Cayman Trough Research Articles

A deep-sea isopod that consumes Sargassum sinking from the ocean's surface.

Most deep-ocean life relies on organic carbon from the surface ocean. While settling primary production rapidly attenuates in the water column, pulses of organic material can be quickly transported to depth in the form of food falls. One example of fresh material that can reach great depths across the tropical Atlantic Ocean and Caribbean Sea is the pelagic macroalgae Sargassum. However, little is known about the deep-ocean organisms able to use this food source. Here, we encountered the isopod Bathyopsurus nybelini at depths 5002-6288 m in the Puerto Rico Trench and Mid-Cayman Spreading Center using the Deep Submergence Vehicle Alvin. In most of the 32 observations, the isopods carried fronds of Sargassum. Through an integrative suite of morphological, DNA sequencing, and microbiological approaches, we show that this species is adapted to feed on Sargassum by using a specialized swimming stroke, having serrated and grinding mouthparts, and containing a gut microbiome that provides a dietary contribution through the degradation of macroalgal polysaccharides and fixing nitrogen. The isopod's physiological, morphological, and ecological adaptations demonstrate that vertical deposition of Sargassum is a direct trophic link between the surface and deep ocean and that some deep-sea organisms are poised to use this material.

Distinct development trajectories and symbiosis modes in vent shrimps.

Most animal species have a singular developmental pathway and adult ecology, but developmental plasticity is well-known in some such as honeybees where castes display profoundly different morphology and ecology. An intriguing case is the Atlantic deep-sea hydrothermal vent shrimp pair Rimicaris hybisae and R. chacei that share dominant COI haplotypes and could represent very recently diverging lineages or even morphs of the same species. Rimicaris hybisae is symbiont-reliant with a hypertrophied head chamber (in the Mid-Cayman Spreading Centre), while R. chacei is mixotrophic with a narrow head chamber (on the Mid-Atlantic Ridge). Here, we use X-ray micro-computed tomography and fluorescence in situ hybridization to show that key anatomical shifts in both occur during the juvenile-subadult transition, when R. hybisae has fully established symbiosis but not R. chacei. On the Mid-Atlantic Ridge, the diet of R. chacei has been hypothetically linked to competition with the obligatorily symbiotic congener R. exoculata, and we find anatomical evidence that R. exoculata is indeed better adapted for symbiosis. We speculate the possibility that the distinct development trajectories in R. hybisae and R. chacei may be determined by symbiont colonization at a "critical period" before subadulthood, though further genetic studies are warranted to test this hypothesis along with the true relationship between R. hybisae and R. chacei.

Late Cretaceous‐Recent Tectonostratigraphic Evolution of the Yucatan Back‐Arc Basin, Northern Caribbean Sea

AbstractThe 110,000 km2 Yucatan Basin in the northern Caribbean Sea is critical for understanding the Late Cretaceous to Recent tectonic evolution of the Caribbean‐North American plate boundary. This study integrates gravity, magnetic, and a 5,500 km grid of 2D seismic data to carry out a tectonostratigraphic analysis of the Yucatan Basin. These data provide the first recognition of 38–102 km‐long spreading ridges that constrain a SW‐NE opening direction in the western Yucatan Basin. The age of this oceanic crust is constrained to be late Paleocene‐middle Eocene (57–42 Ma) based on heat flow measurements, depth‐to‐seafloor, and three sedimentary sequences inferred to be Eocene—Recent in age based on stratigraphic correlations to distant wells. We interpret the Yucatan Basin as a back‐arc basin formed during the northeastward movement of the Caribbean volcanic arc that is now exposed in Cuba, evolved during the early Cretaceous to middle Eocene, and was terminated by collision with the Bahama carbonate platform during the late Paleocene to middle Eocene. We identify regional, left‐lateral strike‐slip faults that extend into the Cuban volcanic arc, as observed in other active back‐arc basins. We propose that the Yucatan back‐arc basin once formed the northwestern extension of age‐equivalent back‐arc basins in Hispaniola, where the basin is inverted, topographically elevated, and strongly shortened, and in the Lesser Antilles where the Paleogene back‐arc basin has remained undeformed and submarine. This once‐continuous back‐arc basin was disrupted and left‐laterally offset by ∼500 km during the Late Eocene‐Recent formation of the Cayman trough strike‐slip system.

