixture of ice from the last interglacial with ice from the beginningof the lastglaciationor from the penultimateglaciation. One of the mostpuzzlingresultsfrom the GreenlandIce Core Project (GRIP) ice core drilled at Summit (central Greenland)concernsthe behaviorof the Eem Interglaciation Vostok record at the very end of MIS-6, from -140 to 130 kyr B.P. The GRIP 518Oastmignalover the last studiedinitially with samplesfrom the Vostok 3G ice core deglaciationagreeswell with similarrecordsfrom GISP2, [Chappellazet al., 1990].

Results

Conclusion