Magnetic polarity sequences from downhole measurements in ODP holes 998B and 1001A,leg 165, Caribbean Sea

The magnetostratigraphy of sediments drilled at two sites (998 and 1001) in the Caribbean Sea during the Ocean Drilling Program (ODP) Leg 165 is established directly from downhole measurements. For the first time in the ODP, the magnetostratigraphy of sediments is determined from both logging magnetic data and paleomagnetic analysis of core material for an age interval of about 16 m.y. across the Cretaceous/Tertiary boundary. The total magnetic induction and the susceptibility of the surrounding sediment were recorded in situ and continuously by the GHMT (Geological High-sensitivity Magnetic Tool). The aim of processing these data is to determine the magnetic polarity. At Site 998 (Cayman Rise), no paleomagnetic results are available from the recovered cores given a hard secondary overprint. Nonetheless, a magnetostratigraphic sequence from early Miocene to late Oligocene is proposed from the analysis of the GHMT data. The comparison with biostratigraphic results shows the accuracy of the resulting magnetic polarity. At Site 1001 (Hess escarpment), cores were recovered in two holes thus allowing the comparison of the magnetostratigraphic results obtained from the GHMT data and from paleomagnetic analysis of core samples over a length of 200 m representing about 16 m.y. from early Eocene to late Cretaceous times. The sedimentation rates computed from the determined magnetochrons are very similar and correspond to the ones obtained from shipboard biostratigraphic results. One of the most striking result obtained from the magnetostratigraphic results, especially the GHMT data, at Site 1001, is the abrupt decrease in the sedimentation rate across the K/T boundary. This suppressed sedimentation lasted at least 4 Ma after the K/T event.     

Late Pliocene Mega Debris Flow Deposit and Related Fluid Escapes Identified on the Antarctic Peninsula Continental Margin by Seismic Reflection Data Analysis

We have obtained improved images of a debris flow deposit through the reprocessing of multichannel seismic reflection data between Drifts 6 and 7 of the continental rise of the Pacific margin of the Antarctic Peninsula. The reprocessing, primarily aimed at the reduction of noise, relative to amplitude preservation, deconvolution, also included accurate velocity analyses. The deposit is dated as upper Pliocene (nearly 3.0 Ma) via correlation to Sites 1095 and 1096 of the Ocean Drilling Program (ODP) Leg 178. The estimated volume is about 1800 km³ and the inferred provenance from the continental slope implies a run out distance exceeding 250 km. The dramatic mass-wasting event that produced this deposit, unique in the sedimentary history of this margin, is related to widespread late Pliocene margin erosion. This was associated with a catastrophic continental margin collapse, following the Antarctic ice sheet expansion in response to global cooling. The seismic data analysis also allowed us to identify diffractions and amplitude anomalies interpreted as expressions of sedimentary mounds at the seafloor overlying narrow high-velocity zones that we interpret as conduits of fluid expulsion hosting either methane hydrates or authigenic carbonates. Fluid expulsion was triggered by loading of underlying sediments by the debris flow deposits and may have continued until today by input of fluids from sediment compaction following the deep diagenesis of biogenic silica.     

长江口外海区DZS2孔第四纪磁性地层

长江河口区外海域是陆源物质沉积以及向外海输送的重要地带。DZS2孔采自长江河口外海域(东海陆架西北部),是我国现今最深,年代最久远的大陆架海底全取心钻孔。对其进行了细致的古地磁研究,结合岩心岩性特征、微体古生物鉴定以及AMS14C测年等的分析,建立了可靠的长江河口外海区年代地层框架序列。磁极性地层研究揭示了清晰的M/B极性转换界线,且沉积地层中出现了多个可与标准极性柱对比的反极性漂移事件,这其中包括Gothenburg、Blake、Emperor、Jaramillo和Olduvai事件,这为钻孔年代约束以及长江三角洲区域地层对比提供了重要的依据。钻孔DZS2沉积物岩性特征较为简单,相对来说泥质层位较多,与长江三角洲陆上典型第四纪钻孔的岩性特征以及沉积演化旋回存在一定差别。这可能与整个长江三角洲区域下覆基岩的起伏有关,根据单道地震剖面的资料,长江河口区西南部有一系列岛屿分布,海底明显存在较高隆起的基岩,松散沉积物厚度较薄,这很可能阻挡了部分长江物质,特别是粗颗粒沉积物向外海的输送。     

