Individual Seamounts Research Articles

Relative Timing of Off‐Axis Volcanism From Sediment Thickness Estimates on the 8°20’N Seamount Chain, East Pacific Rise

AbstractVolcanic seamount chains on the flanks of mid‐ocean ridges record variability in magmatic processes associated with mantle melting over several millions of years. However, the relative timing of magmatism on individual seamounts along a chain can be difficult to estimate without in situ sampling and is further hampered by Ar40/Ar39 dating limitations. The 8°20’N seamount chain extends ∼170 km west from the fast‐spreading East Pacific Rise (EPR), north of and parallel to the western Siqueiros fracture zone. Here, we use multibeam bathymetric data to investigate relationships between abyssal hill formation and seamount volcanism, transform fault slip, and tectonic rotation. Near‐bottom compressed high‐intensity radiated pulse, bathymetric, and sidescan sonar data collected with the autonomous underwater vehicle Sentry are used to test the hypothesis that seamount volcanism is age‐progressive along the seamount chain. Although sediment on seamount flanks is likely to be reworked by gravitational mass‐wasting and current activity, bathymetric relief and Sentry vehicle heading analysis suggest that sedimentary accumulations on seamount summits are likely to be relatively pristine. Sediment thickness on the seamounts' summits does not increase linearly with nominal crustal age, as would be predicted if seamounts were constructed proximal to the EPR axis and then aged as the lithosphere cooled and subsided away from the ridge. The thickest sediments are found at the center of the chain, implying the most ancient volcanism there, rather than on seamounts furthest from the EPR. The nonlinear sediment thickness along the 8°20’N seamounts suggests that volcanism can persist off‐axis for several million years.

Intermittent Beginning to the Formation of Hydrogenous Ferromanganese Nodules in the Vast Field: Insights from Multi-Element Chemostratigraphy Using Microfocus X-ray Fluorescence

Vast ferromanganese nodule fields have been found on the deep-sea floor of all oceans worldwide. They have received attention because they potentially provide high-grade metal resources to develop future high- and green-technology. However, how these vast nodule fields were formed and developed owing to their widespread nature or tendency to be denser with an increasing number of nodules has not yet been established. In this study, the fine-scale inner structure of nodules of various sizes was analyzed on the basis of chemical mapping using microfocus X-ray fluorescence. We found that nodules distributed in the vast field around Minamitorishima (Marcus) Island have several types of innermost layers, which correspond to different chemostratigraphic layers of nodules that have been previously reported by us in this region. As nodules grow in order from the center to the outside, the different types in the innermost layer indicate a difference in the timing of the beginning of their growth. Moreover, because the differences in the chemical features of each layer reflect differences in the composition of the original deep-sea water, our results imply that the beginning of nodule formation occurred intermittently at each time of a water mass replacement due to new deep-sea currents flowing into this region. We recognized that the northern part of the study area was dominated by large nodules that started to grow in relatively earlier times, while the southern part tended to have many nodules that grew in relatively later times. Based on these observations, we hypothesize that the intermittent beginning of nodule formation is governed by the northward inflow of the deep-sea current that originated from the Lower Circumpolar Deep Water for an extended time to form the vast nodule field. Because patterns in the timing of nodule formation were different in the eastern and western regions, we thus further propose that the topographic framework, i.e., the arrangement of individual large seamounts and the cluster of small knolls and petit-spot volcanoes, strongly regulates the flow path of the deep-sea current, even if the position of the entire seamount changes owing to plate motion. The deep-sea current might supply some materials to be nuclei, resulting in the nodule formation at the beginning of the process.

Geomorphological and Spatial Characteristics of Underwater Volcanoes in the Easternmost Australian-Antarctic Ridge

