Recent 2000-year geological records of mud in the inner shelf of the East China Sea and their climat

ARTICLES

Chinese Science Bulletin 2005 Vol. 50 No. 5 466—471 Recent 2000-year geological records of mud in the inner shelf of the East China Sea and their climatic implications XIAO Shangbin1,2, LI Anchun1, JIANG Fuqing1,

LI Tiegang1, HUANG Peng1,2 & XU Zhaokai1,2

1. Institute of Oceanology, Chinese Academy of Sciences, Qingdao

266071, China;

2. Graduate School of the Chinese Academy of Sciences, Beijing 100039,

China

Correspondence should be addressed to Li Anchun (e-mail: acli@)

Abstract AMS14C dating and grain-size analysis for Core DD2, located at the north of the Yangtze River-derived mud off the Zhejiang-Fujian coasts in the inner shelf of the East China Sea, provide us a high-resolution grain-size distribu-tion curve varying with depth and time. Data in the upper mud layer of Core DD2 indicate that there are at least 9 abrupt grain-size increasing in recent 2000 years, with each corresponding very well with the low-temperature events in Chinese history, which might result from the periodical strengthening of the East Asian Winter Monsoon (EAWM), including the first-revealed maximum temperature lowering event at around 990 a BP. At the same time, the finer grain size section in Core DD2 agrees well with the Sui-Tang Warming Period (600—1000 a AD) defined previously by Zhu Kezhen, during which the climate had a warm, cold and warm fluctuation, with a dominated cooling period of 750—850 a AD. The Little Ice Age (LIA) can also be identified in the core. It starts around 1450 a AD and was followed by a subsequent cooling events at 1510, 1670 and 1840 a AD. Timing of these cold events revealed here still needs to be further verified owing to some current uncertainty of dating we used in this study.

Keywords: East China Sea, Yangtze River, mud, coastal current, grain size, East Asian Monsoon, climate.

DOI: 10.1360/03wd0571

Understanding the evolution history of East Asian Monsoon (EAM) is very important for us, because it not only impacts and controls Chinese climate greatly[1,2], but also plays significant role in the global climate system[3,4]. EAWM is the active factor in the East Asian Win-ter-Summer Monsoon system[5]. It not only affects the circulatory flow, weather and climate changes of the East Asia, but also works on a planet scale, which makes the global holding up heat source and divergence centre move eastward, and affects the weather and climate of North America by remote relativity[6]. An[7] pointed out that first of all we should probe the high-resolution proxy of the EAM in the study of East Asian Paleo-Monsoon (EAPM), by disclosing the uncertainty of EAPM’s temporal and spatial distribution, to provide its history background and mechanism for studying the recent monsoon and future climate changes. Furthermore, it is also crucial to treat the East Asian’s continentals, oceans, atmosphere and ice as a whole environment system of EAM.

For a long time, the grain size and susceptibility of the Chinese loess have been regarded as proxies of the East Asian Winter and Summer Monsoon respectively, and many great scientific achievements have already been made in studying the EAM evolution on a long time scale[8—11]. However, it is difficult to use the loess deposits to study the high-resolution climate changes in historical records documented by human being, on centennial or even shorter time scale, due to the restriction of sedimen-tation rate. Thus, ice core[12—15], peat[16—19], pollen[20—23], tree rings[24—28] and lacustrine sediments[29—33]have been gradually used as high-resolution proxies of Chinese pa-leo-climate. Cave carbonate deposits (of which are mainly dripstone) have also been identified as another good proxy[34—37] on land. Together with the major progress of sampling technology and dating method, the resolution of the marine paleo-environment studies can reach centennial scale[38]. However, comparing with those proxies from land, the resolution from oceans is still much lower. We attempt to find a high-resolution proxy from sediments in a continental shelf in this paper, and discuss the historical changes of Chinese paleo-climate from different points.

1 Material and methods

With a sediment discharge of 4.8×108 t/a in history, huge amount of sediments have accumulated at the estu-ary and neighborhood of the Yangtze River, especially at its south side[39]. A major feature of the ECS Coastal Cur-rent is its flow path changing seasonally. That is to say, the coastal current flows northward in summer owing to the southeast monsoon, and flows southward in winter owing to the prevailing north wind[40] (Fig. 1(a)). The modern surface mud in the inner shelf of the ECS, the so called “ECS inner shelf mud”, is mainly derived from suspended sediments from the Yangtze River, and transported south-ward by the winter coastal current1),[41], which is in accord with the distribution of suspended sediments and water masses in this area[42—44] (Fig. 1(b)). The Winter ECSCC will flow more rapidly when the winter monsoon becomes stronger, which will result in coarser deposits. In addition, the weathering process is weak in cold period, which will also make Yangtze River-derived sediments become

1) Xiao Shangbin, Li Anchun, A study on environmentally sensitive grain-size population in inner shelf of the East China Sea, Acta Sedimen-tologica Sinica (in Chinese with English abstract), in press.