Mollusk record of millennial climate variability in the Loess Plateauduring the Last Glacial Maximum

NAIQIN WU, TUNGSHENG LIU, XIUPING LIU AND ZHAOYAN GU

Wu, N. Q., Liu, T. S., Liu, X. P. & Gu, Z. Y. 2002 (March): Mollusk record of millennial climate variabilityin the Loess Plateau during the Last Glacial Maximum. Boreas, Vol. 31, pp. 20–27. Oslo. ISSN 0300–9483.

A high-resolutio n terrestria l mollusk record from the Loess Plateau of China has been studied to characteriz eclimate variability during the Last Glacial Maximum (LGM). The rapid successions in mollusk taxa in theWeinan loess sequence reveal that climate changes occurred at least four times in this period. In the loess re-gion, millennia-scale climate � uctuations existed, as documented in the grain size and weathering intensity re-cords. Our results show such millennia-scal e � uctuations re� ecting changes in both temperature and precipita-tion, rather than a simple cold and warm alternation . Changes in temperature and precipitation were not inphase during the LGM. Temperature varied earlier than precipitation , which could have been the effect ofwinter and summer monsoon interactions . Our data also reveal that the East Asian summer monsoons couldreach the southeast part of the Loess Plateau during the whole of the LGM. The intensi� cation of winter mon-soons during the LGM led to short duration of summer monsoons annually impacting on the Loess Plateau,but the intrinsic intensity of summer monsoons would not have changed signi� cantly, thus providing the ther-mo-hydrologica l conditions for temperate-humidiphilou s mollusks to persistentl y grow and develop in the gla-cial age.

Naiqin Wu (e-mail: luwu@public.east.cn.net) , Tungsheng Liu (e-mail: tsliu@public.bta.net.cn) , Xiuping Liu(e-mail: xpliu@yahoo.com) , Zhaoyan Gu (e-mail: zgu@public2.east.cn.net) , Institute of Geology andGeophysics , Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, P. R. China; received 1st May2001, accepted 29th August 2001.

The present climate in the Northern Hemisphere isrelatively stable compared to that of the Last GlacialMaximum (LGM) about 22000–14000 years ago (e.g.Wright et al. 1993; Ditlevsen et al. 1996). Numerousgeological records reveal that the climatic conditionsduring the LGM were quite different from today,although insolation was similar to the present (Webb& Kutzbach 1998). The boundary conditions (e.g. CO2,vegetation, global ice-volume, sea level, sea-surfacetemperatures) have largely changed (e.g. COHMAPMembers 1988; Wright et al. 1993; Kutzbach et al.1998). Paleoclimate studies from the Chinese loesssequences for the LGM show an accelerated dustaccumulation related to intensi� cation of the wintermonsoon. An expansion of the loess deposition areareached the south of the Yangtze River and off themodern eastern sea coast. The East Asian summer mon-soon was weak in this period (Liu et al. 1985, 1995; Anet al. 1990; Wang & Sun 1994; Rousseau & Wu 1997).

The Loess Plateau is presently located in the EastAsian monsoon zone (Zhang & Liu 1992) (Fig. 1).Climate in this area is characterized by seasonalalternations of the East Asian summer and wintermonsoons. In summer, the warm-humid SE summermonsoon of tropical/subtropical origin leads to abun-dant precipitation. In winter, the cold-dry NW wintermonsoon of sub-arctic origin prevails across the region.Most of the annual rainfall is concentrated in July,August and September (Zhang & Liu 1992). Annualmean temperature (AMT) varies from 14°C in the southto only 4°C in the north (Fig. 2A) along a gradient of

increasing latitude. Annual mean precipitation (AMP)decreases from 650 mm in the SE to less than 200 mmin the NW (Fig. 2B). The Loess Plateau therefore coversthe semi-humid, semi-arid and arid climatic zone (Qian1991).