Underwater volcanoes and their linear distribution on the flanks of mid-ocean ridges are common submarine topographic structures at intermediate- and fast-spreading systems, where sufficient melt supplies are often available. Such magma sources beneath the seafloor located within a few kilometers of the corresponding ridge-axis tend to concentrate toward the axis during the upwelling process and contribute to seafloor formation. As a result, seamounts on the flanks of the ridge axis are formed at a distance from the spreading axis and distributed asymmetrically about the axis. In this study, we examined three linearly aligned seamount chains on the flanks of the KR1 ridge, which is the easternmost and longest Australian-Antarctic Ridge (AAR) segment. The AAR is an intermediate-spreading rate system located between the Southeast Indian Ridge and Macquarie Triple Junction of the Australian-Antarctic-Pacific plates. By inspecting the high-resolution shipboard multi-beam bathymetric data newly acquired in the study area, we detected 20 individual seamounts. The volcanic lineament runs parallel to the spreading direction of the KR1 segment. The geomorphologic parameters of height, basal area, volume, and summit types of the identified seamounts were individually measured. We also investigated the spatial distribution of the seamounts along the KR1 segment, which exhibits large variations in axial morphology with depth along the ridge axis. Based on the geomorphology and spatial distribution, all the KR1 seamounts can be divided into two groups: the subset seamounts of volcanic chains distributed along the KR1 segment characterized by high elevation and large volume, and the small seamounts distributed mostly on the western KR1. The differences in the volumetric magnitude of volcanic eruptions on the seafloor and the distance from the given axis between these two groups indicate the presence of magma sources with different origins.

Monogenetic Near‐Island Seamounts in the Galápagos Archipelago

AbstractRarely have small seamounts on the flanks of hotspot derived ocean‐island volcanoes been the targets of sampling, due to sparse high‐resolution mapping near ocean islands. In the Galápagos Archipelago, for instance, sampling has primarily targeted the subaerial volcanic edifices, with only a few studies focusing on large‐volume submarine features. Sampling restricted to these large volcanic features may present a selection bias, potentially resulting in a skewed view of magmatic and source processes because mature magmatic systems support mixing and volcanic accretion that overprints early magmatic stages. We demonstrate how finer‐scale sampling of satellite seamounts surrounding the volcanic islands in the Galápagos can be used to lessen this bias and thus, better constrain the evolution of these volcanoes. Seamounts were targeted in the vicinity of Floreana and Fernandina Islands, and between Santiago and Santa Cruz. In all regions, individual seamounts are typically monogenetic, but each seamount field requires multigenerational magmatic episodes to account for their geochemical variability. This study demonstrates that in the southern and eastern regions the seamounts are characterized by greater geochemical variability than the islands they surround but all three regions have (Sr‐Nd‐He) isotopic signatures that resemble neighboring islands. Variations in seamount chemistry from alkalic to tholeiitic near Fernandina support the concept that islands along the center of the hotspot track undergo greater mean depths of melting, as predicted by plume theory. Patterns of geochemical and isotopic enrichment of seamounts within each region support fine‐scale mantle heterogeneities in the mantle plume sourcing the Galápagos hotspot.

Hafnium isotopic constraints on the origin of late Miocene to Pliocene seamount basalts from the South China Sea and its tectonic implications

Post-spreading intraplate volcanism has widely affected the South China Sea (SCS) region including Indochina, the northern margin of the SCS, and the SCS basin itself. In the SCS basin, several off- and on-fossil spreading center seamounts formed between 3.8 and 7.9 Ma. Based on previously published geochemical and Sr-Nd-Pb isotopic data, the intraplate volcanism is widely related to the Hainan mantle plume, whose existence has been evidenced by recent geophysical studies. To test this petrogenetic model, new Hf isotope data have been obtained from volcanic rocks from a suite of compositionally representative seamounts in the SCS basin. Compared to published Nd isotope ratios (0.512675–0.512965, 5 εNd units), 176Hf/177Hf ratios span a much larger range (0.282876–0.283097, 8 εHf units), even within individual seamounts (e.g., Zhangzhong seamount). These features, combined with previous studies, clearly confirm mantle heterogeneity beneath the SCS region. Similar to the trends in Sr-Nd-Pb isotope space, Nd-Hf isotope ratio show a relatively narrow, elongate mixing trend between a depleted Indian MORB-type mantle endmember and an enriched EMII-type mantle end member. We propose that this narrow trend is inconsistent with the origin of the enriched end member from a heterogeneous sub-continental lithospheric mantle (SCLM) and instead suggests a plume-related origin. As a conceptual model for the post-spreading tectonic scenario of the Hainan plume affecting the SCS region, we propose that a plume ascends to the bottom of the lithosphere beneath Hainan and its northern Leizhou peninsula at the northern margin of SCS, from where it migrates along sloping rheologic boundary layers to lithospheric faults under an extensional setting towards the central SCS where the magmas erupt at the young spreading centers.

