Enhanced seamount location database for the western and central Pacific Ocean: Screening and cross-checking of 20 existing datasets

Seamounts are habitats of considerable interest in terms of conservation and biodiversity, and in terms of fisheries for bentho-pelagic and pelagic species. Twenty previously compiled datasets including seamount/underwater feature lists, bathymetric maps and emerged feature maps from different sources (ship-derived and satellite altimetry-derived) at different spatial scales (from individual cruise to worldwide satellite data) were gathered in order to compile an enhanced list of underwater features for parts of the western and central Pacific Ocean (WCPO). The KL04 dataset [Kitchingman, A., and Lai, S., 2004. Inferences on potential seamount locations from mid-resolution bathymetric data. Fisheries Centre Research Reports 12 (5), 7–12], listing seamount positions and depths as calculated from satellite altimetry-derived bathymetry, provided the baseline data for this study as it covered the entire region of interest and included summit depth information. All KL04 potential seamounts were cross-checked with other datasets to remove any atolls and islands that had been incorrectly classified as seamounts, to add seamounts undetected by KL04, to update the overall database (geolocation, depth, elevation, and name) and to compile a 12-class typology of the different types of underwater features. Of the 4626 potential seamounts identified in KL04, 719 were multiple identifications of the same large underwater features and 373 (10%) were actually emerged banks, atolls and islands, leaving 3534 actual underwater features. Conversely, 487 underwater features were documented in other datasets but not registered by KL04. The screening of all the potential WCPO seamounts produced a final list of 4021 underwater features with agreed upon position and information. This enhanced list should have many applications in oceanography, biodiversity conservation and studies of the influence of seamounts on pelagic ecosystems and fisheries.     

The physics of gas chimney and pockmark formation,with implications for assessment of seafloor hazards and gas sequestration

Pockmarks form where fluids discharge through seafloor sediments rapidly enough to make them quick, and are common where gas is present in near-seafloor sediments. This paper investigates how gas might lead to pockmark formation. The process is envisioned as follows: a capillary seal traps gas beneath a fine-grained sediment layer or layers, perhaps layers whose pores have been reduced in size by hydrate crystallization. Gas accumulates until its pressure is sufficient for gas to invade the seal. The seal then fails completely (a unique aspect of capillary seals), releasing a large fraction of the accumulated gas into an upward-propagating gas chimney, which displaces water like a piston as it rises. Near the seafloor the water flow causes the sediments to become “quick” (i.e., liquefied) in the sense that grain-to-grain contact is lost and the grains are suspended dynamically by the upward flow. The quickened sediment is removed by ocean-bottom currents, and a pockmark is formed. Equations that approximately describe this gas–piston–water-drive show that deformation of the sediments above the chimney and water flow fast enough to quicken the sediments begins when the gas chimney reaches half way from the base of its source gas pocket to the seafloor. For uniform near-surface sediment permeability, this is a buoyancy control, not a permeability control. The rate the gas chimney grows depends on sediment permeability and the ratio of the depth below seafloor of the top of the gas pocket to the thickness of the gas pocket at the time of seal failure. Plausible estimates of these parameters suggest gas chimneys at Blake Ridge could reach the seafloor in less than a decade or more than a century, depending mainly on the permeability of the deforming near-surface sediments. Since these become quick before gas is expelled, gas venting will not provide a useful warning of the seafloor instabilities that are related to pockmark formation. However, detecting gas chimney growth might be a useful risk predictor. Any area underlain by a gas chimney that extends half way or more to the surface should be avoided.     

Deep-sea mining of seafloor massive sulfides

The potential emergence of an ocean mining industry to exploit seafloor massive sulfides could present opportunities for oceanographic science to facilitate seafloor mineral development in ways that lessen environmental harms.     

An abyssal recipe

Fine- and microstructure observations indicate bottom-intensified turbulent dissipation above rough bathymetry associated with internal wave breaking. Simple analytic representations for the depth profile of turbulent dissipation are proposed here under the assumption that the near bottom wavefield is dominated by a baroclinic tide. This scheme is intended for use in numerical models and thus captures only the gross features of detailed solutions to the energy balance of the internal wavefield. The possible sensitivity of the magnitude and vertical variability of the dissipation rate profile to various environmental parameters is discussed. An expression for the diapycnal buoyancy flux is presented that explicitly treats the difference between the height of an isopycnal above the mean bottom and the actual bottom. This returns a diapycnal velocity estimate that is consistent with both tracer observations of downwelling and a basin scale mass budget that requires upwelling.     