A tale of two tubeworms: taxonomy of vestimentiferans (Annelida: Siboglinidae) from the Mid-Cayman Spreading Centre

The vestimentiferan tubeworm genera Lamellibrachia and Escarpia inhabit deep-sea chemosynthesis-based ecosystems, such as seeps, hydrothermal vents and organic falls, and have wide distributions across the Pacific, Atlantic and Indian Oceans. In 2010–2012 during initial explorations of hydrothermal vents of the Mid-Cayman Spreading Centre (MCSC), both genera were found to co-occur at the Von Damm Vent Field (VDVF), a site characterised by diffuse flow, therefore resembling a ‘hydrothermal seep’. Here, we erect two new vestimentiferan tubeworm species from the VDVF, Lamellibrachia judigobini sp. nov. and Escarpia tritentaculata sp. nov. Lamellibrachia judigobini sp. nov. differs genetically and morphologically from other Lamellibrachia species, and has a range that extends across the Gulf of Mexico, MCSC, off Trinidad and Tobago, and Barbados, and also across both vents and seeps and 964–3304-m water depth. Escarpia tritentaculata sp. nov. is distinguished from other Escarpia species primarily based on morphology and is known only from vents of the MCSC at 2300-m depth. This study highlights the incredible habitat flexibility of a single Lamellibrachia species and the genus Escarpia, and historic biogeographic connections to the eastern Pacific for L. judigobini sp. nov. and the eastern Atlantic for E. tritentaculata sp. nov. ZooBank: urn:lsid:zoobank.org:pub:D9F72BD4-FDE1-4C0A-B84B-A08D06F2A981

Temporal and spatial variation of seafloor spreading at ultraslow spreading ridges: Contribution of marine magnetics

The heterogenous magma supply at ultraslow spreading ridges creates diverse seafloor morphologies and lithospheric structures, which in turn generate a large variability in marine magnetic anomalies. The variability brings difficulties to interpret the evolution of oceanic lithosphere. On the other hand, different magnetic signatures of different seafloors provide an opportunity to identify the modes of seafloor spreading on the ultraslow spreading ridges. Here, we modeled several across-axis magnetic profiles selected from the Gakkel Ridge, Southwest Indian Ridge and Mid-Cayman Spreading Center to explore the lithospheric structure and seafloor spreading processes. Considering conjugate flanks, we observed three modes of seafloor spreading, Magmatic vs Magmatic, Magmatic vs Tectonic, and Tectonic vs Tectonic, on the three ultraslow spreading ridges. These three spreading modes reflect a strong, intermediate, and starved magma supply, respectively. Furthermore, four alternances of the different spreading modes were identified including the Magmatic vs Magmatic to Magmatic vs Tectonic, Magmatic vs Tectonic to Tectonic vs Magmatic, Tectonic vs Tectonic to Magmatic vs Magmatic, and Mixed. These alternances of spreading modes in across-axis direction suggest oscillations of the magma supply at different levels. Variation in the modes of seafloor spreading on four nearby profiles over the Mid-Cayman Spreading Center reveals the evolution of magma supply along the axis of this short ridge.

Active ocean–continent transform margins: seismic investigation of the Cayman Trough-Swan Island ridge-transform intersection

