Science in China: Series D Earth Sciences 2006 Vol.49 No.6 604—614
East Asia Winter Monsoon changes inferred from environmentally sensitive grain-size component records during the last 2300 years in mud area southwest off Cheju Island, ECS XIANG Rong1, YANG Zuosheng2, Yoshiki Saito3, GUO Zhigang2, FAN Dejiang2, LI Yunhai2, XIAO Shangbin1, SHI Xuefa4 & CHEN Muhong1 1. Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Guangzhou 510301, China; 2. College of Geosciences, China Ocean University, Qingdao 266003, China; 3. Geological Survey of Japan, Tsukuba, Ibaraki, 305-8567, Japan; 4. Key Lab of Marine Sedimentology & Environmental Geology, SOA, Qingdao 266061, China Correspondence should be addressed to Xiang Rong (email: [email protected]
Received October 26, 2005; accepted December 27, 2005
Abstract Environmentally sensitive grain-size component (ESGSC) extracted from grain-size data of a sediment core B2, which were retrieved from mud area southwest off Cheju Island (MACI), East China Sea (ECS), can be used to indicate the variations of East Asia Winter Monsoon (EAWM), with high (low) content/mean-size of ESGCS denote to strong (weak) EAWM. Combined with AMS14C datings core B2 provides a continuous high-resolution record of EAWM changes over the past 2300 years, with an average resolution of 13 years. The results show that the variations of EAWM are consistent with temperature changes inferred from historical documents in eastern China over the past 2300 years, from which four climate stages may be identified. In stages before 1900 aBP (50 AD) and 1450―780 aBP (50―1170 AD) the EAWM were comparatively weak, corresponding to warm climate periods in eastern China, respectively. And in stages of 1900―1450 aBP (50―500 AD) and 780―219 aBP (1170―1731 AD) the EAWM were strongly developed, which correspond well to climate changes of two cold periods in eastern China. It is also shown from this study that the stage at 780―219 aBP (1170―1731 AD) was the coldest climate period during the last 2300 years and could be, therefore, related to the Little Ice Age (LIA). Climatic fluctuations appeared obviously in all the four stages, and two climate events of abrupt changes from warm to cold occurred at around 1900 aBP (50 AD) and 780 aBP (1170 AD), of which the latter is probably related to globe-scale changes of atmospheric circulation at that time. Keywords: East China Sea, mud area, environmentally sensitive grain-size component, East Asia Winter Monsoon, Little Ice Age, Late Holocene.
East Asia Winter Monsoon changes inferred from environmentally sensitive grain-size component records
The last 2000 years are an important time span both for IGBP-PAGES and CLIVAR of WCRP. One of the main aims of these projects is to obtain high-resolution records of global change, such as that stored in ice cores, tree rings, speleothems, corals, lakes, marine records, etc., and then use these data to make sound estimates for future global change. To accomplish these projects, we first need to obtain high-resolution geological records and proxies for climatic/environmental changes. The recognition of climate changes over the past 2000 years in China presently is mainly through high-resolution geological records found in land, such as tree rings, ice cores, peat, speleothems, ― historical documents records, etc.[1 11]. However, marine records are comparatively scarce because most ocean sediments have generally low sedimentation rate and consequently low resolution. Mud areas in the continental shelf of the East China Sea have special sediments with high sedimentation rate and preserve high-resolution paleoenvironmental records, which may supply the gap of marine records. Here we report for the first time a continuous high-resolution East Asia Winter Monsoon (EAWM) record covering the past 2300 years according to grain-size analysis of a sediment core B2 from mud area southwest off Cheju Island (MACI), East China Sea (ECS). Grain-size data have been widely used to determine sediment transport patterns and to reconstruct the sedimentary environment. Because the sediments deposited in the ocean are mostly mixtures of materials transported from multi-sources or/and by different sedimentary processes, the grain-size of bulk samples is an incomplete, approximate index for sedimentary environment changes  . Thus, quantitative reconstruction of sedimentary environment with regard to the partitioning of components from bulk grain-size distribution and the identification of their sedimentary implications has become a fundamental task. In recent years, a series of research results have been obtained in this aspect through pre-treatment of samples for grain-size analysis and mathematics analysis of ― grain-size data[12 18]. By these ways, environmentally sensitive grain-size components (ESGSC) or end-members were extracted from the grain-size data, which can be used to indicate the paleoenvironment or/and paleoclimate variations. It shows a good pros-
pect for grain-size data in the study of paleoenvironmental and paleoclimatic changes. MACI was formed under a cyclonic eddy at high sea level with a stable sedimentary environment[19,20]. Its sediments mainly originate from terrigenous suspended particulate mat― ter (SPM)[21 23], which has an apparent macro transport pattern of “summer deposit and winter transport”[22,24,25]. The sediments of MACI were mostly come from winter SPM on the shelf of the ECS and Yellow Sea (YS). And the grain-size of winter SPM has a close relation with the winter interaction between atmosphere and ocean, which also has a close relation with the strength of EAWM. Thus, we can get a good reconstruction of EAWM changes through the ESGSC extraction from grain-size data in MACI. 1
Materials and methods
Sediment core B2 analyzed in this study was retrieved from mud area southwest off Cheju Island (Fig. 1) using a vibrated corer in Sep. 2003, on the cruise of “Investigation of shelf water exchange on the ECS”
Fig. 1. Map shows the location of core B2 and schematic current system of the studied area (after Liu et al., 1999). Shaded areas indicate mud areas distributed in the East China Shelf Seas. ECS: East China Sea; YS: Yellow Sea; KWC: Kuroshio Warm Current; TsWC: Tsushima Warm Current; YSWC: Yellow Sea Warm Current; YSCC: Yellow Sea Coast Current; TaWC: Taiwan Warm Current.
