Deep Ocean Environments Research Articles

Sound propagation in strongly range‐dependent deep ocean environments with application to the Philippine Sea.

Many mid‐latitude deep ocean environments are characterized by strongly range‐dependent sound speed structure associated with mesoscale variability, including internal tides. Often much of this variability is describable as a combination of barotropic and lowest baroclinic mode variability. There are reasons to anticipate that ocean structure of this type leads only to a relatively benign range‐dependent modulation of underwater sound fields. These ideas will be explained and tested using ECCO2 state estimates in the Philippine Sea. [Work supported by the ONR.]

Biostratigraphy of a Paleocene–Eocene Foreland Basin boundary in southern Tibet

This study of the Paleocene–Eocene boundary within a foreland basin of southern Tibet, which was dominated by a carbonate ramp depositional environment, documents more complex environmental conditions than can be derived from studies of the deep oceanic environment. Extinction rates for larger foraminiferal species in the Zongpu-1 Section apply to up to 46% of the larger foraminiferal taxa. The extinction rate in southern Tibet is similar to rates elsewhere in the world, but it shows that the Paleocene fauna disappeared stepwise through the Late Paleocene, with Eocene taxa appearing abruptly above the boundary. A foraminifera turnover was identified between Members 3 and 4 of the Zongpu Formation—from the Miscellanea– Daviesina assemblage to an Orbitolites– Alveolina assemblage. The Paleocene and Eocene boundary is between the SBZ 4 and SBZ 5, where it is marked by the extinction of Miscellanea miscella and the first appearance of Alveolina ellipsodalis and a large number of Orbitolites. Chemostratigraphically, the δ 13C values from both the Zongpu-1 and Zongpu-2 Sections show three negative excursions in the transitional strata, one in Late Paleocene, one at the boundary, and one in the early Eocene. The second negative excursion of δ 13C, which is located at the P–E boundary, coincides with larger foraminifera overturn. These faunal changes and the observed δ 13C negative excursions provide new evidence on environmental changes across the Paleocene–Eocene boundary in Tibet.

A long-term in situ calibration system for chemistry analysis of seawater

An in situ calibration system is a versatile exploration instrument for electrochemical sensors investigating the biochemical properties of the marine environment. The purpose of this paper is to describe the design of an auto-calibrating system for electrochemical (pH) sensors, which permits two-point in situ calibration, suitable for long-term measurement in deep sea aqueous environments. Holding multiple sensors, the instrument is designed to perform long-term measurements and in situ calibrations at abyssal depth (up to 4000 m). The instrument is composed of a compact fluid control system which is pressure-equilibrated and designed for deep-sea operation. In situ calibration capability plays a key role in the quality and reproducibility of the data. This paper focuses on methods for extending the lifetime of the instrument, considering the fluidics design, mechanical design, and low-power consumption of the electronics controller. The instrument can last 46 d under normal operating conditions, fulfilling the need for long-term operation. Data concerning pH measured during the KNOX18RR cruise (Mid-Atlantic Ridge, July-August, 2008) illustrate the desirable properties of the instrument. Combined with different electrodes (pH, H2, H2S, etc.), it should be of great utility for the study of deep ocean environments, including water column and diffuse-flow hydrothermal fluids.

Methanotrophs and copper

Methanotrophs, cells that consume methane (CH(4)) as their sole source of carbon and energy, play key roles in the global carbon cycle, including controlling anthropogenic and natural emissions of CH(4), the second-most important greenhouse gas after carbon dioxide. These cells have also been widely used for bioremediation of chlorinated solvents, and help sustain diverse microbial communities as well as higher organisms through the conversion of CH(4) to complex organic compounds (e.g. in deep ocean and subterranean environments with substantial CH(4) fluxes). It has been well-known for over 30 years that copper (Cu) plays a key role in the physiology and activity of methanotrophs, but it is only recently that we have begun to understand how these cells collect Cu, the role Cu plays in CH(4) oxidation by the particulate CH(4) monooxygenase, the effect of Cu on the proteome, and how Cu affects the ability of methanotrophs to oxidize different substrates. Here we summarize the current state of knowledge of the phylogeny, environmental distribution, and potential applications of methanotrophs for regional and global issues, as well as the role of Cu in regulating gene expression and proteome in these cells, its effects on enzymatic and whole-cell activity, and the novel Cu uptake system used by methanotrophs.

