Deep Ocean Sites Research Articles

Trends in low‐frequency deep ocean ambient noise levels: New results from old data.

Concern about effects of anthropogenic sound generated by ships, seismic exploration, naval operations, and ocean acoustic research on marine life has stimulated new studies of marine animal bioacoustics. An underlying issue in assessing the noise impact establishes the present‐day background noise level and compares it to noise levels from previous times. The most generally accepted prediction of noise level trend at low frequencies (below 500 Hz) is due to Ross, who used noise levels from the 1950s and the mid‐1960s to predict an increase of about 3 dB/decade. This paper addresses the issue of the trend in low‐frequency ambient noise levels in the ocean and presents measurements taken in the intermediate years between the mid‐1960s and the present time. The measurements were made using a calibrated multi‐element volume array (MEVA) at deep ocean sites in the Northeast Pacific from 1978 to 1986. The array provided spectral noise levels and horizontal and vertical directionalities. Compared to the 1965 va...

A global assessment of deep-sea basalt sites for carbon sequestration

AbstractIn recent years, the debate over the most effective means to stabilize greenhouse gas concentrations in the atmosphere has endorsed multiple approaches and a variety of technologies. Assuring secure storage of anthropogenic carbon dioxide is one of our most pressing global scientific challenges that may contribute to achieving a stable solution over the next several decades. Geological sequestration by injection into deep-sea basalt formations provides unique and significant advantages over other potential storage options, including: (a) vast reservoir capacities with high porosity and permeability, sufficient to accommodate centuries-long U.S. production of fossil fuel CO~2~ at locations within a few hundreds of kilometers of populated areas; (b) chemical reactivity of CO~2~ with basalt and in situ fluids to produce stable, non-toxic carbonates; and (c) significant risk reduction for post-injection leakage by geological, gravitational, and mineral trapping mechanisms. We compare independent trapping mechanisms available in deep-sea basalts to those in saline aquifers, which have also been proposed as potential storage environments for anthropogenic carbon dioxide. We suggest that deep-sea basalts offer significant advantages over saline aquifers, in terms of reduced risk of post-injection leakage and storage capacity. Using a global site assessment strategy to highlight the most secure oceanic basalt sites that provide all trapping mechanisms, we initially identify potential target regions that occur in deep-sea basalt and calculate the potential injection volume for each. The largest volumes and most secure basalt sites occur in regions adjacent to intermediate- to fast-spreading seismic ridges as well as deep aseismic ridges. We then use site-specific criteria, such as abundance of ODP and IODP drill sites with basement penetration, permeability and/or porosity data, to refine volume calculations and to prioritize these target regions as promising locations to securely accommodate carbon dioxide injection. Pilot injection studies in deep-sea basalts are necessary to establish the viability of these reservoirs for future CO~2~ sequestration. We suggest that basaltic crust at deep ocean sites offers vast capacity and potential for permanent sequestration of carbon dioxide to mitigate atmospheric build-up of this greenhouse gas.

The effect of submerged plateaux on Pleistocene gyral circulation and sea-surface temperatures in the Southwest Pacific

