NOTES

A climatic reversal at the last interglacial-last glacial transition recorded in the loess section at Zhaitang , Beijing

XlONG Shangfa, LIU Dongsheng (LIU Tungsheng ) and DING Zhongl i Institute of &logy, Chinese Academy of Sciences, k i n g 100029, China

Abstract A weakly developed paleosol was identified at S1-L1 transition of the loess section of Zhai- tang, Beijing. This paleosol may represent a climatic reversal which is associated with the strengthen- ing in summer monsoon and correlated well with the climatic regression event documented in the paleo- climate records from Greenland ice, marine sediments and European pollen at the last interglacial-last glacial transition.

Keywords: last interglacial period, last glacial period, loess, paleosol , climatic reversal.

THE Younger Dryas event (YD event ) is a climatic reversal that occurred at the last glacial-last inter-

Chinese Science Bulletin Vol . 44 No. 6 March 1999

NOTES

glacial transition. It resulted in the rapid shifts in atmospheric temperatures"' and oceanic circulation[21 of the North Atlantic. As an abrupt cooling reversal superimposed on the last deglaciation, YD event has at- tracted extraordinary attentionr3] .

Interestingly, at another climatic transition, the last interglacial-last glacial transition ( LILGT ) , it is also apparently observed in some paleoclinlatic records that a climatic reversal was superimposed on the last glaciation. This climatic reversal, however, is different from the YD event in its warming character. The study of this warming reversal is necessary for understanding of the causes and mechanisms of abrupt climatic shifts, including YD event.

However, it is not yet clear what geographical extent this reversal event affected, and whether or not we can trace it from geological records in east Asia.

Recently, we investigated the loess section of Zhaitang, Beijing, and from there we identified a w e d y developed paleosol at the transition from S1 (last interglacial) to L1 (last glacial), which may represent the warming reversal at LILGT. Analyses of the climatic record of Zhaitang loess would thus contribute to estimate of the geographical extent that the wanning reversal would affect and the character- istics of it, and to understanding of the causes and mechanisms of abrupt climatic shifts occurring at the climatic transitions.

1 Evidence of the climatic reversal at the last interglacial-last glacial transition

The variations in grain size and susceptibility of loess-paleosol sequence are commonly used as the in- dicators of paleomo-nal changesL4, 51. Zhaitang loess sectionL6' consists of two units of deposits. The upper part is named Malan loess and the lower part is a palaesol (S l ) developed in the last interglacial.

The pedostratigraphic unit of S1 (from top to bottom) can be divided into 6 pedogenic beds (figure 1).

A NE Pedostratigraphy

Fig. 1. Zhaitang loess section and the pedostratigraphy , susceptib~llty of S1. 1 , Loess ; 2, palensol; 3, bnght brown paleosol; 4 , brown paleosol; 5, reddish brown paleosol.

0-20 cm. Bright brown; clay loam; with weak, subangular and blocky structure; few carbonate pseudomecelia; boundary smooth, clear.

20-50 cm. Yellowish brown; silty; with weak, subangular and blocky structure; few carbonate pseudomecelia; boundary smooth, clear.

50--80 a. Brown; clayey; with weak, medium and granular structure; roots abundant, with worm pores; boundary smooth, gradual.

80-100 cm. Bright brown; clayey; with moderate, subangular, blocky structure; carbonate pseu- domecelia common; boundary smooth, gradual.

100-145 an. Bright reddish brown; clayey; with moderate, granular structure; clay skins com- mon; boundary smooth and gradual.

145-195 cm. Reddish brown; clayey; with moderate, granular structure; clay skins, Fe-Mn films common; boundary smooth, gradual.

572 Chinese Science Bulletin Vol . 4 4 No. 6 March 1999

The uppermost part of S1 (0-20 cm) is a weakly developed paleosol bed which may be correspond- ing to the climatic reversal at LILGT. Underlying it is a 30-cm loess layer marlung a short climatic cool- ing pulse. The analyses'71 reveal a maximum in susceptibility, total iron oxide (Fet) , free iron oxide (Fed), and a minimum in medium grain size and CaCQ3 at the uppermost part of S1 (fig. 2) . However, the maximum of susceptibility of the uppermost part of S1 is obviously lower than that of layers corre- sponding to the oxygen isotope stages of 5a, 5c, 5e and 3. The minimum of medium grain size of LILGT apparently consists of two pluses of low value, and it appears that the changes in medium grain size and the susceptibility of LILGT are synchronous. The medium grain size variations of loess have been suggest- ed to reflect the fluctuations of winter monsoon, whereas the fluctuations in susceptibility, and contents of Cam, Fet and Fed of loess are considered to be a reflection of changes in strength of summer monsoon. From this perspective, the climatic reversal recorded in Zhaitang loess at LILGT may represent a warming event associated with the strengthening in summer monsoon and the weakening in winter monsoon. The age of this climatic reversal is estimated"] to be 75-68 ka by the correlation between the susceptibility curve of the Zhaitang loess and the deep ocean oxygen isotope curve ( Specmap ) . With its apparent pe- dostratigraphic signature, the climatic reversal in Zhaitang loess section has thus not only a paleoclimatic significance but also a stratigraphc significance.

