High-resolution Bathymetric Map Research Articles

Rapid advancements in large language models for quantitative remote sensing: The case of water depth inversion

This study presents a comparative analysis of two advanced AI models, ChatGPT and ERNIE, in the context of water depth inversion. Utilizing satellite spectral data and in-situ bathymetric measurements collected from Rushikonda Beach, India, we processed and analyzed the data to generate high-resolution bathymetric maps. Both models demonstrated significant accuracy, with ChatGPT slightly outperforming ERNIE in terms of mean absolute error. The study highlights the advantages of AI models, such as efficient data processing and the ability to integrate multi-modal inputs, while also discussing challenges related to data quality, interpretability, and computational demands. The findings suggest that while both models are highly effective for water depth inversion, ongoing improvements in data handling and model transparency are essential for their broader application in environmental monitoring. This research contributes to the understanding of AI capabilities in geospatial analysis and sets the stage for future enhancements in the field.

High-resolution bathymetry coupled with 3D models of hydrothermal vents from opportunistically-acquired imagery: Aurora vent field, Arctic Ocean

ABSTRACT Active venting at the Aurora hydrothermal field was first located in 2014. In July 2023, the AUV/ROV Nereid Under Ice (NUI) expanded the known size of the Aurora hydrothermal field, discovering 7 ‘black smokers’ together with associated lower-temperature flow. In this study, we present a new high-resolution bathymetric map acquired from NUI which has allowed us to identify morphological features previously undetectable from ship multibeam. All known active vents are aligned along a single 230 m-long ridge, parallel to the Gakkel Ridge strike direction and intersected by a scarp following the general trend of the Lena Trough. The new vents were measured at up to 17 m height from 3D models generated using structure-from-motion techniques applied to opportunistically acquired imaging data collected while in exploration mode. The extent of extinct sulfides present, together with the towering height of vents are consistent with a period of sustained high-temperature venting at Aurora.

Automated High-Resolution Bathymetry from Sentinel-1 SAR Images in Deeper Nearshore Coastal Waters in Eastern Florida

Synthetic aperture radar (SAR) imagers are active microwave sensors that could overcome many challenges of passive optical bathymetry inversion, yet their capacity to yield accurate high-resolution bathymetric mapping is not studied sufficiently. In this study, we evaluate the feasibility of applying fast Fourier transform (FFT) to SAR data in coastal nearshore bathymetry derivation in Florida’s coastal waters. The study aims to develop a robust SAR bathymetry inversion framework across extensive spatial scales to address the dearth of bathymetric data in deeper nearshore coastal regions. By leveraging the Sentinel-1 datasets as a rich source of training data, our method yields high-resolution and accurate depth extraction up to 80 m. A comprehensive workflow to determine both the wavelength and peak wave period is associated with the proposed automated model compilation. A novel contour geometry-based spectral analysis technique for wavelength retrieval is presented that enables an efficient and scalable SAR bathymetry model. Multi-date SAR images were used to assess the robustness of the proposed depth-retrieval model. An accuracy assessment against the GMRT data demonstrated the high efficacy of the proposed approach, achieving a coefficient of determination (R2) above 0.95, a root-mean-square error (RMSE) of 1.56–10.20 m, and relative errors of 3.56–11.08% in automatically extracting the underwater terrain at every 50 m interval. A sensitivity analysis was conducted to estimate the uncertainty associated with our method. Overall, this study highlights the potential of SAR technology to produce updated, cost-effective, and accurate bathymetry maps of high resolution and to fill bathymetric data gaps worldwide. The code and datasets are made publicly available.

Geomorphology of coralligenous reefs offshore southeastern Sicily (Ionian Sea)

ABSTRACT Coralligenous (C) includes calcareous build-ups of biogenic origin, formed since the Holocene transgression. Peculiar columnar-shaped C outcrops were documented offshore Marzamemi village (SE Sicily, Ionian Sea), although the actual extension and distribution were not assessed. Project ‘CRESCIBLUREEF’ produced a new, 17 km2 high-resolution bathymetric map, leading to good knowledge about their extent in this area. C bioconstructions are largely distributed along two depth ranges 36–42 m and 86–102 m water depth. By coupling the documented uplift rate in this region and the Holocene sea-level curve, we were able to interpret the distribution of C outcrops over terraces. However, additional investigation is required to understand: (1) the role of the inherited continental shelf landscape, in creating a favorable substrate for the settlement and growth of C habitats during the Holocene, and (2) the extent to which C bioconstructions can impact the evolution of present-day continental shelf landforms and landscapes.

