GEOPHYSICAL RESEARCH LETTERS, VOL. 26, NO. 16, PAGES 2485-2488, AUGUST 15, 1999
Millennial-scale climatic change during the last interglacial
period: Superparamagnetic sediment proxy from paleosol S1,
ß
western
Chinese Loess Plateau
Xiao-Min FangandLi Ji-Jun
Department
of Geography,
LanzhouUniversity,Lanzhou,Gansu,China
SubirK. BanerjeeandMike Jackson
Institutefor RockMagnetism,Department
of GeologyandGeophysics,
Universityof Minnesota,Minneapolis,
Minnesota
Eric A. Oches
Department
of Geology,Universityof SouthFlorida,Tampa,Florida
Rob Van der Voo
Department
of GeologicalSciences,
theUniversityof Michigan,AnnArbor,Michigan
LoessPlateau[Liu, 1985], but has yieldedmore problematic
resultsin other loessareas[e.g., Ochesand Banerjee,1996;
reveal high-resolutionsignalsof summermonsoonchange. Grimley et al., 1998]. Magnetic susceptibilityin 1oessand
and
The last interglacialperiodis represented
hereby the 8-meter paleosolsis largelya functionof the type, concentration
thick SI paleosolcomplex.We have usedlow-temperature grain size of ferrimagneticminerals. The susceptibility
remanencestudiesto analyzevariationsin the concentration contributionsof paramagneticand diamagneticmineralsare
but they become
of superparamagnetic
(SP) grains,which previousstudies relativelyminor in mostof thesesediments,
are low. The
have shownto be largely of pedogenicorigin. The SP con- importantwhen ferrimagneticconcentrations
to climatevariationsis the sumof
centration,interpretedas a proxyfor the extentof pedogene- responseof susceptibility
sis, showsmillennial scalevariationswithin SI. We conclude the separateresponses
of thesediversemineralogies,
and it
that the last interglacialperiodin Asia was characterized
by may thereforebe rathercomplex.
A more selective proxy is the abundanceof ultrafine
rapid climatefluctuations,with at leastone brief returnto
(SP; < 0.03 gm for magnetite[Dunlop,
near-glacialconditionsin the middleof oxygen-isotope
sub- superparamagnetic
1973]) grains. Recent rock magnetic investigationshave
stage5e.
demonstratedthat SP grains are mostly pedogenicin origin
and are largelyresponsiblefor the enhancement
of magnetic
susceptibility
in soil [MaherandTaylor, 1988;Banerjeeet al.,
Introduction
1993; Evans and Heller, 1994]. The contentof SP grains
Climaticinstabilityduringthe last interglacialperiodhas changestremendously
with the degreeof soil development,
becomea subjectof great debatesincecontradictory
results which in turn is controlledby summermonsoonprecipitation
were obtainedfrom two ice coresin Greenland[Dansgaardet
and temperature[Banerjeeet al., 1993; Liu et al., 1995], as
al., 1993;Grooteset al., 1993].Thisdebatehasattractedcon- well as time.
siderable international attention because climate records for
We have carried out a detailedstudyof the variationin
the last interglacialperiodhave fundamentalimportancefor concentration
of SP grainsfor the last-interglacial
SI paleosol
our understanding
of currentclimatetrends.Up to now, the complex in the Jiuzhoutaiwell 1oesssectionin Lanzhou
debatehas been basedmostlyon climaticrecordsfrom the (36øN and 103ø50'E),on the westernChineseLoessPlateau.
North Atlantic and peripheralareas[Thouvenyet al., 1994; As we will show, the resultsallow us to resolve a millennialMaslin and Tzedakis, 1996; Fronval and Jansen,1996; Kukla
scalesummermonsoonhistoryfor the lastinterglacialperiod.
et al., 1997]. A few recordsfrom the ChineseLoessPlateau,
basedon magneticsusceptibility,
carbonateand grain size,
Geologyand Sampling
showthatthe last interglacialAsianmonsoonmay alsohave
Lanzhou is located at the northwestern front of summer
undergonelargevariations[Fanget al., 1995;An and Porter,
1997].
monsoon circulation. This front encompasses
a strong
Bulk magneticsusceptibility
has provento be a relatively gradientbetween warm, humid air massesover the Pacific
preciseproxy of summermonsoonstrengthin the central and Indian Oceans,and cold, dry air massescenteredon
Siberia and West Mongolia. Thus, modem precipitation,
vegetation
and soil biologicalactivity in Lanzhouare very
Copyright1999by theAmericanGeophysical
Union.
sensitive to any summer monsoonalcirculation changes.
