Continental Margin Research Articles

The North Penninic Bündnerschiefer and Flysch of the Prättigau (Swiss Alps) revisited

During the re-mapping of the area for the Geological Atlas of Switzerland, a significant stratigraphic unconformity was discovered in the North Penninic (Valais) Bündnerschiefer and the Flysch series of the northern Prättigau. It separates different units of the Cretaceous Bündnerschiefer from the Palaeogene Flysch. We explain this observation by a basin conversion from extension to compression, which caused the initial deformation of the Bündnerschiefer in an accretionary wedge. Interlinked return-flow has created a new heterogeneous substrate for the flysch sediments and explains the different types of unconformities. The basin conversion coincided with high-grade metamorphism in the vicinity of the the South Penninic suture and the Austroalpine units, and the increased exhumation in the Austroalpine nappe stack. Detrital zircon dating confirms also a change from European to Austroalpine detrital sources in the flysch sandstones. We discuss a palaeotectonic model leading to hP/lT metamorphism of the Bündnerschiefer in the Late Eocene (c. 42 Ma). It appears that the flysch formations were also involved, but to a lesser degree by tectonic deformation from the late Early Eocene onwards, as the pervasive folding characteristic of the Bündnerschiefer is absent. This has been followed by a phase of S-directed backfolding. During the Oligocene and Miocene, more extensive deformation occurred by SE to NW compression and finally by probable westward thrusting and folding. Our main theme is the transition from passive to active continental margins, which in Alpine plate tectonic framework corresponds to the transition to flysch sedimentation by basin conversion. Our results show that the simultaneity of the transition from extension to compression, as indicated by the accumulation of flysch, shifted in time from south to north in the Alpine Tethys.

The role of intra-oceanic arc in the growth of continental crust: Evidence from the Early Paleozoic Bainaimiao arc

The Early Palaeozoic arc-related formations in the northern margin of the North China Craton (NCC) belonging to the Bainaimiao arc belt is an important part of the eastern Central Asian Orogenic Belt (CAOB). The tectonic evolution of this belt remains controversial. Here we present detailed data on the geochronology and geochemistry on a newly identified Early Palaeozoic igneous suite from the Faku area in northern Liaoning. Zircon U–Pb dating of the Faku volcanic and intrusive rocks yielded weighted mean 206Pb/238U ages of ca. 458 Ma and 433–405 Ma, respectively, which represent their crystallization (eruption) times. Zircon Hf isotopic analyses show a low negative to positive εHf (t) values of − 0.33 to + 12.4, implying depleted characteristics of the source. The geochemical features of Laolingshan basaltic andesites display higher TiO2 and lower P2O5 contents, similar to that of mid-ocean ridge basalts, and relatively high Zr/Y and low Ta/Yb ratios, suggesting oceanic island arc basalt affinity. Therefore, we suggest that the Laolingshan basaltic andesites were formed in an intra-oceanic environment. The Bachagou granodiorites, with adakitic geochemical signatures, belong to the calc-alkaline and high-K calc-alkaline series, implying formation in an active continental margin tectonic setting. The Baerhushan granodiorites, which geochemically belong to S-type granites, were probably formed in a syn-collisional environment influenced by arc–continent collision. The Donggou granitoids, which have higher SiO2 and alkali and lower MgO and P2O5, show A-type granite features. Geochemical characteristics of the Donggou A-type granite suggest formation in an extensional setting caused by orogenic intermittent extension. The Baerhushan gabbro diorites, which display properties of both mid-ocean ridge and continental rift basalts, were produced under post-orogenic extensional tectonic setting after the arc–continent collision. Our study traces the Early Palaeozoic tectonic evolution of the south-eastern segment of the CAOB and the related accretionary tectonics.