Uniquely in the Southern Hemisphere the New Zealand micro-continent spans the interface between a subtropical gyre and the Subantarctic Circumpolar Current. Its 20° latitudinal extent includes a complex of submerged plateaux, ridges, saddles and basins which, in the present interglacial, are partial barriers to circulation and steer the Subtropical (STF) and Subantarctic (SAF) fronts. This configuration offers a singular opportunity to assess the influence of bottom topography on oceanic circulation through Pleistocene glacial – interglacial (G/I) cycles, its effect on the location and strength of the fronts, and its ability to generate significant differences in mixed layer thermal history over short distances. For this study we use new planktic foraminiferal based sea-surface temperature (SST) estimates spanning the past 1 million years from a latitudinal transect of four deep ocean drilling sites. We conclude that: 1. the effect of the New Zealand landmass was to deflect the water masses south around the bathymetric impediments; 2. the effect of a shallow submerged ridge on the down-current side (Chatham Rise), was to dynamically trap the STF along its crest, in stark contrast to the usual glacial–interglacial (G–I) meridional migration that occurs in the open ocean; 3. the effect of more deeply submerged, downstream plateaux (Campbell, Bounty) was to dynamically trap the SAF along its steep southeastern margin; 4. the effects of saddles across the submarine plateaux was to facilitate the development of jets of subtropical and subantarctic surface water through the fronts, forming localized downstream gyres or eddies during different phases in the G–I climate cycles; 5. the deep Pukaki Saddle across the Campbell-Bounty Plateaux guided a branch of the SAF to flow northwards during each glacial, to form a strong gyre of circumpolar surface water in the Bounty Trough, especially during the mid-Pleistocene Climate Transition (MIS 22-16) when exceptionally high SST gradients existed across the STF; 6. the shallower Mernoo Saddle, at the western end of the Chatham Rise, provided a conduit for subtropical water to jet southwards across the STF in the warmest interglacial peaks (MIS 11, 5.5) and for subantarctic water to flow northwards during glacials; 7. although subtropical or subantarctic drivers can prevail at a particular phase of a G–I cycles, it appears that the Antarctic Circumpolar Current is the main influence on the regional hydrography. Thus complex submarine topography can affect distinct differences in the climate records over short distances with implications for using such records in interpreting global or regional trends. Conversely, the local topography can amplify the paleoclimate record in different ways in different places, thus enhancing its value for the study of more minor paleoceanographic influences that elsewhere are more difficult to detect. Such sites include DSDP 594, which like some other Southern Ocean sites, has the typical late Pleistocene asymmetrical saw-tooth G–I climate pattern transformed to a gap-tooth pattern of quasi-symmetrical interglacial spikes that interrupt extended periods of minimum glacial temperatures.

Automated real-time detection of cetaceans at the Station ALOHA Cabled Observatory

Hawaiian waters support a wide variety of cetacean species, however, little is known about their occurrence in offshore waters. A cabled ocean bottom observatory located at Station ALOHA (Station ALOHA Cabled Observatory - ACO) provides a unique opportunity to study the occurrence of cetaceans in real-time and over long time scales at a deep ocean research site located 100 km north of Oahu. A retired electro-optical telecommunications cable provides power and broadband Ethernet communications capability to the ACO, allowing real-time continuous acoustic monitoring. The ACO has been operational since February 2007 and includes a hydrophone with a bandwidth of 0.01Hz to 40kHz. Automated methods for detection and identification of cetacean calls have been developed and data are processed in nearly real-time. Several cetacean species have been detected, including: dolphins and humpback, minke, fin, and sperm whales. The relationship between the occurrence of different species and changes in the environment has been explored using near-monthly shipboard oceanographic observations collected at Station ALOHA. In addition, seasonal trends in the occurrence of minke whales, a species that has been especially difficult to study using visual methods due to their cryptic behavior, have been examined.

Seasonal variation in the frequency response of the ear: Implications for communication

Seasonal variation in vocal‐acoustic behavior is well known among nonmammalian vertebrates; however, studies of the neural mechanisms underlying such seasonal variation have focused mainly on the anatomical rather than the neurophysiological correlates of seasonal reproductive periodicity. In this talk, I present the neurophysiological evidence for seasonal variation in the frequency response of the ear in the plainfin midshipman fish (Porichthys notatus). Vocal communication is essential to the reproductive success of the seasonally breeding midshipman fish, which migrates during the late spring and summer from deep ocean sites into the shallow intertidal zone to breed. Female midshipman fish use the auditory sense to detect and locate males that are singing from their nests. The seasonal onset of male mate calling in the midshipman fish coincides with a dramatic increase in the range of frequency sensitivity of the female’s saccule, the midshipman’s main organ of hearing, thus leading to increased encoding of the male’s mate call. I will present evidence for a steroid‐dependent mechanism that leads to this form of auditory plasticity and discuss why this plasticity of peripheral auditory frequency sensitivity may represent an adaptation to enhance the acquisition of auditory information needed for mate identification and localization during the breeding season.