The geology and geophysics of Lake Tarawera, New Zealand: Implications for sublacustrine geothermal activity

The Okataina Volcanic Centre is a large caldera system in the Taupo Volcanic Zone, New Zealand, which includes the Haroharo caldera, where the geothermally active Lake Tarawera is located at its southwestern boundary. Here, we show the results of high-resolution bathymetric mapping of the lake floor combined with gravity, magnetic and heat-flow surveys carried out over the lake, constrained by geochemical sampling. Our combined data elucidate the detailed geometry of the SW structural margin of the Haroharo caldera bisecting Lake Tarawera, and the relationship with the regional faults related with the Taupo Volcanic Zone rift.In this context, the spatial distribution of heat-flow in Lake Tarawera appears controlled by fluid-focusing permeable zones associated with the structural lineaments and by peripheral, gravity-driven circulation of meteoric water associated with cooling of highly permeable rhyolite domes around the lakeshore. The structural boundary of the Haroharo caldera provides first-order control of heat-flow in the lake, with WNW striking normal faults to a lesser degree. An estimated total conductive output of ∼5.3 MW escaping through the floor of Lake Tarawera is refracted by the thick layer of sediments, providing an impermeable layer to the convection of fluids, which in turn diminishes heat output compared to other lakes in this region.Our results contribute to modelling of hydrothermal circulation at the Okataina Volcanic Centre, and when integrated with data from the other lakes and the terrestrial geothermal output, it will contribute to determine the global geothermal output at this caldera system. The relationships to eruptive history and structural setting are the key to modelling similar geothermal system of other silicic calderas worldwide.

Unveiling deep-sea habitats of the Southern Ocean-facing submarine canyons of southwestern Australia

Here we present the outcomes of the first deep-sea remotely operated vehicle study of previously unexplored submarine canyon systems along the southwest Australian continental margin. This was conducted around: (1) the Bremer Marine Park; (2) the Mount Gabi seamount and nearby slope-shelf margin at the interface of the Southern and Indian oceans; with new information from (3) the Perth Canyon Marine Park located in the SE Indian Ocean. These canyons differ from many explored around the world in having no connectivity to continental river systems, thus little detrital input, with the Bremer systems and Mount Gabi facing the Southern Ocean which plays a key role in the global ocean circulation and climate systems. Such studies in the vast deep waters around the Australian continent are rare given the lack of local ROV capability available for research, thus little is known about these environments.Using the resources of the Schmidt Ocean Institute, we characterised the submarine topography from high-resolution bathymetric mapping, geology, physical and chemical oceanography, and provide an overview of these environments including the fauna observed and collected. We show that these Southern Ocean-influenced environments incorporate South Indian Central Water, Subantarctic Mode Water, Antarctic Intermediate Water, and Upper and Lower Circumpolar Deep Water, with Antarctic Bottom Water present in deep water just south of the Bremer canyon systems. The richness in megabenthos, especially along the steep, rocky substrates of the canyon heads and walls around the Bremer canyon systems, contrasts to the comparatively depauperate fauna of the more northerly Perth Canyon. Various corals serve as important substrates for a range of other species and often exhibit particular faunal associations. Especially notable are distinct ecological zones including a bryozoan and sponge-dominated (animal) forest on the shelf edge, spectacular coral gardens along canyon margins, and the occurrence of solitary scleractinians well below the aragonite saturation horizon. Subfossil coral deposits were discovered across all three study areas, reflecting periodic waxing and waning of deep-water Scleractinia throughout this southwest region. Extensive pre-modern assemblages at Mount Gabi contrast markedly with the sparse populations of living species and suggest that it might have once been a major coral hotspot, or whether they reflect long-term coral aggregations is yet to be determined. Nevertheless, stark differences in both living and past coral distribution patterns across our study sites point to at least localised fluctuations in Southern Ocean-derived nutrient and/or oxygen supplies to these deep-sea communities.

