Marine Geology最新文献

Volcanic centers are characteristic features of ultraslow-spreading mid-ocean ridges, the least-explored parts of the global ridge system. Volcanic centers can provide insights into deep magmatic and metamorphic processes at these ridges. Here, we present data from the largest volcanic center on the Gakkel Ridge, the Langseth Ridge, situated at 60–62°E. Langseth is ∼10 km wide, consisting of three peaks that rise to 585 m water depth, some 3–4 km above the surrounding seafloor. It strikes perpendicular to Gakkel's spreading direction and can be traced for ∼40 km, which translates to an age of ∼8 Myr. Seafloor imaging revealed abundant (pillow) basalt but also fissures and geologic faults across the Langseth Ridge. Basaltic rocks were sampled at all summits and diabase at the slope of the northern summit that dips into the rift valley.

Our samples are of normal to depleted mid-ocean ridge basalt composition and exhibit a wide range of major and trace element contents, due to magmatic processes, accumulation of macrocrysts, and hydrothermal alteration. Radiogenic isotope ratios, most notably ¹⁴³Nd/¹⁴⁴Nd and ²⁰⁸Pb/²⁰⁶Pb, trend from typical rift valley compositions to isotopically enriched values with increasing distance to the rift valley. This trend may imply melt pooling from different sources, potentially representing a shift from shallow melting beneath the rift valley to deeper melting of enriched sources and higher degrees of melting underneath Langseth. Mineral compositions and plagioclase sieve textures imply prolonged storage of magma at depth prior to eruption. Hydrothermal alteration occurred over a range of conditions. Basalt from the summits is weakly altered at temperatures ≪100 °C, which likely occurred in situ at the summit sites. Diabase samples experienced chloritization and albitization and display epidote and quartz veins, which formed at >300 °C. These assemblages and temperatures are typical for lower crustal levels and imply uplift of the samples of >1 km. Diabase samples from the Afanasenkov Seamount, another volcanic center on the Gakkel Ridge that we investigated for comparison, were altered under comparable conditions.

Our findings suggest a combined volcanic–tectonic origin of the studied volcanic centers, potentially implying that such complexes may generally form due to the interplay of magmatism and tectonics. Researching volcanic centers has the potential to further our understanding of both deep and shallow crustal processes at ultraslow-spreading ridges, providing further insights into the role of these centers as linkages between lithosphere and hydrosphere and the (deep) biosphere they sustain.

Regional variability of global sea-level rise remains an important area of study given the vulnerability of sediment-starved coastlines to coastal inundation, especially those in proximity to large population centers. Galveston Bay, Texas, is currently experiencing more than double the global rate of sea-level rise and is particularly vulnerable to storm inundation that will further destabilize the coastline. Limitations in instrumental observations necessitate the use of the geologic record preserved offshore modern Galveston Bay to understand how this particular coastline responds to periods of rapid sea-level rise. We present micropaleontological analysis of sediment cores combined with high-resolution seismic data to reconstruct the Holocene paleoestuary offshore Galveston Bay and its evolution since initial inundation ∼10 ka through marine transgression ∼6 ka. We find that despite rapid sea-level rise, the Galveston paleoestuary maintained relatively stable outer boundaries, and within the bay environmental shifts occurred as a result of probable marine incursions due to tidal inlet migrations. Paleoenvironmental changes in the early Holocene coincide with flooding events within other Texas Gulf Coast bays suggesting global sea-level rise played a prominent role. Middle to late Holocene changes occurred when rates of sea-level rise slowed, suggesting regional hydroclimate change played a more dominant role.