A Second Look at Low-Frequency Marine Vertical Seismometer Data Quality at the OSN-1 Site off Hawaii for Seafloor, Buried, and Borehole Emplacements

We improve marine low-frequency (1–100 mHz) vertical seismometer data by subtracting noise generated by tilting under fluid flow and by seafloor deformation under ocean-surface gravity waves. We model the noise from the coherency and transfer functions between the vertical channel and other data channels that are more sensitive to the noise sources: the horizontal seismometer components for tilting and a differential pressure gauge for ocean waves. We subtract noise from three adjacent seafloor broadband seismometer stations at the OSN-1 deep-ocean test site: one sitting on the seafloor, another buried 1 m deep in sediments, and the third clamped in a borehole 248 m beneath the seafloor. Seafloor currents generate the seafloor sensor tilt noise, whereas tidally driven fluid pumping generates the borehole sensor tilt noise. Subtracting the tilt noise reduces the vertical channel noise levels by 35–40 dB between 1 and 60 mHz on the seafloor sensor and by 15–20 dB between 1 and 10 mHz on the borehole sensor. Subtracting the ocean-wave noise further reduces the noise level on all instruments by 5–15 dB between 4 and 20 mHz. After subtracting tilt and ocean-wave noise, the seafloor vertical channel is 5–10 dB quieter than the buried sensor vertical channel at frequencies below 30 mHz. The corrected borehole vertical channel has a similar noise level to the seafloor and buried sites above 10 mHz, but noise increases rapidly at lower frequencies, probably because of vertical strumming under tidally driven fluid flow.

Characteristics and energy rays of equatorially trapped, zonally symmetric internal waves

Characteristic curves of partial differential equations (PDEs) in general differ from short wave energy rays. We give conditions for linear, two dimensional second order PDEs that guarantee an exact correspondence between characteristics and energy rays. The findings are applied to time-harmonic, zonally-symmetric small-amplitude equatorial internal waves. It is shown that for a fairly general model of inertia-gravity waves, an exact characteristic-ray correspondence holds. When characteristics of internal waves, trapped in a meridional plane, are followed over several boundary reflections, a convergence towards a limit cycle (called equatorial wave attractor) can generally be found. The results on the characteristic-ray correspondence help to interpret physically equatorial wave attractors. Recent ideas on energy accumulation by near inertial waves, trapped on wave attractors at deep ocean sites are confirmed by our results, at least in the short wave (WKB) sense.

Diversity and Distribution of Marine Microbial Eukaryotes in the Arctic Ocean and Adjacent Seas

We analyzed microbial eukaryote diversity in perennially cold arctic marine waters by using 18S rRNA gene clone libraries. Samples were collected during concurrent oceanographic missions to opposite sides of the Arctic Ocean Basin and encompassed five distinct water masses. Two deep water Arctic Ocean sites and the convergence of the Greenland, Norwegian, and Barents Seas were sampled from 28 August to 2 September 2002. An additional sample was obtained from the Beaufort Sea (Canada) in early October 2002. The ribotypes were diverse, with different communities among sites and between the upper mixed layer and just below the halocline. Eukaryotes from the remote Canada Basin contained new phylotypes belonging to the radiolarian orders Acantharea, Polycystinea, and Taxopodida. A novel group within the photosynthetic stramenopiles was also identified. One sample closest to the interior of the Canada Basin yielded only four major taxa, and all but two of the sequences recovered belonged to the polar diatom Fragilariopsis and a radiolarian. Overall, 42% of the sequences were <98% similar to any sequences in GenBank. Moreover, 15% of these were <95% similar to previously recovered sequences, which is indicative of endemic or undersampled taxa in the North Polar environment. The cold, stable Arctic Ocean is a threatened environment, and climate change could result in significant loss of global microbial biodiversity.

