Mid-Holocene climate variations recorded by palaeolake in marginal area of east Asian monsoon: A multi-proxy study

IEECAS SKLLQG

Climatic changes in Eurasia and Africa at the last glacial maximum and mid-Holocene: reconstruction from pollen data using inverse vegetation modelling

IEECAS SKLLQG

Sedimentation rates and provenance analysis in the Southwestern Okinawa Trough since the mid-Holocene

As a high-sedimentation rate depocenter along the path of the Kuroshio Current, the southwesternmost part of the Okinawa Trough is a key area to understand the Kuroshio history and sediments transportation. A 34.17-m-long sediment core was obtained by the advanced piston corer of Marco Polo/IMAGES XII MARION DUFRESNE during the May 2005 from the Southern Okinawa Trough at site MD05-2908. The recovered sediments were analyzed by AMS C-14 dating, coarse size fraction (> 63 mu m) extraction and moisture content determination in order to study its sedimentation flux and provenance. The depth-age relationship of core MD05-2908 was well constrained by 17 C-14 dating points. The sediments span across the mid-Holocene (6.8 ka B.P.) and have remarkablely high sedimentation rates between 1.8 and 21-2 m/ka, which is well consistent with the modern observations from sediment traps. We identified five 70-200 a periods of abnormally rapid sedimentation events at 6790-6600 a B.P., 5690-5600 a B.P., 4820-4720 a B.P., 1090-880 a B.P., and 260-190 a B.P., during which the highest sedimentation rate is up to 21-2 m/ka. In general, the lithology of the sediments were dominated by silt and clay, associated with less than 5% coarse size fraction (a parts per thousand << 63 mu m). As the most significant sediment source, the Lanyang River in northeastern Taiwan annually deliver about 10Mt materials to the coastal and offshore region of northeast Taiwan, a portion of which could be carried northward by currents toward the study area. Therefore, we concluded that the 5 abnormally rapid sedimentation events may be related to intensified rainfall in Taiwan and thus increased materials to our study area at that time. However, a few extreme-rapid sedimentation events cannot be explained by normal river runoff alone. The large earthquakes or typhoons induced hyperpycnal discharge of fluvial sediment to the ocean may also act as a potential source supply to the Okinawa Trough.

Records of the East Asian winter monsoon from the mud area on the inner shelf of the East China Sea since the mid-Holocene

AMS(14)C dating and analysis of grain size, major elements and clay minerals were applied to Core MZ01 from the mud area on the inner shelf of the East China Sea. Based on the environmentally sensitive grain size, clay mineral and major element assemblages, the history of the East Asia winter monsoon since the mid-Holocene could be reconstructed. These three proxies, mean grain size (>9.71 mu m), chemical index of alteration (CIA) and ratio of smectite to kaolinite in particular, show similar fluctuation patterns. Furthermore, 10 extreme values corresponding to the contemporary cooling events could be recognized since the mid-Holocene; these extreme values are likely to have been caused by the strengthening of the East Asia winter monsoon. The cooling events correlated well with the results of the delta O-18 curves of the Dunde ice core and GISP2, which therefore revealed a regional response to global climate change. Four stages of the East Asia winter monsoon were identified, i.e. 8300-6300 a BP, strong and unstable; 6300-3800 a BP, strong but stable; 3800-1400 a BP, weak and unstable; after 1400 a BP, weak but stable.

GEO-ECOLOGICAL TRANSECT STUDIES IN NORTHEAST TIBET (QINGHAI, CHINA) REVEAL HUMAN-MADE MID-HOLOCENE ENVIRONMENTAL CHANGES IN THE UPPER YELLOW RIVER CATCHMENT CHANGING FOREST TO GRASSLAND

Geo-ecological transect studies in the pastures of the upper catchment of the HuangHe (99 degrees 30'-100 degrees 00'E/35 degrees 30'-35 degrees 40'N'; 3,000-4,000 in a.s.l., Qinghai province, China) revealed evidence that pastures replace forests. Plot-based vegetation records and fenced grazing exclosure experiments enabled the identification of grazing indicator plants for the first time. The mapping of vegetation patterns of pastures with isolated juniper and Spruce forests raise questions as to the origin of the grasslands, which arc widely classified as "natural" at present. Soil investigations and charcoal fragments of Juniperus (8,153 +/- 63 uncal BP) and Picea (6,665 +/- 59 uncal BP) provide evidence of the wider presence of forests. As temperatures and rainfall records undoubtedly represent a forest climate, it is assumed that the present pastures have replaced forests. Circumstantial evidence arising from investigations into the environmental history of the Holocene effectively substantiates this theory.

