Paleocurrent directions from paleomagnetic reorientation of magnetic fabrics in deep-sea sediments at the Antarctic Peninsula Pacific margin (ODP Sites 1095, 1101)

We have determined the azimuth of bottom-current flow in drift deposit sediments recovered at ODP Sites 1095 and 1101, Antarctic Peninsula, using paleomagnetic reorientation of anisotropy of magnetic susceptibility (AMS) ellipsoids. A total of 38 cores from the two ODP sites have been measured, providing spatial and directional information on the physical record of the ACC (Antarctic Circumpolar Current) in the Plio-Pleistocene. Declination and inclination of the paleomagnetic vector of each core segment were used to reorient the AMS principal axes to the geographic coordinates. The cores were reoriented using the measured direction of the characteristic remanent magnetization (ChRM) with respect to a common reference line for the core, from which we are able to determine the orientation of the paleocurrent flow for Sites 1095 (Drift 7) and 1101 (Drift 4) relative to the geographic coordinates. Both sites have paleocurrent directions trending ~ NW-SE, which in the former locality are parallel to a sediment wave field. Our study shows that a combination of magnetic fabric analysis and paleomagnetism allows deep-sea sedimentary fabric to be used as a long-term proxy of bottom-current flow history.     

ODP204航次钻探与研究最新成果评述

介绍了ODP204航次取得的最新成果与研究认识，评述了在该航次中由于技术体系的创新取得的新的研究成果。     

Grain-size records at ODP Site 1146 from the northern South China Sea: Implications on the East Asian monsoon evolution since 20 Ma

273 samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea (SCS) were analyzed for grain-size distributions using grain-size class vs. standard deviation method and end-member modeling algorithm (EMMA) in order to investigate the evolution of the East Asian monsoon since about 20 Ma. 10–19 μm/1.3–2.4 μm, the ratio of two grain-size populations with the highest variability through time was used to indicate East Asian winter monsoon intensity relative to summer monsoon. The mass accumulation rate of the coarsest end member EM1 (eolian), resulting from EMMA, can be used as a proxy of winter monsoon strength and Asian inland aridity, and the ratio of EM1/(EM2 EM3) as a proxy of winter monsoon intensity relative to summer monsoon. The combined proxies show that a profound enhancement of East Asian winter monsoon strength and winter monsoon intensity relative to summer monsoon occurred at about 8 Ma, and it is possible that the summer monsoon simultaneously intensified with winter monsoon at 3 Ma. Our results are well consistent with the previous studies in loess, eolian deposion in the Pacifc, radiolarians and planktonic foraminifera in the SCS. The phased uplift of the Himalaya-Tibetan Plateau may have played a significant role in strengthening the Asian monsoon at 8 Ma and 3 Ma.     

寻求深海碳酸盐沉积含量的物理标志

运用岩心物理性质求取深海碳酸盐沉积的相对含量，是古海洋学研究中一种“多块好省”的办法，在低分辨率研究中取得成功，但在较高分辨率的研究中精度问题十分突出。采用大洋钻探（ODP）184航次船上颜色反射率和磁化率数据，结合实验室碳酸盐化学分析实测结果，运用最小二乘法等数理分析方法，分别建立颜色反射率和磁化率与碳酸盐含量之间的拟合方程，估算碳酸盐含量。结果表明，根据颜色反射率蓝色光波段值及其平方作为控制变量来拟合的碳酸盐含量，同实测值具有较高的一致性，误差较小；但是，用磁化率推算的碳酸盐含量同实测值比较则误差相当大。用岩心物理性质推测的拟合方程是在开展一定量实测分析的基础上建立的，不同站位的这种拟合关系不能套用，只有当其他影响成分含量相当稳定或有规律变化时，运用岩心物理性质求取深海碳酸盐沉积含量的方法才具有相当高的应用价值。     

Exploring Physical Indicators for Carbonate Contents in Deep Sea Sediments

To determine carbonate contents of deep sea sediments using physical properties is an enjoyable and efficient paleoceanographic way, which has been processed successfully in many studies based on a low resolution, but rather than a high one. A series of mathematical methods including the least square method were used in this study to build multiple regression equations between color reflectance and carbonate content, and between magnetic susceptibility and carbonate content, based on the data of color reflectance and magnetic susceptibility collected via the ODP Leg 184 cruise, as well as measured carbonate contents. We found that percent carbonate estimated with diffuse reflectance is closer to the measured one than that estimated with magnetic susceptibility, and the multiple regression equation is varied in different sites.     

Radiogenic Hf isotopic compositions of continental eolian dust from Asia, its variability and its implications for seawater Hf