Furthermore,the Jiuzhoutaisectionis the highest-resolution
Papernumber1999GL008335.
0094-8276/99/1999GL008335505.00
1oessrecordin the world, containingthe greatesttotal accuAbstract. Detailed magneticanalysesof samplesfrom the
Jiuzhoutai well section on the western Chinese Loess Plateau
2485
2486
FANG ET AL.' MILLENNIAL-SCALE
CLIMATE
CHANGE
ON THE LOESS PLATEAU
mulation(3 l 8 m), whichformedin a relativelyshorttime (1.3 and thesemustbe recognizedand accountedfor separately.
Ma) [Burbankand Li, 1985], yieldinga mean sedimentation Magnetite-bearing
sediments
generallyexhibita sharplossof
overa narrowtemperature
intervalassociated
with
rateof about245 mm/ka.The embeddedS1 paleosolcomplex remanence
formedmainlybetweenabout150 and 75 ka, and it reachesa both the Verwey transition(110-120 K) and the magnetothickness of 8 m near Lanzhou, three to four times thicker crystallineK1 isotropicpoint (~130 K) [Verwey, 1939;
than the corresponding
paleosolsin the centralLoessPlateau DunlopandOzdemir,1997]. Low-temperature
remanence
[Liu, 19851.
carriedby othermineralsmay alsodropsharplyovercharacto N6el pointsor
To obtainfreshsamples,a 40-meterdeepwell wasdugat teristictemperaturerangescorresponding
the top of the Jiuzhoutaisection.The well sectionconsistsof other transitions. Analysisof the thermal decay of lowa distinctivedarkpaleosolcomplex(SO)at the top,the Malan temperatureSIRM thus provides an effective means of
loesses(LI-1 to L1-5) and their embeddedweakpaleosolsof identifyingmagneticmineralsand of quantifyingSP grain
the Sm complex(Sm-I to Sm-4) in the middle, and the Lishi concentration[Hunt et al., 1995].
1oesses
(L2-1 to L2-3) with their embeddedyellowish-brown
The samplesof this studywereanalyzedusinga Quantum
paleosolsof the S1 complex(S 1-a to Sl-c) at the bottom.
DesignsMPMS (magneticpropertymeasurement
system).A
Previous and new systematicdating of the sectionby 2.5 Tesla (T) field was usedto saturatethe samplesat 20
radiocarbon,thermoluminescence
(TL) and paleomagnetism Kelvin (K). The resultantSIRM (carriedby MD, SD and SP
[Li et al., 1992; Fang et al., 1997] showsthat 1oessL0 and grainswith blockingtemperatures
above20 K) was then
paleosolseriesSOare Holocenein age, whereas1oessseries measuredduringwarmingto 300 K in zero field. Between
L I and paleosolsSm were depositedduringthe last glacia- about 100 and 120 K, a sharpdrop in magnetizationis obwith the
tion, and L2 and S1 duringthe last Pleistocene
interglacial servedfor all samples(Fig. 1). This drop,associated
period. Theseassignments
are broadlyin accordwith those Verwey transition,confirmsthat magnetiteis a significant
for the sequencefrom the centralpart of the LoessPlateau remanencecarrierin all samples. No othermagneticmineror sharp variationsin
[Liu, 1985], and they correlatewell with standardmarine als are indicatedby discontinuous
oxygen-isotope
stages(MIS) [Martinsonet al., 1987]:SO,Sm remanenceat other temperatures. Previousstudieshave
and S1 correspondwith warm periods, MIS-I, 3 and 5, shown that the magnetic minerals in Chinese 1oessand
paleosolsare chiefly magnetite,maghemiteand hematite
respectively.
This studyfocuseson the stratigraphic
intervalof depth [Heller and Liu, 1984; Heller and Evans, 1995; Hunt et al.,
with thisconclusion,
although
35.3-38.55m, whichencompasses
thepeakinterglacial
paleo- 1995]. Our dataare consistent
sol (Sl-c) of MIS-5e. Bulk sedimentsamplesfor rockmag- theydirectlyconfirmonly the magnetitecontribution.