南沙群岛海区NS97-13柱样沉积物磁化率各向异性研究

通过南海南沙群岛海区NS97-13柱样磁化率各向异性系统测试、统计计算和测年等分析,给出了沉积物的磁化率椭球几何形态,重建了研究海区的古水流方向、沉积物的沉积形式。研究结果表明:(1)压扁型椭球(E>1)的择优方向位于磁性线理-磁性叶理直线的上方,夹角β的范围为0°<β≤33.14°,拉长型椭球(E<1)的择优方向位于磁性线理-磁性叶理直线的下方,夹角α的范围为33.14°<α<56.86°,指示磁性颗粒沉降时受动力作用的大小及其受力方向不同;(2)由磁化率各向异性参数(磁性线理-各向异性度,磁性叶理-各向异性度)确定的沉积物有改造沉积、再沉积和稳定沉积三种形式,前两种形式掺杂在稳定沉积层中的不同部位,构成部分混合沉积;(3)最大磁化率和中间磁化率的轴向分布表征研究海区距今近62 ka以来古水流方向主要为南北和北东-南西向,总磁化率记录了Heinrich层中的冷事件H1—H6。     

A record of fluctuating bottom currents on the slopes west of the Porcupine Bank,offshore Ireland — implications for Late Quaternary climate forcing

Both local and regional controls on slope sedimentation west of Porcupine Bank are assessed using an array of 25 gravity cores, integrated with shallow seismic, TOBI side-scan and high-resolution bathymetry data. The cores were retrieved from an area of smooth, distally steepened slope (between 52° and 53°N) in water depths of 950 to 2750 m. The slope here is unmodified by gravity failures and is swept by bottom currents that flow from S to N along the margin. The cores reveal a coherent shallow stratigraphy that can be traced along and between transects at upper-, mid- and lower-slope levels. AMS ¹⁴C dating, oxygen-isotopes and carbonate profiles suggest the cored record could extend as far back as 500 ka in the longest cores, with most cores providing details of the slope response to the last interglacial, last glacial and Holocene forcing. The facies indicate deposition was dominated by a combination of bottom currents, ice-rafting and hemipelagic settling, with carbonate-prone deposits during interglacials, and siliciclastic deposits during glacials. Inferred contourites imply that strong currents operated during interglacials, with weaker current reworking during glacial conditions. A pair of erosion surfaces record significant mid- and upper-slope scouring during Marine Isotope Stage (MIS 3) and in the Early Holocene. The lateral facies distribution implies stronger currents at shallower levels on the slope, although there is evidence that the core of the current migrated up and down the slope, and that sand might locally have spilt down-slope. The bathymetry influenced both the wider geometry of the condensed contourite sheet and the local thickness and facies variation across the slope. A significant result of the study is the identification of a pair of thin sand–mud contourite couplets that record enhanced bottom-current reworking corresponding to periods of interstadial warming during MIS 3. The couplets can be correlated to the terrestrial records onshore Ireland and imply that the NE Atlantic margin oceanographic and onshore climate records are strongly coupled at interstadial level.     

Paleomagnetism and paleoclimate change in the South China Sea since the late Pleistocene

Signal processing techniques of cross-correlation, power spectral analysis and cross-power spectral analysis are used to determine the relationships between paleomagnetic properties and the oxygen isotope record of the South China Sea during the late Pleistocene. On the basis of correlation tests between the δ18O time series of cores V3 and A15 with the paleomagnetic properties of core SCS01, five events of susceptibility and intensity of natural remanent magnetization are identified as glacial events corresponding to δ18O positive events. The magnetic properties of the sediments which are independent of the geomagnetic field are sensitive climatic indicators which have frequencies corresponding to the Milankovitch orbital parameters.     