The inorganic silicate fraction extracted from bulk pelagic sediments from the North Pacific Ocean is eolian dust. It monitors the composition of continental crust exposed to erosion in Asia. ¹⁷⁶Lu/¹⁷⁷Hf ratios of modern dust are sub-chondritic between 0.011 and 0.016 but slightly elevated with respect to immature sediments. Modern dust samples display a large range in Hf isotopic composition (IC), −4.70<?_Hf<+16.45, which encompasses that observed for the time series of DSDP cores 885/886 and piston core LL44-GPC3 extending back to the late Cretaceous. Hafnium and neodymium isotopic results are consistent with a dominantly binary mixture of dust contributed from island arc volcanic material and dust from central Asia. The Hf-Nd isotopic correlation for all modern dust samples, ?_Hf=0.78?_Nd+5.66 (n=22, R²=0.79), is flatter than those reported so far for terrestrial reservoirs. Moreover, the variability in ?_Hf of Asian dust exceeds that predicted on the basis of corresponding ?_Nd values (−4.7<?_Hf<+2.5; −10.9<?_Nd<−10.1). This is attributed to: (1) the fixing of an important unradiogenic fraction of Hf in zircons, balanced by radiogenic Hf that is mobile in the erosional cycle, (2) the elevated Lu/Hf ratio in chemical sediments which, given time, results in a Hf signature that is radiogenic compared with Hf expected from its corresponding Nd isotopic components, and (3) the possibility that diagenetic resetting of marine sediments may incorporate a significant radiogenic Hf component into diagenetically grown minerals such as illite. Together, these processes may explain the variability and more radiogenic character of Hf isotopes when compared to the Nd isotopic signatures of Asian dust. The Hf-Nd isotope time series of eolian dust are consistent with the results of modern dust except two samples that have extremely radiogenic Hf for their Nd (?_Hf=+8.6 and +10.3, ?_Nd=−9.5 and −9.8). These data may point to a source contribution of dust unresolved by Nd and Pb isotopes. The Hf IC of eolian dust input to the oceans may be more variable and more radiogenic than previously anticipated. The Hf signature of Pacific seawater, however, has varied little over the past 20 Myr, especially across the drastic increase of eolian dust flux from Asia around 3.5 Ma. Therefore, continental contributions to seawater Hf appear to be riverine rather than eolian. Current predictions regarding the relative proportions of source components to seawater Hf must account for the presence of a variable and radiogenic continental component. Data on the IC and flux of river-dissolved Hf to the oceans are urgently required to better estimate contributions to seawater Hf. This then would permit the use of Hf isotopes as a monitor of past changes in erosion.     

Antarctic environmental variability since the late Miocene: ODP Site 745, the East Kerguelen sediment drift

Characterization of sediment from Ocean Drilling Program Site 745, representing the East Kerguelen Ridge sediment drift, addresses important issues surrounding the timing of Miocene to present East Antarctic ice sheet stability and oceanic environmental change. Our results show three periods of greatly enhanced accumulation of Antarctic-derived sediment, at 6.4-5.9 Ma, 4.9-4.4 Ma and 1.1-0.8 Ma, potentially indicative of warmer, less stable ice sheets at these times. Conversely, the accumulation of Antarctic-derived material is comparatively less during the middle of the Pliocene warm epoch (4.8-3.2 Ma). The deep flow forming the Kerguelen drift was stronger during the latest Miocene and earliest Pliocene and has decreased in intensity continuously since then.     

多维分形奇异功率谱分析与应用

地球物理信号中普遍含有噪声，消除噪声是地球物理信号处理中的关键技术之一.奇异功率谱分析（SSA）是在状态空间（又称相空间）中研究（系统）动力学、非线性科学与混沌现象的方法.本文在状态空间中通过SSA分解，研究、应用地球物理序列的尺度不变性进行多维分形滤波：通过在状态空间的SSA分解，构造了经验正交函数系（EOF）；在EOF子空间中定义了两种尺度与测度后，发现了两种测度与尺度皆在多个尺度范围内存在尺度不变性；利用这种尺度~测度的尺度不变性，设计、实现了多维分形奇异功率谱(MSSA)滤波模型；处理解释了大洋钻探（ODP）1143A孔岩芯自然反射性（NGR）资料；Fourier功率谱分析结果证明，MSSA能有效地压制噪声，提取有用信号.研究得出，嵌入维数对MSSA基本无影响（小于1/1000），多维分形滤波器（MSSA）能有效压制噪声或提取有用信号.     

Inversion of titanomaghemite in oceanic basalt during heating

We studied the change of magnetic behaviour upon laboratory heating of altered mid-ocean ridge basalt (MORB) samples in the age range of 16-35 Ma to determine the influence of titanomaghemite inversion on the thermal demagnetisation of natural remanent magnetisation (NRM) of these basalts. MORB samples were heated to successively higher temperatures and at the same time the temperature dependence of either saturation magnetisation or NRM was monitored continuously. After each heating step, hysteresis loops and remanent magnetisation curves between 10 K and room temperature were measured. With this procedure, it is shown that the dominant magnetic remanence carrier in our MORB samples is cation deficient titanomaghemite. Moreover, it is demonstrated that the titanomaghemite is gradually changing to a Ti-poor titanomagnetite as the final inversion product. During inversion, both the Curie temperature as well as the maximum unblocking temperature of the NRM are gradually increasing. We show that the paradox of unblocking temperatures above the Curie temperatures often observed for altered MORBs is an artefact of this gradual, heating induced inversion process.     

深水沉积物波及其在南海研究之现状

深水沉积物波的研究始于20世纪50年代。根据成因和结构特征,可以将深水沉积物波划分为细粒底流、细粒浊流、粗粒底流和粗粒浊流等类型。不同类型的沉积物波具有不同的形态、物质组成及分布特征。已提出的深水沉积物波的形成模式主要有背流波模式、逆行沙波模式、内波模式及底形和斜坡失稳混合模式等。1994年太阳号95航次和1999年ODP184航次揭示并证实,南海北部东沙岸外1144站所处的深水陆坡区发育有一高速沉积物牵引体。根据最新的地震资料分析发现,该牵引体实际上由一系列逆陆坡向上倾方向迁移的沉积物波组成,这一发现对于南海北部大陆边缘古海洋、古环境和古气候研究,以及南海深水油气勘探具有重要意义。