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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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    Reef-rim structure and building history, Rangiroa, an uplifted Atoll, French Polynesia: The role of morphotectonics and extreme marine hazard events

    Montaggioni, L. F.Baltassat, J. -M.Innocent, Ch.Martin-Garin, B....
    15页
    查看更多>>摘要：Reconstructing the responses of low-lying, atoll-rim islands to future environmental changes due to global warming requires a robust knowledge of the building history of reef rims through time. In the northwest of Tuamotu Archipelago, atolls have experienced opposite vertical motions (uplift versus subsidence) in response to differential tectonic constraints undergone by the underlying oceanic crust. As a result, during Pleistocene high sea levels, some atolls remained emerged whereas others remained flooded. Based on two electrical resistivity tomography profiles across the northern rim-island of the uplifted Rangiroa Atoll and on U/Th dating of corals clasts collected from unconsolidated sand-rubble deposits within two wells through the same island area, the internal structure of the atoll-rim and its mode and timing of building are tentatively reconstructed. The irregularly shaped, upper boundary of the basal resistive unit is interpreted as residual, pinnacle-shaped reliefs of Miocene age. At the seaward rim side, the uppermost highly resistive layers, an about 5 m-thick, were assumed to be reef-related, partly consolidated material emplaced during the Holocene. Thicker resistive layers that infill cavities in the Miocene basement, in the central part of the atoll rim, refer to the fresh water lense in porous, more or less consolidated material (phosphorites?) presumably emplaced during the Mio-Pliocene. In northern Rangiroa, the age of the oldest coral clasts trapped into the atoll-rim islands ranges between about 11,000 and 8000 calendar years when sea level was approximately 60 to 20 m below its present position. This suggests that coral-clast supply from the adjacent forereef slopes was triggered by extreme wave-surge events (presumably, tsunamis generated by island flank failures). The earliest island initiation phase is thought to start by approximately 6000 cal. yr and to be achieved by about 3000 cal. yr. By constrast, on the nearby subsiding Takapoto Atoll, the rim island began to initiate not earlier than 2500 cal. yr. At Rangiroa, morphotectonics is assumed to have played a major role in controlling the internal rim architecture and, associated to extreme marine hazard events, to have promoted the earlier settlement of low-lying, rim islands through an earlier infilling of accommodation space as compared to nearby subsiding atolls.
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    • NSTL
    • Elsevier

    A first appraisal of the seismogenic and tsunamigenic potential of the largest fault systems in the westernmost Mediterranean

    de la Pena, Laura GomezGracia, EulaliaMaesano, Francesco EmanueleBasili, Roberto...
    15页
    查看更多>>摘要：The westernmost Mediterranean hosts part of the plate boundary between the European and African tectonic plates. Based on the scattered instrumental seismicity, this boundary has been traditionally interpreted as a wide zone of diffuse deformation. However, recent seismic images and seafloor mapping studies support that most of the plate convergence may be accommodated in a few tectonic structures, rather than in a broad region. Historical earthquakes with magnitudes M-w > 6 and historical tsunamis support that the low-to-moderate instrumental seismicity might also have led to underestimation of the seismogenic and tsunamigenic potential of the area. We evaluate the largest active faults of the westernmost Mediterranean: the reverse Alboran Ridge, and the strike-slip Carboneras, Yusuf and Al-Idrissi fault systems. For the first time, we use a dense grid of modern seismic data to characterize the entire dimensions of the main fault systems, accurately describe the geometry of these structures and estimate their seismic source parameters. Tsunami scenarios have been tested based on 3D -surfaces and seismic source parameters, using both uniform and heterogeneous slip distributions. The comparison of our results with previous studies, based on limited information on the fault geometry and kinematics, indicates that accurate fault geometries and heterogeneous slip distributions are needed to properly assess the seismic and tsunamigenic potential in this area. Based on fault scaling relations, the four fault systems have a large seismogenic potential, being able to generate earthquakes with M-w > 7. The reverse Alboran Ridge Fault System has the largest tsunamigenic potential, being able to generate a tsunami wave amplitude greater than 3 m in front of the coasts of Southern Spain and Northern Africa.
      原文链接:
    • NSTL
    • Elsevier

    Sedimentary geochemical records of late Miocene-early Pliocene palaeovegetation and palaeoclimate evolution in the Ying-Qiong Basin, South China Sea

