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After the 2011 Tohoku-oki earthquake that occurred off the Pacific coast, significant problems arose concerning the Pacific coastal area (Sanriku Coast) such as the lack of the historical and paleo-tsunami records and the paradox between long-term and short-term vertical crustal movements. The data necessary to solve the issues are also essential for understanding the mechanisms of tsunami generation and earthquakes and assessing the risk of large, low-frequency disasters. Thus, we conducted excavation and coring surveys and reconstructed the paleoenvironment at Onuma on the southern Sanriku Coast, based on sedimentary facies, age data (radiocarbon dating and tephra), fossils (diatoms and mollusks), and topography. First, we discussed the preservation potential of tsunami deposits considering the geomorphologic and geologic evolution of the study area. Subsequently, we identified tsunami deposits and estimated their occurrence ages and inundation height. We discovered six tsunami beds deposited during ca. 5–1 ka when the preservation potential of tsunami deposits was high in the study site. We divided them into two types of deposits—sand and gravel. These deposits reflect different sources and flow processes of paleo-tsunamis. Therefore, we estimated the heights of the paleo-tsunamis that is 5–10 m for sand tsunami deposits (S1–S3 deposits) and > 10 m for gravel tsunami deposits (G1–G3 deposits). The tsunami deposits were dated at 690–1040 BP (S1), 2240–3310 BP (G1), 3320–3630 BP (G2), 3620–4060 BP (G3), 4030–4400 BP (S2), and 4440–5570 BP (S3). This approach and the setting of the study site enabled us to evaluate not only the ages of paleo-tsunami deposits but also the paleo-tsunami heights. Moreover, we estimated the paleo-sea level between 8 and 7 ka based on the sedimentary facies and paleoenvironment deduced from the fossil shells, and calculated the vertical crustal movement during the Holocene at − 0.4 to − 1.0 mm/yr by comparing with the theoretical relative sea-level curve. This information is significant for reconstructing the tectonic evolution along the subduction zone and for paleogeography to estimate the paleo-tsunami magnitudes. [ABSTRACT FROM AUTHOR]

Abstract: Marine geophysical techniques and geological measurement of sea-level change allow detailed mapping of seafloor archeological sites and development of models. In the northwestern Gulf of Maine, the lowstand of sea level is only at −60m depth, but a prolonged stasis during the late Quaternary sea-level rise, herein termed the slowstand, occurred between 11,500 and 7500 calibrated years before present (calyr BP) at depths of 15–25m. This slowdown caused substantial erosion of glacial deposits and formed terrestrial beaches, wetlands and lakes. These landforms attracted people, whose tools were later discovered during present day scallop-fishing activities. Our study areas at the Green Islands and Bass Harbor are within the lowstand and slowstand depth ranges, but at highly exposed and sheltered sites, respectively. The Green Islands site has lost most of its former terrestrial sediment and there is no longer any context for recovered artifacts. At the protected Bass Harbor site, the remains of eroded moraines are associated with drowned spits and a former lacustrine/estuarine basin. Cores at the Bass Harbor location encountered freshwater peat deposits and numerous shallow subtidal shell and plant fossils all deposited during the slowstand. The two most important controls on the preservation of terrestrial landforms and associated human artifacts in the northwestern Gulf of Maine are the rate of sea-level rise and degree of shelter of the site from waves. [Copyright &y& Elsevier]

Abstract: The sedimentation on the Mauritanian shelf off the western Saharan desert is strongly influenced by dust supply and is investigated in this study by combining surface mapping, shallow-acoustic profiling and the analysis of sediment cores. Coarse-grained carbonates dominate wide areas of the open shelf at places where the shelf is exposed to the Atlantic Ocean. The sheltered Golfe d''Arguin, in contrast, is characterized by muddy shoals in its inner area, and locally restricted and wedge-shaped muddy depocenters which extend contour-parallel in mid-shelf settings. In both cases, cross-shelf currents control sediment distribution and accumulation, not location or amount of primary eolian input. A major discontinuity on the outer shelf is located in 85–110 m water plus sediment depth and is refilled by a sedimentary facies succession of shoreface, tidal and open-shelf deposits. This succession has formed during temporary lower sea levels along the last overall regression (MIS-4 and MIS-3), the Last Glacial Maximum, and during the past deglacial sea-level rise. Significant sediment deposition and preservation is limited to very restricted areas. Concerning the overall shelf architecture, sea level was the main factor enabling or suppressing deposition in terms of 1) creating local erosional swales which have favored subsequent trapping and preservation of sediments; 2) determining the type of sedimentary facies refilling these depressions under a variety of nearshore conditions; 3) interacting with the seafloor morphology and, by this, controlling timing of initiation and growth geometry of mud wedges. These mud wedges have formed since ca. 9 cal. ka under a continuous sedimentation regime. These bodies must have formed in hydraulic equilibrium but they are detached from a defined sediment point source which is in contrast to common mudbelt formation on shelves. In general, sediment storage on shelves under hyperarid conditions seems to be very limited due to comparably low sediment input, strong ravinement activity during changing sea levels, significant bypassing and winnowing processes, and a low preservation potential of the non-cohesive materials. These conditions lead to the development of almost sediment-starved systems with low subsidence rates over longer geological timescales. [Copyright &y& Elsevier]

Shorelines respond to rising sea-level through processes such as erosion, landward migration and in-situ drowning (i.e. overstepping). Submerged and preserved shorelines on the continental shelf play a key role in examining coastal response to rising sea levels as they provide important information on how modern shorelines may evolve in time and space within the context of changing climate and post-glacial sea-level rise. This study identifies and assesses the response of a continental shelf to stepped rises in sea level with particular focus on the stepwise evolution of incised valleys and shorelines from the shelf-edge to the inner shelf. Multibeam bathymetry from the mid-outer allows for the analysis of seafloor morphology, including the Protea Banks Reef (a palaeo-shoreline complex), and the adjacent incised, sediment starved continental shelf. Six seismic units and intervening surfaces are identified using interpretations from sub-bottom profiles, these include the incised acoustic basement, variable incised valley fill successions, aeolianite ridges and post-transgressive shoreface and associated sediments that withstood wave ravinement processes. The incised valleys of the outer-shelf are manifested as distinctive seafloor depressions, filled at their bases by fluvial deposits overlain, in the unfilled valley, by deposits derived from cascading subaqueous dunes which comprise the upper-most post-transgressive sediments. A core intersecting the dune material yields a maximum age of deposition of 1191–1263 cal yr BP (68% range), synchronous with a period of higher than present sea-levels in the region suggesting reworking and redistribution of coastal sediment as shelf sediment post-transgression. During the stepped rises in sea level, the shoreface has disconnected from the contemporary shoreline and is preserved by means of topographic barriers formed by antecedent topography as relict shoreface deposits. We provide a new perspective of shoreline response to stepped rises in sea level by integrating the seismic architecture of incised valley fills and shorelines across the continental shelf thus allowing for the assessment of variation in rates of relative sea-level rise since the last glacial maximum. • Unfilled incised-valleys result of reduced sediment supply or rapid increases in rate of sea-level rise. • Aeolianite barriers provide antecedent controls on the preservation of disconnected shoreface deposits. • Preservation of palaeo-shorelines by overstepping is related to the ensuing meltwater pulses 1A and 1B. • During these stepped rises in sea level, several shoreface disconnections from the adjacent shoreline occurred. • Reduced sediment supply during shoreline construction may lead to overstepping of low-gradient coastal areas. [ABSTRACT FROM AUTHOR]