Circulation and variability in the Chile Basin

Current observations at several depths between 250 and 3750m are reported from a 30°S, deep sea site 150km off the Chile coast for the period July 1993–June 2001. These results are used with current observations from a nearby slope site, satellite altimeter data, and hydrographic and Acoustic Doppler Current Profiler data from the WOCE P6E section along 32.5°S to address mean and variable flow in the Chile Basin. Strong current variability in the upper ocean for periods more than 100 days is explained in terms of remotely forced Rossby waves, local baroclinic instabilities of coastal currents and variable wind forcing. Intraseasonal to seasonal current variability at the deep ocean site was greatest during La Niña events while intraseasonal variability at the slope site was greatest during the 1997–1998 El Niño event. Mean westward and poleward flow was observed at all depths at the deep sea site but upper ocean means were not significantly different from zero. There was a well-defined, mean poleward flow of 0.6±0.3cms−1 at 2450m depth there. Geostrophic current calculations are presented for the P6E section with levels of no motion based on our current observations and other constraints. These reference choices yield reasonable and consistent results for the steady-state heat balance of the deep Chile and Peru Basins. Results show a deep equatorward flow of 3–4Sv on the eastern flank of the East Pacific Rise and a deep poleward boundary flow of about 10Sv within 1500km of the Chile coast. Up to half of the total mid-depth outflow of the South Pacific may take place east of the Rise. Thus the deep poleward boundary current off Chile is a major component of the deep circulation of the global ocean but the dynamics of this current remain a puzzle.

Black carbon in deep-Sea sediments

Black carbon (BC) enters the ocean through aerosol and river deposition. BC makes up 12 to 31 percent of the sedimentary organic carbon (SOC) at two deep ocean sites, and it is 2400 to 13,900 carbon-14 years older than non-BC SOC deposited concurrently. BC is likely older because it is stored in an intermediate reservoir before sedimentary deposition. Possible intermediate pools are oceanic dissolved organic carbon (DOC) and terrestrial soils. If DOC is the intermediate reservoir, then BC is 4 to 22 percent of the DOC pool. If soils are the intermediate reservoir, then the importance of riverine carbon in the ocean carbon cycle has been underestimated.

Chemical gradients in sediment cores from an EPA reference site off the farallon islands — Assessing chemical indicators of dredged material disposal in the deep sea

Heavy metal and organic contaminants have been determined in undisturbed sediment cores from the US Environmental Protection Agency reference site for dredged material on the continental slope off San Francisco. As expected, the concentrations are significantly lower than toxic effects guidelines, but concentrations of PCBs, PAHs, Hg, Pb, and Clostridium perfringens (a bacterium spore found in sewage) were nearly two or more times greater in the surface sediments than in intervals deeper in the cores. These observations indicate the usefulness of measuring concentration gradients in sediments at the San Francisco deep ocean disposal site (SF-DODS) where a thin (0.5 cm thick) layer of dredged material has been observed beyond the boundary. This thin layer has not been chemically characterized by the common practice of homogenizing over the top 10 cm. An estimated 300 million cubic yards of dredged material from San Francisco Bay are expected to be discharged at the SF-DODS site during the next 50 years. Detailed depth analysis of sediment cores would add significant new information about the fate and effects of dredged material in the deep sea.

Analysis of total and organic carbon and total nitrogen in settling oceanic particles and a marine sediment: an interlaboratory comparison

Abstract 10 laboratories, using their routine methods, collaborated in a comparison of analyses of total carbon, nitrogen and organic carbon in a single sample of marine sediment and a pooled sample of settling particulate material collected by sediment trap. The standard error of the means from all individual laboratories was ±3% of the mean for all total carbon results, and for total nitrogen in the marine sediment, but was ±7% of the mean for total nitrogen in the sediment trap material. Larger errors (±8% of the mean) were found for organic carbon results from both the marine sediment and the sediment trap material, with a range of results (expressed as a percentage of the untreated dry weight) for the sediment trap sample analyzed here of 5.60–8.03%. When this range is compared with that of particulate organic carbon concentration in sediment trap material obtained by different workers at various deep ocean sites (2.4–10.2%) [Wefer, G., 1989. Particle flux in the ocean: Effects of episodic production, in: W.H. Berger, V.S. Smetacek, G. Wefer (Eds.), Productivity of the Ocean: Present and Past, John Wiley & Sons, pp. 139–154.] it is apparent that a large proportion of the observed oceanic variability could be accounted for by difference in analytical technique. We suggest that the problem stems from the difficulty of accurate separation of the organic and inorganic phases and discuss the inaccuracies involved in the separation. We present evidence that the range of results is partly due to either volatilisation of organic carbon during acid treatment, or incomplete removal of organic matter during thermal treatment. If results from different laboratories are to be compared, as is presently required for basin scale and global studies of ocean carbon cycling and budgets, there is clearly a need for suitable reference materials, rigorous intercalibration and technique development.

