*Corresponding author.Tel.:001-206-543-1166.

E-mail address:scporter @u.washington.edu (S.C.

Porter).

Quaternary Science Reviews 19(2000)743}762

Asynchronous Holocene optimum of the East Asian monsoon

Zhisheng An ,Stephen C.Porter *,John E.Kutzbach ,Wu Xihao ,Wang Suming ,

Liu Xiaodong ,Li Xiaoqiang ,Zhou Weijian

Xi 'an Laboratory of Locss and Quaternary Geology,Academia Sinica,People 's Republic of China

Quaternary Research Center,Uni v ersity of Washington,Seattle,WA 98195-1360USA Center for Climatic Research,Uni v ersity of Wisconsin,Madison,WI 53706USA

Institute of Geomechanics,Ministry of Geology and Mineral Resources,People 's Republic of China

Nanjing Institute of Geography and Lakes,Academia Sinica,People 's Republic of China Lanzhou Institute of Plateau Atmosphere-Physics,Academic Sinica,People 's Republic of China

Abstract

The spatial and temporal distribution of summer monsoon precipitation (or e !ective moisture)during the Holocene has been reconstructed on the basis of geological data,including lake levels,pollen pro "les,and loess/paleosol records.In addition,the summer (July)precipitation increment,e !ective precipitation,and monsoon strength index have been obtained from numerical modeling experiments.Both geological data and numerical modeling indicate that the Holocene optimum,as de "ned by peak East Asian summer monsoon precipitation,was asynchronous in central and eastern China,reaching a maximum at di !erent times in di !erent regions,e.g.,ca.10,000}8000yr ago in northeastern China,10,000}7000yr ago in north-central and northern east-central China,ca.7000}5000yr ago in the middle and lower reaches of the Yangtze River,and ca.3000yr ago in southern China.In southwestern China the maximum appeared ca.11,000yr ago,but probably was related to the maximum landward extension of the Indian summer monsoon.The regional shift in the maximum precipitation belt from northwest to southeast over the past 10,000yr is interpreted as a response to changing seasonality related to orbital forcing of the climate.The southeastward shift of the East Asian summer monsoon maximum is consistent with the progressive weakening of the summer monsoon as the summer solar radiation anomaly decreased progressively through the Holocene and the East Asian monsoon index declined,while the early maximum in southwestern China matches the maximum of the Indian monsoon index. 2000Elsevier Science Ltd.All rights reserved.

1.Introduction

The Asian monsoon is an important component of atmospheric circulation and plays a signi "cant role in the global hydrologic and energy cycles.It has in #uenced signi "cantly the geographic environment of the region it a !ects directly,as well as its marginal zones.The mon-soon region of eastern China di !ers from other dry,subtropical,and temperate areas of the same latitudes in being densely populated.Not only is the livelihood of the people closely linked to the summer monsoon precipita-tion in agricultural regions,but the monsoon rains also generate devastating #oods that can impact tens of thou-sands of people.

A critical question facing the people living in mon-soonal regions is whether,and how,a rapid and signi "-cant change in global climate might a !ect the monsoon system.The past history of monsoon climates is important in answering this question,for it can provide evidence of the past behavior of the monsoons at times when global climate was di !erent from what it is now,and it can provide insights about the nature and regional impacts of future changes in climate.

In the present study,we have synthesized a large body of geologic information related to changes in monsoon climate in China during the Holocene Period,and have compared the data with results from numerical experiments with a Global Climate Model that span the same time interval.The results of the two approaches are consistent,and imply that the zone of peak rain-fall conditions associated with the East Asian summer monsoon shifted latitudinally across China during the Holocene in response to natural variations in solar radiation.

0277-3791/00/$-see front matter 2000Elsevier Science Ltd.All rights reserved.PII:S 0277-3791(99)00031-1