The Chinese loess-paleosol sequence constitutes animportant record of variations in Asian monsoonclimate over the past 2.4 Ma (Liu et al. 1985). For along time, study of the East Asian monsoons largelydepended on such non-biological proxy indicators asmagnetic susceptibility, CaCO3 content, grain-size,chemical weathering index, and so on. Particle sizes,especially the quartz fraction, have been used as ameasure of the strength of the winter monsoon windsresponsible for most of the dust transport (An et al.1991a; Ding et al. 1994; Xiao et al. 1995; Porter & An1995). The magnetic susceptibility of loess and paleo-sols has been regarded as a proxy for summer monsoonintensity (An et al. 1991b). However, magnetic suscept-ibility is only an approximate index of summermonsoon strength, because the cause remains conten-tious due to depositional and pedogenic effects andvegetation changes (Heller et al. 1993; Hunts et al.1995; Lu et al. 1995, 2000; Han et al. 1996; Guo et al.1998; Gu et al. 2000). Molluskan faunas are the mostwidespread fossil remains in the Chinese loess. Thestudy of terrestrial mollusks from the loess sequence ofthe last 240 ka in Luochuan has indicated the variationsin strengthened summer and winter paleomonsoons(Wu et al. 1996, 2000; Rousseau & Wu 1997, 1999), ingood agreement with the variations inferred from

# 2002 Taylor & Francis

pedology, sedimentology and climate modeling. How-ever, little has been known until now about the detailedprocess of climatic change and paleoenvironmentalspatial variability of the LGM on the loess region,because the previous coarse sampling interval did notpermit more precise characterization of the environ-mental changes that terrestrial mollusks can provide.Knowledge of the LGM patterns in the climate andenvironment of the Loess Plateau is important if we areto understand regional climate sensitivity to the changesin global climatic system, and to assess hypothesizedclimate forcing mechanisms, thus providing properboundary conditions for the earth-system models. Forthis reason, we investigated the Weinan loess sequenceat a higher resolution, focusing on the Last GlacialMaximum period.

General setting and methods

The Weinan loess section (34°12’N, 109°31’E) islocated in the southern-most part of the Loess Plateau,about 55 km east of Xian (Fig. 1), in a semi-aridmonsoon climate zone (Qian 1991). The annual meantemperature in this region is about 11.3–13.6°C andannual mean precipitation about 529–638 mm. Thisloess section has been studied in detail over a longperiod (e.g. J. Liu et al. 1994; Liu et al. 1995; Liu &Ding 1998; Ding et al. 1995; Gu et al. 1996, 1997; Guoet al. 1996, 1998; Sun et al. 1997; Lu et al. 1999; Wu etal. 1995, 1999). In the present study, we focus on theloess unit (L1-1) deposited during marine isotope stage2 (MIS 2) (Liu et al. 1995). This loess unit, overlain by

the Holocene soil S0 and underlying the weakly-weathered soil L1-2 (upper part of MIS 3), consists oflight grey-yellow silt sediments with a thickness ofabout 230 cm at depth from 170 to 400 cm (Fig. 3). TheL1-1 loess unit was sampled at 3-cm intervals,equivalent to a time resolution of about 200 years.Eighty-one samples were taken for the mollusk study,each weighing about 15 kg. They were washed andsieved in the � eld with a mesh diameter of 0.5 mm. Themollusk shells were selected under a binocular micro-scope. All identi� able mollusk remains were consideredin the total count of individuals following the methoddeveloped by Puissegur (1976).

The chronostratigraphic framework of the Weinanloess sequence of the past 150 ka has been de� ned onthe basis of AMS radiocarbon and TL ages (J. Liu et al.1994). In this study, we transformed the radiocarbonages into calendar ages following the calculationmethod of Stuiver & Reimer (1993) (Fig. 4).

Mollusk taxa as indicators of changingmonsoon climate

Rousseau & Wu (1997, 1999) demonstrated a closecorrespondence between mollusk assemblage variationsin the loess-paleosol sequence of the Loess Plateau andthe monsoon index deduced from modeling for SouthAsia (Prell & Kutzbach 1987). Wu et al. (2000)subsequently suggested that variations in molluskecological groups were related to changes in the Earthorbital parameters at 41 and 20 ka frequencies. Varia-

Fig. 1. Sketch map of the Central Chinese Loess Plateau and the location of the study site. The solid arrows indicate the direction of theEast Asian summer monsoons, the dashed arrows the pathways of the East Asian winter monsoons.

