查看更多>>摘要：The Gulf of Cadiz Contourite Depositional System (GCCS) developed due to the interaction of the Mediterranean Outflow Water (MOW) with the middle continental slope of the SW Iberian continental margin. The GCCS evolved in a complex tectonic setting within the foreland of the Betic Orogeny and near the Nubia-Eurasia plate boundary. This study used tectonostratigraphic analysis of an extensive 2D multichannel seismic reflection dataset to investigate how inherited basin configuration and tectonic activity controlled sedimentary stacking pattern and evolution of the GCCS. Three regional tectonostratigraphic units (U1-U3) were recognised in the margin. The younger seismic unit 173 corresponds to the Miocene-Quaternary foreland basin system where the contourite system is generated. Seismic analysis also detected the dextral strike-slip Gil Eanes Fault Zone (described herein for the first time), the Cadiz Fault, the Albufeira-Guadalquivir-Donana Basement High and several diapiric structures. Integrated analysis of seismic profiles showing these tectonic structures with thickness and earthquake distribution maps suggest four tectono-sedimentary domains. The distinct characteristics shown by contourite features in the different domains, depends at broad-scale on the tectonic-control of the accommodation space (i.e., subsidence or uplift) and at local-scale on the presence of structural highs and fault-related depressions. Both influence bottom-current circulation and thus the evolution of the contourite deposits through the late Miocene and Quaternary. Three main stages have been recognised in the Gulf of Cadiz evolution: 1) the region was the western continuation of the Betic Corridor until the final re-opening of the Strait of Gibraltar (8-5.3 Ma). In this stage there is a predominance of turbidites or hemipelagic deposits, dependant on tectonic activity; 2) with the final re-opening of the Mediterranean-Atlantic connection there is the onset of the Pliocene-Quaternary contourite depositional system (5.3-2.0 Ma). Short-term changes in sedimentation during this stage, from contourite to turbidite deposits, indicate periods of increased tectonic activity; and 3) after the onset of the transpressive tectonic regime in the area (from 2.0 Ma), sedimentation became more homogeneous suggesting stable conditions (decrease of tectonic activity) with dominant contourite deposition. This work highlights the remarkable influence of structural features and tectonic events in controlling the seafloor relief and in turn in influenced the local oceanic circulation processes that controlling the morphology and sedimentary evolution of contourite systems.
查看更多>>摘要：Forecasting coastal dynamics and sea cliff retreat under different sea level rise scenarios requires a good understanding of the conditioning factors and their relative contribution to cliff stability. The so-called Jurassic Cantabrian Coast extends along 76 km of the coastline of the Asturias region (N Spain) and is well-known worldwide due to its paleontological heritage, in particular the presence of dinosaur remains and footprints. The abundance of stratigraphic, paleontological and tectonic studies contrasts with the scarcity of studies focused on the stability of this rocky coastline where cliffs predominate, sometimes exceeding 120 m in height. In fact, evidence of current and recent instability processes can be observed along the entire coastline. In this regard, continuous monitoring is crucial to understand ongoing instabilities in rocky coastlines, as in these settings some instabilities might initiate as slow movements that induce subtle topographic changes whose detection from either satellite or aerial imagery is problematic due to the spatial and temporal resolutions. This contribution presents a 4D evolution model of a key site, the Tazones Lighthouse landslide, located on the Cantabrian Coast of Asturias (N Spain), which affects subvertical rocky cliffs sculpted in the Jurassic bedrock made of alternating sandstone and marl. A high resolution multiapproach methodology was developed in order to understand its structure and kinematic characteristics, including: i) interpretation of aerial photographs and unmanned aerial photogrammetric surveys (UAV); ii) 22 monthly monitoring campaigns by total station; iii) 5 manual boreholes; iv) geomechanical characterization of the cliff bedrock; v) geomorphological evidence mapping; vi) analysis of landscape deformations obtained from UAV; and vii) precipitation, soil moisture and significant wave height (Hs) data analysis. The results show that the slope evolves by means of a complex-type mass movement, which combines translational and sliding mechanisms, and occupies tens of thousands of square meters. DTM and fieldwork analysis indicate that mass movement is mainly controlled by bedrock discontinuities (SO, 360/15-17; Jl, 262/85; J2 166/75). The most important accelerations of slope movement correlate very well with rainfall, soil moisture and waves. Thus, the largest displacements occurring in January and October-November 2019, coincide with 2 periods of storms (maximum 24-h rainfall of 64.5 mm and 82.1 mm and maximum Hs of 6.54 and 9.09, respectively) and soil moisture values above 90%. Half of the markers moved more than 1 m and one of them exceeded 15 m.