    Hou, DujieDing, WenjingGan, JunJiang, Lian...
    26页
    查看更多>>摘要：The sedimentary geochemical records of palaeovegetation and palaeoclimate variations in late Miocene-early Pliocene sediments in the Ying-Qiong Basin, South China Sea have been studied using higher plant-derived biomarkers, palynological records, trace elements, and macerals. Aliphatic biomarkers including oleanane, des-A-oleanane, bicadinanes, and C-19 tricyclic terpane, and the vitrinite content of the kerogen, show a remarkable decline in the input of the main angiosperms from about 8.2 to 2.7 Ma, corresponding to the deposition of the upper Huangliu Formation (late Miocene) to the lower Yinggehai Formation (early Pliocene). Aromatic parameters including retene/(retene + cadalene), 1,2,5-TMN/1,3,6-TMN, 1-MP/9-MP and 1,7-DMP/ (1,3-+ 3,9-+ 2,10-+ 3,10-DMP) suggest that the upper Huangliu Formation received a greater gymnosperm contribution than older Miocene formations. Trace element ratios (Rb/Sr and Zr/Rb) and the proportion of alginite-amorphous macerals in the shales indicate that a cooler, more arid climate with less seasonal precipi-tation occurred progressively from about 8.2-2.7 Ma, as also supported by higher abundances of Pinus and Alnus and decreasing proportions of ferns in the flora. Lesser input of terrigenous higher plants and the variation in vegetation types occurred during a cooling climate that was likely associated with a weaker East Asian summer monsoon and uplift of the northern Tibet-Plateau from about 8.5 Ma.
      原文链接:
    • NSTL
    • Elsevier

    Geochemical characterisation of offshore New Zealand phosphorites, and mechanisms for their formation

    Frontin-Rollet, Grace E.Handler, Monica R.Wysoczanski, Richard J.Gazley, Michael F....
    16页
    查看更多>>摘要：Phosphate deposits can form in a range of environments, and despite similar P contents, their mineralogy, and hence major and trace element chemistry, can vary substantially. Continental margins with high surface-water productivity are recognised as a major environment for the formation of extensive phosphate deposits. The geochemical characterisation of continental-margin-derived phosphorites from Chatham Rise and south-eastern offshore New Zealand, as well as newly discovered seamount phosphorites from Bollons Seamount (first documented here), offer insights into the different mechanisms of phosphorite formation from these two different environments. Though less studied, seamounts are increasingly recognised for their phosphorite deposits as well as Fe-Mn crusts and Mn nodules. We propose the first model for the formation of nodules at Bollons Seamount, and an updated model for Chatham Rise nodules (and south-eastern offshore NZ phosphorites) is presented. Suboxic-to-anoxic enrichments of U and V are observed in Chatham Rise and south-eastern offshore phosphorites that contrast with oxic enrichments of Mn, Ce, Co, and Cr in Bollons Seamount phosphorites. Differences in uptake of Light Rare Earth Elements (LREE) vs. Heavy Rare Earth Elements (HREE) and variations in mineralogy (e.g., presence of glauconite vs. Fe-Mn crusts), allow phosphate deposits that formed in organic-rich, upwelling continental-margin environments to be distinguished from those that formed in oxic-suboxic organic-poor seamount environments.
      原文链接:
    • NSTL
    • Elsevier

    Glaciomarine sediment deposition on the continental slope and rise of the central Ross Sea since the Last Glacial Maximum

    Ha, SangbeomColizza, EsterTorricella, FiorenzaLangone, Leonardo...
    15页
    查看更多>>摘要：The continental margin of the Ross Sea has been consistently sensitive to the advance and retreat of the Ross Ice Sheet (RIS) between the interglacial and glacial periods. This study examines changes of the glaciomarine sedimentation on the continental slope and rise to the eastern side of Hillary Canyon in the central Ross Sea, using three gravity cores collected at increasing water depths. Besides older AMS C-14 ages of bulk sediments, based on the analytical results, sediment lithology was divided into units A, B1, and B2, representing Holocene, deglacial, and glacial periods, respectively. The sedimentation rate decreased as the water depth increased, with a higher sedimentation rate in the deglacial period (unit B1) than the Holocene (unit A). Biological productivity proxies were significantly higher in glacial unit B2 than in interglacial unit A, with transitional values observed in deglacial unit B1. Biological productivity generally decreased in the Antarctic continental margin during the glacial period because of extensive sea ice coverage. The higher biogenic contents in unit B2 are primarily attributed to the increased transport of eroded and reworked shelf sediments that contained abundant biogenic components to the continental slope and rise beneath the advancing RIS. Thus, glacial sedimentation on the continental slope and rise of the central Ross Sea was generally governed by the activity of the RIS, which generated melt-water plumes and debris flows at the front of the grounding line, although the continental rise might have experienced seasonally open conditions and lateral effects due to the bottom current.
      原文链接:
    • NSTL
    • Elsevier