Sedimentation rates and sediment budgets are important agents to understand the source-to-sink dynamics in marginal seas. As a classical representative of mega-river dominated marginal seas globally, the South China Sea (SCS) receives huge amounts of fluvial input from mega rivers covering different climate zones. Despite its well-documented prevalence, quantifying the spatial distribution of sedimentation rate and sediment budget over the entire SCS remains a challenge due to limited data availability. In this study, we employed a comprehensive approach to quantify the modern sedimentation rates and sediment budget in the SCS. This approach combined ²¹⁰Pb measurements from 409 cores, AMS¹⁴C data from 112 cores, and 33 sediment trap observations. Our results show that higher sedimentation rates >0.3 cm/a mainly occur in deltas, shelf mud areas, and upper continental slope that connects the submarine canyon. In the subaqueous Mekong Delta, for example, the sedimentation rates can exceed 10 cm/a. In contrast, there is no substantial modern sedimentation in sandy and gravelly areas of the shelf due to strong erosion by a combination of waves, tides and ocean currents. We further compare these results with the eastern China seas including the Bohai Sea, Yellow Sea and East China Sea. A similar spatial distribution of sedimentation rates can be observed in the continental shelf of the eastern China seas. The Holocene sedimentation rates in the deep-water regions of the SCS are generally <100 cm/ka. The basin floor experiences the slowest accumulation, with rates below 3 cm/ka. In contrast, sedimentation rates on the eastern island slopes range from 3 to 8 cm/ka, while the northern, western, and southern continental slopes accumulate sediment most rapidly, exceeding 25 cm/ka. Approximately 1191.1 × 10⁶ t of fine-grained sediment is deposited annually in the continental mud areas of the SCS, with a comparable amount of 1185.4 × 10⁶ t/a deposited on the continental shelf of the eastern China seas. The continental slope accumulates sediment at a significantly higher rate (∼161.0–239.4 × 10⁶ t/a) compared to the deep-water basin (∼16.5–20.1 × 10⁶ t/a) in the SCS. Most of the modern sediments (>98%) are deposited on the continental shelf and slope. In the SCS, fluvial inputs dominates modern sediment sources, contributing over 80% of the total. Coastal/seabed erosion and biogenic deposition account for approximately ∼18%, with eolian dust contributing less than <2%. The findings presented here are critical for elucidating the sources, transport pathways, and deposition patterns of modern sediments in marginal seas.

For the past decade, giant deepwater oil discoveries in the pre-salt section of the Campos and Santos basins of Brazil, have brought significant attention to offshore exploration activities along the South Atlantic margins. The prolific Cretaceous coquina deposits in these basins are part of the pre-salt rock record and constitute an effective but complex and heterogeneous hydrocarbon reservoir difficult to predict and model. Parting from this context, an evaluation of the sedimentological, structural and taphonomic criteria for coquinas are essential to better understand and predict the facies distribution and depositional models of the pre-salt coquinas strata. Based on this premise, the present work aims to genetically interpret 133 mixed carbonate-siliciclastic bottom sediments of the Albardão shelf – a modern marine coquina analogue, using facies description, investigating the relationship with hydrodynamic forces, and accessing the influence of morphology and structural framework on their deposition. From these analyses, we recognized a hybrid facies, three modern carbonate facies in analogy to the carbonate rock classification and four siliciclastic facies. These eight facies were then grouped into three facies associations representing high, moderate, and low energy facies. The high energy facies association comprises rudstones (Rf) and grainstones (Gf) with highly fragmented bivalve shells and barnacles abundantly present in the beach system, above the fair-weather wave base limit (FWWB). These facies also occur offshore on bathymetric highs above the storm wave base limit (SWB) but display less reworking than the coastal high energy facies above the FWWB due to wave shoaling. The moderate energy facies association consists of hybrid sand (Hs), sand (S) and muddy sand (mS) occurring between the FWWB and SWB limits in the offshore transition zone with extensive winnowing action and low rate of reworking. The low energy facies association includes sandy mud (sM), mud (M) and micritic mud (Mc), characterized by the decantation of the fine sediments below the offshore SWB limit. The results confirm a bottom facies distribution controlled by depth, shelf profile morphology and energy from incident waves. The fragmented rudstone and fragmented grainstone facies are the best-recognized reservoirs with both having high porosity and high permeability.