查看更多>>摘要：? 2022 Elsevier B.V.One hundred mega-boulders were tagged to survey their movements (RFID) for five winters (2012–2017) in Sainte-Luce, St. Lawrence Maritime Estuary, where the foreshore is frozen for two to four months per year. In addition, four boulders were fitted with accelerometers to determine the exact periods of motion. It was found that 81% of the boulders moved during at least one of the winters studied. The maximum transport recorded in one winter was 151.87 m for a 3.1 t boulder, but most boulders moved shorter distances (2012–2017 average transport: 9.74 m, standard deviation 23.71). The relationships between length/direction of boulder movements and fast-ice activity as well as weather and oceanic conditions were analyzed. 61% of mobility events took place at the end of the ice period during ice-foot break-up by storm waves or melting, but 37% took place at the beginning of the fast-ice phase during storms and/or high tide, which was unexpected. The length and direction of transport was found to depend on conditions during ice break-up (wind, waves, currents, ice thickness), which have large interannual variabilities. However, a greater proportion of boulders moved landward between 2012 and 2017. Two transport modes occurred: short pushing by ice floes and ice-rafting over longer distances. The first is much more common (70 to 90%) while the second seems to be more frequent during cold winters, when ice is thicker. Moreover, we demonstrated that the geomorphological setting exerts a strong control on boulder transport distance. The longest movements took place near mean sea level, where the ice foot is thicker and the rock platform smooth while cobble and boulder pavements inhibit movements.
查看更多>>摘要：? 2022 Elsevier B.V.Cliff erosion is of concern along inhabited coastlines especially under a scenario of increasing sea level rise. Extrapolation of past erosion rates are often used as a first approximation of future rates, which are or form part of the input into numerical models. However, cliff erosion rates in the Holocene are poorly understood. This is due to the generally short record based on historical maps, which can mask or accentuate erosion rates based on different event frequencies, and, for the more recent cosmogenic exposure dating method, it reflects the large number of variables influencing the concentration of cosmogenic nuclides in rocks at the surface of intertidal shore platforms. The study uses historic maps that go back 400 years, updated (to 2020) recent erosion rates, evidence from mid-Holocene terrestrial habitats in front of present-day cliffs, and a geomorphological landscape type interpretation to provide a better understanding of cliff evolution and erosion history for the cliffs in East Sussex (Southeast England). The paper demonstrates that cliff erosion can only have started in the last few centuries with remnants of Eemian cliffs still present in places. This is supported by published 10Be data. The coastline investigated also shows a persistent decrease in erosion rates to less than half the maximum rate observed in the late 19th to early 20th century despite sea level rise. As a consequence, it appears imperative to understand the local drivers of coastal cliff erosion to assess the future consequences of climate change rather than applying general concepts for example in relation to erosion changes with increasing sea level rise.
查看更多>>摘要：? 2022The Cayos Basin is an offshore basin located in the Colombian Caribbean Sea and forms part of the Lower Nicaraguan Rise, a geological province of the western region of the Caribbean Plate. Until now, the origin of the province is still being debated. Advanced research in the study area regarding its composition and structure, from land outcrops, petrology and geochemistry of drilled cores and dredged samples, and geophysical investigations, indicates a volcanic origin for this geological province, and a close relationship to the formation of the Caribbean Large Igneous Province. On the contrary, other studies suggest that the Lower Nicaraguan Rise may be part of the continental Chortis block. In this paper, we present and discuss alternative scenarios for the nature of the underlying crust below the sedimentary sequences in the Cayos Basin. We characterize the basin through the interpretation of magnetic and gravity anomaly grids, and 2D forward modeling, constructed based on three sections, by considering restriction seismic data from previous works. The results show that the Cayos Basin is underlain by geological bodies with high density and higher magnetization. From the gravity and magnetic forward modeling, we estimated the depth to the basement is about 2–6 km, and the Moho discontinuity to have an average of 18 km below, the Cayos Basin. Our investigation implies that, at least, the Cayos Basin is in the oceanic crust domain and shows no evidence of a continental source of the Chortis block.