Coccolithophore and paleoproductivity variations in the subtropical western South Atlantic since the late deglaciation

Here we present a record of coccolith variation over the past 14 kyr BP from core KF-02 (25.84°S, 45.2°W; 827 m water depth) collected from the continental slope of the São Paulo Bight (SPB), located in the subtropical western South Atlantic. Additionally, we estimated primary productivity based on Florisphaera profunda nannoliths and compared it with other published investigations on the productivity based on different proxies. The coccolith assemblages varied and agreed with the main regional oceanic and atmospheric changes since the latest deglaciation, likely in response to the terrestrial influence and nutricline depth variations. Among eutrophic species, Gephyrocapsa oceanica was more abundant during times with enhanced productivity promoted by the input of terrestrial material. Emiliania huxleyi and small Gephyrocapsa dominated the middle Holocene samples and probably benefitted from the more stable oceanic conditions and the enhanced light availability promoted by the Brazil Current. The paleoproductivity estimates indicated that moderate levels of productivity persisted since the latest deglaciation with a decreasing trend after the middle Holocene, delaying the productivity reduction that was indicated by other regional studies based on planktonic foraminifera. Apparently, seasonal regional hydrographic conditions were responsible for changing coccolith assemblages, and these were not reflected by the planktonic foraminifera G. bulloides variations.

Seafloor Sediment Acoustic Properties on the Continental Slope in the Northwestern South China Sea

The acoustic properties of seafloor sediments on continental slopes play a crucial role in underwater acoustic propagation, communication, and detection. To investigate the acoustic characteristics and spatial distribution patterns of sediments on the continental slope, a geoacoustic experiment was conducted in the northwestern South China Sea. The experiment covered two sections: one crossing the shelf and slope in the downslope direction, and the other near the shelf break in the along-slope direction. In situ techniques, sediment sampling, and laboratory measurements were used to acquire data on sediment acoustic properties (such as sound speed and attenuation) and physical properties (including particle composition, density, porosity, and mean grain size). The experimental findings revealed several key points: (1) Acoustic properties of shallow water coarse-grained sediments and deep-sea sediments were higher when measured in the laboratory compared to in situ measurements. (2) Relationships between measured attenuation and physical properties, as well as between sound speed and mean grain size, showed deviations from previous empirical equations. (3) Sediment acoustic and physical properties exhibited significant variations in the downslope direction, while showing gradual variations in the along-slope direction. These variations can be attributed to sedimentary environmental factors such as material sources, hydrodynamic conditions, and water depth.

The 3D stress field of Nordland, northern Norway - insights from numerical modelling

Abstract The Nordland area in northern Norway is the seismically most active area on mainland Fennoscandia. It exhibits patterns of coastal extension, which contrasts with the first-order regional stress pattern that reflects compression aligned with the North Atlantic ridge-push. The regional stress field has been considered to emanate from the interaction of ridge push and glacial isostatic adjustment; while the local stress pattern can be additionally influenced by gravitational, topographic stresses, as well as the flexural effects of erosion and sediment deposition. We employ finite element numerical models at a crustal scale to study the 3D stress field, using existing geometric constraints from previous geophysical studies. Internal body forces, induced by variations in density, topography or Moho depth, already yield significant deviatoric stresses. In the models tested, these can strongly influence the near-surface stress regime, in particular for the continental margin setting we are investigating. In addition, redistribution of rock mass, which occurred mainly under Pleistocene glaciation, can modify the stress field significantly on a semi-regional scale. We consider this process to be the main driver for the coastal extension, in particular in areas where erosion has been high. The northern Atlantic coast of Norway is an area of neotectonic activity and anomalous patterns in seismicity and land uplift. This study aims to improve our understanding of the present-day stress regime in this area. In addition, our study sheds light on the importance of stresses due to redistribution of sediments, a stress driver that is often neglected in stress studies. An earlier version of this manuscript was included in a non-peer reviewed project report (Gradmann et al., 2018).

