Dive Observations Research Articles

Microbialites on the northern shelf of Lake Van, eastern Türkiye#: Morphology, texture, stable isotope geochemistry and age

ABSTRACTLake Van, the world's largest alkaline lake, hosts some of the largest microbialite towers worldwide, which are considered as modern analogues of ancient stromatolites. This study investigates the links between microbialite evolution, geology, climate and hydrology, and the role of biotic and abiotic processes in microbialite growth and morphology. For these objectives, the northern shelf of Lake Van was surveyed by sub‐bottom seismic profiling and diving, and two 9 m and 15 m high microbialite chimneys were sampled at 25 m water depth. Samples were analysed for stable oxygen and carbon isotopes, X‐ray diffractometry, scanning electron microscopy and U/Th age dating. Lake Van microbialites precipitate wherever focused Ca‐rich groundwater flows onto the lake floor to mix with alkaline lake water. Variable columnar, conical and branching morphologies of the microbialites indicate various processes of formation by groundwater channelling within the chimneys. Collectively, our data suggest that the microbialite chimneys have formed within the last millennium, most likely starting during the warm and humid Medieval Climate Anomaly (ca ad 800–1300), when lake level rose approximately to the present level due to enhanced Inputs of riverine Ca‐rich freshwater and groundwater. Our new scanning electron microscopy observations indicate that the internal structure of the microbialites below the outer cyanobacteria‐covered crust is constructed by calcified filaments, globular aggregates and nanocrystals of algal, cyanobacterial and heterobacterial origins and inorganically precipitated prismatic calcite crystals. These textural features, together with dive observations, clearly demonstrate the important role of inorganic carbonate precipitation at sites of groundwater discharge, followed by cyanobacteria and algal mucilage deposition and microbially meditated calcification in the photic zone in the rapid growth of the microbialite chimneys. Considering the close similarities of some textures with those of ancient stromatolites and meteorites, the results of this study provide new insights into the environmental conditions associated with stromatolite formation and extra‐terrestrial life evolution.

Onshore–offshore correlation of central Lake Erie glacial deposits

Dive observations, echogram transects, core sampling, and a seismic profile revealed that the lake bed of north-central Lake Erie is an extensive terrace cut by storm waves and currents. The terrace is an erosional unconformity on which Late Wisconsinan (Port Bruce and Mackinaw) glacial units crop out. Beds of massive diamictons, and glaciolacustrine sediments containing parallel reflections, crop out alternately from west to east, resulting from an oscillatory ice retreat. These beds correlate with the Port Stanley Drift (Port Bruce phase) and Wentworth Drift (Mackinaw phase) exposed in nearby shore bluffs and onshore moraines. The Port Bruce glacier and earlier readvances formed ice tongues and ice shelves in the central basin. Diamicton layers, some with debris flows, constitute the Port Stanley Till (offshore units M and O). A glaciolacustrine unit N was deposited during Port Bruce glacier recessions. Glaciolacustrine unit P lies between Port Bruce unit O and the Mackinaw Wentworth Till, unit Q. A subsequent glaciolacustrine unit R overlaps unit Q. The onshore Galt and Moffat moraines, composed of Wentworth Till, correlate with ridges of the Norfolk moraine unit Q which extend across Lake Erie between the base of Long Point, Ontario, and Erie, Pennsylvania. The onshore Paris moraine appears to have been eroded on the wave-cut terrace and is evident offshore only near the south shore of Lake Erie. Laminated unit S, younger than unit R, occurs in the western part of central Erie basin, and correlates with overflow of Lake Algonquin from the Huron basin.