Implications of optical properties of ocean, lake, and ice for ultrahigh-energy neutrino detection

The collecting power and imaging ability of planned ultrahigh-energy neutrino observatories depend on wavelength-dependent absorption and scattering coefficients for the detector medium. Published data are compiled for deep ice at the South Pole, for deep fresh water at Lake Baikal, and for deep seawater. The effective scattering coefficient is smallest for the clearest deep ocean sites, whereas the absorption coefficient is an order of magnitude smaller for deep ice than for the ocean and lake sites. The effective volume per detector element as a function of energy is calculated for electromagnetic cascades produced by electron neutrinos interacting at the various sites. It is largest for deep bubble-free ice, smallest for shallow bubbly ice, and intermediate for lake and seawater. The effective volume per element is calculated for detection of positrons resulting from the capture of a few megaelectron volt supernova neutrinos by protons in the medium. This volume is proportional to the absorption length and independent of the scattering length; it is larger for ice than for seawater or lake water.

Concepts for sediment restoration on the palos verdes shelf, California

AbstractThis article summarizes a study conducted by the U.S. Army Engineer Waterways Experiment Station to develop technical information and to evaluate the engineering feasibility of restoration alternatives for DDT‐and PCB‐contaminated sediments on the Palos Verdes shelf and slope near Los Angeles, California. The study evaluated the nonremoval alternative of in‐place capping of contaminated sediments on the shelf and slope; removal of contaminated sediments using conventional and specialized dredging equipment and deep ocean mining equipment; treatment of contaminated sediments; and disposal of contaminated sediments in confined (diked) disposal facilities (CDFs), contained aquatic disposal (CAD) sites, upland landfills, and deep ocean basin sites. Cost estimates of the various alternatives were also prepared.This article concludes that restoration of the contaminated sediments is technically feasible. Sediments on the shelf and slope can be removed using available dredging technologies for deep water environments. In‐place capping, CAD, and CDF alternatives are technically feasible. The deep ocean basin disposal alternative is not feasible from the technical or regulatory standpoint. The treatment alternative is not feasible from the implementability and economic standpoint.

Examination of Silk Fibers from a Deep Ocean Site: SEM, EDS, &amp; DSC

ABSTRACTThe research reported herein is directed toward the determination of the microstructure of silk fibers recovered from the shipwreck site of the S.S. Central America. The structural data obtained from examination of these silk materials using SEM, EDS, and DSC was compared to data obtained from the examination of historic silk fibers (ca. 1860–1880) not exposed to the deep-ocean and of modern silk fibers. Marine silks appear more degraded than historic and modern silk when viewed under the SEM and the fiber surfaces are partially obscured by inorganic deposits. The melting endotherm of marine, historic, and modern silk fibers are comparable to each other, but the glass transition observed in the modern silk is not apparent in the DSC trace of any of the marine silks.

Currents in the deep ocean off Chile (30°S)

Results are reported from the first recording current meter observations in the eastern boundary current system off Chile. Currents at 100 m and 3400 m were observed for a 4–6 month period during the austral spring-summer-fall period of 1991–1992 at a deep ocean site 150 km off the Chilean coast and 70 km seaward of the axis of the Peru-Chile Trench. Results indicate energetic inertial oscillations, low eddy kinetic energy at 3400 m, southward flow, decreasing from austral spring to summer, at 100 m and 3400 m, and quite strong eastward flow, increasing from spring to summer, at 100 m. The latter result may help to explain satellite images, showing the zone of high biological production near the coast in the study region to be very narrow during the austral summer despite strong coastal upwelling in that season. The authors current observations, local geostrophic current profiles and abyssal potential temperature distributions indicate a five-layer structure for the alongshore flow in the eastern boundary current system off Chile: northward flow above 100 m, between about 400 m and 1700 m (maximum around 600–700 m, northward transport of Antarctic Intermediate Water) and below 3400 m; southward flow from above 100 m to about 400 m (maximum at 180 m, Peru-Chile Undercurrent) and from about 1700 m to below 3400. This proposed vertical structure is consistent with temperature, salinity, dissolved oxygen and nutrient distributions of the study area and bears resemblance to eastern boundary current simulations ( McCreary et al., 1987).