BOREAS 31 (2002) LGM climate variability in the Loess Plateau, China 21

Fig. 2. A. Annual averagetemperature of the Loess Plateau(°C). B. Annual averageprecipitation of the Loess Plateau(mm) (modi� ed from Qian 1991).Location of the Weinan loesssection is shown.

22 Naiqin Wu et al. BOREAS 31 (2002)

tions in different ecological groups were linked to thesummer and winter insolation at 30°N (Rousseau et al.2000). The most abundant occurrence of thermo-humidiphilous taxa in the loess-paleosol sequenceresulted from the in� uence of the strengthening EastAsian summer monsoon. Puctum orphana is one typicalelement of the thermo-humidiphilous group. Its moderndistribution is mainly limited to the southeast of China,an area presently dominated by the summer monsoon,and where the MAT ranges from 13.0°C to 17.5°C andthe MAP from 615 to 1124 mm. Macrochalamysangigyra, another thermo-humidiphilous species, has asimilar ecological distribution to P. orphana, butrequires a higher temperature and wetter conditions(Chen & Gao 1987). The occurrence of these two fossilspecies indicates warm and humid conditions and is anindication of the northward migration of warm and wetsummer monsoon into the Loess Plateau (Rousseau &Wu 1997).

Wu et al. (2000) also demonstrated that maxima ofthe cold-aridiphilous mollusk group corresponded withthe strengthened winter monsoon, as suggested pre-viously by Rousseau & Wu (1997). The cold-aridiphi-lous mollusk variability re� ects a period of 100 ka (Wuet al. 2001), which is the major periodicity of wintermonsoon driven by the global ice volume (Ding et al.1995). The growth of Vallonia tenera and Pupilla aeoli,two typical representatives of the cold-aridiphilousgroup, is strongly controlled by temperature andmoisture conditions. These species prefer drier and

colder ecological environments, the most optimalranges being 5.8–10.5°C (MAT) and 200–450 mm(MAP) (Chen & Gao 1987). Their modern distributionin China is restricted mainly to the NW continentalinterior, where the current summer monsoon hardlyreaches (Fig. 1), but where the winter monsoon (coldcurrents) predominates for a large part of the year (7–8months) (Liu et al. 1985). Our investigation of modernmollusk assemblages from 60 sites in the Loess Plateauand the surrounding areas indicates that V. tenerahabitats in drier ecological conditions than P. aeoli,generally in a moisture range of 200–350 mm (MAP).The abundance of these two species in a loess-paleosolsequence has been regarded as a proxy for wintermonsoon strength (Wu et al. 2001; Rousseau & Wu1997, 1999).

Mollusk record in the Weinan loess sequenceduring the LGM

Terrestrial mollusks are abundant in the Weinan loesssequence during the LGM. As shown in Fig. 3, twoimportant features are distinct in the record. First, theL1-1 unit shows the highest abundance of mollusk taxa;the number of individuals reaches 1198/15 kg at a depthof 2.78 m and 1196/15 kg at 2.89 m, much higher thanthe L1-2, generally containing an average amount ofabout 500 individuals, and S0 about 200 individuals.

Fig. 3. Variations in mollusk species of the Weinan loess-soi l section, plotted on a depth scale with the stratigraphi c units of the Holocenesoil (S0), Late Pleistocene loess (L1-1) and weakly weathered soil (L1-2). Mollusk species expressed in the absolute abundance (the num-ber of individual s per 15 kg). AMS and TL ages are from J. Liu et al. (1994). The compared marine oxygen isotopic stage (MIS) is fromLiu et al. (1995). 1. Holocene soil; 2. Loess; and 3. Weakly-weathere d soil.

BOREAS 31 (2002) LGM climate variability in the Loess Plateau, China 23

Second, the transitions from the L1-2 to the L1-1 and tothe S0 unit are marked by rapid apparent changes in thecomposition of mollusk fauna, showing completelydifferent scenarios. The L1-2 unit is dominated bywarm-humidiphilous taxa, such as Macrochalamysangigyra, M. sp., Punctum orphana, and Metodontia.Similar to the L1-2, S0 has an analogous composition inmollusk fauna, but increased numbers of Opeasstriatissmum. However, there are fewer species andfewer individuals in S0 than in L1-2, as pedogenicactivity caused much snail shell to dissolve. Thecomposition of the L1-1 unit is characterized by cold-aridiphilous taxa such as Pupilla aeoli, Vallonia tenera,V. cf. pulchella, Gastrocopta armigerella, and Cath-aica. These species show striking variability in theirpeak values in the sequence. Fig. 4 indicates the time-series record of the percentages of these species.

