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Marine Geology
Elsevier Science B.V.
Marine Geology

Elsevier Science B.V.

0025-3227

Marine Geology/Journal Marine GeologySCIISTPAHCIEI
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    Pore structure changes induced by hydrate dissociation: An example of the unconsolidated clayey-silty hydrate bearing sediment reservoir in the South China Sea

    Yao, YanbinQin, XuwenLu, ChengLuo, Wanjing...
    11页
    查看更多>>摘要：The natural gas hydrates (NGHs) in the South China Sea are found in unconsolidated clayey-silty sediments, featured by very weak mechanical strength and low permeability. During gas extraction from such unstable, low permeability reservoirs, hydrate dissociation can result in distinct changes in pore properties and flow capability. Thus, it is extremely important to investigate the pore characteristics and gas flow mechanisms in such reservoirs. In this study, five sediment samples including two from the hydrate bearing layer (HBL) and three from the underlying layer (ULL), were taken from an exploration well in the Shenhu area, South China Sea. The pore morphology, pore specific surface area (SSA), total pore volume, porosity, and pore size distribution of these samples have been analyzed to compare the reservoir physical properties of the HBL (after the hydrate has dissociated and disappeared) with the ULL. Results from scanning electron microscopy reveal that the HBL samples without the hydrate present feature larger grain sizes, larger pore apertures and better connectivity than ULL samples. Results of NMR analysis and low temperature N-2/CO2-adsorption show that both HBL and ULL samples have high total porosities (42.4%-48.8%) and high SSA (21.17-30.09 m(2)/g), while the results from centrifugal experiments demonstrate that the producible porosity (defined as the pore volume available for hydrocarbon emplacement) of the HBL samples is about twice (26.6%-30.3%) that of the ULL samples (15.5%-19.6%). The pores in the two sample types are dominated by intra-particle pores of 80 nm, and inter-particle and inter-aggregate pores of 0.2-3 mu m. The above differences between HBL and ULL can be used to analyze the pore structure changes occurring as a result of the dissociation of NGHs in the HBL. We assume that on the one hand, formation of NGHs reduces pore pressure, thus resulting in higher effective stress, then compacts the pore space and reduces the total porosity. On the other hand, the release of high-pressure gas during hydrate dissociation could increase the pore aperture and improve pore connectivity, and thus increase the producible porosity of the HBL reservoir. It is also assumed that the flow capacity in the methane production zone in the HBL can be improved by the NGHs dissociation, suggesting that not only the hydrate saturation but also the pore structure changes should be considered when estimating the permeability during the gas production process.
      原文链接:
    • NSTL
    • Elsevier

    Shelf-margin architecture and deposition variability across the mid-Pleistocene climate transition, northeastern South China Sea

    Liu, HanyaoZhang, ZhongtaoZhang, BoJiang, Jing...
    17页
    查看更多>>摘要：The shelf-margin prism of the Upper Pliocene to Pleistocene in the northeastern South China Sea has recorded significant signatures of glacio-eustatic cycles and paleo-climatic conditions. Integrated analysis of 3D seismic, well-logging, and borehole dating data in the study area provided a detailed depiction of the high-resolution sequence architecture and depositional evolution as they responded to the glacio-eustatic cycles and climate changes. The shelf-margin section of the Upper Pliocene to Pleistocene is comprised of a composite sequence bounded by a regional unconformity, which can be further divided into five sequences confined by subordinate unconformities. The two lower sequences (S1-S2) with similar and simple internal structures, are displayed as having roughly equal thicknesses (ca. 50 m) and gently seaward-dipping foresets. In contrast, the upper three sequences (S3-S5) are characterized by the development of high (200-600 m) and steep (2-5.) shelf-margin clinoforms, which can be further divided into fifteen clinothems. The sequences generally embody four systems tracts, showing different stratal stacking patterns, and the linking contemporaneous depositional systems including fluvial channels, deltaic and slope fan systems are documented. Based on the chronostratigraphic framework, comparisons have been made between the sequence cycles and the global oxygen isotopic records. The results suggest that the formation of different order sequences may have been in response to the glacio-eustatic cycles paced by the Milankovitch orbital cyclicity. The abrupt changes in the sequence and depositional architectures between S1-S2 and S3-S5 are assumed to be related to the significant increases in the amplitudes and durations of the glacio-eustatic fluctuations across the mid-Pleistocene climate transition (MPT). Intensively variable climatic conditions with strengthened monsoons during and after the MPT could have potentially led to an abundant sediment supply, which may have enhanced the development of shelf to shelf-edge delta systems with the large-scale prograding clinoforms.
      原文链接:
    • NSTL
    • Elsevier