SUMMARY The southern boundary of the Cayman Trough in the Caribbean is marked by the Swan Islands transform fault (SITF), which also represents the ocean–continent transition of the Honduras continental margin. This is one of the few places globally where a transform continental margin is currently active. The CAYSEIS experiment acquired an ∼165-km-long seismic refraction and gravity profile (P01) running across this transform margin, and along the ridge-axis of the Mid-Cayman Spreading Centre (MCSC) to the north. This profile reveals not only the crustal structure of an actively evolving transform continental margin, that juxtaposes Mesozoic-age continental crust to the south against zero-age ultraslow spread oceanic crust to the north, but also the nature of the crust and uppermost mantle beneath the ridge-transform intersection (RTI). The traveltimes of arrivals recorded by ocean-bottom seismographs (OBSs) deployed along-profile have been inverse and forward modelled, in combination with gravity modelling, to reveal an ∼25-km-thick continental crust that has been continuously thinned over a distance of ∼65 km to ∼10 km adjacent to the SITF, where it is juxtaposed against ∼3–4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are only sparsely observed, and, even then, only by a few OBSs located on the continental margin, the 7.5 km s–1 velocity contour is used as a proxy to locate the crust–mantle boundary along-profile. Along the MCSC, the crust–mantle boundary appears to be a transition zone, at least at the seismic wavelengths used for CAYSEIS data acquisition. Although the traveltime inversion only directly constrains the upper crust at the SITF, gravity modelling suggests that it is underlain by a higher density (>3000 kg m–3) region spanning the width (∼15 km) of its bathymetric expression, that may reflect a broad region of metasomatism, mantle hydration or melt-depleted lithospheric mantle. At the MCSC ridge-axis to the north, the oceanic crust appears to be forming in zones, where each zone is defined by the volume of its magma supply. The ridge tip adjacent to the SITF is currently in a magma rich phase of accretion. However, there is no evidence for melt leakage into the transform zone. The width and crustal structure of the SITF suggests its motion is currently predominantly orthogonal to spreading. Comparison to CAYSEIS Profile P04, located to the west and running across-margin and through 10 Ma MCSC oceanic crust, suggests that, at about this time, motion along the SITF had a left-lateral transtensional component, that accounts for its apparently broad seabed appearance westwards.

Behavior of iron isotopes in hydrothermal systems: Beebe and Von Damm vent fields on the Mid-Cayman ultraslow-spreading ridge

It is now clear that, in some parts of the ocean, inputs of hydrothermal iron (Fe) can make a more significant contribution to the Fe inventory than previously thought. While the Fe isotopic signature of seawater has proved useful for distinguishing between inputs of Fe from atmospheric deposition and seafloor sediments, the Fe isotope signature of hydrothermal vent fluids may change during mixing of vent fluids and seawater. To better constrain the processes leading to these changes, the Fe isotopic compositions (δ56Fe) of dissolved and total dissolvable Fe have been determined in high temperature vent fluids and the buoyant hydrothermal plumes at the Beebe and the Von Damm vent fields, which are located along the ultraslow Mid-Cayman spreading center in the Caribbean Sea.Our results show that the δ56Fe value of dissolved Fe in the earliest stages of buoyant plume formation was lower (as low as −4.08‰) than measured in a high temperature, low-Mg, vent fluid sample (−0.28‰). This indicates that the iron isotopic signature of dissolved Fe is principally controlled by oxidation of Fe(II) and precipitation of Fe-(oxyhydr)oxides that preferentially incorporate heavy Fe isotopes. In support of this, the δ56Fe value of labile particulate Fe was higher than the δ56Fe value of dissolved Fe. Nevertheless, at Beebe, the δ56Fe value of total dissolvable Fe increased as the proportion of Fe predicted to have been lost from the plume increased, consistent with preferential fall-out of Fe-sulfides that are enriched in light Fe isotopes. The very low δ56Fe values of dissolved Fe in the Beebe buoyant plume are consistent with (i) the high Fe/H2S ratio of the vent fluids, and (ii) high Fe(II) oxidation rates, relative to other vent sites.

Last interglacial sea levels within the Gulf of Mexico and northwestern Caribbean Sea