查看更多>>摘要：? 2022The mineral ikaite (CaCO3·6H2O) and its pseudomorphs (glendonite) are considered as important paleoclimate indicators in the geological record. Here we examine the youngest known glendonites, their host concretions, and bivalve shells fossilized within these host concretions from the White Sea, north-western Russia. We applied mineralogical, geochemical and isotopic methods to shed new light on the paleogeographic and geochemical environments during ikaite precipitation and ikaite-calcite transformation. We show that White Sea glendonites are so far the youngest known glendonites in the world (4.1 ± 0.4 cal thousand years before present). Ikaite precipitation occurred during the cold winter months, accompanied by the formation of extensive sea ice and presence of decaying organic matter, which resulted in a small negative Ce anomaly and negative δ13C values. Increasing temperature led to ikaite dehydration and subsequent cementation with aragonite or amorphous calcium carbonate, containing elevated concentrations of Ba and Sr. Needle-like cements within glendonites display small negative Ce anomalies and precipitated from pore waters derived from seawater. The high?magnesium calcite forming the host concretion precipitated in the sulfate-reduction zone from pore waters derived from seawater. The geochemistry of these pore waters was modified by interaction with clastic components of the host sediments, resulting in δ18O values comparable to those of dissolved inorganic carbon (DIC) and almost flat Post Archean Australian Shale (PAAS)-normalized rare earth element patterns. Therefore, we show that glendonites can be used as a proxy for near-freezing bottom water temperatures at least seasonally during the cold winter months. Changes in the geochemical composition of carbonate phases within the glendonites and host concretions can be used to determine successive changes in the geochemical environment during their formation.
查看更多>>摘要：? 2022 Elsevier B.V.Shear rates play a critical role in the coagulation-flocculation-sedimentation processes of colloidal particles. Under high shear environments, it is widely accepted that the median floc size at equilibrium (D) decreases with an increase in shear rates (G). For low shear conditions, however, conflicting D-G relationships were measured in previous laboratory experiments, without a clear explanation of the reasons and a reasonable reproduction of the physical processes. In this study, the direct numerical simulation technique was used to mimic the flocculation of two typical colloidal particles (i.e., latex and silica) with different densities but under similar hydrodynamic conditions. Our results show that the previous different observations at low shear are mainly caused by particle gravity, which influences the particle residence time in a device system. They also confirm previous arguments about limited residence time due to gravitational settling being the reason for the observed peak, in accordance with Winterwerp (1998) [J. Hydraul. Res. Vol 36, pp. 309–326]. This study implies that to better investigate the shear-dominated flocculation of high-density particles such as sediments, settling processes should be addressed unless particles are always in suspension under specific conditions.
查看更多>>摘要：? 2022 Elsevier B.V.Tsunamis can cause significant coastal erosion and harbor sedimentation that exacerbate the concomitant flood hazards and hamper recovery efforts. Coupling of the non-hydrostatic model NEOWAVE and the sediment transport model STM provides a tool to understand and predict these morphological changes. The non-hydrostatic model can describe flow fields associated with tsunami generation, wave dispersion as well as shock-related and separation-driven coastal processes. The sediment transport module includes non-equilibrium states under rapidly-varying flows with a variable exchange rate between bed and suspended loads. A previous flume experiment of solitary wave runup on a sandy beach provides measurements for a systematic evaluation of sediment transport driven by shock-related processes. The extensive impacts at Rikuzentakata, Iwate, Japan and Crescent City Harbor, California, USA from the 2011 Tohoku tsunami provide pertinent case studies for model benchmarking. We utilize a self-consistent fault-slip model to define the tsunami source mechanism and field survey data to determine the characteristic grain sizes and morphological changes. The near-field modeling at Rikuzentakata gives reasonable fits with observed large-scale erosion and sedimentation associated with transition of the incoming wave into a surge and formation of a hydraulic jump in the receding flow. The non-hydrostatic module becomes instrumental in resolving tsunami waves at the far-field shore of Crescent City. The results show good agreement with local tide-gauge records as well as observed scour around coastal structures and deposition in basins resulting from separation-driven processes. While the erosion patterns in the laboratory and field cases can be explained by suspended sediment transport in the receding flow, bed load transport can be a dominant mechanism in sediment laden flows and scour around coastal structures.