Spatial distribution and density of the invasive sea urchin Diadema setosum in Turkey (eastern Mediterranean)

AbstractThe sea urchin, Diadema setosum, is a poisonous species that originates in the Indo‐Pacific and the Red Sea. Although this species has been recorded in several areas in the Mediterranean Sea, its habitat selection, density, distribution and ecological impacts have not been comprehensively documented to date. This study combined the diving observations (n = 53 sites) and local ecological knowledge (LEK) of SCUBA divers (n = 100) to provide information on the distribution and density of this invasive sea urchin on the Aegean and Levantine coasts of Turkey between September and October 2020. The results indicated that D. setosum was more prevalent along the southern coast of Turkey compared with the western coast, where it is colder. The highest densities of D. setosum were observed in rocky habitats at a depth range of 0–5 m. Moreover, the densities increased with increasing bottom temperatures (up to 150 ind./100 m2). Recent reports from the Levantine and south Aegean coasts highlighted that the densities of D. setosum increased dramatically during the past year. The results of the present study suggested that D. setosum provides shelter for a total of four fish species, including Chromis chromis, Gobius sp., Cheilodipterus novemstriatus and Thalassoma pavo. This species of urchin poses a threat due to its potential grazing and bioerosion effects in the Mediterranean. Thus, future studies should focus on the feeding ecology of D. setosum and the competition with native species that share the same habitat to obtain an improved understanding of the ecological impacts of this invasive species. Although the present study provides the first comprehensive dataset on the density and distribution of this invasive sea urchin for Turkey in the eastern Mediterranean region, we suggest that its population should be monitored for long periods using direct observations, LEK and citizen science.

Recent Eruptions Between 2012 and 2018 Discovered at West Mata Submarine Volcano (NE Lau Basin, SW Pacific) and Characterized by New Ship, AUV, and ROV Data

West Mata is a submarine volcano located in the SW Pacific Ocean between Fiji and Samoa in the NE Lau Basin. West Mata was discovered to be actively erupting at its summit in September 2008 and May 2009. Water-column chemistry and hydrophone data suggest it was probably continuously active until early 2011. Subsequent repeated bathymetric surveys of West Mata have shown that it changed to a style of frequent but intermittent eruptions away from the summit since then. We present new data from ship-based bathymetric surveys, high-resolution bathymetry from an autonomous underwater vehicle, and observations from remotely operated vehicle dives that document four additional eruptions between 2012-2018. Three of those eruptions occurred between September 2012 and March 2016; one near the summit on the upper ENE rift, a second on the NE flank away from any rift zone, and a third at the NE base of the volcano. The latter intruded a sill into a basin with thick sediments, uplifted them, and then extruded lava onto the seafloor around them. The most recent of the four eruptions occurred between March 2016 and November 2017 along the middle ENE rift zone and produced pillow lava flows with a shingled morphology and tephra as well as clastic debris that mantled the SE slope. ROV dive observations show that the shallower recent eruptions at West Mata include a substantial pyroclastic component, based on thick (>1m) tephra deposits near eruptive vents. The deepest eruption sites lack these near-vent tephra deposits, suggesting that pyroclastic activity is minimal below ~2500 mbsl. The multibeam sonar re-surveys constrain the timing, thickness, area, morphology, and volume of the new eruptions. The cumulative erupted volume since 1996 suggests that eruptions at West Mata are volume-predictable with an average eruption rate of 7.8 x 106 m3/yr. This relatively low magma supply rate and the high frequency of eruptions (every 1-2 years) suggests that the magma reservoir at West Mata is relatively small. With its frequent activity, West Mata continues to be an ideal natural laboratory for the study of submarine volcanic eruptions.

Movement of pulsed resource subsidies from kelp forests to deep fjords.