Abstract. During the last interglacial (LIG) the volume of additional water in the world's oceans was large enough to raise global sea levels about 6–9 m higher than present levels. However, LIG sea levels vary regionally and those regional differences hold clues about the past distribution of ice sheets and local rates of subsidence and tectonic uplift. In this study, I used a standardized database template to review and summarize the existing constraints on LIG sea levels across the northern Gulf of Mexico and Caribbean shoreline of the Yucatán Peninsula. In total, I extracted 32 sea-level indicators including the insertion of 16 U-series ages on corals, 1 electron spin resonance age, 2 amino acid racemization ages, and 26 luminescence ages. Most dated sea-level indicators for the northern Gulf of Mexico are based on optically stimulated luminescence (OSL) ages of beach deposits of a mappable LIG shoreline. This shoreline extends from the Florida Panhandle through south Texas but is buried or removed by the Mississippi River across most of Louisiana. A similar feature is observed in satellite images south of the Rio Grande within the Mexican portions of the Gulf of Mexico but has yet to be dated. Elevations measured on portions of this feature close to the modern coast point to sea levels less than 1 m to ∼5 m higher than present for much of the northern Gulf of Mexico. However, a few, albeit undated, portions of the same shoreline located at more inland locations point to sea levels up to +7.2 m, attesting to up to 7 m of differential subsidence between the inland and coastal sites. Across the Yucatán Peninsula, U-series dating of corals has provided the main index points for LIG sea levels. Other carbonate coastal features such as beach ridges and eolianites have also been described but rely on corals for their dating. The maximum elevation of the LIG coral-based relative sea-level (RSL) estimates decrease from around +6 m across the Caribbean shoreline of the Yucatán Peninsula near Cancún, Mexico, to as low as −6 m to the south beneath the southern atolls of Belize, although discussion continues as to the validity of the ages for these southern corals. If these lower-elevation corals are LIG in age, their below-present elevations may be a result of vertical motion along faults dipping into the Cayman Trough. South of Belize only one purported LIG coral has been dated on the Isla de Roatán off the coast of Honduras at a likely tectonically uplifted elevation of 37.2 m. Thus the elevation of LIG sea levels within the inland siliciclastic shorelines of Guatemala and Honduras as well as the southwestern Gulf of Mexico remains poorly constrained and a potential venue for future research. The database described in this paper is available open access in spreadsheet format as Simms (2020), at this link: https://doi.org/10.5281/zenodo.4556163.

Geomorphology of the Puerto Rico Trench and Cayman Trough in the Context of the Geological Evolution of the Caribbean Sea

Geomorphology of the Puerto Rico Trench and Cayman Trough in the Context of the Geological Evolution of the Caribbean Sea

Calcite twinning fabrics along the Middle America trench, Costa Rica and the Motagua sinistral fault, Honduras and Jamaica: Tectonic implications for the Caribbean plate

The Cretaceous oceanic crust of the Caribbean plate has a long and complex tectonic history as an indentor plate bounded by transcurrent faults (N and S) and subduction margins (W [east slab dip] and E [west slab dip]) with no strain data. Miocene forearc sediments and accreted ophiolites are exposed along the central Pacific coast of Costa Rica and preserve a variety of folding and faulting styles. These clastic sediments, deposited in the forearc as the Middle America trench evolved, contain calcite cements and are cross cut by calcite veins. Analysis of the mechanical twins in these two calcite groups (8 samples, n = 267 twins) at Punta Dominical and the Nicoya Peninsula ophiolite preserve a consistent sub-horizontal shortening strain that is oriented parallel to the current trench axis, as are most of the accretion-related fold axes. Differential stress magnitudes for the calcite twinning strain average −450 bars, and there is little evidence of a twinning strain overprint (low NEVs). Obduction along the Middle America trench involves, in part, a component of trench-parallel horizontal shortening.The Motagua fault originates in the Middle America trench and strikes east as the northern boundary of the Caribbean plate as a sinistral margin. The Roatan Islands are a series of islands composed of south-dipping rocks along the southern side of the Motagua fault. Recent, deformed clastic sediments are drapped over gneisses, schists (40Ar-39Ar biotite age of 14 Ma), conodont-free marbles and ophiolitic fragments of presumed Cretaceous age. Twinning studies (n = 6 samples, n = 425 twins) of these marbles preserve a N–S, horizontal shortening strain, normal to the sinistral plate boundary. Differential stress magnitudes for the calcite twinning strain average −36 MPa. Only in a combined sample (n = 189 grains) is a strain overprint (high NEVs; n = 47) present, which records horizontal shortening strain at 132°, 5°. Jamaica is also along the eastern Motagua fault (Cayman trough) and twinning strain studies in Eocene limestones and younger calcite veins (n = 4 samples; n = 136 twins) preserve a horizontal shortening strain normal to the sinistral plate boundary.Twinning strain results are compared to regional GPS, focal mechanism and SKS anisotropy results around the Caribbean plate, especially along the Middle America trench on the western margin of Costa Rica with some new insights into subduction dynamics.

Complete genome sequence of Methanofervidicoccus sp. A16, a thermophilic methanogen isolated from Mid Cayman Rise hydrothermal vent.