Space–time and hybrid algorithms for the passive acoustic localisation of sperm whales and vessels

In the frame of the European Sea-floor Observatory Network (ESONET) and in preparation of the Demonstration Mission Listening to the Deep-Ocean Environment (LIDO) algorithms for the automated real-time detection, classification and localisation of cetaceans have been developed. Such Passive Acoustic Monitoring techniques have the potential to play a key role in cetaceans’ conservation for they allow a non-invasive study of their behaviour, a better knowledge of their population dynamics, and a better understanding of their dynamic relationship with their environment. This paper investigates the development of efficient and accurate techniques to be used as the basis of a localisation module for an automated real-time Passive Acoustic Monitoring system. An opportunity to assess the capabilities of the developed localisation modules was given by recordings collected with a bottom-mounted (around 2080 m depth) tetrahedral compact hydrophone array located offshore the port of Catania (Sicily) during the NEMO-ONDE campaign in the years 2005–2006 by INFN and CIBRA [2]. A well-known class of methods for acoustic source localisation is based on time differences of arrival (TDOA). Its capabilities have shown to be useful even in adverse situations (i.e., few sensors, high noise levels and/or poor calibration). A second class of methods, the space–time methods, originated in underwater applications such as sonar but reached its most significant achievements over the last 20 years in digital communications with recent progress in the treatment of broadband signals. These developments are here revisited under the scope of the localisation and tracking of cetacean vocalisations. Various broadband space–time methods were implemented and allowed to map the sound radiated during the detected clicks and to consequently localise both sperm whales and vessels. Hybrid methods were also developed which improved the robustness of space–time methods to noise and reverberation and reduced processing time. In most cases, the small variance obtained for these estimates lessened the necessity of additional statistical clustering. Even though not independently confirmed by sightings, the tracks derived in the proposed frame can be considered to be consistent with the known movements of sperm whales and vessels.

Underwater acoustic beam dynamics

Ray- and mode-based theoretical predictions of the spreads of directionally narrow beams are presented and compared to parabolic-equation-based simulations in deep-ocean environments. Both the spatial and temporal spreads of beams are considered. The environments considered consist of a range-independent deep-ocean background sound channel on which a highly structured sound-speed perturbation, associated with either internal waves or homogeneous isotropic single-scale turbulence, is superimposed. The simulation results are shown to be in good agreement with simple theoretical expressions which predict that beam spreading, in both the unperturbed and perturbed environments, is largely controlled by a property of the background sound channel-the ray-based stability parameter alpha or the asymptotically equivalent mode-based waveguide invariant beta. These results are consistent with earlier results showing that wavefield structure and stability are largely controlled by alpha (or beta).

Apatite (U–Th–Sm)/He thermochronology of rapidly cooled samples: The effect of He implantation

Apatite (U–Th–Sm)/He (AHe) thermochronology is increasingly used for reconstructing geodynamic processes of the upper crust and the surface. Results of AHe thermochronology, however, are often in conflict with apatite fission track (AFT) thermochronology, yielding an inverted age-relationship with AHe dates older than AFT dates of the same samples. This effect is mainly explained by radiation damage of apatite, either impeding He diffusion or causing non-thermal annealing of fission tracks. So far, systematic age inversions have only been described for old and slowly cooled terranes, whereas for young and rapidly cooled samples ‘too old’ AHe dates are usually explained by the presence of undetected U and/or Th-rich micro-inclusions. We report apatite (U–Th–Sm)/He results for rapidly cooled volcanogenic samples deposited in a deep ocean environment with a relatively simple post-depositional thermal history. Robust age constraints are provided independently through sample biostratigraphy. All studied apatites have low U contents (< 5 ppm on average). While AFT dates are largely in agreement with deposition ages, most AHe dates are too old. For leg 43, where deposition age of sampled sediment is 26.5–29.5 Ma, alpha-corrected average AHe dates are up to 45 Ma, indicating overestimations of AHe dates up to 50%. This is explained by He implantation from surrounding host U–Th rich sedimentary components and it is shown that AHe dates can be “corrected” by mechanically abrading the outer part of grains. We recommend that particularly for low U–Th-apatites the possibility of He implantation should be carefully checked before considering the degree to which the alpha-ejection correction should be applied.

Coherence in long‐range deep ocean propagation.