A prominent feature of Fig. 4 is the regular variabilityin the peak values of six species during the glacial age.The period from approximately 24 ka BP to 12 ka BPshows a continuous succession in mollusk species in thesequence. Before 20 ka BP, Macrochalamys angigyradominated the whole mollusk fauna, reaching 80%between 24 ka BP and 22 ka BP. Since about 21 ka BP,this species decreases abruptly to less than 10% at about18 ka BP. Coincident with this decline is the sharpincrease in the abundance of Pupilla aeoli and Gastro-copta armigerella, the former attaining its peak value atabout 19.5 ka BP, the latter at around 17.5 ka BP. From17.5 ka BP, both species decrease considerably, with P.aeoli decreasing more slowly than G. armigerella,indicating that the latter is more sensitive to thechanging moisture conditions. Following this change,Vallonia tenera takes over as the dominant molluskassemblage, with the highest peak at 16 ka BP. From

17.5 ka BP to 16 ka BP, this species increases from 20%to 40%. Vallonia cf. pulchella shows a progressivelyincreased abundance during the whole of stage 2, butwith a plateau at about 15 ka BP. Punctum orphana,displaying a low abundance during the LGM period,began to increase signi� cantly at about 15 ka BP andreached its maximum at about 13 ka BP.

The overall quality of mollusk preservation duringthe glacial interval is very good, and so the observedvariability in abundance of six species cannot beattributed to carbonate dissolution. Rather, we interpretthis record as an indication of rapid, large-amplitudechanges in the climate and environment occurringduring the LGM period in the loess region.

Discussion

The observed changes in the abundance of six mollusktaxa have important implications for climate andpaleoenvironmental history in the Loess Plateau duringthe Last Glacial Maximum. We take the modernobserved data (Wu, N. Q. et al. unpublished data) inrelation to climatic parameters as the basis on which toinfer the paleo-climatic variability and thermal andhydrological conditions in this region during the past.As shown in Fig. 4, a very high abundance of M.angigyra before 21 ka BP indicates warmer and wetterclimatic conditions prevailing in this area during thelate MIS 3 – a period of strengthened summer monsoon.After this, the area experienced a series of � uctuationsin climatic conditions during the LGM. Up until 13 kaBP, the summer monsoon was intensi� ed once again, asindicated by a high abundance of the warm-humidspecies P. orphana (Fig. 4). This warming at 14 ka BP

Fig. 4. Rapid variability of 6mollusk species peak values in theWeinan loess section during theLast Glacial Maximum, comparedwith variations in magneticsusceptibilit y (MS) and summerinsolation in the northernhemisphere (35°N, June–August).The time series was establishedfollowing the model of age-depthconversion (J. Liu et al.1994) andthe calculation method of Stuiver& Reimer (1993).

24 Naiqin Wu et al. BOREAS 31 (2002)

implied the onset of the last deglaciation in the LoessPlateau. Comparing the dominant species that occurredin MIS 3 with that in the last deglaciation, we couldassume that the intensity of summer monsoon impactingon this region differed in these two periods. Eithertemperature or moisture during MIS 3 was higher than itwas in the last deglaciation, as shown by the dominanceof M. angigyra, which requires a higher temperatureand more moisture than P. orphana , the dominantspecies of the last deglaciation.

