Marine Geology最新文献

The archipelagic aprons of the large deep-sea seamounts of the northwestern Pacific Ocean (NWPO) show potential areas for significant reserves of ferromanganese nodules (FMNs). This study used datasets such as depth, backscatter intensity (BI), and optical coverage in conjunction with mineralogical, element geochemical, Sr-Nd-Pb isotopic, and chronological analyses of FMNs of the Suda Guyot (SG), which was located on the central area of the Marcus-Wake seamounts, in the NWPO. The results indicated a Y-shaped distribution of the deposit on the northern apron of the SG. Landslides predated the mineralization processes of the FMN deposit, and the ubiquitous channels in the apron had largely minimal influence on the distribution of nodules. Current mineralization of the deposit has been ongoing for ∼10 Myrs. Continuous weakening of the Antarctic Bottom Water (AABW) resulted in a gradual decrease in bottom water oxygen contents around the SG. This in turn resulted in a decrease in cryptocrystalline Fe-vernadite (δ-MnO₂) and elemental contents associated with δ-MnO₂ of FMNs, such as Mo, Te, and Tl. Meanwhile, the contribution of Asian dust to the study area increased, leading to increased Fe, which in turn increased amorphous ferrihydrite (FeOOH), and FeOOH-associated elements such as Ti, Pb, and Th. Productivity gradually increased to its peak value around 4–5 Myrs ago, leading to similar trends in REY, Ba, and U. REY contents exhibited a certain correlation with water depth around the SG. The results of this study suggest that the Carbonate Compensation Depth (CCD) variation resulted in higher content of REY of the FMNs in the shallower apron.

The formation and development of a small Mediterranean deltaic system are investigated through a primary seismic stratigraphic interpretation of a high-resolution seismic profile network, combined with multiple bathymetric data (including multibeam bathymetric imagery) and collated with shallow sediment cores collected with a vibro-corer device.

The submarine delta of the Adra River is divided into a basal patchy seismic unit and five wedge-shaped younger seismic units that are related to the Holocene highstand stabilization. Limited age control indicates that the two uppermost seismic units are very recent, most likely related to a dearth of fluvial fluxes led by channel deviations and by sediment retention. The formation of the three older seismic units is correlated to three humid periods during the Middle Holocene, Late Holocene and Little Ice Age, under a general context of progressive aridification of southeastern Iberia.

The stacking patterns and spatial distribution of individual seismic units document a history of episodic progradation of successive prodeltaic lobes, with a long-term evolution mediated by climatically-induced changes in the river basin and more recent anthropogenic interventions. Overall, the subaqueous deltaic system registers the complete modification of a deltaic system that evolves from a fluvial-dominated delta to recent wave-dominated wedges. In between, the deltaic system exhibits a progressive asymmetric character, due to the instauration of Atlantic waters on the shelf and their subsequent eastward redistribution. The Adra deltaic system is proposed as an outstanding example of a small deltaic system that reacts almost immediately to the complex interaction between natural changes in the system and anthropogenic interventions in the drainage basin.

Estuarine dams and weirs are constructed in estuaries for reasons such as blocking the salt intrusion, securing freshwater, and stabilizing upstream water levels. While they can provide many social benefits, they can also alter estuarine physical and sedimentary processes. How this occurs and their relative importance to global estuaries and deltas are not well understood. To address this, we perform and extensive remote sensing and literature analysis. Remote sensing was conducted based on a global river database of 1531 rivers representing the largest rivers cumulatively draining 85 % of the landmass discharging into the global ocean. It was found that 9.7 % of global estuaries and deltas are currently affected by estuarine dams or weirs acting as the upstream limit of salt, tide, or storm surge intrusion. If we include supplementary examples, overall 220 estuaries with estuarine dams or weirs were identified and confirmed by literature review. These structures are found worldwide and are prominent in developed or developing countries in Asia, Europe, North America, and Oceania. The number of estuarine dams and weirs has increased rapidly since the 1800s with a peak in construction rate in the 1970s, particularly due to construction in Asia. Estuarine dams and weirs are found at the river mouths of both small and large watersheds. Most of these estuarine structures are located at x = 0–100 km inland from the mouth and their discharge intervals can be continuous, daily – weekly, seasonal, or interannual. Based on a quantified classification by geomorphology, estuarine dams and weirs are found most in river mouths which are wave-dominated followed by tide-dominated and then river-dominated. Estuarine dams and weirs can cause significant changes to the quantity and timing of freshwater discharge, tides, stratification, turbidity, sedimentation, oxygen conditions, phytoplankton blooms, and fish migration. We synthesize this current knowledge on estuarine dams and weirs and propose a conceptual model for physical and geomorphological change in mixed wave- and river-dominated and tide-dominated estuaries with estuarine dams.

