Geo-Marine Letters最新文献

Interaction of Anthropic interventions (rigid structures) with coasts is an essential aspect for understanding their geomorphic evolution and incorporating these data into adequate coastal management. This study analyses the short and long-term behavior (1985 to 2019) of shoreline at Santa Marta Bay (Caribbean - Colombia) and their relationship with coastal protection structures. The shoreline variations were analyzed through aerial photographs and satellite images using DSAS tools. The short-term assessment showed initial intense 1985–1991 erosion, with an average retreat speed rate of − 1.6 m·y^− 1. It changed gradually since 2003, due to the construction of rigid structures and beach nourishment, decreasing shoreline retreat and even a progradation rate of 0.2 m·y^− 1 was observed. The 2009–2019 period, despite recording a positive average value, exhibited a high percentage of erosion profiles. Therefore, in a decadal or long-term analysis (1985–2019), despite engineering works present positive results in the short term, new interventions are required. The coastal erosion is not uniform along the shoreline, because the area is a closed bay with small sediment inputs, the variations in erosion and accretion rates change with human intervention and the installation of new structures. Therefore, quantifying the scale and rate of shoreline changes and correlating them with anthropogenic structures is an essential step in assessing shoreline vulnerability.

Abstract

The morphology of coral reefs provides an effective benchmark of past sea levels because of their limited vertical range of formation and good geologic preservation. In this study, we analyze the seafloor morphology around two atolls in the Philippines: Tubbataha Reef, in Palawan, and Apo Reef, in Occidental Mindoro. High-resolution multibeam bathymetry to a depth of 200 m reveals seafloor features including reef ridges and staircase-like terraces and scarps. Depth profiles across the reefs show terraces formed within six and seven depth ranges in Tubbataha Reef and in Apo Reef, respectively. These were further observed through a remotely operated vehicle. The terraces and scarps are interpreted as backstepping reefs that were drowned during an overall rise in sea level from the Last Glacial Maximum (LGM). Terraces are used as indicators of paleo sea level and the separation between terraces as the magnitude of sea-level rises coeval with meltwater pulse events during the last deglaciation. The pattern for both Apo and Tubbataha reefs indicates subsidence, consistent with the absence of Holocene emergent features and their atoll morphologies. Subsidence of up to 17 m since the LGM in Apo Reef is mainly attributed to the downbowing of the crust toward Manila Trench. In Tubbataha Reef, subsidence of up to 14 m is attributed to the continuous cooling of the volcanic crust underlying the atoll. These can be used to fill gaps in the tectonic history of the study sites from the last deglaciation.

The Isla Salamanca coastal barrier on the Colombian Caribbean coast faces significant erosion, driven by climate change-induced, sea level rise and human activities such as highway construction. The Barranquilla-Ciénaga highway, particularly at kilometers 19 and 29, is at risk, with severe consequences for the region’s socio-economic and environmental well-being. Human interventions like the highway construction and seawall installations have disrupted the natural coastal dynamics, leading to increased erosion rates. The study, conducted between 2004 and 2021, reveals that the Isla Salamanca coastal barrier is experiencing substantial transgression, with erosion rates peaking at -16.1 m·yr^− 1. The highway protection measures, with seawall construction, have proven inadequate, exacerbating erosion downstream. The mangrove loss due to hydrological changes and increased salinity is further threatening the fragile ecosystem. The research emphasizes the importance of considering biodiversity loss in the context of rapid erosion rates. The region, declared a Ramsar Site and Biosphere Reserve, hosts vital ecosystems like mangroves and dunes, whose destruction negatively impacts marine biodiversity. The study suggests the relocation of the highway, acknowledging the challenges of preserving wetlands and mangroves in the process. Balancing the need for infrastructure with ecological preservation is essential, and the study proposes comprehensive solutions, including shoreline management, ecosystem-based protection, and community involvement. The goal is to mitigate erosion’s adverse effects on biodiversity, habitat integrity, and the overall health of this ecologically sensitive region.