Fig. 2. Correlations of the climatic reversal event at LILGT in Zhaitang loess section, using contents of Cam( I ) , Fed (2) , Fet (3) , susceptibility (4) and medium grain size ( 5 ) as proxies for paleomonsoonal changes, with medium grain size from Lijiayuan loess section, Gansu Province in chinaLx1 (6 ) , C Q concentration of Vostok ice core from ~ n t a r c t i c ~ ~ ]

(7) , 6180 from Sulu Sea Site 769~["] (8) and s"O of GISP2 ice core from ree en land["] (9). The climatic reversal is indicated by the arrows. The chronology of the inferred records are based on previously published data['-"].

A climatic reversal at the transition from S1 to L1 is indicated by quartz grain size recorded in Luochun section[12] and grain size recorded in Lijiayuan section"31 . In the oxygen isotopic profiles of Guliya ice core[lol , there is also a maximum at the transition from stage 5 to stage 4. The evidence from

Chinese Science Bulletin Vol .44 No. 6 March 1999 573

NOTES Zhaitang loess and other sediments in China suggests that the climatic reversal at LILGT have a regional imprint over east Asia.

2 Correlation and discussion

It has been proposed that the YD event may have a global imprintr3'. Like the YD event, the cli- matic reversal at LILGT has also been seen in various paleoclimatic records over the world"1,147151, and it can be correlated well with that recorded in Zhaitang section (fig. 2 ) . In the profiles of Greenland ice core (GISF2, GRIP)"^], this climatic reversal consists of two pluses, interstadial numbers 19 and 20. In Vostok ice core, the corresponding changes in C Q ~oncentration'~' are larger than that within substage 5a. This means that the fluctuations in C Q concentration were partly responsible for the climatic rever- sal. The North Atlantic core ~ema-23-81"~' recorded a twc-step decreasing in abundance of N. pachy- &m at the transition from stage 5 to stage 4 , reflecting a warming regression in the SST over the North Atlantic.

This climatic reversal event can also be traced in records of the ice-rafting detritus abundance in hgh latitudes['61, 6180 of the Sulu Sea in the tropic[171 , pollen percentages in E U T O ~ ~ ~ ~ ~ and dust concentra- tion reflected by Greenland ice electrical conductivity'18'.

The imprints of the climatic reversal at LILGT in many records over the world suggest that it is also a global event and cooperates with the variations in oceanic circulation and atmospheric ~irculation~'~, l6] .

Meanwhile, its apparent signature in China loess indicates that it is a warming event associated with the abrupt shifts in the strength of summer and winter monsoon over east Asia. Regarding the connection be- tween the monsoon wind and the global dust concentration and atmosphere water vapour, it also means that the climatic reversal is associated with and may partly result from the abrupt fluctuations in the global dust concentration and the atmosphere water vapour.

Like the YD event, the climatic reversal at LILGT has its global impact and is associated with the reorganization of North Atlantic Deep Water ( NADW ) and the shifts of dust concentrationi'" and atmo- spheric circulation, so it is also difficult to explain the causes and mechanisms only by a regional model (for example, the "switch and on" for the thermohaline circulation in North Atlantic ) . However, the regional characters of the climatic reversal imprinted in various records are not negligible because the dif- ferences among the proxy c w e s from Greenland ice, Vostok ice and China loess are so obvious. Until now, we cannot give a plausible explanation of the mechanisms of the climatic reversal due to our igno- rance of the phase relationships among various related processes, such as the variations of NADW, the shifts of the global dust concentration and atmosphere water vapour, and the volcanic eruption event"91. But we could not avoid questing the causes and mechanisms for the abrupt climatic fluctuations including the YD event and the climatic reversal at LILGT, because we are probably faced with a new climatic re- versal-the forward globally climatic change.

Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant Yo 49291100)

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(Received February 13, 1998)

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