Species replacement dominates megabenthos beta diversity in a remote seamount setting

Seamounts are proposed to be hotspots of deep-sea biodiversity, a pattern potentially arising from increased productivity in a heterogeneous landscape leading to either high species co-existence or species turnover (beta diversity). However, studies on individual seamounts remain rare, hindering our understanding of the underlying causes of local changes in beta diversity. Here, we investigated processes behind beta diversity using ROV video, coupled with oceanographic and quantitative terrain parameters, over a depth gradient in Annan Seamount, Equatorial Atlantic. By applying recently developed beta diversity analyses, we identified ecologically unique sites and distinguished between two beta diversity processes: species replacement and changes in species richness. The total beta diversity was high with an index of 0.92 out of 1 and was dominated by species replacement (68%). Species replacement was affected by depth-related variables, including temperature and water mass in addition to the aspect and local elevation of the seabed. In contrast, changes in species richness component were affected only by the water mass. Water mass, along with substrate also affected differences in species abundance. This study identified, for the first time on seamount megabenthos, the different beta diversity components and drivers, which can contribute towards understanding and protecting regional deep-sea biodiversity.

Staying well connected – Lithistid sponges on seamounts

Three species of lithistid sponges,Neoaulaxinia zingiberadix, Isabella mirabilisandNeoschrammeniella fulvodesmuswere collected from deep seamounts off New Caledonia to address questions about their population structure, gene flow and the relative contribution of sexual and asexual reproductive strategies to their populations. The sponges were tested by sequencing the ITS (internal transcribed spacer) and CO1 regions of their genomes. These rare and presumably ancient sponges have a distribution restricted to seamounts in the south-western Pacific. Deep seamounts represent geographically separated islands. Although the sponges could be expected to have sexual reproduction restricted to near neighbours due to low sexual dispersal opportunities via larvae, this study found surprisingly high levels of gene flow between the seamounts. Amongst the specimens ofN. zingiberadixtaken from two seamounts there was no population structure; CO1 resulted in identical genotypes. For the population structure withinN. fulvodesmus, as revealed by ITS, most of the variation was within each individual from the six seamounts on which it occurred and CO1 revealed no difference between individuals or seamounts. The third speciesI. mirabilisshowed four genotypes based on CO1, which were distributed across all the seamounts. Indirect measures of different species showed a range of reproductive strategies from asexual to sexual, but with much higher connection between seamounts than previously thought. Individual seamounts did not show a separate population structure as one might expect from ‘islands’. The conclusion must be that these sponges have mechanisms to attain greater dispersal than previously thought.

Erratum to: Petrology and tectonic significance of seamounts within transitional crust east of Orphan Knoll, offshore eastern Canada

The Early Cretaceous separation of Newfoundland from Iberia–Ireland is a classic example of a magma-poor continental margin with hyperextension and with widespread minor magmatism resulting in seamounts. This study defines the distribution of seamounts east of Orphan Knoll, and documents and interprets the geochemical character of the one recovered lava sample. Video imagery of lava outcrops, and the sample, were obtained by ROV from Orphan seamount, one of a linear series of small seamounts overlying transitional thinned continental crust on the seaward side of Orphan Knoll. New multibeam bathymetry and legacy seismic data show several seamounts that extend irregularly along the fault-bound NE margin of Orphan Knoll. Whole rock geochemistry shows the sample to be highly alkaline basanite or possibly tephrite. Diopside–hedenbergite, kaersutite and K-feldspar phenocrysts were analyzed by electron microprobe and scanning electron microscope, and alteration minerals including kaolinite were identified by X-ray diffraction. The highly alkaline character of the basanite is similar only to Early Cretaceous volcanic and sub-volcanic rocks erupted through thick continental crust of the Mesoproterozoic Grenville Orogeny. The location of the linear set of seamounts is related to margin-parallel faults on the seaward side of Orphan Knoll that provided a pathway for magma, although ENE-trending lineaments in individual seamounts or seamount groups suggest the influence of oceanic fracture zones. A lower gradient crest to Orphan seamount above 2,200 m suggests subaerial erosion, consistent with the presence of kaolinite as an alteration product and the absence of lava pillows at and above this depth.