Methanofervidicoccus sp. A16 is a novel thermophilic and obligate hydrogenotrophic methanogen isolated from a deep-sea hydrothermal vent chimney sample at the Mid Cayman spreading center, Caribbean Sea. Here we report the complete genome of strain A16, which has one circular chromosome of 1,485,358bp with a mean G+C content of 35.01mol%. The complete genome harbors 1442 predicted protein-encoding genes. Genes involved in hydrogenotrophic methane production and N2 fixation were identified in this genome. This study expands our knowledge of methanogenesis at high temperatures and the involvement of these microorganisms in the carbon and nitrogen cycles of deep-sea hydrothermal environments.

Tsunami hazard in the Caribbean coast of Honduras due to large earthquakes occurred along the Cayman Trough at the northwest boundary of Caribbean plate

ABSTRACT In 2018, a large earthquake (Mw7.6) occurred in the Swan Island Fault zone at the northwest boundary of the Caribbean plate. This earthquake generated a small tsunami of 20 cm. However, Puerto Cortes in Honduras is located close to the Swan Island Fault zone. Evaluation of tsunami hazard at Puerto Cortes due to large earthquakes along the fault zone is important. We first estimated the fault parameters of the 2018 Swan Island earthquake using W-phase inversion technique. Then, the moment magnitude of 7.6, the fault length of 134 km, the fault width of 24 km, and the slip amount of 5.1 m were estimated. In addition to those estimates, a small fault dimension of the earthquake, 40 km × 20 km, with a slip amount of 20.8 m was considered. Those two fault models were used to compute tsunami inundation at Puerto Cortes. The tsunami computed from the small fault inundated a large area in Puerto Cortes including the port area. The effect of co-seismic horizontal displacement of ocean floor also enhanced the tsunami inundation at Puerto Cortes. Those results indicate that preparation for future tsunami hazard in Puerto Cortes is important although no significant tsunami was generated historically.

On the systematics and ecology of two new species ofProvanna(Gastropoda: Provannidae) from deep-sea hydrothermal vents in the Caribbean Sea and Southern Ocean

ABSTRACTThe recent discovery and exploration of deep-sea hydrothermal vent fields in the Mid-Cayman Spreading Centre, Caribbean Sea (Beebe Vent Field, 4956–4972 m depth) and the East Scotia Ridge, Southern Ocean (E2 and E9 vent fields, 2394–2641 m depth) have yielded extensive collections of two new provannid species, Provanna beebei n. sp. and P. cooki n. sp. Morphological and molecular taxonomy (530 bp of the mitochondrial cytochrome c oxidase subunit I gene) confirm P. beebei n. sp. and P. cooki n. sp. as distinct species; these species are formally described, and details are provided of their distribution, habitat and species associations. Bayesian and maximum likelihood analyses support the placement of P. beebei n. sp. and P. cooki n. sp within the genus Provanna and show that these two new deep-sea species form a well-supported clade with the abyssal West Pacific P. cingulata. Provanna beebei n. sp. and P. cooki n. sp. represent the first records of Provanna from hydrothermal vents in the Caribbean Sea and Southern Ocean, respectively, and extend the known geographic range of the genus. For the first time, intraspecific phenotypic variation in size and sculpture has been reported for Provanna. At the East Scotia Ridge, shell-size frequency distributions and median shell size of P. cooki n. sp. varied significantly between the E2 and E9 vent fields, as well as between diffuse flow and high-temperature venting habitats within each field. The variation in shell sculpture in relation to habitat was also observed in P. cooki n. sp.

