Marine Geology最新文献

Carbon isotope data is desired to be increased to promote the understanding of carbon cycle throughout in the Earth. Diamond is a key carbonaceous tool to study deep carbon cycle, but most diamond occurrences are limited from kimberlite pipes in the continental region. Recently, micron-sized diamonds have been discovered from the oceanic region and investigated to understand deep carbon cycle in the oceanic mantle. However, some fundamental cautions have been issued on the oceanic diamonds because some of them could be of artificial origin. Hence, alternative oceanic mantle-derived carbonaceous material is needed to increase oceanic carbon isotope data. We report micron-sized calcite vein in a lherzolite xenolith hosted by enriched mantle I (EM1)-type olivine nephelinite from Aitutaki Island, Cook Islands in the southern Pacific. With employing various techniques to determine carbon and oxygen isotope compositions from sub-micrograms of calcite, we demonstrate that carbonaceous fluid originated from EM1-type mantle source exhibited organic carbon signature based on its light carbon isotope composition along with petrographic characteristics of the calcite vein. The oceanic mantle hosts organic carbon in places due to the recycling of surface materials.

Precipitation changes in the East Asia are closely linked to the monsoonal climate in this region and the hydrothermal processes in the western tropical Pacific. However, trends of reconstructed precipitation records for the past millennium are inconsistent and the influencing factors are in dispute. Here we reconstruct a 700-year precipitation record for the northern South China Sea (SCS) using grain size of lagoon sediments. Our data revealed that precipitation increased in the early to middle Little Ice Age, possibly modulated by tropical cyclones and the Walker Circulation. The East Asian Summer Monsoon and Pacific Decadal Oscillation had the major influence on the precipitation changes in the northern SCS. This study provides new insight into the processes and the underlying mechanisms of climate changes in the SCS.

Tropical cyclone (TC) models indicate that continued planet warming will likely increase the global proportion of powerful TCs (specifically Categories 4 and 5 hurricanes), increasingly jeopardizing low-lying coastal communities and resources such as the Pelican Cays, Belize. The combination of increased coastal development and continued relative sea-level rise puts these communities at even higher risk of damage from TCs. The short TC observational record for the western Caribbean hampers the extensive study of TC activity on centennial timescales, which hinders our ability to fully understand past TC climatology and improve the accuracy of TC models. To better assess TC risk, paleotempestological studies are necessary to put future scenarios in perspective. Here, we present a high-resolution reconstruction of coarser-grained sediment deposits associated with TC (predominately ≥ Category 2 hurricanes) passages over the past 1200 years from Elbow and Lagoon Cays, two coral reef-bounded lagoons at the northern and southern end of the Pelican Cays; the most southern Belizean paleotempestological site to date. Coincident timing of historic storms with statistically significant coarser-grained deposits within cay lagoon sediment cores allows us to determine which historic TCs likely generated event layers (tempestites) archived in the sediment record. Our compilation frequency analysis indicates one active interval (above-normal TC activity) from 1740 to 1950 CE and one quiet interval (below-normal TC activity) from 850 to 1018 CE. The active and quiet intervals in the Pelican Cays composite record are anticorrelated with those from nearby and re-analyzed TC records to the north, including the Great Blue Hole (∼100 km north) and the Northeast Yucatan (∼380 km northwest). This site-specific anticorrelation in TC activity along the western Caribbean indicates that we cannot rely on any one single TC record to represent regional TC activity. However, we cannot discount that these anticorrelated periods between the western Caribbean sites are due to randomness. To confirm that the anticorrelation in TC activity among sites from the western Caribbean is indeed a function of climate change and not randomness, an integration of more records and TC model simulations over the past millennium is necessary to assess the significance of centennial-scale variability in TC activity recorded in reconstructions from the western Caribbean.

The steep slopes of carbonate platforms frequently display large-scale sediment destabilization features like rockfalls, mass transport complexes, and slope erosion. The processes and factors triggering such instabilities and how they interact are a matter of ongoing discussion. We use hydroacoustic, sedimentological, and seafloor imaging data to map and characterize slope instabilities and potential controlling factors at the flank of the isolated Tregrosse carbonate bank in the Coral Sea, northeast Australia. Erosion of gullies and submarine valleys is concentrated in slope segments with the platform rim at several 10s of meters of water depth, i.e. where there is potential for sediment transfer from the bank interior to the slope. Gravity core data indicate that most sediment export from the platform occurs during sea-level fall. The toe of slopes neighboring segments with a shallower platform rim are mostly characterized by mass-transport complexes of platform rim and upper slope rocks forming extended block fields. Distal slope areas are dismantled through submarine landslides resulting in scalloped head scarps. The basal detachment surface of these submarine landslides appears to be rooted in several 100 s of meters in the subsurface at a lithological heterogeneity, which is documented by a gamma-ray peak in the downhole logging data from Ocean Drilling Program Site 817. Our findings show that (1) canyon erosion, (2) platform rim and upper slope destabilization as well as (3) lower slope dismantling, largely act independently of each other to destabilize the flanks of the carbonate bank. The complexity of the carbonate platform dismantling processes and the corresponding controlling factors shown in this study should also be considered when interpreting seismic morphological data.