The COSMUS expedition: seafloor images and acoustic bathymetric data from the PS124 expedition to the southern Weddell Sea, Antarctica

Abstract. Between 3 February and 30 March 2021 the research icebreaker RV Polarstern conducted an extensive multidisciplinary research expedition across the southern Weddell Sea, Antarctica. During the Continental Shelf Multidisciplinary Flux Study (COSMUS) expedition (designated research expedition PS124), the Ocean Floor Observation and Bathymetry System (OFOBS) was deployed 20 times to collect high-resolution seafloor image and acoustic data with a 26-megapixel resolution camera, HD (high-definition) video camera, forward-facing acoustic camera and multibeam side-scan system from heights of between 1.5 and 4.5 m above the seafloor. To localise the collected data, the OFOBS was equipped with a Posidonia transponder for ultra-short baseline position triangulation, an inertial navigation system (INS) and a seafloor-tracking dynamic velocity logger (DVL). The 20 deployments were made across contrasting areas of the Weddell Sea, Filchner Trough, Filchner Sill and Filchner Shelf. Here, we present the full image dataset collected from across the various deployments as well as raw side-scan data and derived high-resolution bathymetric maps produced from these acoustic data after the cruise. All images are available from https://doi.org/10.1594/PANGAEA.936205 (Purser et al., 2021d) and https://doi.org/10.1594/PANGAEA.932827 (Purser et al., 2021a). Acoustic data for all deployments are also available from PANGAEA, with the raw side-scan data available at https://doi.org/10.1594/PANGAEA.939322 (Hehemann et al., 2021b) and forward-facing acoustic camera data at https://doi.org/10.1594/PANGAEA.939341 (Purser et al., 2021c). Seafloor topographic maps derived from the side-scan data are available at https://doi.org/10.1594/PANGAEA.939087 (Hehemann et al., 2021a).

Habitat variability and faunal zonation at the Ægir Ridge, a canyon-like structure in the deep Norwegian Sea.

The Ægir Ridge System (ARS) is an ancient extinct spreading axis in the Nordic seas extending from the upper slope east of Iceland (∼550 m depth), as part of its Exclusive Economic Zone (EEZ), to a depth of ∼3,800 m in the Norwegian basin. Geomorphologically a rift valley, the ARS has a canyon-like structure that may promote increased diversity and faunal density. The main objective of this study was to characterize benthic habitats and related macro- and megabenthic communities along the ARS, and the influence of water mass variables and depth on them. During the IceAGE3 expedition (Icelandic marine Animals: Genetics and Ecology) on RV Sonne in June 2020, benthic communities of the ARS were surveyed by means of a remotely-operated vehicle (ROV) and epibenthic sledge (EBS). For this purpose, two working areas were selected, including abyssal stations in the northeast and bathyal stations in the southwest of the ARS. Video and still images of the seabed were usedtoqualitatively describebenthic habitats based on the presence of habitat-forming taxa and the physical environment. Patterns of diversity and community composition of the soft-sediment macrofauna, retrieved from the EBS, were analyzed in a semiquantitative manner. These biological data were complemented by producing high-resolution bathymetric maps using the vessel’s multi-beam echosounder system. As suspected, we were able to identify differences in species composition and number of macro- and megafaunal communities associated with a depth gradient. A biological canyon effect became evident in dense aggregates of megafaunal filter feeders and elevated macrofaunal densities. Analysis of videos and still images from the ROV transects also led to the discovery of a number ofVulnerable Marine Ecosystems (VMEs) dominated by sponges and soft corals characteristic of the Arctic region. Directions for future research encompass a more detailed, quantitative study of the megafauna and more coherent sampling over the entire depth range in order to fully capture the diversity of the habitats and biota of the region. The presence of sensitive biogenic habitats, alongside seemingly high biodiversity and naturalness are supportive of ongoing considerations of designating part of the ARS as an “Ecologically and Biologically Significant Area” (EBSA).

Bathymetric Inversion and Mapping of Two Shallow Lakes Using Sentinel-2 Imagery and Bathymetry Data in the Central Tibetan Plateau