    Drivers of cross-shore chenier dynamics off a drowning coastal plain

    Tas, Silke A. J.van Maren, Dirk S.Helmi, MuhammadReniers, Ad J. H. M....
    15页
    查看更多>>摘要：A chenier is a beach ridge, consisting of sand and/or shells, overlying a muddy substrate. In this paper, we explore the cross-shore dynamics of cheniers in their 'active' phase, i.e. the phase between their formation and their landing on the shore and can no longer be reached by daily wave and tidal influences. While cheniers described in literature are known to only migrate onshore until they reach a stable position with their crest level above tidal influences, observations in Demak suggest the existence of an alternative stable state, highly dynamic on the short term, but stable on the longer term. To explore this alternative stable state, we developed an idealised chenier model to investigate cross-shore chenier dynamics under daily wave and tidal influences. The model is able to predict both onshore and offshore migration; onshore migration is mainly driven by wave action, while offshore migration is induced by a tidal phase lag, or the effect of the storm season. For certain combinations of waves, tide (incl. phase lag) and a storm season effect, the model predicts a dynamically stable chenier. In absence of a phase lag and storm season effect, the model yields a 'classic' stable chenier that welds onto the shoreline by onshore migration.
      原文链接:
    • NSTL
    • Elsevier

    Sedimentary model for mixed depositional systems along the Pacific margin of the Antarctic Peninsula: Decoding the interplay of deep-water processes

    Rodrigues, S.Hernandez-Molina, F. J.Rebesco, M.Hillenbrand, C. D....
    22页
    查看更多>>摘要：Mixed (turbidite-contourite) depositional systems are formed by a complex interplay of deep-water processes. An evaluation of their morphological elements and their lateral and spatial distribution is crucial to better under-stand the interplay of transport and depositional processes, involving along-slope bottom currents and down-slope turbidity currents. This work investigates extensive and still active mixed depositional systems devel-oped along the Pacific margin of the Antarctic Peninsula, which comprise large asymmetric mounded drifts, dendritic channel-complex systems and wide trunk channels. These systems offer a unique setting to investigate diverse morphological elements at a high-resolution spatial scale (10-100 m), using multibeam bathymetry and acoustic sub-bottom profiles. Four main seismic units define distinct evolutionary stages for the Pleistocene to present day record: a) 1.3-1 Ma, characterized by aggradational mounded drifts built by a dominant along-slope bottom current; b) 1-0.6 Ma, built by synchronous interactions between a SW-flowing bottom current and NW-directed turbidity currents; c) 0.6-0.2 Ma, characterized by deposition of thick gravitational deposits across the margin under a weak SW-flowing bottom current comprising modified Lower Circumpolar Deep Water (LCDW); and d) 0.2 Ma - present, when synchronous interactions between the bottom current, characterized by flow speed fluctuations, and ephemeral turbidity currents led to intercalations of turbidites, contourites, reworked turbidite deposits and hemipelagites. Alternations in the stratigraphic stacking pattern suggest cyclic spatial and temporal variations of gravity-driven down-slope processes and along-slope bottom currents, which were responsible for the construction of these modern mixed depositional systems and which themselves were controlled by glacial-interglacial changes. The new results are compared with similar mixed depositional systems to decode the main processes involved in their formation, explore their interactions at short-and long-term time scales, and propose a conceptual sedimentary model.
      原文链接:
    • NSTL
    • Elsevier

    First characterization of the volcanism in the southern Mozambique Channel: Geomorphological and structural analyses

    Berthod, C.Bachelery, P.Jorry, S. J.Pitel-Roudaut, M....
    39页
    查看更多>>摘要：The southern part of the Mozambique Channel is characterized by a cluster of isolated seamounts, including the Bassas da India atoll and Europa Island, ranging in latitude from 20?degrees S to 22 degrees S and from 38 degrees to 40 degrees in longitude, and located at the Rovuma-Lwandle plate boundary. Only Cenozoic carbonate platforms have previously been studied in this region, with very little work done on the volcanic history. We confirm here the volcanic nature of the basement of the Bassa da India/Europa complex by providing important constraints on the setting of this hitherto poorly understood volcanism.Recent bathymetric surveys and dredging operations allowed us to map and date these seamounts, comprising, from west to east, the Hall Bank, the Jaguar Bank, Bassas da India, Ptolemee, and Europa. In addition, we discovered, to the south of Bassas da India, two large new polygenetic volcanic edifices, Pamela Seamount 1 (PS1) and Pamela Seamount 2 (PS2), showing heights and diameters of up to 900 m and 13 km, respectively. Mapping and statistical analysis carried out revealed that the volcanic structures of the Bassas da India/Europa complex are organized along two main alignments with different stages of development: (i) a NE-SW volcanic alignment characterized by volcanic ridges up to 700 m in height, comprising small individual volcanic cones; and (ii) a NW-SE volcanic alignment in which many large and well-developed individual volcanic cones can be found. From this distribution of the volcanism, we suggest that the large volcanic edifices of the Bassas da India/ Europa complex were fed by long-lived magma systems, repeatedly supplied from deep magma reservoirs through a significant network of dykes and faults, with lateral injections of magma guided by a dense network of faults allowing magma to reach the surface along rift-zones. Ar-40/Ar-39 dating confirms that the volcanism covers a period from the Oligo-Miocene to the Pleistocene, and probably extends to the present day. The two volcanic alignments are also consistent with the tectonic features already recognized for the region and are spatially superimposed by active seismicity. Magma ascent is strongly controlled by large pre-existing crustal structures.
      原文链接:
    • NSTL
    • Elsevier