The Caspian Sea (CS) is an endorheic basin located at the boundary of Asia and Europe, which has undergone remarkable environmental transformations since the late glacial period. Its present dimensions and oceanographic features are determined by a complex interaction between climatic change and tectonic processes. However, the link between past climatic variations, especially the dynamics of the Siberian High (SH), and the CS oceanographic characteristics remains poorly constrained, leading to uncertainty in projecting its future rapid changes. In this study, a 1.7 m sediment core from the 584 m of water depth at the eastern side of the southern CS was obtained to reconstruct the climatically-driven environmental changes in the CS region during the last 14,000 years. High-resolution XRF measurements and detailed sedimentological analyses were conducted on the core samples. The outcomes were then systematically compared with a diverse array of extant palaeoclimatological datasets from Central Asia and Europe, regions dominated by two pivotal climatic systems impacting the CS. The objective of this comparative analysis was to clarify the impact of the SH and to define its dynamic interaction with the Westerlies over the CS. It was demonstrated that the SH dynamics exerted a significant influence on the CS environment. When the SH was strong and expanded over a large area, precipitation occurred outside of the CS watershed area and led to sea-level fall. Conversely, when the SH expansion was confined to the CS catchment basin, precipitation over the basin and lower evaporation resulted in sea-level rise.

Numerous investigations into the northern Mid-Atlantic Ridge (the NMAR), a typical slow-spreading mid-ocean ridge, have revealed that NMAR is favorable for the development of long-lived detachment faults and the formation of oceanic core complexes (OCCs). OCCs are often conducive to the development of ultramafic-hosted hydrothermal deposits with significant resource potential. However, as a counterpart of the NMAR on the Southern Hemisphere, the southern Mid-Atlantic Ridge (SMAR), also belonging to the class of slow-spreading ridges, has only received very limited investigation. This prompts the inquiry as to whether the SMAR, like the NMAR, can foster the development of OCC and associated hydrothermal deposit. To address this issue, we present the identification of an OCC (named as Kaifeng OCC) at the intersection of the SMAR and the Martin Vaz transform fault (∼23°S). This discovery is accompanied by evidence detailing a new detachment fault breakaway on an old detachment footwall. Collected samples reveal indications of hydrothermal activity, encompassing (1) residual sulfide containing chalcopyrite within honeycomb-like structures, (2) reddish-brown Fe oxides and atacamite, partially concretized by dolomite, and (3) a dark gray Mn-oxide crust. These mineralogical features indicate the presence of gossans, commonly iron oxide-dominated cover layers that envelope the outer surface of weathered seafloor sulfide deposits, which subsequently undergo modifications due to subsequent hydrothermal activities. Our work proves the existence of OCC and associated hydrothermal deposits at a ridge-transform intersection of the SMAR.

Recent high-resolution multibeam bathymetry and seismic data from the platform-top to the abyssal plain of the Zhongsha Platform allow for a detailed investigation of the morphologies, spatial distribution, and trigger mechanisms of submarine canyons, submarine landslides, and associated sedimentary features along modern isolated carbonate slopes. The newly observed Zhongsha Canyon System provides a natural laboratory for reconstructing the source-to-sink sedimentary processes in a pure carbonate setting. This study reveals that there are thirty-four submarine canyons at water depths between 300 and 4100 m on the northern and western slopes of the Zhongsha Platform. Two morphologically different submarine canyon types are identified: (1) dendritic canyons, which exhibit abundant tributaries with scallop-shaped failures at the canyon heads, and (2) linear canyons, which feature rare tributaries with elongated failures at the canyon heads. The dendritic canyons are more complex in morphology than the linear canyons as a result of the interaction among numerous tributaries. Canyon initiation and evolution pass through three phases: (1) initial stage: off-platform sediment transport and platform margin failures contribute to erosive gravity flows; (2) developmental stage: initiation and incision of submarine canyons along platform margin failures; and (3) mature stage: numerous mature canyons along the platform margin. Off-platform sediment transport, density cascading, gravity flows, monsoon currents, and deep circulation play an essential role in shaping the slope morphologies. In addition, submarine landslides are extensively observed along the entire slope of the Zhongsha Platform at water depths of 600 to 4200 m, including canyon-wall failures, slope landslides, canyon-front landslides, and slope-toe failures based on their location and genesis. These processes can steepen the platform slopes by upward retrogressive and downward progressive erosion. On a larger scale, the persistent submarine canyons and occurrence of landslides around the Zhongsha Platform contribute to the uniqueness of this landscape among modern carbonate slopes. The morphologies and evolutionary processes of Zhongsha Canyon System present significant differences from the global carbonate submarine canyons in terms of their dimensions and trigger mechanisms. The findings of this work provide novel insights into the morphological features and sedimentary processes of submarine canyons in modern isolated carbonate platform settings.