High-accuracy lake bathymetry and mapping are crucial for estimating lake water storage on the Tibetan Plateau (TP). In this study, we constructed traditional empirical (TE) models and machine learning (ML) models to compare the prediction accuracy and remote sensing bathymetric mapping performance by using Sentinel-2 satellite imagery and in situ measured water depth from Caiduochaka (CK) and QiXiang Co in the central TP. We analyzed the relationship between the band reflectance and depth and explored the universality of the model in different lakes. The results indicated that when using the TE model, the mean absolute percentage error (MAPE) varied between 14.5% and 26.5% for the test dataset at different study sites. When using the ML models, the MAPE varied between 7.6% and 18.9%, and it was the better choice overall. For the test dataset of the random forest (RF) model with the highest accuracy, in the CK with the maximum depth of approximately 16 m, the mean absolute error (MAE) and root mean square error (RMSE) were 0.54 m and 0.89 m, and the precision was the highest with an MAE of 1.13 m and RMSE of 1.67 m in QiXiang Co with a maximum depth of approximately 28 m, whereas the portability of the model was not satisfactory. Overall, the results indicated that the ML model can obtain bathymetric maps with high accuracy, good visual performance, and reliability, outperforming the TE model. It can be used effectively for deriving accurate and updated high-resolution bathymetric maps for shallow lakes.

Submarine Geomorphological Features and Their Origins Analyzed from Multibeam Bathymetry Data in the South China Sea

We processed the raw multi-beam bathymetry data acquired in the central and northeastern part of the South China Sea by eliminating noise and abnormal water depth values caused by environmental factors, and a high resolution bathymetric map with a 20-m grid interval was constructed. Various scales of seafloor geomorphological features were identified from the data, including an image of Shenhu canyon, which is located in the northern continental margin of the South China Sea; submarine reticular dunes in the north of the Dongsha atoll; submarine parallel dunes in the northeast of the Dongsha atoll; and several seamounts in the southwest sub-basin and in the east sub-basin. In the processing step, various anomalies in the multi-beam bathymetry data were corrected. The optimal swath filtering and surface filtering methods were chosen for different scales of seafloor topography in order to restore the true geomorphological features. For the large-scale features with abrupt elevation changes, such as seamounts (heights of ~111–778 m) and submarine canyons (incision height of ~90–230 m), we applied swath filtering to remove noise from the full water depth range of the data, and then surface filtering to remove small noises in the local areas. For the reticular dunes and parallel dunes (heights of ~2–32 m), we applied only surface filtering to refine the data. Based on the geometries of the geomorphological features with different scales, the marine hydrodynamic conditions, and the regional structure in the local areas, we propose that the Shenhu submarine canyon was formed by turbidity current erosion during the Sag subsidence and the sediment collapse. The submarine reticular dunes in the north of the Dongsha atoll were built by the multi-direction dominant currents caused by the previously recognised internal solitary waves around the Dongsha atoll. The submarine parallel dunes in the northeast of the Dongsha atoll were built by the repeated washing of sediments with the influence of the tidal currents and internal solitary waves. The conical, linear and irregular seamounts identified from the bathymetry data were formed during the spreading of the southwest sub-basin and the east sub-basin. The identified seamounts in the multi-beam bathymetry data are correlated to deep magmatic activities, the Zhongnan transform fault and the NE-trending faults.

Subaqueous geomorphology and delta dynamics of Lake Brienz (Switzerland): implications for the sediment budget in the alpine realm

Non-invasive techniques such as seismic investigations and high-resolution multibeam sonars immensely improved our understanding of the geomorphology and sediment regimes in both the lacustrine and the marine domain. However, only few studies provide quantifications of basin wide-sediment budgets in lakes. Here, we use the combination of high-resolution bathymetric mapping and seismic reflection data to quantify the sediment budget in an alpine lake. The new bathymetric data of Lake Brienz reveal three distinct geomorphological areas: slopes with intercalated terraces, a flat basin plain, and delta areas with subaquatic channel systems. Quasi-4D seismic reflection data allow sediment budgeting of the lake with a total sediment input of 5.54 × 106 t sediment over 15 years of which three-quarter were deposited in the basin plain. Lake Brienz yields extraordinarily high sedimentation rates of 3.0 cm/yr in the basin plain, much more than in other Swiss lakes. This can be explained by (i) its role as first sedimentary sink in a high-alpine catchment, and by (ii) its morphology with subaquatic channel-complexes allowing an efficient sediment transfer from proximal to distal areas of the lake.