Cold event at 5 500 a BP recorded in mud sediments on the inner shelf of the East China Sea

A 700-year record (1.0-1.5 a resolution) of the East Asian winter monsoon (EAWM), based on grain-size analysis and AMS(14)C dating of Core EC2005 from the inner-shelf mud wedge of the East China Sea (ECS), was compared with the Dongge stalagmite delta O-18 record during the mid-Holocene. The upper muddy section of Core EC2005 has been formed mainly by suspended sediments derived from the Changjiang (Yangtze) River mouth since 7.3 ka BP. High precipitation and a strengthened EAWM might have played key roles in the high sedimentation rate (1 324-1 986 cm/ka) between 5.9-5.2 ka BP. The EAWM strengthened when the Asian summer monsoon weakened, especially around 5 500 a BP, which corresponded to a worldwide cold event. The EAWM during the mid-Holocene shows statistically significant solar periodicities at 62 and 11 a. The 5 500 a BP cold event might be resulted from orbital forcing and changes in solar activity.

Flux and fate of Yangtze river sediment delivered to the East China Sea

Numerous cores and dating show the Yangtze River has accumulated about 1.16 x 10(12) t sediment in its delta plain and proximal subaqueous delta during Holocene. High-resolution seismic profiling and coring in the southern East China Sea during 2003 and 2004 cruises has revealed an elongated (similar to 800 km) distal subaqueous mud wedge extending from the Yangtze River mouth southward off the Zhejiang and Fujian coasts into the Taiwan Strait. Overlying what appears to be a transgressive sand layer, this distal clinoform thins offshore, from similar to 40 in thickness between the 20 and 30 m water depth to < 1-2 in between 60 and 90 m water depth, corresponding to an across shelf distance of less than 100 km. Total volume of this distal mud wedge is about 4.5 x 10(11) m(3), equivalent to similar to 5.4 x 10(11) t of sediment. Most of the sediment in this mud wedge comes from the Yangtze River, with some input presumably coming from local smaller rivers. Thus, the total Yangtze-derived sediments accumulated in its deltaic system and East China Sea inner shelf have amounted to about 1.7 x 10(12) t. Preliminary analyses suggest this longshore and across-shelf transported clinoform mainly formed in the past 7000 yrs after postglacial sea level reached its mid-Holocene highstand, and after re-intensification of the Chinese longshore current system. Sedimentation accumulation apparently increased around 2000 yrs BP, reflecting the evolution of the Yangtze estuary and increased land erosion due to human activities, such as fanning and deforestation. The southward-flowing China Coastal Current, the northward-flowing Taiwan Warm Current, and the Kuroshio Current appear to have played critical roles in transporting and trapping most of Yangtze-derived materials in the inner shelf, and hence preventing the sediment escape into the deep ocean. (c) 2006 Elsevier B.V. All rights reserved.