Becausethe sharplossof remanencearound110 K is due
netic analysiswere taken at 2.5-cm intervals,providinga
nominalresolutionof ca. 180years.SinceTL ageshavelarge to the structuralphasetransition,andnot to thermalunblockerrors,we follow the conventionalmethod[An and Porter, ing, it mustbe tallied separatelyin the estimationof total SP
1997] of refining the chronology:first we correlatethe content. A simplegraphicalmethod[Banerjeeet al., 1993;
boundariesof L 1-5/Sl-a, L2-2/S 1-c and S 1-c/L2-3 to thoseof
Hunt et al., 1995] was usedto partitionthe low-temperature
MIS 4/5 (74 ka), MIS 5d/5e(114 ka) andMIS 5/6 (130 ka), SIRM into three fractions:the stableremanenceremaining
respectively
[Martinsonet al., 1987];thenwe applyan agedepthmodelbasedon grain size data [Fanget al., 1995] to
calculateages for levels betweenthe boundaryages. This
1.2E-2
method is better than linear interpolationbetween dated
horizons,becausethe winter monsoonand 1oessdeposition
rateshavechangedsharplyin the past[Liu, 1985].
ß
1.0E-2
•'
Magnetic Measurements
The purposeof our detailedmagneticstudieswasto isolate
the contributionsof different magneticmineralsand different
size fractionsto the room-temperature
low-field susceptibility
signal in the Jiuzhoutaisection. In particular,we aimed to
quantifythe ultrafineSP fractionof magnetiteandmaghemite
grains,which have been linkedby previousstudiesto pedogenieprocesses.
Low-temperaturemagnetic measurementsare the most
definitivemeansof quantifyingthe concentration
of magnetic
grainsthat are SP at roomtemperature[Hunt et al., 1995]. A
low-temperaturesaturationisothermalremanentmagnetization (SIRM) will be partially thermally demagnetized
warmingin zero field. The unblockingtemperature
spectrum
is closelyrelatedto the distributionof particlesizesin the SP
fraction,and the total remanencelossby thermalunblocking
is a direct measureof the total massof SP materialpresent
[Banerjeeet al., 1993;Hunt et al., 1995].
In addition to the continuous loss of remanence due to
thermalunblocking,there are severalothermechanisms
that
may contributeto remanencelossduringzero-fieldwarming,
IE
.o_
JZT9-4 (Sl-c)
ß
JZT9-5 (Sl-c)
ß
JZT10-2 (Sl-c)
•-
JZTll-2
A
JZT2-10 (L2-2)
[]
JZT12-5(L2-3)
•
JZT13-11(L2-3)
8.0E-3
6.0E-3
JZTI-1 (Sl-b)
•
(Sl-c)
._N
=
4.0E-3
2.0E-3
0.0E+0
0
50
100
150
200
250
300
Temperature (Kelvin)
Figure 1. Thermaldemagnetization
of low-temperature
SIRM
acquiredat 20 K, for a few representative
1oess
andpaleosol
samples.Superparamagnetic
(SP) contentcan be determined
by an extrapolation
of a fitted remanencedecaycurve(see
text for details).Note the two groupsof curvesseparate
clearlythe paleosols
(solidsymbols)and1oesses
(opensymbols)exceptfor soil sampleJZT10-2,whichmarksa significant fluctuationduring the mid-last interglacialmaximum.
Locationsof the samplesarein Figure3.
FANG ET AL.' MILLENNIAL-SCALE
CLIMATE
CHANGE
ON THE LOESS PLATEAU
2487
MIS
lOO JZT1-1
5c
JZT2-10
11o
-
5d
JZT9-4
JZT9-5
12o
5e2 •'
V2
-
•
5el
•"'
JZT10-2____
V4
JZT11-2
5e4
P5
130
5e3
P3
5e5
JZT12-5
--
JZT13-11
I
10
2o
x(10
3o
-8
(a)
4o
3
m /kg)
5o
0
2
4
2
Msp(Am/kg)
(b)
6
'
-40
I
-35
a•80(%o)
(c)
Figure 2. Comparisonof stratigraphic
variationsin mass-specific
magneticsusceptibility(a) and remanencecarriedby
SP grains(b) to the oxygen-isotope
climaticrecordfromthe GreenlandGRIP ice core(c). Asterisksshowthe locationsoœ
sampleswhosethermalremanencedecaycurvesare shownin FigureI.