A downscaled bathymetric mapping approach combining multitemporal Landsat-8 and high spatial resolution imagery: Demonstrations from clear to turbid waters

High spatial resolution bathymetric maps of coral reefs can show the details of terrain. However, most satellite-based imagery with a spatial resolution < 10 m has only three visible bands and one near-infrared (NIR) band. When in situ bathymetric data are unavailable, it is difficult to map bathymetry from high spatial resolution imagery with spectral matching or empirical models for clear or turbid waters. In this study, we developed a downscaled bathymetric mapping approach (DBMA) that uses the water depth estimated from multitemporal Landsat-8 data to calibrate the empirical model for high spatial resolution imagery (e.g., Sentinel-2A/B, GaoFen-1/2, ZiYuan-3, and WorldView-2) in the absence of in situ bathymetric data. Our results show that DBMA provides high accuracy for depth ranging from 0 to 12 m for clear waters (0 m to 5 m for turbid waters), with a root mean squared error (RMSE) smaller than 2 m. Relative to empirical models (calibrated with in situ data), DBMA underestimates water depth more for depth >12 m for clear waters (5 m for turbid waters) while slightly overestimates more for depth < 4 m for clear waters (3 m for turbid waters). Nevertheless, DBMA performs better than the empirical models for depth between 4 m and 12 m for clear waters (3 m and 5 m for turbid waters). Furthermore, the good performance of DBMA is also demonstrated by the finding that the scaling effect on the DBMA is limited. DBMA presents a reliable solution to obtain high spatial resolution bathymetric maps without leaving out small regions in the absence of in situ data.

High-resolution multibeam bathymetry of the northern Mid-Atlantic Ridge at 45–46° N: the Moytirra hydrothermal field

ABSTRACT This work presents a new high-resolution multibeam bathymetric map of a segment of active deep sea-floor spreading in the Atlantic Ocean, the northern Mid-Atlantic Ridge (MAR) at 45–46º N. New high-resolution bathymetry data were acquired using an Atlas multibeam echosounder onboard the research vessel Sarmiento de Gamboa during the EXPLOSEA-2 survey in 2019. The final map of the MAR (50 m cell grid size) at the original scale of 1:200,000 shows a segment of 140 × 35 km of the MAR, at water depths from 715 to 3700 m. This new high-resolution bathymetric map allows to better defining the submarine morphology of the Moytirra hydrothermal active field, the only high-temperature field identified between the Azores Archipelago (Portugal) and Iceland. ROV submarine observations reaching the deepest part of the system for the first time show giant anhydrite-sulfide chimneys up to 20 m high, active strong black smokers and polymetallic massive sulfides.

GAS SIPS AND SHALLOW ACCUMULATIONS OF GAS ON THE SHELF OF TURKEY IN THE BLACK SEA

It has been suggested that shelf and slope sediments of high deposition rate are methane sources, whereas the deep basin is methane sink. The methane production and migration in sediments may cause massive slope failures so methane is geologically important. Methane production is also economically important as methane seeps may indicate the presence at depth of hydrocarbon reservoirs, and methane hydrate may be an important source of energy. Recent studies in marine geology indicate potential geo-resources in the Turkish coast of Black Sea. The Black Sea sediments are rich in calcite and organic carbon, the latter showing a high degree of preservation due to anoxia in the waters below 100-150 m. Different marine geophysical surveys at different times were carried out in order to understand the sedimentary features of gas-saturated sediments in the Black Sea. Multibeam, side scan sonar, sub-bottom profiler and multi-channel seismic data were collected to make both high-resolution bathymetric and reflectivity maps of the seafloor. In some cruises, deep-tow combined side scan sonar and subbottom profiler was used to obtain acoustic images of both the seafloor surface and subbottom sediments. Several different structures were observed in the Black Sea basin as slumps, pockmarks, faults, gas chimneys, shallow gas accumulations and dome-like structures. Structures, which contain gas hydrates, are present on the seismic sections as strong acoustic reflections.