    The precision and accuracy of measuring micro-scale erosion on shore platforms

    Yuan, RunjieKennedy, David M.Stephenson, Wayne J.Finlayson, Brian L....
    12页
    查看更多>>摘要：Shore platforms evolve over millennial scales but understanding precisely how instantaneous processes scale over these longer periods is a major scientific quandary. A reason for this, is that erosion often occurs on the granular scale and measurement of magnitude and frequency of this decay is difficult. Over the past 50 years, a number of techniques have been used to quantify the micro-scale (0.01-10 mm) erosion of coastal bedrock, from direct surface contact techniques to those which involve remote sensing. The micro-erosion meter (MEM) and traversing micro-erosion meter (TMEM) are the most common techniques, recording erosion rates from <0.1 to 5 mm/yr on rock surfaces of c. 50 cm(2). The traversing erosion beam (TEB) is specifically designed to measure rapid erosion (>10 mm/yr) while the Swantesson laser scanner (SLS) and close-range Structure-from-motion (SfM) photogrammetry have been used for observation of larger rock surfaces of 0.2 m(2). The rock tablet method is used to investigate the individual processes operating on downwearing. By comparing their advantages and disadvantages, the T/MEM is suggested to continue to play an important role in monitoring surface lowering in conditions where the total erosion is 20 mm over the monitoring period, with close-range SfM recommended as ideal for quantifying erosion of 2-50 mm. However, all the instruments are biased in the field conditions under which they can be operated, and it is important for researchers to contextualise their measured surfaces in regards to a whole platform. Future studies will benefit from the combination of the ground-based methods with new techniques that provide finer morphological features at the regional scale or interpret the evolution of shore platforms over long timescales.
      原文链接:
    • NSTL
    • Elsevier

    Investigating the resetting of IRSL signals in beach cobbles and their potential for rock surface dating of marine terraces in Northern Chile

    Brill, DominikAgeby, LucasObert, ChristinaHollerbach, Rolf...
    17页
    查看更多>>摘要：Inactive shorelines represent valuable records for sea level change, shoreline variations and tectonics if we can constrain the timing of their formation. Where the associated beaches are cobble dominated, luminescence rock surface dating is a promising alternative to established dating approaches, since unlike other techniques it offers the potential to identify clasts unaffected by inherited ages. While luminescence rock surface dating has successfully been used on Holocene and Late Pleistocene beach ridges previously, in this study the potential of IRSL rock surface dating is evaluated for the magmatic cobbles of uplifted Pleistocene terraces along the tectonically active coast of northern Chile. Cobbles from an active beach were used to investigate the influence of cobble lithology on IRSL signal properties and the effectiveness of IRSL signal resetting in the rock. While alkaline and andesitic cobbles yield low IRSL intensities and limited signal resetting due to strong light attenuation, more favourable characteristics for dating were observed for some diorite and granite cobbles. Their IRSL signals were well reset in the uppermost few mm without any systematic difference between upper and lower surface. Some of them revealed bleaching plateaus with inherited ages close to zero after correction for laboratory residuals. For dating, cobbles from three Pleistocene marine terraces, for which new uranium-thorium and ESR control ages on molluscs provide age control, were targeted. None of the associated IRSL rock surface burial ages agrees with the MIS 5 control ages of the terraces. Most of the selected cobbles are either too dark to allow for effective signal resetting or yield IRSL properties unsuitable for dating. Only one of the targeted cobbles shows both signs of signal resetting at its surface and sensitive IRSL signals, but its signal was already in field saturation due to dose rates >6 Gy/ka. In conclusion, our data indicate that beach cobbles with granitic to dioritic lithology combine appropriate IRSL properties and sufficient IRSL signal resetting for dating Holocene landforms. Last interglacial terraces may already be beyond the limit of IRSL dating for most cobbles of this lithology since they show large dose rates compared to IRSL sediment dating.
      原文链接:
    • NSTL
    • Elsevier