LITHO-STRATIGRAPHIC FEATURES OF THE LOWER CRETACEOUS DEPOSITS OF THE SKYBA AND DUKLA-CHORNOGORA NAPPES OF THE UKRAINIAN CARPATHIANS AND PROSPECTS FOR THEIR OIL AND GAS POTENTIAL

Lower Cretaceous sediments are widespread within the Skybа and Dukla-Chornorgora Nappes (Spas and Shypot Formations) over a considerable area. These deposits contain sandstones that can be industrial reservoirs for hydrocarbons. Such sandstones are described both on the surface and identified in deep wells. Thick sandy strata in the middle part of the Spas and Shypot formations formed as deposits of gravity flows at the second level of avalanche sedimentation, at the foot of the continental slope of the northern continental margin of the Tethys Ocean. Their formation is correlated with global regression in Aptian time. Porous sandstones of the Spas formations are present on the surface in the areas of the villages of Tershiv and Busovysko. They sometimes form bundles up to 60 m thick. They are exposed in the Oryv and Berehyv Skybas by deep wells 1-Lugy, 1-Shevchenko, and 4-Maksymivka. During the drilling of the 1-Shevchenko well from a depth of 6,900 meters, a film of oil and gas continuously flowed into the drilling mud. According to the results of industrial and geophysical studies, the section of the Spas formations of the second (lower) scale in the interval of 6,940–7,520 m was recognized as the most promising in terms of oil and gas potential. Two promising objects in the intervals of 7020–7070 and 6945–6985 m were identified, the oil and gas saturation coefficients of which are 65–70%. Similar sandstones are noted in the Hoverla sub-Nappe of the Dukla-Chornogora Nappe along the Balzatul River and its tributaries (Bila Tysa River Basin) as part of the Shypot Formation. Sandstones with an open porosity of 8–12 % were identified in the 1-Hrynyava and 1-Semakiv wells in the deposits of the Shypot Formation of the Skupiv sub-Nappe according to the results of industrial-geophysical research. When testing them in the 1-Semakiv well, from the total interval of 473–569 m, they received inflows of combustible gas and light oil with flow rates of 3,000 m3/day and 2.64 m3/day, respectively. The analyzed materials testify to the prospects of the Spas and Shypot formations in relation to the discovery of industrial accumulations of hydrocarbons.

Influence of sedimentary processes and fault tectonics on the evolution of submarine canyons in the East Andaman Basin: Insights from high-resolution seismic data analysis

The Andaman Sea Basin, a well-known back-arc spreading centre and a mature petroliferous basin, presents a unique and complex scenario for understanding the interplay of sedimentary processes and fault tectonics on seafloor geomorphology evolution, particularly in the eastern shelf region. This study, using high-resolution 3D seismic data and published exploratory well-seismic interpretation results, aims to establish a comprehensive stratigraphic framework for the Tanintharyi region, spanning the Lower Miocene, Middle Miocene, Upper Miocene, Pliocene, and Quaternary. A submarine canyon system was identified and investigated along the Tanintharyi continental slope within the East Andaman Basin, comprising thirteen slope-confined canyons arranged predominantly in an east-west vertical slope orientation. By analysing seismic reflection features, five distinct seismic facies have been identified, including basal lag (BL), slump and debris-flow deposits (SDFDs), canyon confined sheets (CCSs), laterally inclined packages (LIPs), and channel-levees (CLs). The evolution of this canyon system, a vital focus of this study, can be categorised into three phases based on the morphology and sedimentary structures observed: canyon initiation, canyon extension, and erosion-sedimentation. The findings of this study highlight the critical controlling role of faults in canyon evolution, with the intensity of fault activity displaying a negative correlation with canyon size, indicating the direct impact of faults on geomorphological changes. In addition, the geomorphological changes induced by fault activity have resulted in changes in sediment supply, dispersal, and sedimentation rates, thus significantly impacting the spatial distribution and dimensions of slope-confined canyons within the East Andaman Basin, with far-reaching implications.