Acoustic seabed segmentation from direct statistical clustering of entire multibeam sonar backscatter curves

A fast, simple method is presented to obtain acoustic seabed segmentation from multibeam sonar backscatter data, for situations where processed backscatter curves are already available. Unsupervised statistical clustering is used to classify multibeam sonar backscatter curves in their entirety, with the curves essentially treated as geometrical entities. High variability in the backscatter curves is removed by along-track averaging prior to clustering, and no further preprocessing is required. The statistical clustering method is demonstrated with RESON 8125 multibeam sonar data obtained in two bathymetrically complex environments. These are a sandwave field in Keppel Bay, Queensland, and an area of inter-island sand, reef, seagrass, and rhodolith beds in Esperance Bay, Western Australia. The resulting acoustic charts are visually compelling. They exhibit high spatial coherence, are largely artifact free, and provide spatial context to comparatively sparse grab samples with relatively little effort. Since the backscatter curve is an intrinsic property of the seafloor, the mappings form standalone charts of seafloor acoustic properties. In themselves they do not need ground truthing. Conceptually, use of the full angular backscatter curve should form the primary means of obtaining acoustic seabed segmentation. However, this is dependent on the scale and configuration of seabed backscatter features compared to the dimensions of the averaged swathe used to obtain reliable realisations of the backscatter curve.     

Fine-scale population structure in a deep-sea teleost (orange roughy, Hoplostethus atlanticus)

Microsatellite and otolith chemistry variability were analysed to assess fine scale genetic structure in the deep-sea teleost orange roughy (Hoplostethus atlanticus). The Porcupine Bank located on the continental shelf west of Ireland, comprises a complex system of mounds and flat areas that are broken up by canyons. Orange roughy form spawning aggregations on mounds and flat areas, and were heavily fished until the resource was depleted. By analysing adults in spawning condition and juvenile orange roughy from six mounds and one flat area, shallow but significant genetic population structure was evident (F_ST=0.0031, D_est across loci=0.0306 and G-test). Most of the structure was accounted for by inclusion of a sample from the flats (six of ten significant pairwise F_ST estimates and G-tests, and five of the highest D_est estimates included the flat sample). While the flat sample contributed most to the genetic structure, there was still significant (albeit weaker) structure among mound samples. The observed structure was supported by otolith analyses. Fish caught as late juveniles in either the flat or mound areas showed consistent differences in chemistry at the otolith core throughout the initial 10 years of growth, which could indicate site fidelity. We hypothesise that seafloor topographic structures (mounds and flats) may provide discrete spawning areas for orange roughy and that the limited gene flow between these spawning areas is insufficient to counteract genetic drift.     

Accuracy assessment of GPS/Acoustic positioning using a Seafloor Acoustic Transponder System

The accuracy of GPS/Acoustic positioning is crucial for monitoring seafloor crustal deformation. However, the slant range residual is currently the only indicator used to evaluate the precision of positioning seafloor transponders. This study employs a unique Seafloor Acoustic Transponder System (SATS) to evaluate the accuracy of GPS/Acoustic seafloor positioning. The SATS has three transponders and an attitude sensor in a single unit, which provides true lengths of transponder baselines and true attitude of the SATS to ensure assessment reliability and validity. The proposed approach was tested through a GPS/Acoustic experiment, in which an off-the-shelf acoustic system was used to collect range measurements. Using GPS/Acoustic geodetic observations, the positions of three transponders on the SATS were estimated by an optimization technique combined with ray-tracing calculations. The accuracy of the GPS/Acoustic seafloor positioning is assessed by comparing the true baselines and attitude with the results derived from the position estimates of the three transponders. A sensitivity analysis is conducted to investigate the robustness of the GPS/Acoustic positioning results to changes of sound speed. Experimental results demonstrate that the use of the SATS can help to assess the validity of the GPS and acoustic travel time measurements in the GPS/Acoustic seafloor positioning.     

北阿拉伯海的光纤海洋观测网络:成功、挑战和机遇

2005年夏,一个先进的海洋观测网络——包括实时的光纤海洋观测系统和内部存储的自动化观测系统——被投放在了阿曼海和北阿拉伯海并运行至今.在2010年初,其中的自动化观测系统被升级到了新的深水光纤观测系统.这个海洋观测网络是在阿曼农业和渔业部的资助下,由美国的Lighthouse R&D公司设计、开发、安装和维护的.这2个观测系统作为一个整体已经连续工作了7年多的时间.所采集数据包括海流、温度、盐度、压力、溶解氧和浊度等.该海区是一个多水团的汇合区,波斯湾的高盐水和阿拉伯海的低盐水在这里汇合并蔓延南下到印度洋.对采集的数据研究表明,这一观测网络对研究该区域的物理和生物过程具有重要价值.在此,将系统介绍整个观测网络,并简要阐述已经完成和接近完成的4个研究主题:①对阿拉伯海有记载以来最强热带气旋“古怒”的海洋响应的研究;②阿曼海北部的季节性缺氧现象的季节及年际变化和成因分析;③深海声散射层的时空演变;④阿曼海和北阿拉伯海的高温高盐现象的成因.该观测网络采集的长期、连续的时间序列对这一地区的海洋动力研究、水文的季节性变化,以及气候的长期变化等研究都有很大帮助.此外,如果观测网络可以完成25年的设定观测目标,这将对验证和改进海洋环流模式和海气耦合模式具有重要意义.     

In situ hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexico

Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct–Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C₁ through C₅) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg^− 1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH₄. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH₄ concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.