Resource subsidies in the form of allochthonous primary production drive secondary production in many ecosystems, often sustaining diversity and overall productivity. Despite their importance in structuring marine communities, there is little understanding of how subsidies move through juxtaposed habitats and into recipient communities. We investigated the transport of detritus from kelp forests to a deep Arctic fjord (northern Norway). We quantified the seasonal abundance and size structure of kelp detritus in shallow subtidal (0‒12m), deep subtidal (12‒85m), and deep fjord (400‒450m) habitats using a combination of camera surveys, dive observations, and detritus collections over 1year. Detritus formed dense accumulations in habitats adjacent to kelp forests, and the timing of depositions coincided with the discrete loss of whole kelp blades during spring. We tracked these blades through the deep subtidal and into the deep fjord, and showed they act as a short-term resource pulse transported over several weeks. In deep subtidal regions, detritus consisted mostly of fragments and its depth distribution was similar across seasons (50% of total observations). Tagged pieces of detritus moved slowly out of kelp forests (displaced 4‒50m (mean 11.8m ± 8.5 SD) in 11‒17days, based on minimum estimates from recovered pieces), and most (75%) variability in the rate of export was related to wave exposure and substrate. Tight resource coupling between kelp forests and deep fjords indicate that changes in kelp abundance would propagate through to deep fjord ecosystems, with likely consequences for the ecosystem functioning and services they provide.

Reworking processes and deposits in coral reefs during (very) high-energy events: Example from a Pleistocene coral formation (125 ka), La Désirade Island, Lesser Antilles

Abundant literature has documented the ecological impact of very high-energy (hurricane-type) events and high-energy (storm-type) events on modern coral reefs and sediment transport. In previous studies, sedimentary processes are often deduced from geophysical image analysis and scuba diving observations. Few field-based studies have been devoted to the impact of (very) high-energy disturbance on the structure and ecology of Holocene and Pleistocene coral reefs, and very few to the sedimentary processes acting around these reefs.This research examines the structure of the late Pleistocene (end of MIS5e stage) reefs of La Désirade Island, Guadeloupe Archipelago, Lesser Antilles, and focuses on the poorly known reworking processes and related coral-breccia sedimentation surrounding these reefs. Fieldwork was performed to document two coral-fragment-rich breccia interbedded with coral patches. The breccia predominantly show the same petrography but differ in their internal structure, showing a “massive breccia” type and a “stratified breccia” type. This work infers that these breccia are the result of gravity-induced high-density mass flows triggered by high to very-high energy events such as storms and hurricanes. Damage and reworking vary according to the type of (very) high-energy event. Prior to transport, storms are thought to have broken the heads of branching and columnar coral colonies, and broken and overturn isolated lamellar and massive colonies. Hurricanes (particularly their eye walls) may have reworked much more sediment than storms, and may have also destabilised entire several metre-sized columnar colonies from the flank of coral patch reefs. We propose the name “coral debrite” for these coral breccia deposits caused by gravity-induced debris flows.The reworked deposits are interbedded between two coral reef levels, of which the main coral-species vertical succession should record the end of Marine Isotope Stage 5e sea-level rise. Thus, reworking is believed to occur during the transitional stage between a glacial and an interglacial period.

Innovative method for optimizing Side-Scan Sonar mapping: The blind band unveiled

Over the past few years, the mapping of Mediterranean marine habitats has become a priority for scientists, environment managers and stakeholders, in particular in order to comply with European directives (Water Framework Directive and Marine Strategy Framework Directive) and to implement legislation to ensure their conservation. Side-scan sonar (SSS) is recognised as one of the most effective tool for underwater mapping. However, interpretation of acoustic data (sonograms) requires extensive field calibration and the ground-truthing process remains essential. Several techniques are commonly used, with sampling methods involving grabs, scuba diving observations or Remotely Operated Vehicle (ROV) underwater video recordings. All these techniques are time consuming, expensive and only provide sporadic informations. In the present study, the possibility of coupling a camera with a SSS and acquiring underwater videos in a continuous way has been tested. During the ‘PosidCorse’ oceanographic survey carried out along the eastern coast of Corsica, optical and acoustic data were respectively obtained using a GoPro™ camera and a Klein 3000™ SSS. Thereby, five profiles were performed between 10 and 50 m depth, corresponding to more than 20 km of data acquisition. The vertical images recorded with the camera fixed under the SSS and positioned facing downwards provided photo mosaics of very good quality corresponding to the entire sonograms's blind band. From the photo mosaics, 94% of the different bottom types and main habitats have been identified; specific structures linked to hydrodynamics conditions, anthropic and biological activities have also been observed as well as the substrate on which the Posidonia oceanica meadow grows. The association between acoustic data and underwater videos has proved to be a non-destructive and cost-effective method for ground-truthing in marine habitats mapping. Nevertheless, in order to optimize the results over the next surveys, certain limitations will need to be remedied.