Petrofabric Investigation of Gabbros from the Oman Ophiolite: Comparison between AMS and Rock Fabric

The anisotropy of magnetic susceptibility was measured on 42 gabbros sampled across a complete plutonic sequence from the Oman ophiolite. The rock fabrics, investigated in the field and through plagioclase crystallographic fabric measurements, were compared to the magnetic fabrics. This comparative study reveals that from the paleo-Moho to the top of the foliated gabbros level, 73% of the rocks display a good correspondence in orientation, between the magnetic and rock fabric orientation. In these rocks, the AMS is controlled by secondary magnetites located in the fracture network of the olivines, and probably, but to a lesser extent, by secondary magnetites located in the exsolution lamellae of the clinopyroxenes. The high correlation between the AMS ellipsoid orientation and the rock fabric orientation is explained by the fact that the magnetic foliation is essentially constrained by the orientation of the olivine fracture planes, which is in turn constrained by the orientation of the overall magmatic rock fabric. In contrast to the primary mineral phases, the orientation of magnetite crystals in these gabbros is not due to their alignment in a flowing magma, so their preferred orientation, although usually mimicking that of the rock fabric, does have not the same origin. Furthermore, given that the preferred orientation of the anisometric secondary magnetites is much less perfect than the preferred orientation of the plagioclases, no correlation between the shape and magnitude of the AMS and plagioclase fabrics can be established. In the uppermost levels of the sequence there is no correspondence between the magnetic and rock fabric orientation. The magnetism of these rocks is mainly carried by primary magnetite and ilmenite grains. These minerals occur as small and scattered interstitial grains that exhibit neither alignment nor parallelism with the pre-existing rock fabric. Hence, the anisotropy, shape and orientation of the AMS ellipsoid are independent of the rock fabric ellipsoid. Although in the Wadi Al Abyad gabbros, just like in other magnetite bearing rocks (Rochette et al., 1992; Archanjo et al., 1995), the AMS cannot be used to evaluate the shape and strength of the finite strain ellipsoid, it can be reliably used to get the orientation of the rock fabric ellipsoid when the AMS is controlled by secondary magnetites.     

Tying seismic data to geologic information from core data: an example from ODP Leg 177

The integration of seismic data with core data should provide ground-truth to a structural interpretation of seismic data. The main difficulty in such an integration effort is the correct translation of physical property measurements on cores to a form which can be used in seismostratigraphic interpretation. In the absence of down-hole well data and check-shots, required knowledge of the velocity structure at the drilling locations can be obtained directly from measurements of the physical properties of core samples. This involves upscaling of the data from physical properties of cores to the sample interval used in the seismic data. In the present study, three of the seven drill-sites of ODP (Ocean Drilling Program) Leg 177 in 1997/1998, located on the Agulhas Ridge in the south-eastern Atlantic (sites 1088–1090), were connected with eight seismic profiles. Physical properties data measured on the cores from the various holes at each site were combined to create a single continuous log and used to construct synthetic seismograms. The synthetics generally show a good agreement with real seismic data in terms of amplitude and waveform. Some reflections in these generated traces may have a time-shift due to sections with incomplete or spurious P-wave velocity measurements in the ODP datasets. The main reflectors identified in the real seismic data correspond to hiatuses or periods of reduced sedimentation rates, and correlate well with density variations. One particular hiatus, clearly observable in the real seismic data, was not unequivocally identifiable in the various types of core data, and tying core data to seismic data can confirm its existence in the core data, showing the benefit of including seismic data in an interpretation of core log data. On the other hand, core data provide a calibration tool for the geological timescale of seismic data and information about the lithology, needed in the interpretation of seismic data.     

