Geo-Marine Letters最新文献

This study focusses on the processes that control the morphodynamics of the seabed in the shoaling wave domain, widely known as the shoreface. Shoreface morphodynamics is not only controlled by the local wave climate but also by associated wave- and wind-driven currents, the grain-size, and antecedent geomorphology. This paper provides basic information on sedimentological action of monochromatic waves at the seabed in the shoaling zone to the non-specialist without him-/herself having to solve the complex equations before arriving at workable solutions. It is emphasized that the traditional oceanographically defined wave base (0.5L₀) is not relevant in a geological/sedimentological context and should be replaced by a grain-size dependent effective wave base (EWB). Furthermore, open ocean swell-dominated shorefaces have to be distinguished from those of marginal seas dominated by local wind-generated waves. For convenience, a collection of tables and diagrams are provided in the electronic supplementary material (ESM) attached to this paper. They highlight the depths of effective wave bases for a range of grain sizes, wave heights and wave periods between 2 and 16 s.

The oblique subduction of the Indian Plate beneath the Sunda Plate has resulted in the formation of diverse morphotectonic structures and major changes to the seafloor topography in the Andaman Sea, northeastern Indian Ocean. Inner volcanic arc, sliver fault system, narrow oceanic basins and backarc basins are the principal morphotectonic elements that influenced the tectonic setting and geodynamics of the Andaman backarc region. The volcanoes and fault systems in Myanmar and Sumatra are well studied, but we know relatively less about the morphotectonics of the submarine volcanoes and sliver fault networks in the Andaman Sea, that connect the volcanoes and fault systems in Myanmar and Sumatra. In the present study, we compiled the all-available high resolution bathymetry data which covers an area of approximately 140,000 km² in the Andaman backarc region and provided a detailed morphotectonic analysis of each of the tectonic elements. The data show 33 submarine volcanoes of varying dimension in the Andaman Sea, that stretches from the dormant Narcondam volcano to the north of Sumatra Island. In addition, the major fault systems such as Great Sumatra Fault, West Andaman Fault and Andaman Nicobar Fault that starts from northern Sumatra and ends with Andaman Backarc Spreading Centre delineated from the high-resolution bathymetry data are analysed. Northward extension of the Great Sumatra Fault furcates into several branches, which produce narrow oceanic basins. The existence of submarine volcanoes situated in the middle of these basins indicate that the Andaman-Nicobar-Sumatra volcanic arc traverses through this narrow basinal area. The occurrence of well-developed cratered volcanoes, frequent earthquake swarms and gas emanations through the flanks of the cratered seamount suggest that the off Nicobar region between 6ºN to 8ºN is the most active part of the volcanic arc during the recent past.

This study analysed long-term shoreline change, the influence of erosion, and adaptation strategies in coastal Ghana. The change between 1972 and 2021 was analysed using Landsat satellite images and the Digital Shoreline Analysis System (DSAS v.5.0), and adaptation strategies were revealed through field observations. The End Point Rate (EPR), Linear Regression Rate (LRR), and Weighted Linear Regression (WLR) were used to estimate the rate of change, whereas the Net Shoreline Movement (NSM) and Shoreline Change Envelope (SCE) were used to calculate the distances of change. The coefficient of shoreline armouring (K) index was used to evaluate the grade of artificial or human interventions on the coast. The erosion rates for the EPR (− 107.6 m/year, or 95.0%) and LRR (− 75.7 m/year, or 99.3%) were higher than the accretion rates of (28.5 m/year, or 5.0%) and (33.6 m/year, or 0.7%), respectively. The NSM recorded maximum erosion (− 14,080 to − 10,840 m) and accretion (1107 to 2135 m) with an average distance of − 4943.1 m. The SCE estimated a maximum (14,080.5 m) and minimum (813.8 m) distances with an average distance change of 5557.9 m. The central coast experienced erosion at average rates of − 119.0 m/yr, − 89.6 m/yr, and − 94.0 m/yr, according to EPR, LRR, and WLR statistics. The eastern coast observed lower erosion rates than the central coast, with rates of − 75.3 m/yr, − 53.2 m/yr, and − 41.9 m/yr for the EPR, LRR, and WLR statistics, respectively. Since 1972, there has been a significant increase in artificial coastal structures on Ghana's central and eastern coasts. The central coast has reached a maximal level of shoreline armouring index, while the eastern coast has reached a minimal level. Although hard protective measures have been implemented on most parts of the coast for adaptation, improved policy initiatives on soft and nature-based protection measures are encouraged based on their favourable ecological impact and economic effectiveness.

Near coasts and shelves are strongly affected by the hydrodynamic forces and involves important information about the long term and permanent changes. Understanding the past dynamic transformations of shelves and near coasts can shed the future projections about effects of climate changes. High-resolution seismic and multibeam bathymetry data shed light on sedimentation since the last sea level lowstand in the Bandırma Bay, southern shelf area of the Marmara Sea. Five different seismic units were identified from the bottom upward based on seismic stratigraphic analysis, which are bounded by the reflection surfaces. Differential hydrological and depositional processes in the bay are shown by the definition and mapping of these unique seismic units and surfaces in the shallow sedimentary record: The water level in the basin crossed the threshold between the two basins, and advancing flows began from the north to the western basin. The basin continued to fill with water, and progressive depositions were interrupted with the onlaps of U3–4. The water level rose above the threshold in both basins, resulting in the two lakes becoming a single lake and the creation of marine conditions. The initial deposition in the eastern basin was fluvial, deposited in the form of regressive systems tracts. The overall morphology and stratigraphic settings observed in Bandırma Bay have nearly the same characteristics as those observed in the Sea of Marmara (like northern and western Marmara shelf, Gemlik Bay, Büyükçekmece Bay etc.) oceanography, implying that similar hydrodynamic conditions and erosional and depositional processes are controlled mainly by sea level changes controlled by climate changes related to morphological properties.