Comunidades de equinodermos en la región de Tunantal, Golfo de Cariaco, Venezuela

Echinoderms are relevant in the structure of marine benthic assemblages, both due their diversity and their ecological niche. However, studies related with occurrence, abundance and patterns of distribution of echinoderms in Venezuela are still scarce. In the present study we describe the echinoderms in shallow-waters habitats (corals patches, Thalassia beds, sandy bottom, rocky subtidal shore, and mouth of river) at Tunantal bay, Golfo de Cariaco, Venezuela, an area that face threats related with the increase of urban development. Samples were performed during September - October 2010 and February - April 2011, using quantitative (1 m2 plots) and qualitative diving observations. Measurements include density, number of species, evenness, Shannon diversity and similarities between habitats based on Bray-Curtis and Sorensen Indexes. Differences in the structure of echinoderm assemblages were detected between habitats. The overall number of echinoderm species in the area was estimated in 40 ± 3.02. Coral patches and rocky shores showed more species and abundance, evenness and Shannon diversity than others habitats. Although the former habitats are similar in evenness and Shannon diversity, differences in the structure of the assemblages were detected. In coral habitats Echinometra viridis, Ophiactis savignyi and Ophiothrix angulata were the dominants species of the assemblages, meanwhile in rocky shores E. lucunter and Ophiocoma echinata were dominants in abundance followed by Ophionereis reticulata. Other habitats of the bay (Thalassia beds, sandy bottom and mouth of river) showed very low diversity and abundance of echinoderms, in some cases only occasional occurrence. Substrate heterogeneity is proposed as the main driving factor of the diversity and structure of the echinoderms assemblages in the bay. Moreover coral patches and rocky shores offer a substratum to dominant species (E. viridis, E. lucunter and O. echinata), but habitat differences in terms of substratum, depth and wave stress could explain the differences in the echinoderm assemblage. The overall echinoderm diversity is promoted by the differences of habitats in the bay and the changes of species composition between habitats.

Accumulation of an earthquake-induced extremely turbid layer in a terminal basin of the Nankai accretionary prism

Seismic shaking is a major trigger for sediment redistribution in subduction zones, and clouds of dilute suspended sediment have been reported in association with large earthquakes. Dive observations in a basin on the slope of the central Nankai accretionary prism soon after the 2004 off-Kii Peninsula earthquakes documented a layer of suspended sediment with extremely high turbidity. We estimated the thickness of this bottom turbid layer to be more than 2.5 m by comparison of seafloor depths between surveys in 2004 and 2010 and about 2.6 m from instrumental evidence involving the submersion of a heat-flow probe. A high-resolution subbottom profiling survey across the basin revealed acoustically transparent layers thicker than 2 m. Because the slope basin is a terminal basin completely enclosed by topographic highs, we examined the possibility that the uppermost transparent layer was deposited as a consequence of the 2004 earthquakes. Considering the sediment source area and the volume of the basin fills, the mobilization and redeposition of thin surface sediments on the prism slope can account for the volume of the transparent layer. We conclude that the 2004 earthquakes caused widespread disturbance of the prism slope and concentrated surface sediments in this terminal basin. Our results emphasize the utility of a terminal basin in a subduction zone as an earthquake recorder.