During the LGM, the climate in the Weinan regionunderwent at least four apparent changes. First, theclimatic became colder at about 19.5 ka BP, as re� ectedby a high abundance of P. aeoli. Judging from theabundance of mollusk species during the LGM, wefound that modern Pupilla aeoli faunas, most similar tothis glacial assemblage, are associated with tempera-tures of about 8–10°C and precipitation of about 300–500 mm. If the same fauna temperature and faunaprecipitation relationships existed during the glacialperiod, it would indicate that the temperature in theWeinan region around 19.5 ka BP was at least about 3–5°C and precipitation about 100–300 mm lower thantoday. Second, as this low temperature continued,however, the precipitation pattern changed andincreased by 100–150 mm from 19.5–17 ka BP. Theconditions at about 17.5 ka BP were the wettest, withprecipitation of 450–550 mm in this region based on theobserved results of the modern Gastrocopta armigerellapopulation. Third, the moisture decreased rapidly atabout 17 ka BP and the climate became dry. The driestconditions occurred at about 16 ka BP, as indicated bythe highest abundance of V. tenera. The modernpopulations of V. tenera similar to the glacial assem-blages indicate that the precipitation at about 16 ka BPwas only about 200–350 mm, approximately corre-sponding to the modern precipitation conditions alongthe northwestern boundary of the Loess Plateau (Fig.2B). Finally, after 16 ka BP, both temperature andmoisture rose, as re� ected by a plateau occurrence of V.cf. pulchella and a progressive increase of P. orphana atabout 15 ka BP. After 14 ka BP, this region entered thewarming episode of the last deglaciation. These four� uctuations correspond to signi� cant changes in glacialtemperature and precipitation, changes that have notbeen reported previously in studies of pollen and phyto-lith for the same period of this region owing to largersampling intervals (Sun et al. 1997; Lu et al. 1999).

The rapid successions in mollusk taxa during the LastGlacial Maximum suggest that climatic conditions inthis area were highly unstable. A millennia-scaleclimatic � uctuation existed in the Loess Plateau at theLGM period. Recent studies of loess grain-size andpaleo-weathering intensity also revealed rapid changesin climate during the last glaciation (Porter & An 1995;Guo et al. 1996; Ren et al. 1996). Loess depositsrecorded the Dansgaard-Oeschger cycles occurring athigh latitudes in Greenland in periods of 2–5 ka (Ren et

al. 1996). These � uctuations were therefore consideredrelated to changes in North Atlantic climate. Our resultsfrom the Weinan loess sequence suggest that suchmillennia � uctuations, which occurred during the LGM,are not a simple cold-dry and/or warm-humid alterna-tion in climate. The variability in mollusk taxa showsthat the changes in temperature and precipitation are notnecessarily in phase during this period. Temperatureapparently changed earlier than precipitation, becausethe coldest interval in this region was present from 19.5to 17 ka, and driest at about 16 ka. What is the reason?One possible interpretation is the effect of winter andsummer monsoon interaction over this region. Duringthe LGM, when winter monsoonal circulation over theNorthern Hemisphere continents had become enforced,cold and dry air from high latitudes was rapidly broughtto the Loess Plateau and resulted � rst in a decrease insurface temperature. The variations in the East Asiansummer monsoons, however, could have respondedslowly to this change because of the effect of the hugeheat capacity of the oceans – the major source of watervapor to the Loess Plateau (Fig. 1). Smaller heatcapacity of the land causes the atmospheric temperatureto respond much more quickly than that of the ocean(Kutzbach & Webb 1993). Thus, variations in mon-soonal precipitation might lag behind continental temp-erature on a millennia scale. On the contrary, whenclimate became warmer after about 15 ka BP, tempera-ture was the � rst to increase and then precipitation.

The continuous low abundance of Punctum orphanaduring the LGM indicates that the East Asian summermonsoons could affect this area even in the coldeststage of the last glacial period. Rather than beinguniformly cold or dry as previously thought (An et al.1990), there were periodic warm-humid air incursionsto the Loess Plateau in the LGM period. Pollen data ofabout this period from the same sequence showed amoist meadow environment composed of many types ofCompositae, Gramineae and Polygomum (Sun et al.1997). The phytolith study (Lu et al. 1996) revealed thatthe variations in July precipitation in the southern partof the Loess Plateau did not show a distinct decreasesince the marine isotopic stage 5, varying from 100 to150 mm. Moreover, precipitation in July has similarvariability to the changes in strength of the IndianOcean monsoon (Clemens & Prell 1990). The dominantcycle that affects the summer monsoon (July precipita-tion) of the Loess Plateau during the last 130 ka isconsistent with the solar isolation in low latitudes (Lu etal. 1996; Rousseau et al. 2000). This implies that theAsian summer monsoon has not changed its intrinsicintensity during the glacial interval. The small changesin summer SST in the tropical oceans (CLIMAP 1981;Thompson 1981; Wang & Sun 1994; Wang et al. 1999)and seasonal solar radiation similar to today (Prell &Kutzbach 1987) also document sustained summermonsoon intensity during the LGM (Fig. 4), whichprovided appropriate thermo-hydrological regimes for a