Science in China: Series D Earth Sciences
executed by R/V “Dongfanghong 2” of China Ocean University. Core B2 (125°45′E, 31°45′N), retrieved from a water depth of 64 m, is 4.03 m long. It is mainly composed of green gray clay silt, with a distinct sand layer between 3.27―3.47 m (Fig. 2). Samples used for grain-size analysis were taken at 2-cm intervals and a total of 201 samples were obtained from core B2. Because biogenic component of the normally deposited sediments (mainly benthic foraminifera) have a very low content, averaging 0.3% in the component >63 µm (the main size range for benthic foraminifera), and one of the main sediment sources of MACI―Huanghe sediments have a relatively high carbonate content in the fine fraction, therefore in the pre-treatment of samples for grain-size analysis, we only add 10% H2O2 solution to get rid of the organic matter, and retained the carbonate component. Grain-size analyses were carried out on a Britain Malvern 2000 grain-size analyzer in College of Geosciences, China Ocean University, with a measurement range of 0.02―2000 µm and a size resolution of 0.01φ. The repeated measuring error is within 3%. Mixed benthic foraminifera from 6 horizons were picked out for AMS14C dating, which were measured at Beta Analyses Company, USA (Fig. 2). Raw radiocarbon dates were converted to calibrated calendar ages following Stuiver et al. using the CALIB4.3 program which include a 400-year reservoir correction, almost
the same value with the local reservoir age. In this paper, ages of core B2 are all expressed as calendar ages. 2
The frequency distribution curve of sediment grain-size directly reflects the grain size character it includes. Sediments of the upper 3.27 m of core B2 consist of homogeneous clay silt (Fig.2), samples from which have very similar grain-size distribution curves, characterized by a distinct single peak (Fig. 3(a)), indicate that this layer was continuously deposited under a comparatively stable hydrological environment. Sediments between 3.27―3.47 m is a sandy silt layer, samples from which have distinct high values of magnetic susceptiblity, and two obvious fine and coarse peaks can be easily identified from the frequency distribution curves of grain-size in this layer (Fig. 3(a)), therefore we speculate that this layer was deposited by storm or other similar disaster event deposits, and further studies are needed to verify its origin. Sediments between 3.47―3.91 m consist also of clay silt, however, sediments of this layer contain more sand than that of the upper 3.27 m, and AMS14C data at 3.47― 3.49 m are obviously older than the core bottom one, suggesting that this layer was also affected by the disaster event deposits. Sediments at 3.91―4.03 m show
Fig. 2. Lithology and AMS14C data of core B2.
East Asia Winter Monsoon changes inferred from environmentally sensitive grain-size component records
Fig. 3. Result of grain-size analysis in core B2. (a) Grain-size distribution curves of sediments at different core depths; (b) diagram of standard deviation/grain-size classes. Numbers 1―4 represent the 4 grain size components respectively, and the italic numbers indicate the mode size and boundary size for each component.
similar grain-size distribution curves with that of the upper 3.27 m, suggesting that it was deposited under a normal eddy environment. This paper mainly discusses the paleoenvironment and paleoclimate variations recorded in the upper 3.27 m of core B2 which was deposited under a stable eddy environment and preserved a continuous sediment record. The chronology of the sequence is based on 4 radiocarbon datings of mixed benthic foraminifera samples collected from the core. Age between age control points was obtained by interpolation. Core top sediments, however, were missed when sampling, hence the age above the first age point was obtained by extrapolation. Age below 2.49 m was determined by interpolation between age control points B2-4 and B2-6, which didn’t account the disaster deposits layer of 3.27―3.91 m, and arrived at an age of 2278 aBP (328 BC) for 3.27 m. Thus, the upper 3.27 m of core B2 provides a high-resolution sediment record from 2278 (328 BC) to 219 aBP (1731 AD), with a mean resolution of 13 years. Sediment transport processes and sedimentary environment changes have been inferred by the proportions and size ranges of grain-size components of sediments from the South China Sea[13,14] and Arabian Sea. In the extraction of the ESGSC, for the Arabian Sea, end-member modeling of the grain-size data was used to calculate the numbers of end-members; as for the South China Sea, the grain-size components were identified by calculating the standard deviation
of grain-size content in samples from a sediment core via each grain-size class. The grain-size components calculated by the two methods reflect the same number, suggesting that both methods can obtain reliable results in the grain-size data analysis. Standard deviation/grain-size classes diagram mainly reflects the grain-size content variability of one sample group via each grain-size class, with peak values indicating high variability of grain-size content in the sample group at the corresponding grain-size classes, and low values implying less important changes. From the standard deviation/grain-size classes plot, the component numbers and size range of each grain-size component of one sample group can be easily identified, which have a close relation with sediment sources and/or sedimentary environment. In this study, the standard deviation/grain-size classes diagram was used to extract the ESGSC in core B2 (core B2 mentioned below specially refers to the upper 3.27 m of core B2). Standard deviation values vs. grain size classes of core B2 are displayed on Fig. 3. Four peaks are observed in this plot, at 0.8, 5.1, 21.9 and 522 µm, respectively, which represent the mode size of each grain-size component. According to foraminiferal analysis, component 4 is mainly composed of benthic foraminifera and can be neglected in this study for its very low content, averagely content only 0.3% in the >65.6 µm fraction in grain-size data of core B2. The size range of the other 3 main grain-size components are 65.6―10.5, 10.5―1.3 and