冲绳海槽西南端中全新世以来的沉积特征及其物源与环境意义

作为大陆向大洋的过渡带，由于享有得天独厚的沉积环境，具有独特的构造特征以及与黑潮主流之间的密切关系，一直以来，冲绳海槽都是中外学者研究的重点靶区。2005年5月，由我国与法国联合主持的IMAGES 航次在台湾东北海域获取MD05-2908柱状岩芯（24º48.04′N,122 º29.35′E,水深为1275米），该柱状岩芯为一34.17米长高质量的连续沉积记录，岩性以深灰色粘土质粉砂为主，含水量较高，粘性、可塑性强，含有数层厚度不等的夹层。岩芯年龄模式依据17个AMS 14C定年数据建立，岩芯底部年代约6.8ka，为中全新世以来的沉积。在实验室对样品按照2cm的间隔进行分割后分别进行了粒度分析、粘土矿物提取与测试、碎屑矿物提取与鉴定、常微量元素和稀土元素分析等实验。 粒度分析结果显示，MD05-2908岩芯沉积物粒度垂向上总体比较均一，以细颗粒的粘土与粉砂质为主，但不同层位也稍有差别，表现为底部层位粒度较粗，含砂量较高，说明底部沉积环境比较复杂。粘土粒级（<2µm）矿物主要由四种粘土矿物和少量石英、长石碎屑组成。其中，粘土矿物相对含量变化中，伊利石（～68％）与绿泥石（～17%）构成主要成分，含有蒙皂石（～10％）和高岭石（～5％）。结合台湾东北外海表层沉积物的研究，利用粘土矿物伊利石/蒙皂石和绿泥石/高岭石比值得出岩芯粘土矿物主要为陆源碎屑粘土矿物，其源岩主要为台湾中央山脉的变质岩与台湾东部的沉积岩。重矿物分析共选取了41个层位，对63~250μm粒级的样品在实体镜和偏光显微镜下进行鉴定，结果显示，岩芯重矿物主要由绿泥石（29%）、普通角闪石（22%）、白云石（10%）、黑云母（8%）、绿帘石（7%）、白云母（7%）、褐铁矿（5%）等组成。稳定矿物少，矿物成熟度低。碎屑矿物风化程度低，磨蚀不明显，分选较差，表明沉积物来自于近源，后期改造作用不明显。常量元素分析结果表明，SiO2 、Al2O3和Fe2O3是岩芯沉积物中的最主要组分，这三种组分占沉积物总量的82%左右。 整个岩芯自下而上各常量组分变化不大，其平均值与东海陆架沉积物基本接近。微量元素变化比较明显， Ba、Cr、Cu、Zn元素的含量比东海陆架沉积物中的含量要高，而Sr的含量明显低于东海陆架。对常微量元素的R型因子分析表明，常量元素SiO2、Al2O3、Fe2O3、MgO和K2O，微量元素Cr、Cu、Ni、Zn、Pb、Rb和Mn可代表陆源物质；常量元素CaO和微量元素Sr、Ba可代表生物源物质。岩芯沉积物以陆源物质为主，生源物质的贡献起次要作用。岩芯沉积物中稀土元素总量平均为169.87×10-6，并且轻稀土含量均高于重稀土，LREE/HREE平均值为10.14，表明了轻稀土对稀土总量的贡献远高于重稀土，沉积物富集轻稀土，反映了沉积物的陆源特征。 岩芯MD05-2908中全新世以来平均5m/ka的高沉积速率主要源于丰富的物质供应和适宜的沉积环境。岩芯细粒级沉积物中，地球化学特征表明沉积物主要来源于陆源碎屑物质，粘土矿物特征与台湾东部陆源物质相同；粗粒级沉积物中，重矿物含量及矿物特征也表明岩芯沉积物粗颗粒组分主要来自于近源沉积。台湾宜兰境内的兰阳溪每年携带约一千万吨冲积物入海成为研究区重要的物质来源。由于受到黑潮的强烈影响，逆时针涡流及底层反向流的存在是岩芯高沉积速率重要控制因素。因此，利用动力分选的粉砂组分可以用来示踪古洋流强度，结果显示，6.8ka以来黑潮的强弱波动频繁，并表现出一定的旋回性变化，频谱分析表明，其具有的千年尺度周期（1500a）、百年尺度周期（604a、242a、192a、153a、133a）与十年尺度周期（22a）的周期性变化均与太阳辐射量变化有密切关系，因此，黑潮的强弱变化在大背景上是由太阳活动所控制的。 根据测年资料可以识别出岩芯存在5期快速堆积事件，这与区域性降水增加有关，降雨量增加导致陆源物质输入的增加。另外，岩芯位于大陆斜坡区，附近存在有三支海底峡谷，并且地震活动频繁，沉积在宜兰陆架及东海陆架处的浅海沉积物由于受到地震、风暴等活动的影响而受扰动崩塌、因重力作用而向低处输送，产生二次侵蚀并经由海底峡谷搬运到冲绳海槽南段堆积,使得沉积环境更为复杂，但同时也为冲绳海槽提供了丰富的物质供应。

Seasonal pattern of reproduction of Hizikia fusiformis (Sargassaceae, Phaeophyta) from Nanao Island, Shantou, China

The maturation pattern of sexual reproduction in Hizikia fusiformis (Harvey) Okamura (Sargassaceae, Phaeaophyta) was examined in 2003 at Yunao Bay, Nanao Island, Shantou, China. Maturation began in mid-April (seawater temperature 19-21 degrees C), reached the peak in mid-May (maturation rate ca. 70%, and seawater temperature 23.5-25 degrees C) and finished in late-June (seawater temperature 27.5-30 degrees C). The Hizikia plants continued to gain the length from the beginning of maturation season to reach a maximum mean length of 34.8 cm in mid-May, after which the mean length was reduced drastically due to the senescence and rupture of the larger plants in size. The major portion of the mature plants belonged to the larger plants between April and May, but to the smaller ones in June. It is suggested that the plant must achieve a critical size before reproductive maturation occurred. There was a positive relationship between the number of receptacles (NR), as well as the reproductive allocation (RA), and the plant size of Hizikia population, with the recorded maximum values of NR and RA being 1220 and 64.3% respectively, for a single plant.