    Benthic foraminifera as indicators of recent mixed turbidite-contourite sediment transport system in the Eastern Mediterranean upper continental slope, offshore Israel

    Ashkenazi, L.Katz, O.Abramovich, S.Almogi-Labin, A....
    18页
    查看更多>>摘要：Nile derived siliciclastic sediments are the main source of sedimentation along the Levant continental margins of the Mediterranean Sea. These sediments are transported along the southeastern Mediterranean coast via well documented longshore currents, mainly operating along the shelf. However, the cross shelf component of sediment transport, responsible for conveying sediments toward the upper slope, is less known. To better understand the cross-shelf vs. the longshore component of sediment transport we studied a-6 m long piston core (DOR280) sampled on the upper continental slope (280 m water depth) and analyzed benthic foraminiferal assemblages and their shell taphonomy alongside the particle size distribution and mineralogy of bulk sediments, to document both the source and the transport mechanism of those upper continental-slope sediments. The radiocarbon dating at the core-base is-650 Cal. Yrs BP, indicating an exceptionally high average sedimentation rate of-800 cm/kyr. DOR280 consists of two alternating distinct sedimentary facies: (1) laminated (L) intervals up to-40 cm thick each and showing a high ratio of allochthonous vs. autochthonous (allo/auto) benthic foraminiferal species and a high percentage of broken shells, indicating contribution of transported sediments originating from mid-shelf habitats; (2) non-laminated (NL) intervals up to-200 cm thick each and showing a low allo/auto ratio and low percentages of broken shells, indicating mostly in-situ hemipelagic deposition. The L intervals are interpreted here as turbidites. The sedimentation rates calculated only for the NL intervals are still exceptionally high, thus excluding hemipelagic sedimentation as the sole mechanism for the NL deposition. Therefore, a contour bottom current transported component is suggested here as a source of the NL sedimentation. We conclude that a mixed contourite-turbidite system actively prevails along the upper continental slope of the Levant coast, offshore Israel. The mid-shelf to upper slope sediment transport is nearly continuous and presents occasional turbidite events of average decadal reoccurrence time.
      原文链接:
    • NSTL
    • Elsevier

    The response of foraminifera to rapid sea-level rise from tidal restoration of Ni-les'tun marsh, Oregon, USA

    Milker, YvonneDura, TinaHorton, Benjamin P.
    13页
    查看更多>>摘要：Foraminifera preserved in saltmarshes are widely used to reconstruct relative sea-level change (RSL), and inferred from this, coseismic vertical coastal motions following an earthquake. However, how quickly foraminifera respond to rapid changes in RSL is poorly understood. Here, we present a six-year foraminiferal study of the tidal restoration of Ni-les'tun marsh and a comparison of modern and fossil assemblages. We installed eight stations on the marsh and sampled these stations for live foraminifera prior to and during the first six years after tidal restoration (2011-2017), and we extruded one short core at station 1 in 2016. At stations 1 to 7, tidal flat/ low marsh assemblages, dominated by Miliammina fusca, colonized 10 months to 2.5 years after tidal restoration. At station 8, the first living mixed assemblage of foraminifera, dominated by Haplophragmoides wilberti, was found 2 years after tidal restoration. Our observations suggest that M. fusca, and to some extent H. wilberti, are opportunistic species (r-strategists), able to increase their standing crop rapidly after invading a new habitat (to up to ~3600 specimens per 10 cm3 sediment volume). Potential causes for the delay in foraminifera colonization include their reproductive cycle and/or limited food availability due to the slow response of the vegetation community and soil development to tidal restoration. However, the similarity among assemblages and concentrations of agglutinated foraminifera between the fossil and modern sediments, indicate that postdepositional taphonomic processes have minimal influence when incorporated in the stratigraphic record. Although foraminifera have shown a delayed response to tidal restoration in the Ni-les'tun marsh, the similarities between the modern and fossil assemblages indicate that the delayed response of foraminifera to tidal restoration is undetectable in study areas with low sedimentation rates (in this case 3.3 mm/yr) after tidal restoration. In the case of high post-earthquake sedimentation rates, sampling a few cm higher rather than immediately above an earthquake contact could avoid uncertainties of coseismic vertical motions when foraminifera have a delayed colonization.
      原文链接:
    • NSTL
    • Elsevier