Feb 27, 2000 - Twelve glacial dust samples extracted from the Greenland Ice Core ... The clay mineralogy of the ice core dust shows a weak climatic de-.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D4, PAGES 4637-4656, FEBRUARY 27, 2000
Characterization of late glacial continental dust in the Greenland Ice Core Project ice core Anders Svensson Departmentof Geophysics,Universityof Copenhagen, Denmark
PierreE. Biscaye Lamont-DohertyEarth Observatoryof ColumbiaUniversity,Palisades,New York
FrancisE. Grousset• D6partementde G6o!ogieet Oc6anographie, Universit6Bordeaux1, Talence,France
Abstract. Twelveglacialdustsamplesextractedfrom the GreenlandIce CoreProjectice corehavebeencharacterized in termsof mineralogy,Sr andNd isotopiccomposition,and rareearthelementconcentrations. The ice samplesare takenfrom the YoungerDryas, the BOlling,the LastGlacialMaximum,andbothcold andmild glacialperiodsback to 44 kyr B.P. A comparisonwith samplesfrom possibledustsourceareasdetermineseasternAsia as beingthemain sourceareaat all times,herebyconfirmingandextendingearlierresults (Biscayeet al., 1997). This findingis partlyin agreementwith recentatmosphericgeneral circulationmodelsimulationsof glacialatmospheric dusttransport(Andersenet al., 1998; Mahowaldet al., 1999). A low intersamplevariabilityin tracercharacteristics is discussed in thecontextof a volcanogenic influence,grainsizedependence, andvariabilityin the continentaldustsourceareas.The clay mineralogyof the ice coredustshowsa weak climatic dependence,while the isotopiccompositionof Sr andNd doesnot correlatewith climate.This is interpretedas a geographicalvariationof sourceareason a regionalscalewithin Asia. Two ice dustsamplesweresize-fractionated at 2 gm, andthe grainsizedependence on tracer characteristics is in accordance with the literature.
trast,duringthe lastglacialperiodthe generallycoldclimate is interruptedby significantlymilder interstadials(IS), the
1. Introduction 1.1. Dust in the Greenland
Ice Sheet
The two deep ice cores drilled 28 km apart at Summit, Greenland,the GreenlandIce Core Project (GRIP) and the GreenlandIce SheetProject 2 (GISP2) ice cores,have provided the most detailed paleoclimaticice core recordsfrom the Northern Hemisphere (Greenland Summit ice cores, GreenlandIce SheetProject2 and GreenlandIce Core Project, Journal of GeophysicalResearch,102(C12), 1997). The GRIP ice core [GreenlandIce Core Project (GRIP) Members, 1993] coversmore than 115 kyr of continuoussnow accumulation andpossiblyanother150 kyr for which the stratigraphy is disturbedto somedegree[Johnsenet al., 1997]. Throughout the core the concentration of insolublemicroparticles,or dust,in the ice showsa closerelationshipto the climate signal. During the Holoceneperiod, which is characterizedby a
Dansgaard-Oeschger events,andthe dustconcentration varies upward from the Holocene value over almost 2 orders of magnitude(Figure 1). Generally,cold climateconditionsare characterizedby a high dust concentrationin the ice, while
milder periodsare accompanied by lower concentrations. AroundtheLastGlacialMaximum(LGM), -23-21 kyr B.P.,
/5180 reaches itsminimum, andthedustconcentration displays anextreme variation andreaches values closeto 10mgkg'1. In the previousinterglacialperiod,the Eemian,-125-115 kyr B.P., where the climate was comparableto that of the Holoceneclimate, the dust concentrationwas as low as during the Holoceneperiod [Steffensen,1997]. The dustprofile thusexhibits variationboth on a long timescale,associatedwith the glacialcycle, and on a shortertimescalerelatedto the abrupt climaticeventsoccurringduringthe last glacialperiod.In ad-
relatively stable/5180 signal, thedustconcentration is gener- dition, the dust concentration has a seasonal variation et al., 1978].Although notasregular asthe/51•O ally low and hasbeenmeasuredfor selectedice core sections [Hammer annualcycle, the generalpatternis the appearanceof one or more pronounceddust peaksduring springand a generally higherconcentrationlevel duringsummerthan duringwinter. •AlsoatLamont-Doherty EarthObservatory of Columbia Univer- Thispatternsuggests thatat leastduringtheHolocenemostof sity,Palisades,New York. the continentaldustdepositionin centra. 1 Greenlandoccurs during a few stormeventsin spring.The GRIP dust concenCopyright2000 by theAmericanGeophysical Union. tration profile is in good accordancewith the GISP2 [Ram and Koenig, 1997] and the Dye 3 (GISP1) [Hammer et al., Paper number 1999JD901093. 0148-0227/00/1999 JD901093509.00 1985] dustprofiles.
to be 2 and by a smectitecontentthat exceeds10%. These resultsare confirmedby other recent studies[Tomadin et al., 1996; Avila, 1997] and are in accordancewith the clay mineralogy of recent (postglacial)North Atlantic sedimentscollected off the West African coast [Biscaye, 1965]. An XRD spectrumof a typical, smectite-rich,kaolinire-rich, aeolian samplecollectedin Mauritania is shown in Figure 4e. Some authors report that Saharan dust may contain significant amountsof calcite, dolomite, and palygorskiteand that the distribution
of those minerals
within
the Sahara Desert is re-
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SVENSSON
ET AL.: GREENLAND
ICE CORE PROJECT
ICE CORE DUST
144--
I
64_, (a)GRiPLGM(GR#05B) t•/, 144(b) PSA Illinois (248-8) 64
16 !'P :.-..•' 0
I '1 I • I I I ol
256
--
144
--
.
.
64-16--
144
--
64 -- (d) PSA Shaanxi (270-2)
16 --' 64--
16--
0
10
20
30
40
20(deg)
50
Figure 4. ComparisonbetweenselectedXRD spectraof GRiP andPSA samples.The intensitiesare in arbitrary unitsshownon an exponentialscale(0, 1, 4, 16.... ). Solid linesindicateair-driedsamples,and dashed linesindicateglycolatedsamples:(a) GRiP samplefrom the LGM, (b) typicalloesssamplefrom the midcontinentalUnitedStateswith highsmectiteandlow chloritecontent,(c) till samplefrom Michigan,(d) Chinese glacialloesssamplefrom Lantian,and (e) typicalSaharanaeoliansamplewith high smectiteand kaolinite content.For peakidentificationseeFigure2. The mineralogyof the samplesis summarizedin Table 3.
gionallydependent [Pye, 1987;Ganorand Foner,1996;Molinaroli, 1996]. Althoughthe studiesof Saharandustin the
rangeof theGreenland icecoredust,theNd is lessradiogenic thantheicecoredustby morethanoneENd(O ) unit(Figure5). literature haveapplieddifferenttechniques for sampling,Although thesizefractionof themarinesediments is larger measurement, and dataanalysisand although the analyses thanthatof theicecoredust,thevariability in theNdisotopic havebeencarriedout on differentsizefractionsof dust,it is composition in thegrainsizerangeof fine-grained sediments clearthatthe clay mineralogydetermined for recentSaharan is small [Goldsteinet al., 1984] and is believednot to influ-
dustis significantly differentfromthatof theglacialGreen- encethe ratiosimportantly. The Nd isotopesthereforealso landicecoredust.In contrast to theSaharan dust,theclay precludeSaharaas an importantdustsourcefor Summitdurmineralogy of theice coredustis characterized by smectite ing the late glacial. contents of 2gm I
0.716
