Ocean Water Column Research Articles

Monterey Bay acoustic environmental monitoring system.

Over the past few decades sound has been used with increasing success to measure and monitor a variety of oceanographic features and processes−biological populations, water structure, internal waves, bathymetry, and climatic change to name just a few. Monterey Bay and surrounding ocean regions have recently been proposed as a Marine Sanctuary. Scientists see this region as a natural marine laboratory. In an effort to unobtrusively monitor features and processes in this ocean region a group of local scientists have initiated the design of an Acoustic Environmental Monitoring System (AEMS). A preliminary design meeting was held 28–29 February 1992 by a group of interested scientists and engineers at the Monterey Bay Aquarium. This paper reports on the preliminary design and objectives of the AEMS based on that meeting. The AEMS will consist of three monitoring sites within Monterey Bay, one near the head of Monterey Canyon, and two on the adjacent shelf areas. The sites will be connected to shore by high bandwidth telemetry links. Each site will consist of an instrumentation platform, with quick connect sensor mountings. While acoustic instrumentation will constitute the backbone, the monitoring systems will have provisions to support other oceanographic instruments. Hardware and software interface standards will be established for accommodating these sensors. The acoustic instrumentation will consist of hydrophone arrays, projectors, and acoustic Doppler current meters. The intent is to provide a long-term monitoring capability and a test bed for a variety of environmental monitoring systems. Some of the initial objectives for the AEMS are: provide a long-term acoustic research facility available to the civilian scientific community; provide a reception site for global-warming experiments and other long-range ocean propagation research; monitoring activity in the Marine Sanctuary; marine mammal research; monitoring geological processes; ocean circulation and water column processes. The purpose in presenting this paper at the May 1992 meeting of the Acoustical Society of America is to gain input to this ambitious project from the largest gathering of knowledgeable scientists and engineers possible.

Barite formation in the Southern Ocean water column

The intensity of past biological activity can be correlated with the occurrence of suspended and sedimented barite (BaS04). To date, there is a lack of information on the processes controlling the production of pelagic barite. During the EPOS-2 cruise (November–December 1988 ), situated in the Scotia Sea-Weddell Sea Confluence, suspended matter samples were taken for analysis of total Ba by inductively coupled plasma-atomic emission spectroscopy ( ICP-AES ) and for scanning electron microscopy-electron microprobe (SEM-EMP). The vertical profiles of particulate barium generally show a Ba maximum between 200 and 500 m. SEM-EMP investigations on Scotia-Weddell Sea Confluence profiles provide a possible explanation for the origin of this subsurface Ba maximum. In the surface waters (i.e. above 200 m) barite is mainly contained within large bioaggregates. The fact that in the first 10–20 m of the water column the barite particles in the bioaggregates appear as amorphous entities without a clear crystalline habit (in contrast to their form at greater depths) probably reflects the evolution from BaSO 4 precipitation to barite crystallization. Below this surface layer barite in bioaggregates is present as microparticles with a crystalline habit. Deeper in the water column, below the first few hundred meters, barite crystals occur as free discrete particles. This suggests that in the subsurface zone (below 200 m) the “aggregates” are dispersed, possibly as a result of bacterial activity on the organic matrix, thereby releasing the individual barite crystals. As these microcrystals, set free in the water column, settle much more slowly than the carrier-aggregate and as barite dissolves relatively slowly, local accumulation of barite can result. In some cases high Ba concentrations are present in the first 10–20 m of the water column, occasionally coinciding with biomass parameter maxima. This correlation suggests active precipitation by the living phytoplankton cell (i.e. intravacuolar barite formation), as already observed by others for cultures of marine algae (Pavlovales). Our SEM-EMP investigations on INDIGO 3 and EPOS-2 samples did not reveal any association of barite with identifiable phytoplankton individuals. Thus, SEM-EMP evidence favours passive precipitation in saturated microenviron-ments as the process involved in formation of pelagic barite.

A hybrid method to simulate acoustic/elastic wave scattering from a rough seafloor.

A hybrid scheme to simulate 2-D wave propagation through a vertically heterogeneous ocean and local scattering from a highly irregular elastic seafloor is presented. The hybrid scheme is based on the combination of Gaussian beam, finite-difference, and Kirchhoff integral solutions of the wave equation. Gaussian beam method, used to propagate the source field through a vertically heterogeneous water column to the scattering region, is regular in the critical and caustic regions, making it ideally suited to accommodate the gradational velocity variations within the oceanic water column, and is numerically efficient. Finite differencing is used to compute the complex acoustic/elastic interaction of this wave field with the rough seafloor. A fourth-order space, second-order time 2-D finite-difference algorithm permits calculation of high-frequency (150–300 Hz) acoustic/elastic scattering from fine-scale (1–10 m) roughness. The backscattered field is extrapolated to a receiver array distant from the scattering locale by means of the Kirchhoff integral, with the Gaussian beams being used in the Kirchhoff kernel.