Identifying Ecologically or Biologically Significant Areas (EBSA): A systematic method and its application to seamounts in the South Pacific Ocean

The Convention on Biological Diversity (CBD) has adopted a scheme of using scientific criteria for identifying ‘Ecologically or Biologically Significant Marine Areas’ (EBSAs) in need of protection in open-ocean and deep-sea habitats. To date, expert opinion collated during regional workshops has been the main method to identify regional EBSAs. In this paper, we propose a new method that could complement this process by adding more objective and transparent analyses. There are four main steps: 1) identify the area to be examined, 2) determine appropriate datasets and thresholds to use in the evaluation, 3) evaluate data for each area/habitat against a set of criteria, and 4) identify and assess candidate EBSAs. The method can be applied to any habitat, but offshore seamounts were chosen as a test habitat to develop and evaluate it. Several options for various combinations of criteria are presented, with one being proposed as the most appropriate to identify a tractable number of seamounts that satisfied the EBSA criteria and which could be combined into larger areas that represent meaningful ecological and practicable management units. This option selects seamounts that meet any one of the 5 “biological” criteria (i.e. unique/rare, diverse, productive, threatened species, critical habitat) and which contain environmental features that are vulnerable to human activities but not yet significantly impacted by them. This selection process resulted in 83 seamounts being identified from over 3000 evaluated in the South Pacific Ocean. The priority seamounts group into 10 areas, consisting of 5 clusters of seamounts, and 5 individual seamounts. The primary strength of the method is the adoption of a transparent, and logically sequential, selection process that is conceptually transferrable to other habitat types and regions beyond our model system. We contend that in a global EBSA context it can be a useful tool to assist deep-sea management.

A global assessment of seamount ecosystems knowledge using an ecosystem evaluation framework

Seamounts are ubiquitous habitats of the deep-sea, collectively forming an area as large as Europe. Their characteristics have led marine scientists to hypothesize a series of ‘seamount effects’ enhancing numbers of endemic species, rates of production and bentho-pelagic trophic transfers. These effects have profound implications for deep-sea management; collating the existing body of seamount knowledge to describe potential effects on individual seamounts is therefore of paramount importance. In the course of this study, relevant literature was searched for key geological, oceanographic and ecological seamount attributes, and assembled in a ‘Google Earth’ map and in an online database (the Seamount Ecosystem Evaluation Framework, SEEF, www.seamounteef.org), comprising 597 seamounts located in the Atlantic, Pacific, Southern and Mediterranean basins. Data collated were described both in terms of quality and quantity, and the status of past and present global seamount knowledge was assessed. In addition, we investigated to what extent the available information supports seamount functioning hypotheses. The analysis confirms that seamounts remain largely unexplored, with only 0.4–4% of the total seamount population directly sampled for scientific purposes. Some of the seamount hypotheses tested are better supported than others, for example, some seamounts may represent ‘oases’ of the abyssal plains and some may play a role in connecting benthic and pelagic communities. However, seamounts present heterogeneous geophysical settings, suggesting that not all seamounts affect the food webs and biogeochemical fluxes in the surrounding ocean in the same way. Therefore, SEEF constitutes a tool to identify features playing a key role in deep-sea ecosystems.

Seamount benthos in a cobalt‐rich crust region of the central Pacific: conservation challenges for future seabed mining

AbstractAimThe benthic fauna of seamounts typically includes organisms that are slow‐growing, long‐lived and sensitive to mechanical disturbance, making seamounts susceptible to anthropogenic impacts. Such impacts may arise from mining cobalt‐rich crusts, envisaged for seamounts in the central North Pacific; this scenario requires that environmental guidelines for mining operations on seamounts be developed. Here, we provide the biological information essential for effective conservation planning of deep‐sea features targeted for such mining.LocationCentral North Pacific, Hawaiian Seamount Chain.MethodsSpatial analysis of seamount benthos using a large biological dataset (> 600 taxa) obtained from 144 submersible dives (depth range: 113–1985 m) on 27 seamounts covering a distance of over 2200 km of ocean.ResultsBenthic assemblages of invertebrates are structurally different between seamounts located inside and outside a region with cobalt‐rich crusts. This spatial contrast results from variations in species composition and relative abundance of species, rather than differences in species richness, challenging historical notions of an impoverished cobalt‐rich crust fauna in the region. Seamount assemblages also have high species turnover with depth and distance at the scale of individual seamounts, but geographic separation was a poor predictor of ecological separation for the region at large.Main conclusionSeveral implications for the design of spatial management and conservation tools with respect to mining emerge: (1) conservation of seamounts outside the cobalt‐rich crust region is unlikely to capture the full range of ecological features found inside the region; (2) conservation areas need to encompass a broad bathymetric gradient; (3) ideally, mining blocks on individual seamounts should not exceed 2 km in length. Overall, the life history characteristics and morphological traits of the deep‐water invertebrate fauna typical of seamounts in the region imply that any recovery from mechanical impacts is likely to be very slow.