陆相断陷湖盆沉积物磁组构特征及环境意义--以泥河湾盆地为例

选取陆相湖盆泥河湾盆地的郝家台和东谷坨两剖面,共取样２８６块,研究喾物磁组构参数特征及其环境意义。结果表明,沉积物磁组构参数的变化与沉积物粒度参数、磁化率的变化密切相关,蕴含着环境变化的信息。磁化率各向异性度（Ｐ）、磁线理（Ｌ）、沉积物磁化率的高值和细粒物质含量的低值指示湖水平面下降,气候较为干燥;相反,则说明湖水平面上升,气候较为湿润。据此,将泥河湾盆地沉积过程中湖水平面的变化过程划分为２个大的     

南极陆缘天然气水合物特征及资源前景

首先,根据地震反射剖面的似海底反射特征、深海钻探计划(DSDP)和大洋钻探计划(ODP)钻孔沉积物的高甲烷含量、高有机碳含量以及孔隙水盐度、氯离子浓度和硫酸根离子浓度异常等地球物理和地球化学证据推测,南极陆缘有7个潜在的天然气水合物分布区,它们分别为南设得兰陆缘、南极半岛的太平洋陆缘、罗斯海陆缘、威尔克斯地陆缘、普林斯湾陆缘、里瑟-拉森海陆缘和南奥克尼群岛东南陆缘等。其次,从气源条件、沉积条件、热流及温压条件和地质构造条件等对南极陆缘天然气水合物的成藏条件进行了分析,认为该陆缘具备天然气水合物形成和赋存的有利地质条件。最后,对南极陆缘天然气水合物的资源前景进行了探讨,认为其资源量非常可观。     

A 23 Myr magnetostratigraphic time framework for Site 1148, ODP Leg 184 in South China Sea and its geological implications

The Ocean Drilling Program (ODP) Leg 184 recovered a continuous sedimentary sequence since Miocene at Site 1148 in the northern South China Sea (SCS), which provided a unique record for better understanding the tectonic, environmental and climatic evolution of the SCS. In this study, we report a detailed paleomagnetic study on 881 discrete samples from the upper 460 m cores of Hole 1148A through stepwise alternating field demagnetization (AF) or thermal demagnetization (TD). Rock magnetic results from isothermal remanent magnetization (IRM) acquisition, temperature dependence magnetic susceptibility (κ-T) and hysteresis loop parameters of representative samples indicate that the main magnetic mineral is low coercivity pseudo-single domain magnetite. The characteristic remanent magnetization (ChRM) with moderate downward or upward inclinations was identified after removing a low-stability drilling-induced remanence with steep downward inclinations. A new 23 Myr magnetostratigraphic time framework was established by correlating the magnetic polarity sequence to the Geomagnetic Polarity Time Scale 2012 (GPTS 2012; Ogg, 2012) in combination with biostratigraphic data. The mean sediment accumulation rates for lithostratigraphic Units V and IV, Unit III and lower Unit II, upper Unit II, Unit IB and Unit IA are calculated as ∼20.8 m/Myr, ∼15–17 m/Myr, 24.18 m/Myr, 17.07 m/Myr and 49.01 m/Myr, respectively.     

珠江口全新世沉积物粒度与磁化率的变化特征及其所反映的气候环境变化

通过对来自珠江口万顷沙W2孔的沉积物进行AMS 14 C测年,结合古地磁测试以及孢粉分析结果,并与其他相关记录综合对比,建立了该地区全新世以来的年代框架。使用粒度组合特征以及磁化率等环境代用指标,获取了该地区约6000cal yr BP以来的气候环境变化信息。经过分析与对比,认为该区自中全新世以来,经历了暖干-凉湿-温湿3个气候环境变化阶段。在晚全新世中国南方地区气候(尤其是降雨量)发生了重要的变化,这与北半球太阳辐射减弱,气温降低,亚洲季风减弱,降雨带南移有着一致的对应关系,同时还很可能与这一时期厄尔尼诺—南方涛动活动加强有着紧密的联系。     