Acoustic wavefield structure, stability, and coherence properties in deep ocean environments will be discussed. Properties of transient wavefields will be emphasized. Observations, simulations, and some theoretical considerations will be presented. Some important experimental milestones, beginning approximately in 1979, will be noted. It will be argued that the last 30 years have been characterized by a number of pleasant surprises relating to the stability and coherence of transient wavefields in deep ocean environments. [Work supported by ONR.]

Passive detection in deep water using the reliable acoustic path.

The detection of quiet targets in deep water environments is of increasing interest for Navy applications, and detection performance depends critically on efficiently exploiting the frequency-dependent signal and noise structure. Previous measurements in the deep ocean environment of the northeast pacific has demonstrated the significant depth and frequency dependence of the noise, which can also vary with local wind conditions [Gaul et al., IEEE JOE (2007)]. The signal structure depends both on the spectrum of the target itself, but also on the propagation characteristics of the channel. In the concept of a reliable acoustic path (RAP), a receiver placed at or below the critical depth can detect shallow sources out to moderate ranges independent of near-surface water conditions or bottom interaction (hence, the identification as a “reliable” path). In this presentation, we present analysis of noise and signal structure for deep water channels from historic data, propagation modeling, and more recent experimentation. This structure is then used to compute signal excess to quantify detection performance as a function of receiver array placement and topology. Implications of the results on future Navy systems is discussed.

Ray-based description of normal modes in a deep ocean acoustic waveguide

Modal structure of the wave field in a deep ocean environment with sound speed fluctuations induced by random internal waves is considered. An approximate analytical description of the modal structure at megameter ranges is derived by combining two known results: (i) relations expressing mode amplitudes through parameters of ray paths and (ii) stochastic ray theory. For a monochromatic wave field, a simple analytical estimate has been obtained for a coarse-grained distribution of acoustic energy between normal modes. Significant attention has been paid to the investigation of the mode pulses, that is, sound pulses carried by individual modes. Analytical estimates for the spread of mode pulse and bias of its mean travel time in the presence of internal waves are derived.

Low mode‐number scattering of transient signals in deep ocean.

A modal description of the propagation of transient signals in deep ocean environments is considered. The environment is assumed to consist of a range‐independent background sound channel with a single sound speed minimum on which a structured range‐dependent perturbation, due, for example, to internal waves, is superimposed. Mode coupling in such environments is predominantly local in mode number, leading, to an excellent approximation, to the diffusive spreading of energy in mode number with increasing range. Because mode number is non‐negative, however, the simple diffusive spreading of energy in mode number requires modification for low mode numbers (corresponding to the near axial sound energy). Simulations of distributions of energy in (low) mode number and time are presented and compared to estimates based on the theoretical work of Virovlyansky et al. [J. Acoust. Soc. Am. 121, 2542–2552 (2007)]. The results presented are relevant to the analysis of measurements made during the recent long‐range acoustic propagation experiment. [Work supported by ONR.]

Resonant forward scattering of sound in deep ocean environments.

Resonant forward scattering of sound in deep ocean environments occurs when a ray cycle (double loop) distance is equal to the wavelength of a spectral component of the sound speed perturbation field or, more generally, when these quantities are rationally related. In the presence of a sound speed perturbation field with a broad spectrum (e.g., an internal‐wave‐induced perturbation field), many resonances are excited and these can be expected to overlap everywhere. In spite of this complexity, there are theoretical reasons that lead to the expectation that forward scattering, in general, and mode coupling, in particular, are largely controlled by resonant scattering. Relevant theory will be briefly described and numerical simulations (ray, mode, and PE) will be presented to illustrate important concepts. [Work supported by ONR.]

Software-driven sensor networks for short-range shallow water applications

Most existing underwater networks target deep and long range oceanic environments, which has led to the design of power hungry and expensive underwater communication hardware. Because of prohibitive monetary and energy cost of currently over-engineered communication hardware, dense deployments of shallow water sensor networks remain an elusive goal. To enable dense shallow water networks, we propose a network architecture that builds on the success of terrestrial sensor motes and that relies on the coupling of software modems and widely available speakers and microphones in sensor motes to establish acoustic communication links. In this paper, we analytically and empirically explore the potential of this acoustic communication system for the underwater environment. Our experimental approach first profiles the hardware in water after waterproofing the components with elastic membranes. The medium profiling results expose the favorable frequencies of operation for the hardware, enabling us to design a software FSK modem. Subsequently, our experiments evaluate the data transfer capability of the underwater channel with 8-frequency FSK software modems. The experiments within a 17 × 8 m controlled underwater environment yield an error-free channel capacity of 24 bps, and they also demonstrate that the system supports date rates between 6 and 48 bps with adaptive fidelity.