Synrift and post-rift thermal evolution of rifted margins: a re-evaluation of classic models of extension

Abstract The thermal evolution of continental rifted margins is key to understanding margin subsidence and hydrocarbon prospectivity. Observed heat-flow values, however, do not always comply with classic rifting models. Here, we use 2D numerical models to investigate the relationship between rifting, sedimentation and thermal history of margins. We find that during the synrift, the basement heat-flow and temperature are not only controlled by extension factor, but also by synrift sediment thickness and the evolution of deformation. As this progressively focuses oceanward, the proximal sectors thermally relax, while the distal sectors experience peak temperatures. In the post-rift, the lithosphere under the hyperextended margins does not return to its original state, at least for c. 100 Myr after break-up. Instead, it mimics that of the adjacent oceanic plate, which is thinner than the original continental plate. This results in heat-flow increasing oceanward at post-rift stages, when classic rifting theory predicts complete thermal relaxation. Our models also predict slightly increased heat-flows in the adjacent oceanic crust, potentially extending hydrocarbon plays into distal margins and oceanic crust, previously discarded as immature. Finally, our models indicate that commonly used temperature approximations to calculate heat-flow during rifting may strongly differ from those occurring in nature.

Unveiling taxonomic diversity in the deep-sea fish genus Notacanthus (Notacanthiformes: Notacanthidae) with description of Notacanthus arrontei n. sp.

Notacanthid fishes constitute a common part of benthopelagic deep-sea fish communities on seamounts and continental slopes around the world. However, their highly conserved morphology and the usual lack of information on deep-water organisms make it difficult to appropriately address their biodiversity. A multidisciplinary approach combining morphological data with a DNA-based species delimitation analyses was used to explore the taxonomy of Notacanthus species. For this purpose, morphological and molecular data were obtained from 43 individuals, and the resulting information was combined with the available data. The results showed the occurrence of Notacanthus arrontei n. sp. from the Iberian Peninsula and highlighted several taxonomic conundrums regarding the Notacanthus genus. For instance, no significant differences were found between Notacanthus indicus and the recently described Notacanthus laccadiviensis, questioning its taxonomic status. Similarly, the result of the species delimitation molecular analysis coincided with previous DNA barcoding studies supporting the snubnosed spiny eel Notacanthus chemnitzii as a species complex that requires further research. Moreover, two unidentified records from the Indian Ocean were confirmed to belong to an unknown species pending formal description, and barcoding data show for the first time the occurrence of the shortfin spiny eel Notacanthus bonaparte in the Australia-New Zealand area. This research confirms the existence of important gaps in the knowledge of notacanthid fishes and represents a step forward toward a better understanding of their biological diversity.

Stereo-video landers can rapidly assess marine fish diversity and community assemblages

Climate change and anthropogenic stressors affect the distribution, abundance, and diversity of fish communities across the world. To understand rapidly changing biotic communities, resource managers need a method to quickly and efficiently document temporal and spatial changes in community assemblages across various spatial scales. In this study, we describe the use of new video lander tools to survey fish communities in rocky marine habitats on the continental shelf and slope in California, USA. We evaluate how fish diversity metrics determined by video surveys vary among geographically distinct areas. Our results demonstrate that species diversity, evenness, and richness vary spatially across the coast. Furthermore, community assemblages differ at both broad and fine spatial scales because of differences among habitats. Length frequencies and densities of species in this study were similar to those reported in previous studies. As community assemblages and biodiversity metrics shift in response to changing stressors, it is increasingly important to develop tools and methodologies to detect and rapidly monitor these changes.

Fluvial deposits of the Patlanoaya Group in southern Mexico: the stratigraphic record of an Early Mississippian supradetachment basin formed along the margin of Gondwana during the closure of the Rheic Ocean