after warmingto 300 K; the remanenceunpinnednear 110 K ture. Note that this is a measure of absolute concentration of
by the magnetocrystalline
and structuraltransitionsof mag- SP particles,ratherthanthe proportionof ferrimagneticmatenetite;andthe continuous,
quasi-exponential
decayassociated rial in the SP state. The SP contentdiffersstronglyin 1oesses
with thermalunblockingof the particlesthat are SP at room andpaleosols,
withrespective
means
of2.0x 10-3Am2/kg
and
3.9 x 10'3 Am2/kg.
Themostimpressive
feature
in theSP
temperature(Fig. 1).
Oxidation of magnetite(maghemitization)suppresses
the concentrationcurveis the largedrop (V4) in the centerof SlVerweytransition[Ozdemiret al., 1993]. The lossof c, where it falls nearly to the levels characteristicof the
remanencenear 110 K is thus attributablemainly to near- 1oesses
(Fig. 2b). The 1ow-SPinterval spansapproximately
stoichiometricmagnetite. Maghemitemay contributeboth to 3.5 ka, with a centralage of ca. 123 ka. This featureis also
the SP fractionthat unblockson warmingto 300 K and to the clearly visible in both volume- and mass-specificmagnetic
stablefractionthat survivesat room temperature[Sun et al., susceptibilities
(Fig. 2a).
1995]. For our purposes,it is not important how the SP
A smaller decrease of shorter duration (V2) can be
fraction is partitionedinto magnetiteand maghemitecontri- observedin the SP concentrationin the upper part of Sl-c
butions;we assumethat SP particlesof both speciesresult (Fig. 2). This drop,thoughlessdramaticthan V4, represents
from pedogenesis
[Verosub,1994], andthereforethe total SP a significantchangein the pedogenicsignal. It is not seenin
fraction representsthe signal of soil formation. We also the susceptibility, which exhibits a continuous broad
assumethat enoughof the fine SP fractionsurvivesto pre- maximum over the same interval. The source of the enhanced
susceptibilityhere is not yet understood;all threeof the lowservethis signal.
temperatureremanencecomponentsare somewhatdiminished
in V2 samples. The sharp minimum in SP concentration
Summer monsoonchangessuggestedby SP
arguesagainstthe conventionalpictureof pedogenicsuscepgrains
tibility enhancementin this interval.
The two d.ropsV2 and V4 divide the Sl-c recordinto three
Figure 2 shows the stratigraphicvariation in SP grain
abundance, in terms of the contribution made to the lowpeaks(P1, P3 and P5), two of which are characterizedby a
temperatureSIRM by particlesthat are SP at room tempera- rapid initial increaseat the baseand a more gradualupward
2488
FANG ET AL.' MILLENNIAL-SCALE
CLIMATE
decline (Fig. 2c). Peak 5 is the most strikingof these.The
threepeakshave durationsof about 1-3 ka, with centralages
estimatedat ca. 115, 120 and 127 ka, respectively.
These variationsin SP contentsuggestthat the summer
monsoonduring 5e underwentseveral large changes:an
enhancement
at the transitionfrom the penultimateglaciation
(MIS 6) to the lastinterglacialstage(MIS 5), with its warmest
climateat about127 ka; a gradualweakeningof the summer
monsoonand/ora temperature
dropto a nearlyglaciallevel at
about120 ka; and at leastonemoresimilarcycleof increase
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Acknowledgment: We gratefullyacknowledgehelp in initial well
digging and samplingby Wang Jintai, Edward Derbyshire,Dai
Xuerongandothers,andconstructive
reviewsby JosephRosenbaum,
NicolasThouvenyand an anonymousreviewer. This work was cosupportedby Ministry of Educationof China (grant for TransCentury Excellent Researchersand the Key Program), Visiting
Fellowship Fund of IRM, the National Key Project for Basic
Researchon Tibetan Plateau, the NSFC (No. 49871010), and the
INQUA 1997 Project. The IRM is fundedby the Instruments
and
Facilities Program, Earth Sciences Division, National Science
Foundation,and by the W. M. Keck Foundation. This is IRM
contribution number 9904.
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