The 1998 eruption of Axial Seamount: New insights on submarine lava flow emplacement from high‐resolution mapping

Axial Seamount, an active submarine volcano on the Juan de Fuca Ridge at 46°N, 130°W, erupted in January 1998 along 11 km of its upper south rift zone. We use ship‐based multibeam sonar, high‐resolution (1 m) bathymetry, sidescan sonar imagery, and submersible dive observations to map four separate 1998 lava flows that were fed from 11 eruptive fissures. These new mapping results give an eruption volume of 31 × 106 m3, 70% of which was in the northern‐most flow, 23% in the southern‐most flow, and 7% in two smaller flows in between. We introduce the concept of map‐scale submarine lava flow morphology (observed at a scale of hundreds of meters, as revealed by the high‐resolution bathymetry), and an interpretive model in which two map‐scale morphologies are produced by high effusion‐rate eruptions: “inflated lobate flows” are formed near eruptive vents, and where they drain downslope more than 0.5–1.0 km, they transition to “inflated pillow flows.” These two morphologies are observed on the 1998 lava flows at Axial. A third map‐scale flow morphology that was not produced during this eruption, “pillow mounds,” is formed by low effusion‐rate eruptions in which pillow lava piles up directly over the eruptive vents. Axial Seamount erupted again in April 2011 and there are remarkable similarities between the 1998 and 2011 eruptions, particularly the locations of eruptive vents and lava flow morphologies. Because the 2011 eruption reused most of the same eruptive fissures, 58% of the area of the 1998 lava flows is now covered by 2011 lava.

One orphaned marine manatee calf´s feeding, breeding and behavior in captivity (Trichechus manataus, Linnaeus, 1758)

In Brazil, although there are records of marine manatee in captivity, few information report the management of this species. As a way of reducing this gap, this work aims to report the obtained results through the feeding control, the behavior observation and the monitoring of a female orphaned Trichechus manatus and compare with similar studies conducted on animals in captivity of the same species, populations Florida (USA). The specimen of manatee, female, was captured in a fishing corral and transferred to a coastal lagoon of fresh water and subsequently kept in a pool. During the period in captivity, the food provided was given using bottles composed of dried milk and vitamin complex, and at times tested the addition of vegetable oil. Furthermore, there was an attempt to enter the food items of plant origin, such as reed (Eleocharis spp.), rush (Eichhornia crassipes), lettuce, cabbage, marine algae (Gracilaria sp. Vidalia sp. Hipaeae sp. Laurencia sp.) and marine phanerogamic (Halodule sp.). The records of the behavior of the calf on the shifts in the nursery, body position and dive time observations were made using specific "ad libitum" with the aid of binoculars and a digital stopwatch. For growth studies, morphometric measures and weight were taken. About the food supply, it was recorded gradual increase in milk consumption, but the calf did not show active interest in any item of plant origin that was offered. Except the behavior conditioned to the signals for feeding, or when it was captured for the biometrical tests (when it swam quickly), the calf was extremely docile and slow, spending many hours a day resting, floating passively, or submerged at the bottom of the bed, coming to surface only to breathe. We observed the use of the pectoral fins to direct the movements and perform small displacements along the bottom, while the caudal fin was the main cause of long strokes and fast. Two typical positions of rest were observed, as well as calling on the occasions of management. During the period in captivity, the calfcalf grew in 33 cm total length and gained 46 kg, which can be regarded as a normal growth pattern. As a result of a process congestive visceral, the calf died approximately one year of life.

A multi-method approach for benthic habitat mapping of shallow coastal areas with high-resolution multibeam data