An ecosystem evaluation framework for global seamount conservation and management.

In the last twenty years, several global targets for protection of marine biodiversity have been adopted but have failed. The Convention on Biological Diversity (CBD) aims at preserving 10% of all the marine biomes by 2020. For achieving this goal, ecologically or biologically significant areas (EBSA) have to be identified in all biogeographic regions. However, the methodologies for identifying the best suitable areas are still to be agreed. Here, we propose a framework for applying the CBD criteria to locate potential ecologically or biologically significant seamount areas based on the best information currently available. The framework combines the likelihood of a seamount constituting an EBSA and its level of human impact and can be used at global, regional and local scales. This methodology allows the classification of individual seamounts into four major portfolio conservation categories which can help optimize management efforts toward the protection of the most suitable areas. The framework was tested against 1000 dummy seamounts and satisfactorily assigned seamounts to proper EBSA and threats categories. Additionally, the framework was applied to eight case study seamounts that were included in three out of four portfolio categories: areas highly likely to be identified as EBSA with high degree of threat; areas highly likely to be EBSA with low degree of threat; and areas with a low likelihood of being EBSA with high degree of threat. This framework will allow managers to identify seamount EBSAs and to prioritize their policies in terms of protecting undisturbed areas, disturbed areas for recovery of habitats and species, or both based on their management objectives. It also identifies seamount EBSAs and threats considering different ecological groups in both pelagic and benthic communities. Therefore, this framework may represent an important tool to mitigate seamount biodiversity loss and to achieve the 2020 CBD goals.

Deep-Sea Fish Distribution Varies between Seamounts: Results from a Seamount Complex off New Zealand

Fish species data from a complex of seamounts off New Zealand termed the “Graveyard Seamount Complex’ were analysed to investigate whether fish species composition varied between seamounts. Five seamount features were included in the study, with summit depths ranging from 748–891 m and elevation from 189–352 m. Measures of fish species dominance, rarity, richness, diversity, and similarity were examined. A number of factors were explored to explain variation in species composition, including latitude, water temperature, summit depth, depth at base, elevation, area, slope, and fishing effort. Depth at base and slope relationships were significant with shallow seamounts having high total species richness, and seamounts with a more gradual slope had high mean species richness. Species similarity was modelled and showed that the explanatory variables were driven primarily by summit depth, as well as by the intensity of fishing effort and elevation. The study showed that fish assemblages on seamounts can vary over very small spatial scales, in the order of several km. However, patterns of species similarity and abundance were inconsistent across the seamounts examined, and these results add to a growing literature suggesting that faunal communities on seamounts may be populated from a broad regional species pool, yet show considerable variation on individual seamounts.

Evolution of Macquarie Ridge Complex seamounts: Implications for volcanic and tectonic processes at the Australia–Pacific plate boundary south of New Zealand