查看更多>>摘要：? 2021This is the first systematic high-resolution rock magnetic study complemented by mineralogical and petrological observations conducted on sediment cores representing shallow (active cold seep; SSD-45/Stn-4/GC-01) and deep-seated (NGHP-01–15A) gas hydrate systems in the Bay of Bengal which addresses two key questions: (i) how magnetic minerals respond to the geochemical environment at two different diagenetic settings experiencing variable fluid sequence, and (ii) elucidates the control on the magnetite and greigite authigenesis in sulfidic, methanic, and gas hydrate bearing sediments. Titanomagnetite is the dominant detrital magnetic mineral identified in the studied cores from the Krishna-Godavari (K-G) basin along with diagenetic (pyrite) and authigenic (magnetite, greigite) minerals. Large detrital titanomagnetite and silicate-hosted (fine-grained) magnetic inclusions survived diagenetic dissolution during high sedimentation events. Magnetic proxies (ARM/SIRM and SIRM/χlf) provided useful insights on the diagenetic and authigenic formation and preservation of magnetic particles in the studied cores. Preferential diagenetic dissolution of finer (detrital) magnetic particles in sulfidic and authigenic formation of magnetite in methanic environment is clearly evident in the ARM/SIRM record of cores from both sites. Elevated values of SIRM/χlf in cores SSD-45/Stn-4/GC-01 (sulfidic and hydrate bearing) and NGHP-01–15A (methanic) indicate formation and preservation of ferrimagnetic authigenic (SP) greigite particles. Based on the magnetic proxies, we demonstrated that the formation of authigenic magnetite in the methanic environment of both sites is tightly linked with the microbial iron-reduction process. Multiple occurrences of authigenic carbonate provided evidence on the episodic intensification of anaerobic oxidation of methane (AOM) at active seep and silicate weathering coupled to microbial methanogenesis at deep-seated gas hydrate site respectively.
查看更多>>摘要：? 2021The sediment core (MGS11/02), collected from the lower boundary of the Bay of Bengal (BoB) oxygen minimum zone (890 m) is studied to understand the influences of summer monsoon variations on the terrigenous sediment influx and its relation to bioproductivity and early diagenetic changes in the southwest BoB during the Late Quaternary. The high content of terrigenous controlled metals increased, kaolinite/chlorite increased, and K/Rb decreased after 19 cal kyr BP, indicating a simultaneous increase in the terrigenous influx and intensity of chemical weathering during this period. The variations in summer monsoon and land influx observed in the core sediments are also reflected in the sources of organic matter. High values of δ13C observed at 40 to 19 cal kyr BP suggests higher influence of marine organic matter and/or presence of C4 terrestrial plant. The negative shift of δ13C between 19 and 6 cal kyr BP could be the result of the change in terrestrial plant biomass to C3 photosynthesis and increased marine organic matter. Rock magnetic properties and redox-sensitive metals distribution suggests the prevalence of reduced environment during 52 to 19 cal kyr BP and oxic condition during 19 cal kyr BP to present. The low sedimentation during glacial period allowed sufficient time for pyritization which resulted in the low magnetic signals during 52 to 20 cal kyr BP and led to reducing conditions in the sediment interval. The δ15N values in the core sediments are suggesting the absence of denitrification in the water column during last 52 cal kyr BP. Previous studies suggest that the BoB is a geochemical ‘tipping point’ because the slight changes in water column denitrification by anthropogenic or climatic impact can accelerate BoB nitrogen contribution to the global nitrogen budget. This study indicates the absence of water column denitrification signals during last 52 cal kyr BP even though drastic climatic changes happened in the BoB in the Late Quaternary.
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