Reversals in wave-generated longshore currents in surf zones occur at different temporal and spatial scales. Along coastlines with tidal channel openings with well-developed mouth bars or ebb-tidal deltas, the reversal mechanism is often attributed to the mouth bar – induced refraction of the shoaling waves. This reversal mechanism is characterized by convergence of longshore currents from the adjoining surf zones at the mouth of the tidal channel. Simultaneous half-hourly monitoring of wave-generated longshore currents over 50 successive (daylight) semi-diurnal tidal cycles in beach surf zones adjoining the Qua Iboe River estuary, S.E coast of Nigeria showed the above reversal pattern during flooding stage only. The converse pattern, where the surf zone longshore currents diverged away from the mouth of the estuary, was observed during ebbing stage. Both surf zones showed flow direction inversion with respect to each other, with velocity vector correlation coefficient r > − 0.8 in over 80 % of the data set. Instances of comparable flow direction (<10 %) were also recorded. Tidal processes are implicated in the documented results. Direction-averaged longshore current velocities, typically in the 15–60 cm/s range, attained highest values in both surf zones at about spring tide phase. Also, tidal cycle-residual longshore current maximum and minimum velocities occurred close to spring and neap tide, respectively. Only 30 % of the residual velocities were eastward directed in the up-drift surf zone as against 80 % in the down-drift counterpart. Given the prevailing southwesterly waves, the present results negate the assertion that reversal in longshore current direction in this offset shoreline setting is exclusively a consequence of wave refraction by mouth bar morphology. The reversing pattern of the longshore currents over a tidal cycle is well explained by incorporating interacting effects of shoaling waves with tide-induced oscillations in water level as well as the estuarine flow.

The evolution of oceanic temperatures between the Turonian and the K/T boundary indicates a long-term cooling coincident with a decrease of atmospheric CO₂ levels, yet the cause of climate cooling at that time still remains debated. In this study, we evaluated the possible implication of enhanced silicate weathering as a sink for atmospheric CO₂ by applying paired NdHf isotope measurements to detrital clay records from the West African margin. The use of this novel proxy for chemical weathering intensity (Δɛ_Hf(t)clay) was complemented by additional mineralogical and major-trace element analyses in order to investigate the variability of mechanical erosion patterns and further explore potential linkages between tectonics, weathering and climate during the late Cretaceous.

Our Δɛ_Hf(t)clay data suggest more intense silicate weathering on the West African Craton during the Santonian to the middle Campanian period, coincident with enhanced physical erosional inputs as inferred from higher Quartz/Clays and Feldspar/Clays ratios. This observation suggests that the shift towards intensified chemical weathering at that time was driven by enhanced mechanical erosion, possibly related to a moderate tectonic event on the West African craton. Evidence for increasing kaolinite contents and higher Δɛ_Hf(t)clay values during the Maastrichtian point towards more hydrolysing conditions, inducing either destabilization of older Mesozoic lateritic material or favouring the development of kaolinite-rich soils.

Overall, this study was compared with several new data of chemical weathering evolution along the south Atlantic margins, adding new insights on tectonic-weathering-climate interactions during the late Cretaceous, suggesting a possible link between silicate weathering feedbacks and global cooling at that time.