The Macquarie Ridge Complex (MRC) coincides with the Australia–Pacific plate boundary south of New Zealand and is characterised by a rugged bathymetry that comprises numerous seamounts along its length. Tectonic plate reconstructions show that the Australia–Pacific plate boundary evolved from a spreading centre to a transpressional boundary from ca. 40 to 6Ma. However, limited sampling and high-resolution mapping of the region have meant that the composition and morphology of the MRC remain relatively unknown. Here we present results from multibeam mapping, photographic investigation and petrologic and geochemical analyses from 10 seamounts along the MRC. Dredge samples collected from the seamounts are alkaline to sub-alkaline basalts, containing 45.1–50.8wt.% SiO2, 3.5–12.0wt.% MgO and 1.9–4.9wt.% total alkalis. All samples are enriched mid-ocean ridge basalts that have K/Ti>0.15, which originated as low degree partial melts during the late stages of mid-ocean ridge volcanism. This oceanic crust has been sheared and accreted along the plate boundary since ca. 6Ma by cumulative transpressional relative plate motion, such that lavas from distinct magma chambers have been juxtaposed at individual seamounts along the ridge. MRC seamounts have formed as discrete elevations as a result of dip-slip and strike-slip faulting of the ridge axis. Three guyot-type seamounts have flat-top summit plateaux that were presumably formed by wave erosion via relative sea-level lowering due to uplift of the ridge. These seamounts have since subsided due to extension associated with the step-over fault geometry of the transform plate boundary along the McDougall and Macquarie ridge segments. While the seamounts are of volcanic origin, they are manifestations of tectonic and geomorphic processes. Seamounts distal from the ridge axis, however, are not subjected to plate boundary deformation and instead have morphologies formed through volcanic processes.

Deep ocean circulation and transport where the East Pacific Rise at 9–10°N meets the Lamont seamount chain

We report the first 3‐D numerical model study of abyssal ocean circulation and transport over the steep topography of the East Pacific Rise (EPR) and adjoining Lamont seamount chain in the eastern tropical Pacific. We begin by comparing results of hydrodynamical model calculations with observations of currents, hydrography, and SF6 tracer dispersion taken during Larval Dispersal on the Deep East Pacific Rise (LADDER) field expeditions in 2006–2007. Model results are then used to extend observations in time and space. Regional patterns are pronounced in their temporal variability at M2 tidal and subinertial periods. Mean velocities show ridge‐trapped current jets flowing poleward west and equatorward east of the ridge, with time‐varying magnitudes (weekly average maximum of ∼10 cm s−1) that make the jets important features with regard to ridge‐originating particle/larval transport. Isotherms bow upward over the ridge and plunge downward into seamount flanks below ridge crest depth. The passage (P1) between the EPR and the first Lamont seamount to the west is a choke point for northward flux at ridge crest depths and below. Weekly averaged velocities show times of anticyclonic flow around the Lamont seamount chain as a whole and anticyclonic flow around individual seamounts. Results show that during the LADDER tracer experiment SF6 reached P1 from the south in the western flank jet. A short‐lived change in regional flow direction, just at the time of SF6 arrival at P1, started the transport of SF6 to the west on a course south of the seamounts, as field observations suggest. Approximately 20 days later, a longer‐lasting shift in regional flow from west to SSE returned a small fraction of the tracer to the EPR ridge crest.

Assemblage structure, but not diversity or density, change with depth on a northeast Pacific seamount

AbstractAlthough depth‐related patterns in assemblage structure are documented in several deep‐sea systems, variation in diversity, assemblage structure, and abundance with depth on individual seamounts remains unexplored. Knowledge of alpha‐ and beta‐diversity on single seamounts is needed for any robust generalization about large‐scale biodiversity patterns on seamounts. Here, we explore bathymetric variation in benthic megafauna, based on ROV video transects, on Davidson seamount (1246–3656 m) in the Northeast Pacific Ocean. We found that substantial change in assemblage structure can occur over vertical scales on an individual seamount. Changes of 50% in assemblage composition (beta‐diversity, faunal turnover) were observed over as little as a ∼1500 m depth interval down the flanks of the seamount, although bathymetric clines in composition were not uniform across major taxa. Diversity and density exhibit no consistent bathymetric pattern and can vary greatly on a single isobath. Our findings suggest that ecological and evolutionary processes may vary considerably on a single seamount. As such, seamounts should be viewed as patchworks of habitats where high beta‐diversity may ultimately increase total biodiversity.