ABSTRACT The Rheic Ocean closure stratigraphic record in southern Mexico is largely exposed in the Paleozoic Acatlán Complex. One of the most distinctive characteristics of the Acatlán Complex is the presence of high-pressure rocks, interpreted as remnants of the Rheic Ocean and the adjacent continental margin of Gondwana, which were subducted and then exhumed to the surface. Despite its importance in reconstructing the dynamics of the westernmost Rheic Ocean closure, the timing and mechanisms of exhumation of these high-pressure rocks remain debated. Some authors have tentatively interpreted the late Paleozoic, fluvial to marine succession of the Patlanoaya Group in southern Mexico as the sedimentary response to the exhumation of high-pressure rocks of the Acatlán Complex. Therefore, these sedimentary rocks may be particularly helpful in understanding the timing and evolution of high-pressure rocks exhumation during the Rheic Ocean closure. In this work, the tectono-sedimentary evolution of the continental deposits of the Patlanoaya Group was reconstructed, integrating sedimentologic, petrographic, and U-Pb detrital geochronological data to test the different scenarios proposed for the exhumation of the Acatlán Complex high-pressure rocks. Our data suggest that the lower Patlanoaya Group was deposited in an Early Mississippian basin with a two-stage evolution: a phase characterized by debris flow deposits derived from small-scale reliefs composed of low-grade metamorphic rocks and a subsequent phase marked by an NNE-trending fluvial system deposits sourced from low-metamorphic grade rocks of a mylonitic zone. Therefore, the lower Patlanoaya Group deposition was controlled by a major NNE-trending mylonitic shear zone associated with the development of a supradetachment basin linked to the progressive exhumation of high-pressure rocks during an Early Mississippian major extensional event. This work highlights the importance of sedimentary deposits in reconstructing the late Paleozoic history of the Acatlán Complex during the Rheic Ocean closure and the final stage of the Pangea assembly.

Observations of Near-Inertial Internal Waves over the Continental Slope in the Northeastern Black Sea

The article presents observations of near-inertial internal waves (NIWs) in the slope waters of the Black Sea in winter and summer. Rotary spectral analysis of a time series of sea current velocity measurements revealed the prevailing anticyclonic component of the motions near the local inertial frequency f. The clockwise rotation of the velocity vector with depth implies that the NIWs propagate downwards. The amplitude of NIWs usually was 0.1–0.2 m s−1. NIWs were observed in the layer of the permanent pycnocline and the seasonal pycnocline, which attenuate below depths of 160 m and 80 m in winter and summer, respectively. The amplitude of the near-inertial kinetic energy (NIKE) showed a close relationship with vertical stratification. During winter, NIKE exhibited maximum values in the layer of the permanent pycnocline, whereas, in summer, it was primarily observed in the seasonal pycnocline layer. The near-inertial oscillations were generally more energetic in winter.

Characteristics and paleoenvironmental conditions of the well-preserved Early Mesoproterozoic Fengjiawan Formation stromatolites in the southern North China Craton

Stromatolites in the lower part of the Early Mesoproterozoic Fengjiawan Formation in the southern North China Craton (NCC) contain abundant microbial dolostones with well-preserved primary structures. This study comprehensively investigates the macromorphological, petrographical, mineralogical, and geochemical characteristics of these pure stromatolite-dominated microbialites for the first time. The results reveal diverse macromorphological structures distributed from the bottom to the top as conical (lower intertidal to upper subtidal zones), hemispherical domal (lower supratidal zone), spherical domal (upper to middle intertidal zone), and more abundant columnar (upper to middle subtidal zone) stromatolites. The mineral composition of these stromatolites indicates that they are pure dolostones, predominantly consisting of dolomite (avg. 94.24 wt%). These stromatolite-dominated microbialites were subjected to minimal detrital contamination and weak post-depositional alteration as documented by different geochemical multiproxy evaluations. This indicates that they have effectively retained the composition of the seawater in which they were initially formed. Geochemical indicators, such as Y/Ho, La/La*, Gd/Gd* and Ce/Ce* strongly suggest deposition of the stromatolites in an open shallow marine environment characterized by a passive continental margin setting. Different elemental ratios, such as Ce/Ce*, MoEF, UEF, MoEF/UEF, imply the stromatolites formation under a suboxic redox environment, where the photosynthesis of cyanobacteria likely played a significant role in the slight oxygenation of the seawater during their formation. These oxygenation levels decreased as water depth increased, with more nutrient supply promoting from deeper waters. The formation and growth of the stromatolite-dominated microbialites were significantly influenced by various depositional conditions, including sea level fluctuations, biological processes, suboxic redox environment, low detritus contamination, and weak post-depositional alteration. These conditions all contributed to the excellent preservation of the Fengjiawan Formation stromatolites, providing valuable insights into deep time microbial carbonates.