Constraining the maximum depth of brittle deformation at slow- and ultraslow-spreading ridges using microseismicity

AbstractThe depth of earthquakes along mid-ocean ridges is restricted by the relatively thin brittle lithosphere that overlies a hot, upwelling mantle. With decreasing spreading rate, earthquakes may occur deeper in the lithosphere, accommodating strain within a thicker brittle layer. New data from the ultraslow-spreading Mid-Cayman Spreading Center (MCSC) in the Caribbean Sea illustrate that earthquakes occur to 10 km depth below seafloor and, hence, occur deeper than along most other slow-spreading ridges. The MCSC spreads at 15 mm/yr full rate, while a similarly well-studied obliquely opening portion of the Southwest Indian Ridge (SWIR) spreads at an even slower rate of ∼8 mm/yr if the obliquity of spreading is considered. The SWIR has previously been proposed to have earthquakes occurring as deep as 32 km, but no shallower than 5 km. These characteristics have been attributed to the combined effect of stable deformation of serpentinized mantle and an extremely deep thermal boundary layer. In the context of our MCSC results, we reanalyze the SWIR data and find a maximum depth of seismicity of 17 km, consistent with compilations of spreading-rate dependence derived from slow- and ultraslow-spreading ridges. Together, the new MCSC data and SWIR reanalysis presented here support the hypothesis that depth-seismicity relationships at mid-ocean ridges are a function of their thermal-mechanical structure as reflected in their spreading rate.

Diversity, Distribution and Phylogeny of Hesionidae (Annelida) Colonizing Whale Falls: New Species of Sirsoe and Connections Between Ocean Basins

Whale falls are important environments contributing to biodiversity, connectivity and evolutionary novelty in deep-sea ecosystem. Notwithstanding, most of this knowledge is based in studies from NE Pacific basin. Interestingly, the only known natural whale fall on the SW Atlantic has faunal composition affinities with carcasses from other deep-ocean basins. In this carcass, annelid worms belonging to Hesionidae are abundant and species-rich, and include some shared species with NE Pacific Ocean. Here we evaluate the diversity of Hesionidae on the SW Atlantic using new information of implanted whale bones and explore whether some species have interbasin distribution or if they represent cryptic species in different basins. We described, using morphological and molecular data, a total of 10 new hesionid species and report of a new lineage Sirsoe ‘BioSuOr,’ not formally described herein. Two hesionids found exclusively in deep-sea chemosynthetic environments, SirsoePleijel (1998) and VrijenhoekiaPleijel et al. (2008), are primarily distinguished from each other by the presence of a median antenna on the former and its absence on the latter. However, our analyses showed that Vrijenhoekia should be synonymized with Sirsoe and for this reason we emended the diagnosis of Sirsoe. We also emphasized the presence of Sirsoe balaenophila comb. nov. and S. sirikos in SW Atlantic whale falls confirming their interbasin distribution. Moreover, COI and 16S rDNA data reveal that S. balaenophila comb. nov. also comprises cryptic species on the SW Atlantic (S. pirapuan sp. nov. and S. ypupiara sp. nov) and perhaps also in the Pacific Ocean (herein named as S. balaenophila lineage-2). The new species, S. maximiano, is shared between whale falls from SW Atlantic and vent sites from Mid-Cayman Spreading Center. Our data adds to the growing literature showing species are shared between deep ocean basins and among cognate deep-sea environments. Zoobank registration publication LSID - urn:lsid:zoobank.org:pub:7E891B1D-DCE4-45C8-83F4-8586D286B327.

262 Voyages Beneath the Sea: a global assessment of macro- and megafaunal biodiversity and research effort at deep-sea hydrothermal vents.

For over 40 years, hydrothermal vents and the communities that thrive on them have been a source of profound discovery for deep-sea ecologists. These ecosystems are found throughout the world on active plate margins as well as other geologically active features. In addition to their ecologic interest, hydrothermal vent fields are comprised of metallic ores, sparking a nascent industry that aims to mine these metal-rich deposits for their mineral wealth. Here, we provide the first systematic assessment of macrofaunal and megafaunal biodiversity at hydrothermal vents normalized against research effort. Cruise reports from scientific expeditions as well as other literature were used to characterize the extent of exploration, determine the relative biodiversity of different biogeographic provinces, identify knowledge gaps related to the distribution of research effort, and prioritize targets for additional sampling to establish biodiversity baselines ahead of potential commercial exploitation. The Northwest Pacific, Southwest Pacific, and Southern Ocean biogeographic provinces were identified as high biodiversity using rarefaction of family-level incidence data, whereas the North East Pacific Rise, Northern East Pacific, Mid-Atlantic Ridge, and Indian Ocean provinces had medium biodiversity, and the Mid-Cayman Spreading Center was identified as a province of relatively low biodiversity. A North/South divide in the extent of biological research and the targets of hydrothermal vent mining prospects was also identified. Finally, we provide an estimate of sampling completeness for each province to inform scientific and stewardship priorities.