The Nansha Trough (NT) is part of the southern continental margin boundary of the South China Sea (SCS). It has undergone complex tectonic superposition and evolutionary processes involving the subduction demise of the Proto-SCS and subsequent spreading of the SCS. This study provides the first systematic identification and analysis of igneous bodies and seamounts along the NT, based on a multi-channel seismic profile (NDL1) recently acquired along it. The seamounts within the trough are of magmatic origin and the carbonate build-ups observed at the summits of some seamounts exhibit a substantial thickness. Igneous bodies within the trough are consistently associated with high P-wave anomalies. Furthermore, at the eastern and western sides, there are distinct gravity-magnetic-anomaly patterns. On the eastern side, Yinqing Seamount, Nanle Hill and volcanic mounds show high gravity and strong negative magnetic anomalies. In contrast, on the western side, Jinghong Seamount, Yangshu Hill and intrusive bodies show less pronounced magnetic anomalies. This difference may be related to differences in magmatic periods. Unlike the extensive post-spreading magmatism in the SCS's northern margin and deep basin, the most widespread magmatic activity in the NT occurred at ca. 16 Ma before decreasing during the Miocene. This decrease may be closely related to subduction cessation in the Proto-SCS and the collision between the Nansha Block and Borneo. The identification and analysis of NT igneous bodies and their evolutionary processes help delineate the southern boundary of magmatism at the SCS margin. They also provide crucial information for constraining the magmatic processes of Proto-SCS subduction termination and SCS spreading evolution.

Although variable well log resolution and its control on saturation estimation has been studied, it has not been directly applied to a specific location to explore the nature of gas hydrate within a sand reservoir. We applied in-situ measurements of resistivities, neutron porosity, and gamma ray at two sites in the Qiongdongnan Basin, South China Sea (QDN-W05–2021 and QDN-W08–2021) to investigate the reservoir parameters of a hydrate-bearing sand reservoir. Our results show that gas hydrate is distributed in 5 zones with a total thickness of 10.7 m and an average saturation of 69% at the QDN-W05–2021 site, while they are distributed in 2 zones with a total thickness of 4.3 m and an average saturation of 49% at the QDN-W08–2021 site. We found that variances in saturations estimated from lateral-extra deep button (RX), phase shift (P40H-P40L), and attenuation (A40H-A40L) resistivities within the laterally mapped continuous sand body were affected by the nature of gas hydrate occurrences. Results indicate gas hydrate forms and accumulates at the center of the sand layer and tends to be less or not present toward the top and base. Integrated with seismic data, the in-situ measurements provide insights in the evolution of a mushroom-shaped, hydrate-gas reservoir system. In the system, free gas is likely horizontally transported from the top-center of the gas chimney to the surrounding areas in the early stage dominated by a warm-gas environment, whereas hydrate forms in the opposite pathway starting from the surrounding areas in the following stage with temperature reducing. Our study suggests that high-resolution in-situ measurements not only are a tool to identify the physical properties, but also can be used to help explain the physical process of hydrate growth and accumulation.

Intertidal flats are important shallow-water habitats and buffers against coastal erosion. Strong, short-lasting meteorological events, such as storms and rainfall, are the main mechanisms of transporting (in)organic materials and sediments. Two in-situ mooring systems were installed simultaneously in the tidal channel and mudflat of Jeungdo, Korea, to understand the dynamic behaviors of suspended sediment and chlorophyll-a (chl-a) under the episodic events. During fair-weather periods with a distinct tidal cycle, the sediment in the mudflat was resuspended during the flood and then advected to the tidal channel during the ebb. The maximum suspended sediment concentration (SSC) and chl-a under storm event were approximately 9 and 2 times higher than those under fair-weather periods, respectively. Under rainfall event, the maxima were approximately 7 and 1.2 times higher than fair-weather, suggesting that sediment and microphytobenthos were highest resuspended by the meteorological events. In addition, a time lag (∼ 1.5 h) between SSC and chl-a occurred in the tidal channel during ebb tide with a rainfall event. During the post-rainfall periods, the SSC and chl-a increased, showing a positive relationship with the bed shear stress, suggesting that the rainfall event could reduce sediment stabilization.