Electronic tags reveal high migratory diversity within largest Atlantic halibut (Hippoglossus hippoglossus) stock

Growing evidence suggests that complex spatial structure occurs within the Scotian Shelf and southern Grand Banks (SSGB) Atlantic halibut stock, yet large knowledge gaps remain about migratory and spawning behaviors. Here, 71 pop-up satellite archival tags were deployed on large Atlantic halibut (FL: 87 - 166 cm) between 2012 and 2020. Migration tracks were successfully reconstructed for 43 fish using a hidden Markov geolocation model, and temperature and depth time series were available for nine fish (total n = 52). Five migratory behaviors were identified: shelf residency, slope residency, shelf-channel migration, shelf-slope migration, and dispersal. The high-resolution data for four of 20 physically recovered tags provided evidence for putative female spawning behavior in deep channels on the continental shelf and along the continental slope between January and February. Additionally, four halibut displayed previously undocumented periods of sustained, oscillatory vertical movement along the continental slope between November and February. The high migratory diversity observed in the SSGB stock supports the existence of multiple resident and migratory contingents in the stock in the apparent absence of significant genetic structure.

The complex interaction between channel–levee systems and mass transport complexes in the Pliocene–Quaternary Rakhine Basin, offshore Myanmar

Channel–levee systems (CLSs) and mass transport complexes (MTCs) are prevalent in deep-water basins, yet their interplay remains enigmatic. This study uses high-resolution 3D seismic data to investigate the architecture of CLSs and MTCs, aiming to explore how CLSs influence the distribution of MTCs in the Rakhine Basin, Bay of Bengal. Two models are built to illustrate the interaction between CLSs and MTCs. In the first model, large-scale aggradational CLSs created unfilled relief in their overbank environments, which acted as spatially confined topography for subsequent mass-wasting deposits. In this model, the interaction between large-scale CLSs and confined MTCs controls the distribution of confined MTCs. In the second model, laterally migrating CLSs do not create such prominent topographic relief in their overbank environments. This characteristic renders them inconsequential in influencing the distribution of subsequent mass-wasting processes. Consequently, spatially extensive MTCs develop within unconfined settings. This configuration gives rise to lithological traps, which might be common and represent potential drilling targets on continental margins. This study could contribute to a better understanding of the interplay between CLSs and MTCs and have some enlightenment on looking to provide some insight into the search for lithological traps in the Rakhine Basin.

Origin of a high-velocity layer: Insights from seismic reflection imaging (South China Sea)

Magmatism and crustal structural characteristics are well-known phenomena that support understanding the marginal sea's tectonic evolution. The South China Sea (SCS) has become a natural laboratory for researchers to investigate the evolution of the marginal sea because of its complex tectonic background and multiple phases of magmatic activities. However, due to limitations in resolution and depth of observations, the evolutionary process of the SCS is still controversial. We use the latest processed multi-channel seismic reflection profile Line AB to evaluate the sedimentary sequences, crustal tectonics, and the Cenozoic magmatism in the southern SCS. The gravity modeling results indicate a reduction in crustal thickness from 18 km in the proximal domain to 7 km in the Continent-Ocean Transition (COT). The COT is roughly 35 km wide, representing a rapid break up process of the SCS. A high-velocity layer (HVL) of about 3 km thickness was formed in the continental slope crust, interpreted as post-spreading magmatic underplating. In addition, four sedimentary sequences, post-spreading magmatic intrusion, and the possible magma conduits were identified in the profile. Combining with previous ocean bottom seismometer (OBS) data, multi-channel seismic data, fault system data, and petrologic observations, we indicate the deep mantle magmatic material upwelled to the crust and further intruded into sediments along the hyper-extended crust or detachment faults during the post-spreading stage. We contend that cooling and heat shrinking within the SCS oceanic lithosphere have the potential to initiate decompression melt deformation in the continental slope. In summary, we construct a model of the origin and migration mechanism of extensive post-spreading magmatism in the SCS. Our research reveals the Cenozoic evolution and the HVL of the southern SCS, and provides a possible genesis for the anomalously young magmatism observed at the continental margin.