Early interglacial bottom-current sedimentation on the eastern Reykjanes Ridge

Unusual late deglacial/early interglacial sedimentation rates following the last two glaciations have been detected from three cores on the eastern Reykjanes Ridge. Morphologic, textural, and mineralogic evidence pinpoint bottom currents as the process responsible for these unusual depositional rates, which rise at first to 10–40 times the regional mean, and then subside slightly to values still 2–5 times the norm. A portion of the lutite excesses in the interglacial sediment of these cores is accounted for by input of fine detritus eroded from subaerially exposed and isostatically uplifted Icelandic basalts. For the most part, however, the extraordinary depositional rates late in the deglacial periods are caused by erosion and redistribution of previously deposited glacialage sediments. We infer that reinitiation of overflow of deep water into the Atlantic due to deglacial removal of the Norwegian Sea ice cover triggered a strong bottom-current flow that dislodged sediment from exposed sites. The deglacial intensification of the deep circulation thus left a major sedimentological imprint on the sediments of the eastern Reykjanes Ridge.     

西菲律宾海GX149柱状样磁性地层学

对取自西菲律宾海西部的GX149重力活塞柱状样沉积物进行系统的岩石磁学和古地磁学研究,结果表明:沉积物中主要载磁矿物为低矫顽力的磁铁矿,粒径以假单畴(PSD)为主。天然剩磁强度分布范围为(1.07~6.51)×10-7 Am2,平均值为3.33×10-7 Am2。经过0~80 mT交变场退磁后,几乎所有样品均可获得稳定的特征矢量方向,且磁倾角绝大部分为正值,表明整个GX149柱状样处于布容正极性期。样品的磁倾角普遍存在浅化现象,且从顶到底,磁倾角变浅程度逐渐增强,与沉积物的压实作用有关。通过与相邻海域柱状样的对比,估算出GX149柱状样的平均沉积速率在0.79~2.42 cm/ka之间,其底界年龄在256.2~781 ka之间。     

Seismic analyses of Cenozoic contourite drift development in the Northern Norwegian Sea

Four drift accumulations have been identified on the continental margin of northern Norway; the Lofoten Drift, the Vesterålen Drift, the Nyk Drift and the Sklinnadjupet Drift. Based on seismic character these drifts were found to belong to two main groups; (1) mounded, elongated, upslope accretion drifts (Lofoten Drift, Vesterålen Drift and Nyk Drift), and (2) infilling drifts (Sklinnadjupet Drift). The drifts are located on the continental slope. Mainly surface and intermediate water circulation, contrary to many North Atlantic and Antarctic drifts that are related to bottom water circulation, and sediment availability have controlled their growth. Sediments were derived both from winnowing of the shelf and upper slope and from ice sheets when present on the shelf. The main source area was the Vøring margin. This explains the high maximum average sedimentation rate of the nearby Nyk (1.2 m/ka) and Sklinnadjupet (0.5 m/ka) Drifts compared with the distal Lofoten (0.036 m/ka) and Vesterålen (0.060 m/ka) Drifts. The high sedimentation rate of the Nyk Drift, deposited during the period between the late Saalian and the late Weichselian is of the same order of magnitude as previously reported for glacigenic slope sediments deposited during glacial maximum periods only. The Sklinnadjupet Drift is infilling a paleo-slide scar. The development of the infilling drift was possible due to the available accommodation space, a slide scar acting as a sediment trap. Based on the formation of diapirs originating from the Sklinnadjupet Drift sediments we infer these sediments to have a muddy composition with relatively high water content and low density, more easily liquefied and mobilised compared with the glacigenic diamictons.     

珠江口全新世沉积物粒度与磁化率的变化特征及其所反映的气候环境变化（英文）

通过对来自珠江口万顷沙W2孔的沉积物进行AMS14C测年,结合古地磁测试以及孢粉分析结果,并与其他相关记录综合对比,建立了该地区全新世以来的年代框架。使用粒度组合特征以及磁化率为环境代用指标,获取了该地区约6000 cal yr BP以来的气候环境变化信息。经过分析与对比,认为该区自中全新世以来,经历了暖干-凉湿-温湿三个气候环境变化阶段。在晚全新世中国南方地区气候（尤其是降雨量）发生了重要的变化,这与北半球太阳辐射减弱,气温降低,亚洲季风减弱,降雨带南移有着一致的对应关系,同时还很可能与这一时期厄尔尼诺—南方涛动活动加强有着紧密的联系。     