    Characteristics and possible formation process of ferromanganese beachrock in an intertidal zone of East China Sea influenced by buried ancient woods

    Yang, ZhongkangZhang, ZezhouSun, LiguangYuan, Linxi...
    6页
    查看更多>>摘要：The ferromanganese (Fe-Mn) deposits could be found in many shallow water and deep-ocean places. Lots of studies have been performed to explore the occurrence, formation and economic value of Fe-Mn nodules or crusts on the deep ocean floor. However, the occurrence and formation of Fe-Mn beachrocks at the coastal zones has rarely been reported. In the present study, we collected two types of dendritic Fe-Mn beachrock samples from an intertidal zone of Zhoushan Archipelago, East China Sea, where was significantly amended by an ancient wood layer formed during 5900-5600 cal. yr B.P. Then the microscale analysis including electron microprobe analyzer (EMPA), X-ray powder diffraction (XRD), and scanning electron microscopy-energy dispersive X-ray spectrometer (SEM-EDS) were performed to study the microscale characteristics on these Fe-Mn beachrocks. Combined with our previous studies, a possible formation process was proposed: (1) Buried ancient woods produced enough organic acids to accelerate the weathering of nearby bedrock via chemical or biological reactions; (2) The seepage water could be regarded as a bridge to connect the ancients woods and the intertidal zones, and large amounts of dissolved Fe2+ and Mn2+ can be transported to the sandy beach and intertidal zones; (3) Precipitation of insoluble Fe-Mn oxides occurred with the involvement of microbial processes and direct inorganic chemical sorption under mixing of seepage water and seawater; (4) After that, these Fe-Mn oxides would experience nucleation, crystallization growth, and then diagenetic process to form the Fe-Mn beachrocks. The present study spotted a light on the microscale features, formation process and biogeochemical cycling of nearshore/coastal diagenetic Fe-Mn deposits.
      原文链接:
    • NSTL
    • Elsevier

    Modern dynamics, morphology and habitats of slope-confined canyons on the northwest Australian margin

    Post, Alexandra L.Przeslawski, RachelNanson, RachelSiwabessy, Justy...
    18页
    查看更多>>摘要：Repeat multibeam mapping of two slope-confined canyons on the northwest Australian margin provides new understanding of the processes that are active in shaping these environments. The Cape Range and Cloates canyons initiate on the mid to lower continental slope but are now known to be connected to the shelf via small channels and gullies. Both canyons have areas of steep walls, with evidence of slides, and large depressions on the canyon floors. These canyons were first mapped systematically with multibeam sonar in 2008 and were remapped in 2020 during a biodiversity survey that also collected high-resolution imagery and biological samples. Comparison of seabed features between these two time periods indicates active sliding, minor headwall retreat and continued excavation of deep depressions on the canyon floor. Significantly, intact blades of displaced seagrass imaged at various depths up to 4200 m throughout both canyons indicates that material sourced from the adjacent continental shelf is being transported through these canyon systems. Turbidity currents are actively modifying canyon walls and floor depressions, while also providing a sediment source that has resulted in minor accretion on the canyon floor. Sedimentation likely regulates benthic communities in these canyons, with imagery showing highest densities of sessile invertebrates in habitats protected from sedimentation (e.g. rock overhangs, cliff edges). Since steep canyon habitats are rare within these canyons, and support high benthic abundance, they likely represent biologically significant areas of the Gascoyne Marine Park. Repeat mapping provides an understanding of the dynamics of these canyons and a context for assessing and monitoring the stability of the seabed habitats within this marine reserve.
      原文链接:
    • NSTL
    • Elsevier

    Morphology and evolution of submarine canyons on the northwest South China Sea margin