Seamount megabenthic assemblages fail to recover from trawling impacts

AbstractBecause the nature, tempo and trajectories of biological changes that follow the cessation of trawling are unknown for seamounts, it is unclear whether closing them to trawling will lead to a recovery of the fauna and, if so, over what time scales. This paper reports on a ‘test of recovery’ from repeated towed camera surveys on three seamounts off New Zealand in 2001 and 2006 (5 years apart) and three off Australia in 1997 and 2006 (10 years apart). In each region, seamounts where trawling had ceased were compared to adjacent seamounts where trawling was still active, and to seamounts that had never been trawled. If recovery signals existed, the likelihood of detecting them was high because the seamounts were relatively small and topographically simple, and because quantitative survey methods were employed. Multivariate patterns showed no change in the megafaunal assemblage consistent with recovery over a 5–10 year timeframe on seamounts where trawling had ceased. Results based on the number of species and diversity were equivocal, with some cases of increase and decrease on seamounts where trawling had ceased. A few individual taxa were found at significantly higher abundance in the later surveys where trawling had occurred. We suggest this may have resulted from their resistance to the direct impacts of trawling (two chrysogorgid corals and solitary scleractinians), or from protection in natural refuges inaccessible to trawls (unstalked crinoids, two chrysogorgid corals, gorgonians, and urchins). Alternatively, these taxa may represent the earliest stages of seamount recolonisation. They have potential to be dominant for long periods because the pre‐trawling composition of benthic assemblages on seamounts includes taxa that grow slowly and/or have an association with ‘thickets’ of a single keystone stony coral (Solenosmilia variabilis) that has generated biogenic habitat over millennia. Resilience of seamount ecosystems dominated by corals is low compared to most other marine systems subject to disturbance by bottom trawling because there are no alternative habitats of the same value for supporting associated species, and because trawling typically removes coral habitat from large areas of individual seamounts. Management to conserve seamount ecosystems needs to account for changing oceanographic conditions (ocean acidification), as well as the direct impacts of human activities such as bottom trawling. Networks of spatial closures that include intact habitats over a range of depths, especially <1500 m, and on clusters and isolated seamounts, may be effective by maintaining the resilience of seamount benthic communities.

Seamount Ecosystem Evaluation Framework (SEEF): A Tool for Global Seamount Research and Data Synthesis

The Seamount Ecosystem Evaluation Framework (SEEF) is an innovative multidisciplinary tool developed to standardize the parameters by which seamounts are characterized, to report the extent of knowledge about individual seamounts, and to identify and assess threats to them, and to address other issues. Initially developed by Pitcher and Bulman (2007) and Pitcher et al. (2007), SEEF identifies critical gaps in knowledge and may be used as a guide to develop future research plans for a specific seamount. By systematizing sets of seamount data, it can also promote and assist consistent seamount ecosystem modeling, meta-analysis, and, for management, development of ecosystem-based plans. SEEF is available to the scientific community and general public through the SEEF web site (http://www. seamounteef.org), and through the Seamounts Online initiative (http://seamounts.sdsc.edu; Stocks, 2009). using this interface, individuals can update the existing knowledge of a specific seamount, thus contributing to an improved framework. The SEEF version presented here incorporates revisions to the original schema made by a group of participants at the March 2009 Seamount Biogeosciences network workshop held at Scripps Institution of Oceanography (http://earthref.org/ events/SBn/2009/). SEEF incorporates three different parts. The first part (Part A in Figure 1) scores the extent of our knowledge about individual seamounts by listing their most important attributes, including geological, oceanographic, and ecological features that are found on seamounts that may contribute to their role in enhancing local biomass and biodiversity. The second part of SEEF (Part B in Figure 1) scores the relative severity of threats posed by human activities to the abundance and diversity of seamount resources, principally fishing and other extractive exploitation. Effectively, it identifies seamounts in different conservation states. Parts A and B are scored by experts in the field; scoring disagreements can lead to productive research and useful expressions of uncertainty. The left-hand side of Figure 1 lists attributes and issues. A full description is available on the SEEF web site. The third part of SEEF (C), currently under development, aims to quantify each attribute and threat using actual values, or categories of values, in a fuzzy logic system. In doing so, SEEF will determine the extent of local enhancements in the food web, biodiversity, or biomass, and at the same time will quantify the different threats posed to seamount ecosystems. Scores can be color coded, so that scanning or mapping a set of SEEFs immediately reveals the extent of our ecological ignorance as well as the location of threatened seamount areas. At the same time, it highlights gaps in understanding of general seamount ecosystems, promotes seamount data synthesis, and thus uses and contributes to existing online data sets such as Seamounts Online.