This study investigates seismic anisotropy in the northeastern region of the Indian plate, including the Eastern Himalayan front, Eastern Himalaya Syntaxis (EHS), Indo-Burmese subduction zone, Shillong Plateau, Assam foredeep, and Bengal basin. Variations in azimuthal anisotropy are interpreted in terms of pre-existing lithospheric structures, mantle flow movement, and dynamic lithospheric stresses. Analysis of shear-wave splitting (SWS) in the waveforms recorded at 64 stations yielded 305 splittings (SKS, SKKS, and PKS phases) and 386 Null measurements. Results reveal an average delay time ((delta )t) of 0.95 ± 0.32 s, indicating significant anisotropy. Modeling the back-azimuthal dependence of the splitting parameters indicates two-layer anisotropy along the Eastern Himalaya, Shillong Plateau, and south of the Dauki fault contiguous with the Indo-Burmese arc. Application of the spatial coherency technique localizes the depth of the anisotropic layers in different tectonic subdivisions. Stresses and lithospheric strain associated with Absolute Plate Motion (APM) of India explain the deformation patterns gleaned from splitting measurements. A vertically coherent crust-mantle deformation is proposed at the Himalayan collision front, where east-west-oriented extensional shear stresses result in north-south compressive strains. APM-related stresses forge anisotropy in the Assam foredeep region that shows a coupled crust-mantle deformation. East-west-oriented fast polarization directions (FPDs) beneath the Shillong Plateau indicate localized mantle flow along the Dauki fault. The fast axes of anisotropy in the Indo-Burmese subduction zone align parallel to the arc. These findings enhance the knowledge of mantle dynamics in the subduction and continent-continent collision zones.

Local earthquake tomography was performed to image the subsurface structure beneath the seismically active Cachar fold and thrust belt in lower Assam, northeast India. A total of 3341 P-phases and 1833 S-phases corresponding to 180 local microearthquakes recorded over 11 months by a temporary, dense network of 76 stations were used to simultaneously locate the hypocenters and estimate the P- and S-wave velocity structure in three dimensions. The entire crust down to a depth of ≈ 40 km is seismically active with about 70% of the microearthquakes ranging in magnitude from 0.5 to 3.5 occurring in the upper crust. The velocity tomograms reveal alternating patterns of high and low velocity anomalies down to a depth of ≈8 km that correlate with the NE-SW trending en-echelon pattern of the anticlines and synclines of the Cachar fold belt (CFB). These structures are distinctly demarcated up to depths of ≈ 4 km revealing the deformation of the low velocity sediments (Vp ≈ 1.8 to 4.3 km/s) beyond which they are correlatable with the relief in the high velocity medium (Vp ≥ 5 km/s). The sediment (Vp < 5 km/s) thickness varies from ≈ 4 km above the anticlinal structural highs to ≈ 8 km in the broad synclines. The shallow (≤ 10 km) microseismicity is aligned in a NE-SW direction correlating predominantly with the major anticlines. The study reveals the geometry and depth extent of the structural features of CFB and their association with seismicity indicating faulting resulting from deformation due to the compressional tectonics in northeast India.

The Bengal Fan, the largest submarine fan in the world, spanning the whole Bay of Bengal resulted from the India-Asia collision event and subsequent Himalayan orogeny. It is a significant depositional feature formed by the major river systems: the Ganges and Brahmaputra. Previous studies revealed the occurrence of an extensive channel-levee system in the Bengal Fan through various marine geoscientific investigations. In the present study, we have successfully mapped a submarine channel-levee segment in the lower Bengal Fan using hull-mounted multibeam bathymetry data collected for the first time. In addition, we conducted a pioneering study of a submarine channel segment using an Autonomous Underwater Vehicle (AUV) within the identified channel. The identified channel, characterized by moderate sinuosity, maintains a smooth morphology throughout its course, with the channel incision increasing towards the lower course. The microbathymetric observations revealed fine-scale features such as mass wasting, sediment waves and scour/depressions indicating the direct interaction between turbidity currents and the channel floor. The newly identified channel is proposed to result from the channel bifurcation of the previously identified channel W6 described by Curray et al. (2003). This study showcases the importance of near-bottom observations in complex submarine channel-levee systems for a better understanding of the formation processes involved in such systems.