    Li, ShuangAlves, Tiago M.Li, WeiWang, Xiujuan...
    12页
    查看更多>>摘要：Submarine canyons are observed along both passive and active continental margins, but the factors controlling their complex morphology are still poorly understood. Here, we use high-resolution multibeam bathymetric and 2D seismic data to investigate an area of the northwest South China Sea in which 48 submarine canyons are identified. These previously unstudied submarine canyons incise the continental shelf, being located at a water depth between 200 m and 2200 m. Canyon morphology varies from southwest to northeast, namely in what their length and incision depth are concerned. We therefore divide these canyons into four main types: a) Types A, B and C showing a predominant NW-SE direction, and b) Type D canyons striking to the north. By analysing their internal architectures, we propose that submarine canyons along the northwest South China Sea margin were initiated in the Late Miocene by retrogressive slope failure in response to the gradual build-up of sediment on the continental slope. Differences in sediment supply and fault activity are recognised here as the main factors controlling the morphology of the investigated submarine canyons. In addition, recurrent mass-transport deposits (MTDs) fed sediment from the northwest South China Sea margin into the study area, accelerating the filling of the Central Canyon system, a giant submarine canyon located to the south of the investigated continental slope. The discovery of gas fields (LS22-1, LS17-2) and a gas hydrate drilling zone (GMGS5) in the Central Canyon system proves that MTDs comprise good reservoir intervals. Our results contribute to a better understanding of the origin and development of submarine canyons and highlight the role of sediment supply and tectonic events in controlling canyon morphology.
      原文链接:
    • NSTL
    • Elsevier

    New insights on the Alboran Sea basin extension and continental collision from magnetic anomalies related to magmatism (western Mediterranean)

    Tendero-Salmeron, VictorGalindo-Zaldivar, JesusD'Acremont, EliaCatalan, Manuel...
    14页
    查看更多>>摘要：In the Alboran Sea there are a few well exposed Neogene and Quaternary volcanic zones, often geographic highs, that are generally associated with magnetic anomalies. In this paper, we present a characterization of these magnetic anomalies based on a recent and accurate magnetic data compilation for the Abloran Sea area. The anomalies reveal the distribution of magmatism and shed light into the discussion about the origin and evolution of the westernmost Mediterranean. One of the most relevant magnetic anomalies is the Nador dipole, which extends from the Gourougou volcano to the Chafarinas Islands, and is related to an E-W crustal scale intrusion. However, the main NE-SW elongated continuous dipoles of the central Alboran Sea are not related to any surface structure, but they are parallel to the Alboran Ridge, which is the main volcanic high in the Alboran Sea, and are located to the north of it. These anomalies extend discontinuously eastward along the NW-SE dipoles located along the Yusuf fault zone. The results of our 2D magnetic forward modeling suggest that the causative bodies of these main magnetic dipoles are deep igneous bodies. According to the tectonic evolution of the region, and the high magnetic susceptibility values obtained, these igneous bodies probably are made of a basic igneous rocks. Their emplacement may represent the westward tip of the rift axis of the AlKaPeCa Domain, which is related to the Oligocene-Miocene NW-SE extension, and associated with the southern slab retreat stage and oceanic spreading of the Algerian basin. Afterwards, these bodies were displaced toward the west, together with the Alboran Domain, and affected by the STEP fault located at its southern limit. Since the Late Miocene, the north Alboran Ridge elongated intrusions acted as a backstop that conditioned the folding and uplift of the Alboran Ridge in a tectonic indentation setting. In this setting, the STEP fault is deformed and the eastern part of the bodies were segmented along the Yusuf transtensional fault system. Simultaneously, the E-W crustal body related to the Nador magnetic dipole was emplaced, possibly evidencing a slab tearing process. The deep seated basic igneous bodies constitute main crustal heterogeneities that reveal and drive the Alboran Sea tectonic inversion.
      原文链接:
    • NSTL
    • Elsevier

    Source-to-sink pathways of clay minerals in the cadiz contourite system over the last 25 kyrs: The segregational role of mediterranean outflow water