BOREAS 31 (2002) LGM climate variability in the Loess Plateau, China 25

few temperate-humidiphilous mollusks to grow in theLoess Plateau. However, as other boundary conditionsand climatic forcings (e.g. CO2, global ice-volume, sealevel, SST) were very different from today during theLGM (Webb & Kutzbach 1998), intensi� cation of theNW winter monsoon led to enhanced aridity and eolianprocesses on the Loess Plateau, and the seasonality wasmuch stronger than it is now (An et al. 1990; Wang &Sun 1994). As a consequence, duration of the wintermonsoon should have been longer than today. Eventhough the summer monsoons in the glacial periodmaintained their intrinsic strengths, their arrival at theLoess Plateau was delayed and their duration in thisregion was shortened, leading to a decreased environ-mental impact. The discontinuous appearance ofthermo-humidiphilous mollusks in the Luochuan loesssequence (about 200 km north of the Weinan) in thesame period indicated that the summer monsoons wereweaker there (Wu et al. 1996; Rousseau & Wu 1997). Ithas been considered that the shift from the wintermonsoon-dominated season to the summer monsoon-dominated season over East Asia depends on theatmospheric adjustment over a larger area (Chen et al.1991). Under the strong in� uence exerted by ice sheetson the high-latitude atmospheric conditions, there is alikelihood that adjustment of the atmospheric back-ground to the summer monsoon circulation is signi� -cantly delayed in summer seasons (Liu & Ding 1998).We suggest that global ice volume through the effects ofwinter monsoons must affect the duration of summermonsoon on a given area at millennia scales and, as aresult, controlled the annual climatic pattern of theLoess Plateau. But, it did not change the initialperiodicity and intensity of summer monsoon asso-ciated with the low-latitude summer solar insolation.

Conclusions

Our study on the Weinan loess sequence in the SE LoessPlateau clearly shows that high-resolution investigationof terrestrial mollusk assemblages provides morereliable and detailed paleoclimatic information thanpreviously utilized physical and chemical proxies suchas magnetic susceptibility. The sensitivity of terrestrialmollusks to changes in environment (e.g. humidity andtemperature) allows them to respond to short-livedphases of climatic amelioration and can be used as asensitive indicator of paleoclimatic changes.

A series of rapid � uctuations in climate, as indicatedby the changes in mollusk taxa, has been documented inthe loess deposits of the LGM. The succession ofmollusk species re� ects the changing processes of theLGM climate in the Loess Plateau and the relationshipsbetween temperature and precipitation. Changes intemperature and precipitation in this region are not inphase. The temperature decrease or increase leadsprecipitation. Consequently, the climate in the Weinan

region has been cold, cold-wet, cold-dry, temperate-dry,and temperate-humid since 22 ka BP, and is attributedto the effect of winter and summer monsoon interac-tions on the loess region.

Our results also suggest that during the entire LGMthe East Asian summer monsoons could reach the SEpart of the Loess Plateau. Intensi� cation of the wintermonsoon at that time led to short duration summermonsoons impacting on the Loess Plateau, such thatseasonality was strengthened. However, the summermonsoons reaching other loess regions could sustaintheir intrinsic intensity and thus supply thermo-hydro-logical conditions suf� cient for a few temperate-humidiphilous mollusks to persistently grow anddevelop in the glacial age.

Acknowledgements . – This work was supported by the NationalNatural Science Foundation of China (nos. 40024202 , 49894170 ,49771078) and the Chinese Academy of Sciences (no. KZ951-A1-402). We are grateful to Z. T. Guo, J. M. Han and H. Y. Lu fortheir helpful discussions and valuable suggestions , J. T. Han forimproving the English. We thank Denis-Didier Rousseau and IanSmalley for their helpful reviews of this paper.

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