Seismic investigation of an active ocean–continent transform margin: the interaction between the Swan Islands Fault Zone and the ultraslow-spreading Mid-Cayman Spreading Centre

SUMMARY The Swan Islands Transform Fault (SITF) marks the southern boundary of the Cayman Trough and the ocean–continent transition of the North American–Caribbean Plate boundary offshore Honduras. The CAYSEIS experiment acquired a 180-km-long seismic refraction and gravity profile across this transform margin, ∼70 km to the west of the Mid-Cayman Spreading Centre (MCSC). This profile shows the crustal structure across a transform fault system that juxtaposes Mesozoic-age continental crust to the south against the ∼10-Myr-old ultraslow spread oceanic crust to the north. Ocean-bottom seismographs were deployed along-profile, and inverse and forward traveltime modelling, supported by gravity analysis, reveals ∼23-km-thick continental crust that has been thinned over a distance of ∼70 km to ∼10 km-thick at the SITF, juxtaposed against ∼4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are not widely observed, the 7.0 km s−1 velocity contour is used to define the Moho along-profile. The apparent lack of reflections to the north of the SITF suggests that the Moho is more likely a transition zone between crust and mantle. Where the profile traverses bathymetric highs in the off-axis oceanic crust, higher P-wave velocity is observed at shallow crustal depths. S-wave arrival modelling also reveals elevated velocities at shallow depths, except for crust adjacent to the SITF that would have occupied the inside corner high of the ridge-transform intersection when on axis. We use a Vp/Vs ratio of 1.9 to mark where lithologies of the lower crust and uppermost mantle may be exhumed, and also to locate the upper-to-lower crustal transition, identify relict oceanic core complexes and regions of magmatically formed crust. An elevated Vp/Vs ratio suggests not only that serpentinized peridotite may be exposed at the seafloor in places, but also that seawater has been able to flow deep into the crust and upper mantle over 20–30-km-wide regions which may explain the lack of a distinct Moho. The SITF has higher velocities at shallower depths than observed in the oceanic crust to the north and, at the seabed, it is a relatively wide feature. However, the velocity–depth model subseabed suggests a fault zone no wider than ∼5–10 km, that is mirrored by a narrow seabed depression ∼7500 m deep. Gravity modelling shows that the SITF is also underlain, at >2 km subseabed, by a ∼20-km-wide region of density >3000 kg m−3 that may reflect a broad region of metamorphism. The residual mantle Bouguer anomaly across the survey region, when compared with the bathymetry, suggests that the transform may also have a component of left-lateral trans-tensional displacement that accounts for its apparently broad seabed appearance, and that the focus of magma supply may currently be displaced to the north of the MCSC segment centre. Our results suggest that Swan Islands margin development caused thinning of the adjacent continental crust, and that the adjacent oceanic crust formed in a cool ridge setting, either as a result of reduced mantle upwelling and/or due to fracture enhanced fluid flow.

Methanofervidicoccus abyssi gen. nov., sp. nov., a hydrogenotrophic methanogen, isolated from a hydrothermal vent chimney in the Mid-Cayman Spreading Center, the Caribbean Sea.

A novel hydrogenotrophic methanogen, strain HHBT, was isolated from a deep-sea hydrothermal vent chimney sample collected from Beebe Vent Field at the Mid-Cayman Spreading Center, Caribbean Sea. The cells were non-motile regular to irregular cocci possessing several flagella. The novel isolate grew at 60-80 °C, pH 5.0-7.4 and with 1-4 % of NaCl (w/v). The isolate utilized H2/CO2 as the only substrates for growth and methane production. The results of phylogenetic analyses of both 16S rRNA and mcrA gene sequences and comparative genome analysis indicated that HHBT represented a member of the order Methanococcales, and was closely related to the members of the genera Methanothermococcus and Methanotorris. The most closely related species were Methanothermococcus okinawensis IH1T and Methanotorris igneus Kol 5T in comparison of 16S rRNA gene sequences (each with 93 % identity), and Methanotorris formicicus Mc-S-70T in the case of deduced amino acid sequence similarity of mcrA genes (92 % similarity). The ANI and AAI values between HHBT and the members of the genera Methanothermococcus and Methanotorris were 69-72 % and 66-70 %, respectively. Although many of the morphological and physiological characteristics were quite similar between HHBT and the species of the genera Methanothermococcus and Methanotorris, they were distinguishable by the differences in susceptibility to antibiotics, formate utilization, growth temperature and NaCl ranges. On the basis of these phenotypic, phylogenetic and genomic properties, we propose that strain HHBT represents a novel species, of a novel genus, Methanofervidicoccus abyssi gen. nov., sp. nov. The type strain is HHBT (=JCM 32161T=DSM 105918T).