Neodymium isotope composition and rare earth element distribution of East Antarctic continental shelf and deep waters

Circumpolar Deep Water (CDW) and Antarctic Bottom Water (AABW) play key roles in the Earth's climate system. Both water masses form critical components of the Antarctic Circumpolar Current and Meridional Overturning Circulation and therefore directly influence the large-scale redistribution of heat, nutrients and carbon. Reconstruction of past CDW transport and AABW production and export has been a key target in palaeoceanography. One promising proxy to achieve this has been the neodymium (Nd) isotope composition of seawater. The biogeochemical processes controlling Nd in the ocean, however, remain underconstrained, and modern observations of Nd isotopes in the Southern Ocean are still geographically limited.To overcome this limitation, 61 seawater samples were collected for Nd isotope and rare earth element (REE) analysis at nine stations along the Wilkes Land continental margin and in the Australian-Antarctic Basin (65°S 125°E) near East Antarctica. The results show that the different water masses have the following Nd isotope characteristics: Antarctic Surface Water (AASW), εNd = −9.0 ± 1.0 (2SD; n = 22); Modified CDW (MCDW), εNd = −8.8 ± 0.8 (2SD; n = 22); AABW, εNd = −8.3 ± 0.5 (2SD; n = 17).There is no evidence of continental REE inputs to surface waters on the Wilkes Land margin. Observed zonal variability of Nd isotope composition in AASW can be attributed to seasonal competition between the poleward flow of warm AASW from the AAG and the westward export of cold surface shelf waters by the Antarctic Slope Current.In terms of deep and bottom waters, mixing of upwelled CDW with AASW and AABW exclusively controls the Nd isotope composition of MCDW, with no indication of boundary processes modifying the Nd isotope composition of MCDW as it encroaches the shelf or slope. The regional Nd isotope signature for AABW is intermediate between published data for the Atlantic sector AABW (εNd = −9.1 ± 0.7) and Pacific sector AABW (εNd = −7.4 ± 0.9). In the absence of active convection, this Nd isotope signature can be explained by a mixture of advected AABW from the Adélie Land coast with a more dominant component of local MCDW and CDW.The data presented in this study constitute the first Nd isotope data set in this part of the Southern Ocean and suggest conservative behaviour of Nd in proximity to the old East Antarctic continent.

The impacts of profile concavity on turbidite deposits: Insights from the submarine canyons on global continental margins

Submarine canyons are primary conduits for turbidity currents transporting terrestrial sediments, nutrients, pollutants and organic carbon to the deep sea. The concavity in the longitudinal profile of these canyons (i.e. the downstream flattening rate along the profiles) influences the transport processes and results in variations in turbidite thickness, impacting the transfer and burial of particles. To better understand the controlling mechanisms of canyon concavity on the distribution of turbidite deposits, here we investigate the variation in sediment accumulation as a function of canyon concavity of 20 different modern submarine canyons, distributed on global continental margins. In order to effectively assess the isolated impact of the concavity of 20 different canyons, a series of two-dimensional, depth-resolved numerical simulations are conducted. Simulation results show that the highly concave profile (e.g. Surveyor and Horizon) tends to concentrate the turbidite deposits mainly at the slope break, while nearly straight profiles (e.g. Amazon and Congo) result in deposition focused at the canyon head. Moderately concave profiles with a smoother canyon floor (e.g. Norfolk-Washington and Mukluk) effectively facilitate the downstream transport of suspended sediments in turbidity currents. Furthermore, smooth and steep upper reaches of canyons commonly contribute to sediment bypass (i.e. Mukluk and Chirikof), while low slope angles lead to deposition at upper reaches (i.e. Bounty and Valencia). At lower reaches, the distribution of turbidite deposits is consistent with the occurrence of hydraulic jumps. Under the influence of different canyon concavities, three types of deposition patterns are inferred in this study, and verified by comparison with observed turbidite deposits on the modern or paleo-canyon floor. This study demonstrates a potential difference in sediment transport efficiency of submarine canyons with different concavities, which has potential consequences for sediment and organic carbon transport through submarine canyons.