Autonomous Vehicles Mapping Plitvice Lakes National Park, Croatia

Plitvice Lakes National Park is the largest national park in Croatia and also the oldest from 1949. It was added to the UNESCO World Natural Heritage List in 1979, due to the unique physicochemical and biological conditions that have led to the creation of 16 named and several smaller unnamed lakes, which are cascading one into the next. Previous scientific research proved that the increased amount of dissolved organic matter (pollution) stops the travertine processes on Plitvice Lakes. Therefore, this complex, dynamic but also fragile geological, biological and hydrological system required a comprehensive limnological survey. Thirteen of the sixteen lakes mentioned above were initially surveyed from the air by an unmanned aircraft equipped with a survey grade GNSS and a full frame high-resolution full-screen camera. From these recordings, a georeferenced, high-resolution orthophoto was generated, on which the following surveys by a multibeam sonar depended. It is important to mention that this was the first time that these lakes had ever been surveyed both with the multibeam sonar technique and with such a high-resolution camera. Due to the fact that these thirteen lakes are difficult to reach and often too shallow for a boat-mounted sonar, a special autonomous surface vehicle was developed. The lakes were surveyed by the autonomous surface vehicle mounted with a multibeam sonar to create detailed bathymetric models of the lakes. The missions were planned for the surface vehicle based on the orthophoto from the preliminary studies. A detailed description of the methodology used to survey the different lakes is given here. In addition, the resulting high-resolution bathymetric maps are presented and analysed together with an overview of average, maximum depths and number of data points. Numerous interesting depressions, which are phenomena consistent with previous studies of Plitvice Lakes, are noted at the lake beds and their causes are discussed. This study shows the huge potential of remote sensing technologies integrated into autonomous vehicles in terms of much faster surveys, several orders of magnitude more data points (compared to manual surveys of a few decades ago), as well as data accuracy, precision and georeferencing.

Termination of a Trench-Linked Strike-Slip Fault Zone in the Sumatra–Java Forearc Basin and Accretionary Wedge Complex

This paper presents a review of several published seismic reflection and seismicity data and analyzes of high-resolution bathymetry data to revise the exact location and reveal detail characteristics of a strike-slip fault zone that formed the southernmost segment of the Sumatran Fault (SF). Previous works interpreted this fault segment as a horst structure to the south of a pull-apart basin. We observe a clear linear trace of dissected seafloor parallels to SF in the high-resolution bathymetric map. This structure extends from the south of a pull-apart basin in the northwest to the Sunda accretionary wedge farther southeast. This lineament exhibits a narrow valley and a linear ridge that in the subsurface are interpreted as negative and positive flower structures, respectively. The structure exhibits a vertical fault plane and appears to have deformed the accretionary wedge sediments and basement at depth. A cluster of shallow seismicity is observed along this NW-trending fault zone, indicating the activity of this zone. Here, we proposed this strike-slip fault as the Ujung Kulon Fault that marks the southeasternmost segment of the SF zone. This segment deformed the area of the Sumatra-Java forearc basin and terminated in accretionary wedge near the trench. The accumulated strain within UKF may trigger large earthquake in the future, close to the highly populated areas in the coast of Sumatra and Java.Keywords: Strike-slip fault, Sumatra Fault, Ujung Kulon Fault, segmentation, earthquake.

Methodology for Bathymetric Mapping Using Open-Source Software

Abstract The modern technologies not only provide an opportunity to collect water depth data much more efficiently than in the past, but also there is a wide range of opportunities to process these data to develop bathymetric maps and perform various calculations in the computer environment. This research aims to construct a high-resolution bathymetric map by using open-source software. Lake Zebrus was selected as the study site, where fieldwork was performed to obtain the depth data and subsequently process to find optimal methodology for creating a bathymetric map. The methodology used in this research is modifiable for different purposes and is based on obtaining the best results from using open-source resources and knowledge of cartography. Lake Zebrus is an example how to perform accurate and state of the art bathymetric survey in a short time period. The area of Lake Zebrus is 417.57 ha. The lake has a small crooked coastline. Lake Zebrus has a maximum depth of 4.45 m and an average depth of 1.48 m. Results from the bathymetric survey can be used for further research based on depth data. The research tasks are accomplished as the methodology selected is repeatable in the other lakes. The research results prove that this methodology can be used for a bathymetric survey and it is possible to obtain high-definition results using open-source resources.

Syn-eruptive soft-sediment deformation structures in a deep submarine caldera: Havre, 2012

The 2012 rhyolitic eruption of Havre submarine caldera in the Kermadec arc generated an assemblage of new, soft-sediment deformation structures on the caldera floor. These structures are the first documented examples of syn-eruptive, soft-sediment deformation structures within a submarine caldera. They are exquisitely portrayed on a high-resolution (~1 m) bathymetric map made in 2015. Their timing has been resolved by means of stratigraphic relationships with an overlying giant pumice deposit that correlates with the pumice raft first observed on 18th July 2012 UTC. Weeks to months earlier in 2012, the largest lava was erupted from a vent on the southwestern caldera wall, flowed down to the caldera floor and advanced almost 1 km farther. A surficial layer of the caldera-floor sediment surrounding the leading edge of the lava failed, most likely in response to load and heat transfer from the lava. Failure produced a slump sheet comprising folds with arcuate crests at the toe, an undeformed domain in the middle and a basin-and-ridge domain at the head, adjacent to the lava. Subsequently, (1) the lava ruptured on two main faults that propagated across the caldera floor, and (2) strike slip on the faults moved the lava that was in between forward, simultaneously offsetting segments of the slump sheet. These two events were probably triggered by the earthquake swarm recorded on the 17th July 2012 UTC, a day before the pumice raft appeared. The leading edge of the lava then sank slowly into the underlying sediment. Sinking was accommodated by sideways expulsion of sediment from beneath the lava, modifying the topography around the edge of the lava. The caldera-floor sediment was also disturbed when part of the southwestern caldera wall collapsed, generating a small debris avalanche which is represented by a cluster of hummocks on the caldera floor. Collapse was a response to shallow intrusion of a rhyolite dyke into the southwestern caldera wall.