Volcanic-Tectonic Structure of the Mount Dent Oceanic Core Complex in the Ultraslow Mid-Cayman Spreading Center Determined From Detailed Seafloor Investigation.

The flanks of the ultraslow‐spreading Mid‐Cayman Spreading Center (MCSC) are characterized by domal massifs or oceanic core complexes (OCCs). The most prominent of these, Mount Dent, comprises lower‐crustal and upper‐mantle lithologies and hosts the Von Damm vent field ~12 km west of the axial deep. Here, presented autonomous underwater vehicle‐derived swath sonar (multibeam) mapping and deep‐towed side‐scan sonar imagery lead to our interpretation that: (i) slip along the OCC‐bounding detachment fault is ceasing, (ii) the termination zone, where detachment fault meets the hanging wall, is disintegrating, (iii) the domed surface of the OCC is cut by steep north‐south extensional faulting, and (iv) the breakaway zone is cut by outward facing faults. The Von Damm vent field and dispersed pockmarks on the OCC's south flank further suggest that hydrothermal fluid flow is pervasive within the faulted OCC. On the axial floor of the MCSC, bright acoustic backscatter and multibeam bathymetry reveal: (v) a volcanic detachment hanging wall, (vi) a major fault rifting the southern flank of Mount Dent, and (vii) a young axial volcanic ridge intersecting its northern flank. These observations are described by a conceptual model wherein detachment faulting and OCC exhumation are ceasing during an increase in magmatic intrusion, brittle deformation, and hydrothermal circulation within the OCC. Together, this high‐resolution view of the MCSC provides an instructive example of how OCCs, formed within an overall melt‐starved ultraslow spreading center, can undergo magmatism, hydrothermal activity, and faulting in much the same way as expected in magmatically more robust slow‐spreading centers elsewhere.

Regional transect across the western Caribbean Sea based on integration of geologic, seismic reflection, gravity, and magnetic data

We analyze western Caribbean structural styles and depositional controls associated with Late Cretaceous–Cenozoic deformational events using a 1600-km (994-mi)-long, regional, northwest–southeast transect extending from the Cayman Trough in Honduras to northern Colombia. Different structural provinces defined along the transect include (1) the Cayman Trough and adjacent Honduran borderlands marking the North American–Caribbean transtensional plate boundary characterized by late Eocene–Holocene fault-controlled depocenters; (2) the Nicaraguan Rise that includes continental Paleocene–Eocene rocks deposited in sag basins, which are overlain by relatively undeformed Miocene–Holocene carbonate and clastic shelf deposits of the northern Nicaraguan Rise, following a Late Cretaceous convergent phase; (3) the Colombian Basin that includes thick Miocene clastic depocenters and the localized presence of Upper Cretaceous rocks overlying the basement and where much of the subsidence is likely isostatic and flexurally driven given its proximity to the subduction zone of northern Colombia; (4) the south Caribbean deformed belt, an active, accretionary prism produced by the subduction of the Caribbean large igneous province beneath the South American plate, which has deformed the Cenozoic prism and fore-arc section and produced thrust-fault–controlled accommodation space for upper Miocene–Holocene piggyback deposits; and (5) the onshore Cesar–Rancheria Basin in northern Colombia, which has recorded the uplift of its bounding mountain ranges, the Sierra de Santa Marta massif to the west and Perija Range to the east. Plate reconstructions place the various crustal provinces along the transect into the context of the Late Cretaceous–Cenozoic deformation events that can be partitioned into strike-slip, convergent, and extensional components.