    Moal-Darrigade, PaulDucassou, EmmanuelleBout-Roumazeilles, VivianeHanquiez, Vincent...
    17页
    查看更多>>摘要：Despite major advances in our understanding of the interactions between bottom currents and sedimentary deposits over the last forty years, few studies have focused on the nature of fine particles in contourite depositional (CDS). XRD analyses of marine sediments can be used to improve our understanding of finegrained sediment sources and settling processes. This work presents a detailed sedimentological study of sediment cores collected over the middle slope of the Gulf of Cadiz as part of Integrated Ocean Drilling Program Expedition 339 and the 2001 CADISAR cruise. We performed high-resolution clay mineral analyses to reconstruct the pathways of fine-grained particles from their sources to their deposition along the contourite depositional system of the Gulf of Cadiz (source-to-sink approach). The clay mineral associations reflect the major contribution of the Guadalquivir River and North African rivers/dusts to fine particles settling over the middle slope. Our results suggest that size segregation deposition processes along the path of the Mediterranean Outflow Water (MOW) are responsible for the distinct clay mineral associations between sites located under the upper MOW and the lower MOW. We observed changes of sedimentation rates over the contourite depositional system throughout the last 25 kyrs. We propose that these changes are due to temporal variations in the vertical distribution of the upper and the lower MOW whose concentrations in suspended particulate matter are drastically different. Sea-level and large scale atmospheric changes (e.g., ITCZ migration) over this period induced major variations in the distance of river mouths to the Gulf of Cadiz CDS, and in the amount of Northwest African dust delivered to this depositional system, respectively. Climate changes therefore modified fine particle sources and pathways, which considerably influenced clay minerals settling in the middle slope of the Gulf of Cadiz since the Last Glacial Maximum.
      原文链接:
    • NSTL
    • Elsevier

    A reassessment of Nd-isotopes and clay minerals as tracers of the Holocene Pacific water flux through Bering Strait

    Liu, YanguangWang, WeiguoHuang, YuanhuiSong, Tengfei...
    14页
    查看更多>>摘要：The flux of low-salinity Pacific water (PW) through Bering Strait (BS) weights on the freshwater budget of the Arctic Ocean, which, in turn, impacts the Atlantic Meridional Overturning Circulation (AMOC). Through time, this PW flux has been tightly controlled by the bathymetry of the Strait (similar to 50 m water depth at present), thus by sea-level (SL) changes, as documented in several papers based on a large array of sedimentological and geochemical tracers. The present study aimed at reassessing the robustness of these tracers of the PW-flux, based on five surface samples collected along a south-north transect from BS to the Chukchi Plateau, within the bathymetric range of the PW-mass dispersal. We also estimated their variation in relation to the post-glacial SL-rise in a sedimentary core (ARC4-R09), from the Chukchi shelf, spanning the last similar to 10 cal kyr. Potential indicators of PW-flux include sortable silt, clay minerals, and the particulate and exchangeable Nd-isotope composition of specific grain-size fractions. The results indicate that epsilon Nd-values in leachates, and to a lesser extent, the smectite abundance, both relating to volcanic rocks of the Bering Sea, are sensitive indicators of the PW-flux, whereas sortable silts and other clay minerals relative abundances are also influenced by other processes (e.g., west-east shelf currents, sea-ice rafting). The modern distributions of epsilon Nd-value and smectite abundance illustrate mixing between two endmembers: i) a northern Bering Sea endmember, characterized by similar to 14% of smectite (vs all clay minerals) and a epsilon Nd-value of the exchangeable fraction of similar to-2; ii) a Chukchi Sea endmember, characterized by similar to 8% of smectite and a epsilon Nd-value ranging from -7 to -8. The Holocene sedimentary record from the study core indicates a progressive increase in Bering Sea supplies through time, linked to an enhanced inflow of PW, which we associate with the deepening of the Strait due to the rising SL. Following a major reorganization of the Arctic Ocean circulation at similar to 8 cal kyr BP, a steady increase in PW flux prevailed until similar to 4 cal kyr BP. A tenuous but continuous positive trend towards modern epsilon Nd values followed during the late Holocene. The middle Holocene steep increase of warm, low salinity PW-flux through BS, led to important changes in the Arctic Ocean, in particular of its salinity budget. Assuming that the BS bathymetry is the main parameter governing the PW-flux towards the Arctic Ocean, a first-order estimate of PW-flux under a higher than modern SL can be made using the SL-elevation-epsilon Nd relationship of the major middle Holocene shift.
      原文链接:
    • NSTL
    • Elsevier