The coastal waters of the Maltese Islands, central Mediterranean Sea, sustain a diversity of marine habitats and support a wide range of human activities. The islands’ shallow waters are characterised by a paucity of hydrographic and marine geo-environmental data, which is problematic in view of the requirements of the Maltese Islands to assess the state of their coastal waters by 2012 as part of the EU Marine Strategy Directive. Multibeam echosounder (MBES) systems are today recognised as one of the most effective tools to map the seafloor, although the quantitative characterisation of MBES data for seafloor and habitat mapping is still an underdeveloped field. The purpose of this study is to outline a semi-automated, Geographic Information System-based methodology to map the distribution of habitats in shallow coastal waters using high-resolution MBES data. What distinguishes our methodology from those proposed in previous studies is the combination of a suite of geomorphometric and textural analytical techniques to map specific types of seafloor morphologies and compositions; the selection of the techniques is based on identifying which geophysical parameter would be influenced by the seabed type under consideration.We tested our approach in a 28km2 area of Maltese coastal waters. Three data sets were collected from this study area: (i) MBES bathymetry and backscatter data; (ii) Remotely Operated Vehicle imagery and (iii) photographs and sediment samples from dive surveys. The seabed was classified into five elementary morphological zones and features – flat and sloping zones, crests, depressions and breaks of slope – using morphometric derivatives, the Bathymetric Position Index and geomorphometric mapping. Segmentation of the study area into seagrass-covered and unvegetated seafloor was based on roughness estimation. Further subdivision of these classes into the four predominant types of composition – medium sand, maërl associated with sand and gravel, seagrass settled on sand and gravel, and seagrass settled on bedrock – was carried out through supervised classifications of morphometric derivatives of the bathymetry and textural indices of backscatter, based on information from training stations. The resulting morphologic and seabed composition maps were combined to plot the distribution of the predominant habitats in the coastal waters offshore NE Malta, some of which are of high conservation value. Ground-truthing of the habitat map using ROV imagery and dive observations confirms that our approach produces a simplified and accurate representation of seafloor habitats while using all the information available within the MBES data sets.

Unraveling the tilting history of the submerged reefs surrounding Oahu and the Maui‐Nui Complex, Hawaii

There is considerable uncertainty concerning the history of lithospheric flexure of Oahu and the Maui‐Nui Complex associated with the Hawaiian hot spot. Previous reconstructions based on models and sparse observational data place the zone of maximum uplift (forebulge) either under Oahu or between Oahu and Molokai. To address this issue we have compiled and analyzed all existing and new high‐resolution multibeam bathymetry, dive observations, and chronologic data from submerged terraces in the region. We have identified 89 separate submarine terraces and mapped their distribution across the volcanic flanks of Oahu and the Maui‐Nui Complex. These data are used to systematically measure the direction and amplitude of terrace tilting within the Maui‐Nui Complex and to provide new constraints on lithospheric flexure associated with volcanic loading at the hot spot. The terraces are divided into six separate geographic regions (Oahu, Molokai, Penguin Bank, Lanai, Maui, and East Maui) defined by significant discontinuities in either terrace morphology or depth. Following the sequential development of new substrate as the new volcanoes formed throughout the development of the MNC, 18 new and 51 published 87Sr/86Sr isotopic ages show that the terraces in these regions were also initiated at different times, and the tilting data show that they also responded to lithospheric flexure independently. The general pattern of terrace tilting records the plate's response to lithospheric flexure with each region continually tilting toward the zone of maximum subsidence since their formation. Our individual tilt calculations are based on the most complete and highest‐resolution bathymetric data available, and they show that plate response to volcanic loading is a gradual process. Finally, we demonstrate that the shallowest terraces we identified in the Oahu and Molokai regions tilt toward the north, away from the current zone of maximum subsidence, indicating that these regions have recently passed the zone of maximum uplift. We compare our results with two separate published numerical models and differentiate between these, resolving a long‐standing uncertainty in the position of the forebulge.

Feeding behavior and follower fishes of Myrichthys ocellatus (Anguilliformes: Ophichthidae) in the western Atlantic

This paper described aspects of feeding behavior of Myrichthys ocellatus in the Western Atlantic, using diving observations and additional data from literature. The feeding behavior of M. ocellatus was characterized by scanning the sand surface and searching for buried preys, using its head and tail as a searching tool. Twelve species of reef fish were detected as followers of M. ocellatus, including three new records of follower species. The food overlap index between M. ocellatus and most follower species was low.