The Shetland Islands (UK) are a seminal location for investigating palaeo-tsunami deposits. Onshore evidence suggests three tsunami have occurred during the Holocene: the Storegga tsunami ca. 8150 cal yr BP, the Garth tsunami ca. 5500 cal yr BP and the Dury Voe tsunami ca. 1500 cal yr BP. However, little research has been published on the impact of tsunami on the subtidal shelf where a large amount of North Sea hydrocarbon infrastructure is located. Here, we test the hypothesis that Holocene tsunami impacted shelf sediments, using radiocarbon dating and sedimentological characterization of cores recovered from the Fetlar Basin, offshore east Shetland. The cores contain distinct sand and shell lenses within a Holocene mud sequence, indicating a sudden change in hydrodynamic conditions. Radiocarbon dates bracketing the sand lenses overlap with the published dates for the Storegga event. Dates within the deposit are older (>9 cal. yr BP) which is consistent with reworking and redeposition of earlier sediments. Particle size analysis, ITRAX and MSCL data evidence increases in mean grain size, a reduction in sorting capacity, increased shell concentrations and peaks in associated elements (log(Ca/Fe), log(Ca/Ti) and Sr). These attributes indicate transport of allochthonous material from the inner shelf, and are typical of tsunami backwash-generated submarine debris flows. No evidence was found within the cores for any later Holocene tsunami, which may be due to either bioturbation, active currents, or lack of an initial deposit. The disturbance of sediments, and generation of a submarine debris flow within the Fetlar Basin by the Storegga event highlights the need to assess the potential impact of any future tsunami on planned and existing infrastructure at seabed. Erosion and deposition of allochthonous older marine sediment by the Storegga event also has consequence for interpretation of the coeval 8.2 ka cold event in marine sedimentary records in the tsunami affected region.

The Gulf of Corinth represents an ideal setting for studying the impact of sea level changes and regional climate on a semi-enclosed, syn-rift basin. Here we investigate the stratigraphic and paleoceanographic variability recorded in the sedimentary succession of the basin over the Marine Isotope Stage (MIS) 5 period when global sea level and climatic conditions along the eastern Mediterranean exhibited pronounced fluctuations. We used sedimentological (granulometry, composition), micropaleontological (planktic and benthic foraminifera), and isotopic (stable δ¹⁸O, δ¹³C, and clumped isotope) proxies on core samples from site M0079A (IODP Expedition 381) combined with additional data from the expedition overview and records from the surrounding area. The sedimentary succession comprises an alternating pattern of a) bioturbated, biogenic-rich deposits associated with increased hemipelagic sedimentation with b) partly bedded, detrital-rich sediments attributed to intercalated sediment gravity flows within the hemipelagic background under low oxic sea-surface conditions, and c) aragonite-rich laminated deposits, indicating either transitional conditions between marine and isolated environment or a highly stratified seawater column and low oxygen seafloor conditions. We find that the Gulf of Corinth lay under marine conditions for nearly the entire MIS 5 period, while the Rion sill would have been possibly shallower, even 10 m, than the current depth. Nevertheless, water exchange was restricted during the MIS 5a – MIS 4 transition when the sea level fluctuated very close to the sill height. The hydrological conditions within the Gulf during most of the highstands MIS 5a, 5c, and 5e reflect higher oxygen levels and/or increased nutrient availability compared to the Holocene and present-day regime. The combined effects of Ionian Sea inflows and enhanced riverine runoff led to increased water column stratification and low oxygen, eutrophic seafloor conditions in the Gulf of Corinth during times of high precipitation in southern Europe and deposition of sapropels S3, S4, and S5 throughout the eastern Mediterranean. In contrast, during periods of widespread cold and arid conditions in the eastern Mediterranean, water column mixing was intense within the Gulf. Prevalent marine conditions are also proposed during the MIS 5b and 5d lowstands, yet associated with predominately bedded-detrital sediments in the Gulf. A complementary investigation in the adjoining Patras Gulf is suggested to fully comprehend the dynamics of climate and sea level changes in complex rift systems.