Multiple forward scattering through an ocean waveguide with 3D random inhomogeneities

Analytic expressions have been derived for the mean and spatial covariance of the acoustic field multiply forward scattered though a stratified ocean waveguide containing 3D random surface or volume inhomogeneities [Ratilal and Makris, J. Acoust. Soc. Am. 118, 3532-3559 (2005)]. These expressions are further used to derive the temporal coherence of an acoustic signal propagated through 3D random inhomogeneities. Field moments are given in terms of moments of the scatter function density of the 3D random inhomogeneities, which enables straightforward application to a broad range of 3D scatterers. Here we give examples of the attenuation, dispersion and loss of temporal coherence expected after multiple forward scattering through (1) random internal waves in both continental shelf and deep ocean environments, (2) fish schools, and (3) random wind-generated bubbles in continental shelf and surf-zone area. We show that 3D scattering effects become important when the Fresnel width exceeds the cross-range coherence scale of the inhomogeneities, and can lead to substantial power loss.

Rock magnetic and geochemical analyses of surface sediment characteristics in deep ocean environments: A case study across the Ryukyu Trench

Abstract Magnetic minerals in marine sediments are often dissolved or formed with burial depth, thereby masking the primary natural remanent magnetization and paleoclimate signals. In order to clarify the present sedimentary environment and the progressive changes with burial depth in the magnetic properties, we studied seven cores collected from the Ryukyu Trench, southwest Japan. Magnetic properties, organic geochemistry, and interstitial water chemistry of seven cores are described. Bottom water conditions at the landward slope, trench floor, and seaward slope are relatively suboxic, anoxic, and oxic, respectively. The grain size of the sediments become gradually finer with the distance from Okinawa Island and finer with increasing water depth. The magnetic carriers in the sediments are predominantly magnetite and maghemized magnetite, with minor amounts of hematite. In the topmost sediments from the landward slope, magnetic minerals are diluted by terrigenous materials and microfossils. The downcore variations in magnetic properties and geochemical data provided evidence for the dissolution of fine-grained magnetite with burial depth under an anoxic condition.

Modal group time spreads in weakly range-dependent deep ocean environments

The temporal spread of modal group arrivals in weakly range-dependent deep ocean environments is considered. It is assumed that the range dependence is sufficiently weak that mode coupling is predominantly local in mode number. The phrase "modal group arrival" is taken here to mean the contribution to a transient wave field corresponding to a fixed mode number. There are three contributions to modal group time spreads which combine approximately in quadrature. These are the reciprocal bandwidth (the minimal pulse width), a deterministic dispersive contribution that is proportional to bandwidth and grows like range r, and a scattering-induced contribution that grows approximately like r(32). The latter two contributions are shown to be proportional to the waveguide invariant beta, a property of the background sound speed profile. The results presented, based mostly on asymptotic theory, are shown to agree well with full-wave numerical wave field simulations and available exact mode theoretical results. Simulations are shown that correspond approximately to conditions during the LOAPEX acoustic propagation experiment.

Identification of natural sunken wood samples

Sunken woods are abundant in deep oceanic environments, housing a huge faunal diversity. Studies on that substrate firstly focused on the associated organisms, but since a few years, identification of wood is a further aim. The purpose is to appreciate its degradation state, diversity, geographical origin and to identify specific associations between colonizing organisms and substrates. The first determinations were made on sunken woods from Taiwan/Philippines, the Vanuatu Archipelago, and the Mediterranean Sea. Samples’ identification was based on histological studies. Different preparation techniques were used, depending on their degradation state. Detailed anatomy descriptions were made and compared to the native flora and the introduced species. Wood samples were well preserved. Diversified species were encountered, seemingly originating from local floras. In situ settlements of known wood species will enhance the knowledge of degradation and colonization degrees.