A submersible study of the Mariana Trough back-arc spreading center at 17.DEG.N

During the JAMSTEC R/V Yokosuka YK08-08 Leg-1 cruise, the submersible Shinkai 6500 survey of the central Mariana Trough at 17°N was carried out to study a formation process of oceanic lithosphere in the active back-arc spreading center. Three dives were devoted to the median valley in the 17°N segment center. Visual observation of geology, lava flow morphology, and fault structure provides ground references for the backscattering imagery obtained from the deep-towed side-scan sonar survey conducted in 2003 to discuss volcanotectonic features. Smooth surfaces of side-scan imagery with high backscattering intensities are found to be jumbled-wrinkled, folded sheet, or lobate lava flows. The visual observation proves that the 17°N segment is covered with sheet lava flows of high effusion rates. Quantifiable degrees of sedimentation superposed on the lavas suggest relative age differences of formation. Sheet lava flows in the axial portion of the median valley are considered to be the youngest among the three dive observations. Sedimentation more or less gradually increases toward the western margin of the median valley. The observed eastern margin of the median valley is covered with sedimentary layers of ∼0.5-2 m thickness, thus the area is regarded as the oldest in our dive observations. North-south trending tectonic structures (i.e. faults, fissures), oblique to the median valley direction, could be younger than valley-parallel NNW-SSE trending structures because they show relatively little accumulation of sediment, and they cut the other structures in some places. Basaltic rock samples were collected at a total of 22 stations where several types of lava flow morphology are shown. Spatial variations in the rock faces, suggesting variations of chemical characteristics and rock ages, were found (phyric/aphyric, manganese coating, vesiculation). Hemipelagic sediment samples including nanofossils were also collected at 6 stations.

Relationship between soak time and number of enmeshed animals in experimentally lost gill nets

In order to determine the effects of ghost fishing by lost gill nets, the relationship was examined between soak time and number of enmeshed animals in experimentally lost gill nets by using diving observations. Two experimental gill nets were set at 13 m depth in Tateyama Bay, Chiba Prefecture, Japan for approximately 200 days. One gill net was deployed in a small trough surrounded by artificial reefs, and the other was deployed on an adjacent open sandy bed. Twelve species of crustaceans, six species of gastropods, and five species of bony fish were enmeshed in the experimental gill nets. The number of enmeshed animals in the artificial reef gill net was substantially larger than that in the sandy bed gill net. The number of enmeshed animals in the experimental gill nets increased rapidly within one month after deployment, and then declined gradually showing fluctuations caused by the decrease in newly enmeshed animals, and the drop off from gill nets caused by the decomposition of dead animals. The decrease in the number of enmeshed animals was expressed by logarithmic equations, and based on these equations, the duration of capture function for the lost gill nets was calculated to be 284-561 days in the artificial reef gill net and 200 days in the sandy bed gill net. The duration of capture function for the lost gill nets for non-commercial by-catch species such as small crustaceans and gastropods was longer than for commercial species such as Japanese spiny lobster and bony fish.

Geology and hydrothermal evolution of the Mothra Hydrothermal Field, Endeavour Segment, Juan de Fuca Ridge

Detailed characterization of the Mothra Hydrothermal Field, the most southern and spatially extensive field on the Endeavour Segment of the Juan de Fuca Ridge, provides new insights into its geologic and hydrothermal development. Meter‐scale bathymetry, side‐scan sonar imagery, and direct dive observations show that Mothra is composed of six actively venting sulfide clusters spaced 40–200 m apart. Chimneys within each cluster have similar morphology and venting characteristics, and all clusters host a combination of active and extinct sulfide structures. Black smoker chimneys venting fluids above 300°C are rare, while more common lower‐temperature, diffusely venting chimneys support dense colonies of macrofauna and bacterial mat. Hydrothermal sediment and extinct sulfide debris cover 10–15 m of the seafloor surrounding each vent cluster, obscuring the underlying basaltic substrate of light to moderately sedimented pillow, lobate, sheet, and chaotic flows, basalt talus, and collapse terrain. Extinct sulfide chimneys and debris between the clusters indicate that hydrothermal flow was once more widespread and that it has shifted spatially over time. The most prominent structural features in the axial valley at Mothra are regional (020°) trending faults and fissures and north‐south trending collapse basins. The location of actively venting clusters within the field is controlled by (1) localization of fluid upflow along the western boundary fault zone, and diversion of these fluids by antithetic faults to feed vent clusters near the western valley wall, and (2) tapping of residual magmatic heat in the central part of the axial valley, which drives flow beneath vent clusters directly adjacent to the collapse basins 70–90 m east of the western valley wall. These processes form the basis for a model of axial valley and hydrothermal system development at Mothra, in which the field is initiated by an eruptive‐diking episode and sustained through intense microseismicity and non‐eruptive diking events.