Annually 5–6 typhoons strike the Yangtze Estuary (YE) as extreme events. However, their high energy and importance for sediment transportation and geomorphic changes are still not fully understood. In this study, high-resolution observations of wind, wave, flow velocity, and suspended sediment concentration (SSC) at two in-situ stations were carried out during the 2022 Hinnamnor typhoon. Additionally, we simulated the change in SSC, estuarine bed erosion/deposition, and flow and sediment transport with and without a typhoon in the YE using MIKE3 numerical model. The findings revealed that the Hinnamnor typhoon-induced waves increased the SSC of the turbidity maximum zone (TMZ) by a factor of 5.6 times (maximum is 2.8 kg/m³). The TMZ area also extended by 2.68 times (maximum is 7880km², 70.4% of YE) in the YE. Moreover, the typhoon caused a dramatic change in sediment transport and bed erosion/deposition in the YE. First, in the delta front area where the mean water depth is >5 m, the typhoon significantly increased the southward flux of residual flow and sediment, causing sediment transport into Hangzhou Bay to abruptly increase 26.3 times (increase of 52 million tons, accounting for 1/3 of the present annual flux of the Yangtze River (150 million tons)) during a single spring-neap period. The net erosional area and volume extended to 6770km² (60.4% of YE) and 91.18 × 10⁶ m³. Second, in the delta shoals (where the mean water depth is <5 m, including east Chongming Shoal, Hengsha Shoal, Jiuduansha Shoal, and east-south Nanhui Shoal), residual flow and sediment flux decreased northward from the typhoon and resulted in the erosion of the shoal. Third, in channels with trumpet-shaped mouths (North Branch (NB), North Channel (NC) and South Passage (SP), except for North Passage (NP)), the upward flux of residual flow and sediment increased due to the typhoon, resulting in bed deposition in these channels (NB, NC and SP). This study highlights the important influence of typhoons on flow and sediment transport and bed erosion in estuarine areas.

Barrier islands are rarely preserved on continental shelves following sea-level rise. Proxies like overwash deposits, tidal inlets, and wave ravinements identify the location of paleo-barrier islands through time. Barrier island remnants are potential sand resources for beach nourishment to combat shoreline erosion from increasing rates of sea-level rise. Additionally, understanding the conditions that lead to barrier island drowning can be used to advise coastal policy makers. This study aims to identify barrier island signatures and deposits to understand the coastal processes that maximize preservation of paleo-barrier island remnants. We employed high resolution chirp sub-bottom data coupled with legacy sediment cores collected over Heald and Sabine Banks, on the east Texas shelf, which have been identified as possible preserved barrier island associated facies. Heald Bank exhibits a predominantly homogenous, low-amplitude facies with few low-amplitude internal horizons overlying the transgressive ravinement, whereas Sabine Bank consists of high-amplitude, landward-dipping reflectors beneath this surface, likely indicative of preserved subaqueous overwash deposits. This stratigraphy suggests Sabine Bank includes barrier island associated facies, whereas Heald Bank is mostly a marine sand bank. The overwash unit of Sabine Bank displays landward-thinning and landward-dipping deposits with reflections increasing in amplitude and displaying lower slopes to the NW. We hypothesize that higher slopes to the SE indicate proximity to the former barrier island. The Sabine River paleo-valley is mostly filled with estuarine sediment, leaving only ∼4 m of antecedent accommodation in a limited area of the NE portion of the paleo-valley. The low shelf gradient, which increases accommodation, and initially high sediment supply that decreased during the drowning of Sabine Bank are the major factors controlling partial preservation of the subaqueous portion of the paleo-barrier island.

The East China Sea (ECS) is located between the Eurasian continent and the Pacific Ocean with a wide continental shelf, which acts as a potential source of reactive iron in the Western Pacific. However, the source and fate of reactive iron in continental shelf sediments of the ECS remain poorly constrained. Here, we examined the influence of the depositional environment on the fate of reactive iron on the continental shelf of the ECS since the last deglaciation. The contents of redox-sensitive elements (U and Mo) indicate that the sediments in the ECS inner shelf have primarily deposited in oxic and suboxic environments since 18.5 ka. The ratio of reactive iron to total iron (Fe_HR/Fe_T) ranges from 0.24 to 0.41, and the ratio of total iron to aluminum (Fe_T/Al) is approximately 0.55 ± 0.11. These ratios suggest that the majority of reactive iron is derived from fine-grained terrestrial sediments discharged by the Changjiang River. The contents of Fe_py and Fe_carb exhibit opposite trends with depth in the core, indicating competition between carbonate (bicarbonate) ions and sulfide ions for ferrous ions. This competition is primarily controlled by the depositional environment and redox state since 18.5 ka. The Fe_carb is the dominant iron speciation throughout the core sediments, but its abundance declined since 13.2 ka when the ECS inner shelf was influenced by seawater transgression due to deglacial sea-level rise. The Fe_py content reached its maximum when the ECS inner shelf was fully flooded. Our study highlights the depositional control on the source-sink processes of reactive iron, providing new insights into the fate of reactive iron on continental shelves in response to environmental evolution.