Evidence for climatic and oceanographic controls on terrigenous sediment supply to the Indian Ocean sector of the Southern Ocean over the past 63,000?years

Multiple proxy studies on sediment core SK 200/22a from the sub-Antarctic sector of the Indian Ocean revealed millennial-scale fluctuations in terrigenous input during the last 63,000?years. The marine isotope stages 1 (MIS 1) and MIS 3 are characterized by generally low concentrations of magnetic minerals, being dominated by fine-grained magnetite and titano-magnetite. Within the chronological constraints, periods of enhanced terrigenous input and calcite productivity over the last 63,000?years are nearly synchronous with the warming events recorded in Antarctic ice cores. An equatorward shift of the westerly wind system in association with a strengthening of the Antarctic Circumpolar Current (ACC) system may have promoted wind-induced shallow-water erosion around oceanic islands, leading to enhanced terrigenous input to the core site. Major ice-rafted debris events at 13?C23, 25?C30, 45?C48 and 55?C58?ka BP are asynchronous with ??¹⁸O and carbonate productivity records. This out-of-phase relation suggests that ice-sheet dynamics and ACC intensity were the primary factors influencing ice rafting and terrigenous input to the Indian sector of the Southern Ocean, with only limited support from sea-surface warming.     

晚更新世末期地磁短期游移的研究

本文根据对胶州湾至丁字湾岸段的6个柱状岩芯古地磁标本系统测量结果,结合地层、古生物、古气候及~(14)C年代测定,确定了该区晚更新世末期哥德堡短期游移的存在。     

Impact of gamma densitometry on the luminescence signal of quartz grains

Most of the marine sediment cores collected in the past 10 years by the ODP and IMAGES programs have been processed using a multi-sensor core logger (MSCL, Geotek). This system estimates the density of sediment using the γ-ray attenuation of a ¹³⁷Cs source. To date, optically stimulated luminescence (OSL) dating has not been applied to sediments from cores treated in this way, because of the possible impact of this exposure on the existing luminescence signal. We have investigated the effect of such γ-ray irradiation by test exposure, measurement and estimation, and conclude that the usual ¹³⁷Cs exposure from γ-ray attenuation densitometry has no impact on the luminescence signal of quartz grains in sediment cores. For longer exposure to the ¹³⁷Cs source, a formula is provided to calculate the received dose. This result will permit the OSL dating technique to be applied to the voluminous body of marine cores already collected and processed by MSCL.     

Are Antarctic Peninsula Ice Sheet grounding events manifest in sedimentary cycles on the adjacent continental rise?

The direct record of Late Miocene–Early Pliocene Antarctic Peninsula Ice Sheet expansions from a previously published seismostratigraphic study of the outer shelf at Ocean Drilling Program Site 1097 is compared to the glacial history we deduced from published proxy evidence within coeval sections on the adjacent continental rise. The proxies are sedimentary structures (laminated vs. massive/bioturbated facies) and clay minerals (predominantly smectite and chlorite contents) from Ocean Drilling Program Site 1095 located on the distal part of a large drift. The comparison shows that more sedimentary cycles are evident on the continental rise for three of the four diatom biozones we considered. This indicates that the continental-rise sedimentology may indeed be related to local or regional paleoenvironmental variability, including Antarctic Peninsula Ice Sheet grounding events on the adjacent outer continental shelf. If correct, this would be a promising result because unlike the outer continental shelf sequences drilled thus far, the continental rise record is relatively continuous and can be dated using paleomagnetic and biostratigraphic data. However, our study also shows that no objective criteria provide direct linkages between the glacial history we deduced from the two continental rise proxies and that previously derived from the continental-shelf seismic stratigraphy. Furthermore, the two sedimentologic proxies on the continental rise do not always provide a consistent picture of glacial history when compared against each other.