Support for the Giant Wave Hypothesis: evidence from submerged terraces off Lanai, Hawaii

The origin of subaerial coral conglomerate deposits on the Hawaiian islands of Lanai and Molokai is controversial, primarily because these deposits are difficult to interpret and the vertical motion of these islands is poorly constrained. Based on bathymetry, dive observations, sedimentary and radiocarbon data from coralline algal dominated deposits from two submerged terraces at −150 and −230 m off Lanai, Lanai has experienced relatively little vertical movement over the last 30 ka. Using internally consistent age versus depth relationships, paleowater depths, and published sea level data, we estimate that Lanai has experienced maximum rates of uplift of 0.1 m/kyr or subsidence of 0.4 m/kyr over this period. Our analysis of possible uplift mechanisms, published geophysical, numerical modelling, and recent tide data suggests that this is also the maximum uplift rate for the last several hundred thousand years. Taken together these data support the interpretation that coral conglomerates at elevations higher than +35 m on Lanai are tsunami deposits with a minimum wave run up > 170 m, rather than shoreline deposits formed during the last two interglacials, then uplifted to their present elevations.

Drowning of the −150 m reef off Hawaii: A casualty of global meltwater pulse 1A?

We present evidence that the drowning of the 2150 m coral reef around Hawaii was caused by rapid sea-level rise associated with meltwater pulse 1A (MWP-1A) during the last deglaciation. New U/Th and 14 C accelerator mass spectrometry dates, combined with reinterpretation of existing radiometric dates, constrain the age of the coral reef to 15.2-14.7 ka (U/Th age), indicating that reef growth persisted for 4.3 k.y. following the end of the Last Glacial Maximum at 19 ka. The drowning age of the reef is roughly synchronous with the onset of MWP-1A between 14.7 and 14.2 ka. Dates from coralline algal material range from 14 to 10 cal ka (calibrated radiocarbon age), 1-4 k.y. younger than the coral ages. A paleoenvironmental reconstruction incorporating all available radiometric dates, high-resolution bathymetry, dive observations, and coralgal paleobathymetry data indicates a dramatic rise in sea level around Hawaii ca. 14.7 ka. Paleowater depths over the reef crest increased rapidly above a critical depth (30-40 m), drowning the shallow reef-building Porites corals and causing a shift to deep- water coralline algal growth, preserved as a crust on the drowned reef crest.

Study of the Saldanha Massif (MAR, 36°34′ N): Constrains from rock magnetic and geophysical data

We present a study of the magnetic properties of a group of basalt samples from the Saldanha Massif (Mid-Atlantic Ridge – MAR – 36° 33′ 54′′ N, 33° 26′ W), and we set out to interpret these properties in the tectono-magmatic framework of this sector of the MAR. Most samples have low magnetic anisotropy and magnetic minerals of single domain grain size, typical of rapid cooling. The thermomagnetic study mostly shows two different susceptibility peaks. The high temperature peak is related to mineralogical alteration due to heating. The low temperature peak shows a distinction between three different stages of low temperature oxidation: the presence of titanomagnetite, titanomagnetite and titanomaghemite, and exclusively of titanomaghemite. Based on established empirical relationships between Curie temperature and degree of oxidation, the latter is tentatively deduced for all samples. Finally, swath bathymetry and sidescan sonar data combined with dive observations show that the Saldanha Massif is located over an exposed section of upper mantle rocks interpreted to be the result of detachment tectonics. Basalt samples inside the detachment zone often have higher than expected oxidation rates; this effect can be explained by the higher permeability caused by the detachment fault activity.