Intrinsically Episodic Antarctic Shelf Intrusions of Circumpolar Deep Water via Canyons

Abstract The structure of the Antarctic Slope Current at the continental shelf is crucial in governing the poleward transport of warm water. Canyons on the continental slope may provide a pathway for warm water to cross the slope current and intrude onto the continental shelf underneath ice shelves, which can increase rates of ice shelf melting, leading to reduced buttressing of ice shelves, accelerating glacial flow and hence increased sea level rise. Observations and modelling studies of the Antarctic Slope Current and cross-shelf warm water intrusions are limited, particularly in the East Antarctica region. To explore this topic, an idealised configuration of the Antarctic Slope Current is developed, using an eddy-resolving isopycnal model that emulates the dynamics and topography of the East Antarctic sector. Warm water intrusions via canyons are found to occur in discrete episodes of large onshore flow induced by eddies, even in the absence of any temporal variability in external forcings, demonstrating the intrinsic nature of these intrusions to the slope current system. Canyon width is found to play a key role in modulating cross-shelf exchanges; warm water transport through narrower canyons is more irregular than transport through wider canyons. The intrinsically episodic cross-shelf transport is found to be driven by feedbacks between wind energy input and eddy generation in the Antarctic Slope Current. Improved understanding of the intrinsic variability of warm water intrusions can help guide future observational and modelling studies in the analysis of eddy impacts on Antarctic shelf circulation.

Origin and Geological Implications of Monzogranites and Rhyolitic Porphyries in the Wunugetu Porphyry Copper–Molybdenum Deposit, Northeast China: Evidence from Zircon U-Pb-Hf Isotopes and Whole-Rock Geochemistry

The Wunugetu deposit, a large-scale porphyry copper–molybdenum deposit, is located in the southern Erguna block. Its ore bodies are primarily found within monzogranites, granite porphyries, and biotite monzogranites. Additionally, the deposit contains late-stage intrusive dykes of rhyolitic porphyries. This study examined the deposit’s monzogranites and rhyolitic porphyries using lithogeochemistry, zircon U-Pb dating, and Hf isotopic analysis. The main findings include: (1) Zircon U-Pb dating showed that the monzogranites formed around 209.0 ± 1.0 Ma, whereas the rhyolitic porphyries in the northern portion formed around 170.49 ± 0.81 Ma, suggesting magmatic activity in the deposit spanned from the Late Triassic to the Middle Jurassic. (2) The monzogranites exhibited high silicon content (73.16–80.47 wt.%) and relatively low aluminum content (10.98–14.37 wt.%). They are enriched in alkalis (content: 3.42–10.10 wt.%) and deficient in magnesium and sodium, with aluminum saturation indices (A/CNK) ranging from 1.1 to 2.9. In addition, the monzogranites are enriched in large-ion lithophile elements (LILEs) such as Rb, K, and Ba and deficient in high-field-strength elements (HFSEs) like Nb, P, and Ti. (3) The monzogranites have low Zr + Nb + Ce + Y contents of (151.3–298.6 ppm) × 10−6 and 10,000 × Ga/Al ratios varying between 1.20 and 2.33, suggesting that they are characteristic of I-type granites. (4) Positive zircon εHf(t) values ranging from +0.3 to +7.6 in both rhyolitic porphyry and monzogranite samples, increasing with younger emplacement ages, imply that the deposit’s rocks originated from magmatic mixing between mantle-derived mafic magmas and remelts of the juvenile crust. Considering these results and the regional geological evolution, this study proposes that the Wunugetu deposit was formed in an active continental margin setting and was influenced by the Late Triassic–Middle Jurassic southeastward subduction of the Mongol-Okhotsk Ocean.