The current study is an effort to understand the relationship between the average shell size and Limacina Dissolution Index (LDX) of pteropod species, Heliconoides inflatus as a metric for shell calcification using several cores, ranging in age from recent to 1.2 Myr. The current study is based on the variability of H. inflatus average shell size, LDX, and fragmentation ratio (FR) and their correlation among different spatial and temporal sediment core records from the Northern Indian Ocean. Results suggest that in the cores collected above the Aragonite lysocline (Aly) and the Aragonite Compensation Depth (ACD) (SPC 05, 06, 09, 11, 12, 13, 14, and NGHP-17), the average shell size values exhibited larger shells during the stadials/glacial periods (Little Ice Age (LIA), Marine Isotope Stages (MIS) 3, 6, 10 − 9 transition etc.) which corresponds to lower LDX values. However, the cores beyond the ACD (SK168, AAS11, and RVS2) show larger shell size values during the warm interstadials (e.g., Bølling–Allerød) with higher FR. The variability in shell size and LDX shows an indication of the carbonate ion saturation in the water column over glacial/interglacial time scales and the impact of changing atmospheric CO₂ in the atmosphere. However, the factors adding to the carbonate ion saturation within the water column could be varied physiographically. The calcification proxy complements the dissolution proxies and reveals that the most intense aragonite dissolution occurred during the Holocene and interstadials/interglacials.

The Coastal Vulnerability Index (CVI) has been widely applied around the world. This study provides a review of the suitability of the variables and mathematical expression of the CVI and proposes a new Integrated Coastal Vulnerability Index (ICVI), comparing both indices for 4 study areas in the southern Caribbean. The ICVI assesses vulnerability to sea level rise by integrating the Environmental Vulnerability Index (EVI) and the Socioeconomic Vulnerability Index (SVI). Regarding the variables that constitute the indices, it is noted that the CVI includes vulnerability and hazard variables and therefore it should be considered a risk index rather than a vulnerability index. The EVI includes geomorphological vulnerability variables, like the CVI, but also ecological ones. Regarding the mathematical expression, the use of the arithmetic mean versus the formula proposed for the CVI is discussed based on the comparison of the results obtained for EVI and SVI in the 4 study areas. In addition, the use of absolute (between 0 and 1) or relative (based on percentiles) limits in these indices, and the use of weights or not, are also discussed. The conclusion is that the use of relative thresholds necessarily forces the identification of very low to very high vulnerability zones for any study, and the use of weights on the variables increases the subjectivity of the assessment, all of which impedes the comparability of the index at a global level. Therefore, the ICVI, with the formula based on the arithmetic mean, with absolute limits between 0 and 1 and without variable weightings, is preferable to the CVI for use at the global level.

On the southeastern South American shelf, the discharge of the Río de la Plata and those from the Uruguayan and southern Brazilian lagoons forms a low-salinity water mass named Plata Plume Water (PPW). The northward penetration of this water mass into the SW Atlantic presents a clear seasonal regime, determined more by the intensity of the southerly winds than by river discharge. In this sense, understanding the plume’s variation is a valuable tool for recognizing Holocene wind dynamics in the Southwest Atlantic. This study aims to identify the PPW’s onset and intensity variations on the southeastern South American shelf by analyzing the grain-size characteristics of the Holocene deposits of the southern Brazilian shelf. End-member modeling was used to recognize the sediment population representing the PPW input on the southeastern South American shelf. The results led us to recognize a silty population whose characteristic is coherent with the PPW input. Moreover, this grain size population presents a behavior that agrees with other independent proxies from other sediment cores in the same area, especially after ca. 2,800 cal BP, concurrent with establishing the late Holocene climatic and oceanographic conditions. Finally, we could recognize a ca. 300 years periodicity, similar to other works in the southern hemisphere, indicating that external (solar) forcing played an important role in the observed century-scale climate variability during the Holocene of the Southwest Atlantic.

Cusp formation and related processes are well-studied all over the world, but research along the Indian coast is scanty and fragmentary. This paper provides a detailed description of the geomorphology, characteristics, and development of a series of cusps after the cyclone “Vardah” crossed along the east coast of India on 12^th December 2016. Observations of instantaneous shoreline, cusp dimension, and related processes such as wind, wave, littoral environmental observation (LEO), sediment texture, and total suspended sediment load in the surf zone were obtained for three discrete observations spanning 20 days, revealing a clear insight into the post cyclonic beach processes and the cusp development. The cusp development shows an accretionary environment (12 m in 20 days) and sediment characteristics show to coincide with cusp evolution as coarser sediment at horns than the bays. After the storms, the mean grain size decreases and the sorting value increases along the coast. The spacing of observed cusps ranged from 15 to 40 m with a dominant spacing of 20–30 m (80%) while cusp depths are mostly 4–6 m (55%) and range from 2 to 8 m. The cusps are formed under the influence of low wave energy, anticipated from numerical modeling study; wave height (H_s) is 0.6 m, wave period (T_s) is 6 s, and wave direction is 90 deg. The present study reveals that cusp formation along this region is a better fit with self-organization theory.

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.