Detailed volcanic and tectonic morphology of Nakadomari Hill in the southern Okinawa Trough

Nakadomari Hill is an unexplored edifice in the southern Okinawa Trough back-arc basin where crustal extension by continental rifting occurs and few submarine eruptions associated with back-arc volcanism have been mapped in detail. High-resolution bathymetric mapping with an autonomous underwater vehicle (AUV) reveals that Nakadomari Hill is a submarine volcanic complex with recent lava flow eruptions. The high-resolution (2 m) bathymetry shows striking contrast between rugged terrain in the central area and the adjacent smooth terrain. The cone-shaped features in the rugged terrain are covered by ropey fabric with high acoustic reflectivity and are devoid of tectonic deformation, thus they are interpreted to be relatively younger lava domes. The cone-shaped features in the smooth terrain show low reflectivity and are dissected by ENE–WSW trending tectonic lineaments, suggesting they were formed during older volcanism. Nakadomari Hill developed through multi-stage eruptions before and after back-arc rifting and faulting. The ropey and curtain-folded morphology with high acoustic reflectivity are interpreted to be a field of lava flows and they are relatively young. The area considered to be lava flows is estimated to be over 13 km2, showing that eruptions in the southern Okinawa Trough have been extensive. An active volcanic system is supported by indicators of high-temperature hydrothermal venting. Hydrothermal plumes with concurrent sharp temperature increases of up to 0.17 °C were present even at the vehicle altitude of ~100 m above the bottom. The alignment of volcanic cone-shaped features at Nakadomari Hill is in the NNW–SSE direction which is clearly different from the orientation of local rifting (ENE–WSW). At a larger scale, the volcanic edifices west of Kume Island from the arc to back-arc are also aligned roughly in the NNW–SSE direction and they are sub-parallel to a major tectonic structure in the arc, the Kerama Gap. The agreement of the local volcanic and tectonic trends implies that magmatic focusing may have taken place along the tectonic structures of the Kerama gap and in turn caused the extensive back-arc volcanism in this area.

Investigating the Norse Harbour of Igaliku (Southern Greenland) Using an Integrated System of Side-Scan Sonar and High-Resolution Reflection Seismics

This study presents the results of a marine geophysical survey performed in the Igaliku fjord in southern Greenland in order to understand the harbour setting of the former Norse settlement Garðar (modern Igaliku). The aims of the survey were (a) to reconstruct the former coastline during the first centuries of the Norse settlement period (c. 11/12th centuries) and (b) to search for archaeological remains on the seabed connected to maritime traffic and trade. In order to approach these goals, we used an integrated marine survey system consisting of a side-scan sonar and a reflection seismic system. The system was designed for lightweight transport, allowing measurements in areas that are logistically difficult to access. The side-scan sonar data revealed no remains of clear archaeological origin. Bathymetric data from seismic seabed reflection and additional Differential GPS height measurements yielded a high-resolution bathymetric map. Based on estimates of Holocene relative sea level change, our bathymetry model was used to reconstruct the shift of the high and low-water line since the early Norse period. The reconstructed coastline shows that a small island, which hosts the ruins of a tentative Norse warehouse at the mouth of the present harbour, was connected to the shore at low tide during the early Norse period. In addition, reflection seismics and side-scan sonar images reveal a sheltered inlet with steep slopes on one side of the island, which may have functioned as a landing bridge used to load ships. We also show that the loss of fertile land due to sea level rise until the end of the Norse settlement was insignificant compared to the available fertile land in the Igaliku fjord and is thus not the reason for the collapse of the colony.