Messinian palaeoenvironments and hydrology in Sicily (Italy): The dinoflagellate cyst record

Hydrological conditions prevailing before, during and after the Messinian salinity crisis in Sicily have been approached using dinoflagellate cyst records. The synthetic sequence considered is based on five classical sections from the Caltanissetta Basin. Our interpretations are based on the recognition of autochthonous, allochthonous and reworked population among the dinocyst assemblages. For the first time, sea-surface temperatures and seasonal salinity contrasts were tentatively reconstructed using a “Mutual Climatic Range Method”. Sicilian late Tortonian deposits correspond to marine environment with significant terrestrial inputs favourable to eutrophic dinocyst species. Immediately after the beginning of the Messinian Stage, euryhaline assemblages took place, followed by meso-hyperhaline taxa, within a general trend to shallowing. At the end of the Tripoli diatomitic Formation, environment appears confined, with regular oceanic inflows. Such marine inflows remain persistent during the deposition of the salt Member, witnessing the probable persistence of nearby normal marine sea-surface water conditions in the Mediterranean Sea but with possible reduced hydrological circulation and/or low nutrient component. Such inflows are slightly decreasing up to the top of the Sicilian Upper Evaporites. At the same time, river inputs appear weak during the salt deposition, as the consequence of a rather dry climatic context. As shown by reworking activity, terrestrial inputs increase progressively from the base of the Upper Evaporites. During the Lago Mare period, while climate remains rather dry (absence of freshwater algae inputs, very low amount in trees requiring humid conditions), local deposition environment is confined with mesohaline to hypohaline sea-surface waters, in a context with relatively high seasonal sea-surface salinity contrast (up to 6‰). The very high reworking observed in the Arenazzolo silts, in addition to slightly increasing water depth, led us to consider the presence of a discontinuity between the Lago Mare and the Arenazzolo Fms. We consider the Arenazzolo Formation as a transgressive facies following a deep downcutting period. The Arenazzolo Formation presents a two-step development. The first one, correlative with the presence of a G. etrusca (a species with Paratethyan affinities), corresponds to a relative high-stand sea-level with oceanic influxes, low seasonal sea-surface salinity contrast and probable more humid context, as revealed by the important freshwater algal inputs. During the second step, salinity becomes much more variable, with a clear increase of seasonal sea-surface salinity contrast, a possible slight mean sea-surface temperature increase (only few degrees) and a clear weakening of the river inputs. Taking the proposed Zanclean position of the Arenazzolo Fm. into consideration, we state that mean sea-surface temperature did not change significantly from the base of the Messinian to the earliest Zanclean. At 5.33 Ma, the suddenly achieved flooding restored a fair, deep oceanic environment characterized, at the beginning, by a clear mean sea-surface temperature cooling (up to 6–7 °C) and a nutrient depletion, associated with the basins starvation. Sea-surface salinities were normal, with very low seasonal contrast. Hydrodynamics then nutrient supply became then quite normal from c.a. 5.08 Ma. The status of the Sicilian Caltanissetta Basin as a marginal basin although fastly deepening and the stratigraphical location of the Messinian discontinuity at the base of the Arenazzolo is the scenario that best matches our dinocyst record.

Mode processing of transient signals using a deficient vertical receiving array, with application to LOAPEX measurements

Mode processing of transient signals in a deep ocean environment using a deficient vertical receiving array is considered. Here, ‘‘deficient’’ implies that the vertical array is too sparse to resolve modes and/or has gaps. It is well known that mode processing of cw signals using a deficient array leads to significant modal cross‐talk, i.e., nonzero projections of mode n energy onto mode m, m≠n. It is shown, using simulated wave fields, that when phase‐coherent processing is employed, transient signals are less sensitive to modal cross‐talk than their cw counterparts. An explanation of this behavior is provided. Mode processing of a subset of the measurements made during the LOAPEX experiment at approximately 1 Mm range with f0=75 Hz, Δf=35 Hz is shown to give robust estimates of modal group time spreads for mode numbers 0≤m≤70. [Work supported by ONR.]

Modal group time spreads in weakly range-dependent deep ocean

Modal group time spreads in deep ocean environments are investigated. The environments considered consist of a range-independent background on which a range-dependent perturbation, due for example to internal waves, is superimposed. Modal group time spreads are defined to be the temporal spread of acoustic energy with a fixed mode number across a nonvanishing frequency band. It is shown that there are three contributions to modal group time spreads that combine approximately in quadrature. These contributions are (1) the recriprocal bandwidth, (2) a deterministic dispersive contribution, and (3) a scattering-induced contribution. Contributions 2 and 3 are shown to be controlled by the waveguide invariant β(m,f), a property of the background sound speed structure. Numerical simulations are shown to be in good agreement with theoretical predictions. [Work supported by ONR.]