The Quaternary sediments in the northwestern South China Sea (NW-SCS) provide valuable information about provenance, climate and sea level changes. However, the correlation between the geochemical records in marine sediments and these influencing factors remains less understood in the NW-SCS. Two wells penetrated Quaternary sediments on the shelf and shelf margin of the NW-SCS and provide an excellent dataset. In this study, the major, trace, and rare earth elements in the sediments were analyzed to reveal the Quaternary provenance changes that correlate with the climatic Middle Pleistocene transition (MPT). These results indicate that the core sediments were mainly derived from felsic source rocks and the degree of chemical weathering in the source areas is relatively low (CIA averaged at 58). The Quaternary provenance of the NW-SCS did not undergo significant changes, recording mixed sedimentation from the Red River, Hainan Island, and central Vietnamese sources. The felsic source rocks and negative Eu anomaly indicated the shelf area received sediments primarily from the Red River and Hainan Island. However, a positive Eu anomaly on the shelf margin suggests additional contributions from mafic rocks sourced from central Vietnam. During the MPT (1.3–1.4 to 0.8–0.9 Ma), the dominant periodicity of climate cycles changed from a 40 kyr obliquity cycle to a 100 kyr eccentricity cycle; the cooling climate led to a slight weaking in chemical weathering and a minor decrease in sediment supply from terrestrial sources. After the MPT, the mixed sourced sediments in the shelf and shelf margin areas of the NW-SCS likely suffered from multiple hydrodynamic forces under the influence of the Pleistocene high-frequency and high-amplitude sea level fluctuations. These climate induced changes led to significant fluctuations in element content that provide new insights into Quaternary sources and climatic events in the NW-SCS.

Seagrasses impact on sedimentary intertidal and subtidal ecosystems by affecting local hydrodynamics, geomorphology and sediment properties. Their influence on hydrodynamics is to reduce flow velocities in their canopies, and this leads to increased net sedimentation rates and reduction of the grain size. Most investigations of the seagrass-hydrodynamics-sediment feedback system has been carried out over silt and fine-sand beds under tide-dominated conditions, mostly in the intertidal zone. Here, we use sedimentological data from a relatively wave-exposed and subtidal seagrass (Zostera marina) meadow in the Isles of Scilly with a fine-to-medium sand bed and show that the sand within the seagrass meadow is indeed finer than in adjacent, unvegetated regions. However, in contrast to previous studies, this is not due to increased mud/silt content within the seagrass meadow, but an almost 0.1-mm shift in the median sediment size across the sand fraction from 0.25 mm (fine to medium sand) to 0.35 mm (medium sand). The studied seagrass meadow is characterised by a distinct spatial structure comprising of vegetated ‘ridges’ and bare sand ‘valleys’. Even the bare sand valleys within the seagrass meadow are characterised by significantly coarser sand than the adjacent vegetated ridges, providing further confirmation of the efficiency of sediment sorting by wave processes that takes place within seagrass meadows. It is concluded that any numerical modelling involving sediment transport processes associated with seagrass environments must account for variability in the textural characteristics.

The impacts of coastal changes and human land use on depositional processes, ecological conditions, geomorphic evolution and harbor works at the archaeological site of Lattara, one of the oldest cities of the northwestern Mediterranean built in a deltaic environment, were investigated from a multi-proxy approach based on sedimentological, biological and geochronological analyses. A distributary channel connected to the ancient harbor of Lattara was deepened and channelized around 200 cal BCE. The drastic increase in water depth caused by channelization was associated with increased flow competence and bedload transport, and could have improved navigation in the harbor area. By contrast, high accumulations of anthropogenic deposits in the channelized stream from the second century CE seem to have negatively affected sediment transport conditions by reducing bedload flux. The construction of a cobble pavement on the western bank of this channelized stream in the fourth century CE was contemporaneous with a sharp decrease in bedload transport showing an abrupt transition to a low energy environment such as in abandoned channels. A drainage ditch was then dug in the deposits of the channelized stream during the Medieval Warm Period, in a context of land use intensification and increased river flooding that led to the deposition of coarser sediments.