Broadband geoacoustic inversion in shallow Arctic water

A broadband technique used successfully to localize acoustic sources and determine water depth and array position at a deep Pacific Ocean site [P. C. Mignerey, Oceans ’90 Proc., 121–124] is used to determine geoacoustic parameters at a shallow water site in the Arctic. The technique minimizes differences between calculated impulse response times and pulse arrival times measured on a vertical array. Initially the method is used to localize the source (SUS) and determine bottom depth and array orientation. The method is then applied to pulse arrivals that penetrate the upper sedimentary layers of the shallow bottom. The inversion provides compressional speed profiles and sediment layer thicknesses of the bottom near the array site. Differences between calculated arrival times and data show a significant decrease, i.e., improvement, when using the geoacoustic profile as determined by the inversion technique. Broadband matched‐field processing is used to demonstrate inversion effects on processor performance. The results demonstrate the broadband method to be useful for geoacoustic surveys of shallow water environments.

X-Ray Diffractometric Measurement of Microcrystallite Size, Unit Cell Dimensions, and Crystallinity: Application to Cellulosic Marine Textiles

Procedures were developed to enable standard x-ray diffractometric equipment to be used to study and compare textile fiber crystallographic characteristics. The approach was based on fitting diffractometric data to the Gaussian distribution by weighted regression analysis, which also supported an assessment of error propagation. Crystallite dimensions were obtained through the Scherrer method using analytically derived spectral line breadths. Percent crystalline component was determined by summing background-corrected amorphous and crystalline components of the spectra. A dif fractometric analysis was made of samples of rare textiles recovered from a 133-year old shipwreck, as well as modem fabrics submerged for three months at the same deep ocean site. The microcrystallite size of the historic textile materials increased, while the percent crystalline component decreased and unit cell dimensions remained con stant. The crystallite sizes of modern linen immersed for three months with iron, copper, or alone did not increase, while the percent crystalline component decreased and the unit cell dimensions remained constant.

Wind dependence of deep ocean ambient noise at low frequencies

A series of experiments has been carried out using a vertical line array to measure low-frequency ambient noise at deep ocean sites in the Northeast Pacific Ocean. Data in the band from 13 to 300 Hz were processed to obtain the array response directed vertically upward in order to study the noise due to local winds. The results indicate that there are two regimes of behavior depending on the wind speed v, with the transition occurring at the onset of wave breaking, v∼10 kn. The noise level (NL) for a specific wind noise process was related to a power n of the wind speed according to the relationship NL=B+20n log v, and the model parameters were obtained by fitting the data. The average value over the band was n=0.3 for wind speeds &amp;lt;10 kn and n=1.33 for higher speeds. This latter value is significantly larger than the value n=1, which is observed in ambient noise data at higher frequencies above 1 kHz. This result suggests that the noise process in breaking waves may be different at low frequencies from that at higher frequencies. Assuming a uniform distribution of surface dipole sources, the source level for the wind-generated noise was determined over the low-frequency band for wind speeds from 10–30 kn. The values are consistent with those of a recent analysis of available noise data [Kewley et al., ‘‘Low-frequency wind-generated ambient noise source levels,’’ J. Acoust. Soc. Am. 88, 1894–1902 (1990)], and the model provides good agreement with wind noise measurements obtained with omni hydrophones in the southern oceans.

Transport of Sewage Sludge in a Mixed Water Column

Abstract Sewage sludge from four publicly-owned treatment works was sampled and characterized in terms of parameters affecting transport at the 106-mile deep ocean disposal site as part of the US Environmental Protection Agency's site monitoring programme. Samples from treatment plants in Passaic Valley, Rahway, and Elizabeth, New Jersey and New York City were characterized in terms of dynamic size distribution, suspended solids and density. the transport characteristics of sludge particles were measured using a 2 metre computer-interfaced laboratory settling column. Experiments were conducted at constant salinity (35 ppt) while varying hydrodynamic mixing, sludge type and concentration using a modified factorial experimental design. Hydrodynamic power dissipation was varied so that the vertical dispersion and rms fluid shear rate ranged between 0-6 cm2S−1 and 0-30s−1 respectively. Results indicate that at least 80% of suspended sludge particles will eventually settle under mixed conditions. the average s...