Deep-sea octocorals and antipatharians show no evidence of seamount-scale endemism in the NW Atlantic

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 397:25-35 (2009) - DOI: https://doi.org/10.3354/meps08318 Deep-sea octocorals and antipatharians show no evidence of seamount-scale endemism in the NW Atlantic Jana N. Thoma*, Eric Pante*, Mercer R. Brugler, Scott C. France** University of Louisiana at Lafayette, Department of Biology, PO Box 42451, Lafayette, Louisiana 70504, USA *Jana N. Thoma and Eric Pante have contributed equally to this project**Corresponding author. Email: france@louisiana.edu ABSTRACT: Seamounts are undersea mountains commonly characterized by accelerated currents, exposed hard-substrates, and relatively high biomass and biodiversity. Hydrographic features associated with seamounts have led authors to hypothesize that benthic invertebrate populations from geographically separated seamounts (and the continental slope) may experience varying degrees of genetic isolation, resulting in high levels of endemism. While this hypothesis has been tested for multiple taxonomic groups in the Pacific, it has rarely been addressed in the Atlantic. We tested the null hypothesis that the geographic ranges of corals from NW Atlantic seamounts are restricted to individual seamounts. We examined 188 octocoral and 50 antipatharian colonies (representing 6 and 2 genera, respectively) from 14 seamounts, spanning 1700 km, and the adjacent continental margin and estimated their genetic variation using mitochondrial loci (msh1 for all octocorals, as well as an intergenic region for isidids, and 3 multi-gene spanning segments for antipatharians). Well-sampled haplotypes were not geographically isolated on individual seamounts, thus refuting the hypothesis of local endemism of coral fauna on the New England and Corner Seamounts. The narrow geographic distribution of rare haplotypes is most likely due to undersampling rather than endemism. Our results do not preclude that cryptic variation and endemism not revealed by mitochondrial DNA may become evident should more variable markers be developed. KEY WORDS: Endemism · Biogeography · Marine connectivity · Dispersal · Chrysogorgiidae · Paramuricea · Black coral · Anthozoa Full text in pdf format Supplementary material PreviousNextCite this article as: Thoma JN, Pante E, Brugler MR, France SC (2009) Deep-sea octocorals and antipatharians show no evidence of seamount-scale endemism in the NW Atlantic. Mar Ecol Prog Ser 397:25-35. https://doi.org/10.3354/meps08318Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 397. Online publication date: December 17, 2009 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2009 Inter-Research.

Intraplate volcanism with complex age‐distance patterns: A case for small‐scale sublithospheric convection

Many volcano chains in the Pacific do not follow the most fundamental predictions of hot spot theory in terms of geographic age progressions. One possible explanation for non–hot spot intraplate volcanism is small‐scale sublithospheric convection (SSC), and we explore this concept using 3‐D numerical models that simulate melting with rheology laws that account for the effects of dehydration. SSC spontaneously self‐organizes beneath relatively mature oceanic lithosphere. Whenever this lithosphere is sufficiently young and thin, SSC replaces the shallow layer of harzburgite, which was formed by partial melting at the mid‐ocean ridge, with fresh peridotite. This mechanism enables magma generation without any preexisting thermochemical anomalies. However, the additional effect of melting‐induced dehydration to stiffen the harzburgite requires lower background viscosities to allow for vigorous SSC, overturn of the compositional stratification, and related magmatism. The intrinsic stiffness of the dehydrated harzburgite furthermore restricts penetration of SSC into very shallow and cooler levels. On the one hand, such a restriction precludes high degrees of melting, but on the other hand, it slows asthenospheric cooling and thus prolongs the duration of melting (to ∼25 Ma). Volcanism over such an elongated melting anomaly continues for at least 10–20 Ma and occurs on seafloor ages of ∼20 to ∼60 Ma. These seafloor ages increase with increasing mantle temperature due to the effect of forming a thicker harzburgite layer from more extensive mid‐ocean ridge melting. The long durations of volcanism predicted reconcile observations of extended activity of individual seamounts and synchronous activity over great distances along some volcanic chains. SSC thus gives an explanation for previously enigmatic volcano ages along the Line Islands and the Gilbert and Pukapuka ridges, as well as along the individual subchains of the Wakes, Marshalls, and Cook‐Australs.