Algal and bacterial hydrocarbons in particulate matter and interfacial sediment of the Cariaco Trench

particulate matter samples (<53 μm and > 53 μm size fractions) and sediment from the sediment-water interface were collected in the Cariaco Trench, an anoxic marine basin on the continental shelf of Venezuela, and analyzed for hydrocarbons. Concentrations of hydrocarbons were highest (10–20 ng/1) in oxic surface waters (0–150 m) and at the top of the anoxic zone (350–600 m), and were low (3–4 ng/l) in the low oxygen zone above the oxic-anoxic interface (150–250 m) and anoxic bottom waters (600–1150 m). Branched C 25-trienes and -tetraenes dominated the hydrocarbon distributions in <53 μm particles in the oxic zone, while the isoprenoid C 25- and C 40-alkanes, 2,6,10,15,19-pentamethyleicosane and lycopane, dominated in the <53 μm particles in the anoxic waters; petrogenic C 17–C 40 n- alkanes dominated the > 53 μm particles. Particle size and depth distributions of the hydrocarbons demonstrate the importance of algal hydrocarbons in surface waters, coupled with increased abundances of bacterial hydrocarbons, especially in the <53 μm particles, in the anoxic zone. These observations illustrate the importance of oxic-anoxic interfaces, even in the oceanic water column, as zones of intense microbial alteration of organic matter and as sources of some of the organic compounds found in sediments.

210Pb scavenging in the North Atlantic and North Pacific Oceans

The radionuclide 210Pb shows significant geographic variations in the extent of its removal from the open ocean water column. This “texture of scavenging” is defined by mapping: (1) the integrated deficiency of 210Pb in the water column, relative to its supply from the atmosphere and from in situ decay of dissolved 226Ra, and (2) inventories of excess 210Pb in deep-sea sediments. The ratio of 210Pb deficiency to its supply, termed the scavenging effectiveness, is ∼ 20% in the North Equatorial Pacific and ∼ 50% in the North Atlantic. This variation is related to the combined effects of uptake of 210Pb onto sinking particles and lateral transport of 210Pb to areas of more intense removal. Sediment inventories of excess 210Pb, normalized to the 210Pb deficiency in the overlying water column, permit evaluation of the relative importance of these effects. In the North Equatorial Pacific virtually all of the 210Pb removed from the water column is present in the underlying sediments but in the mid-latitude North Atlantic, the sediments comprise only about 50% of the 210Pb removed. The deficiencies of 210Pb in the mid-latitude North Atlantic sediments south of 50°N are qualitatively offset by surpluses in high-latitude sediments north of 50°N. Higher primary productivity and new production in the surface waters of the high-latitude North Atlantic and North Equatorial Pacific, relative to the oligotrophic central North Atlantic, may account for the greater fluxes of 210Pb to bottom sediments in those areas.

Upward fluxes of particulate organic matter in the deep North Pacific

The flux of particulate matter through the oceanic water column is a primary component in elemental cycling and is generally perceived as being in one direction: downward1, 2. The organic matter constituting these particles is produced through photosynthesis in surface waters and either sinks directly as phytoplankton and products3, 4 or undergoes various trophic transformations through the water column. A large proportion of the particulate organic matter produced in surface waters is regenerated in the euphotic zone5–7. A fraction of this organic matter, however, leaves the surface waters and settles through the water column, generally decreasing in quantity and changing in quality with increasing distance from the surface8–11. Although the net transport of organic matter must be downward to fuel the lower portions of the water column, there is also an upward component to transport. Positively buoyant particles, including lipid-rich eggs, larvae and, possibly, carcasses of deep-sea animals are examples of particles which undergo upward transport12–13. A previous attempt to quantify the upward mass flux indicated rates of 1–4% of the downward mass flux14. Here we report the first evidence that there is a significant upward flux of particulate organic matter, up to 66.7% of the concurrently measured downward flux, at two stations in the deep North Pacific. Given this magnitude, the previously ignored upward flux of such organic matter must be considered in models of carbon and nitrogen cycling in the open ocean.

Thermocline ventilation of anthropogenic lead in the western North Atlantic

Massive industrial aerosol emissions from North America have over the past century overwhelmed the natural distribution of lead in the western North Atlantic thermocline. The resultant features to 2000‐m depth have been mapped near Bermuda for the year 1984. Since earlier reported measurements have largely been rejected on grounds of sample contamination, the twentieth‐century evolution of anthropogenic Pb in the oceanic water column is unknown. Coupling of the 1984 water column Pb distribution to historical records in reef‐building corals and tracer‐derived thermocline ventilation rates (Jenkins, 1980) has permitted a model reconstruction of Pb penetration into the Sargasso Sea. The model is constrained by three distinct Pb distributions (Pb concentration, stable Pb isotopes, and 210Pb) with adjustable scavenging and regeneration parameters to account for nonconservative behavior. The results of the model substantiate the importance of lateral isopycnal ventilation while pointing out the necessity of dissolved‐particulate exchange (i.e., scavenging τΓ of the order of 70 years in the deep thermocline; minimum regeneration of 10% particulate Pb between surface and 1700 m). Residual discrepancies between model isotopic predictions and observations below 800 m are likely due to a combination of the following factors: (1) presence of nonradiogenic background Pb in intermediate depth waters, (2) geographic variability in North American industrial source signatures, (3) Mediterranean Pb inputs of European origin, and (4) along‐isopycnal mixing effects.

Simulation of the global air‐sea transfer velocity field of helium

Air‐sea exchange is important in constructing a budget of He in the upper oceanic water column. This exchange of He is controlled by the transfer velocity (KwHe) and the air‐sea concentration gradient. Here, the global transfer velocity field of He is computed for January and July with user‐defined derived fields based on control runs of the NCAR (National Center for Atmospheric Research) CCM1 (Community Climate Model), a 12 layer atmospheric general circulation model. The spatial resolution of these calculations is 4.5° × 7.5° latitude‐longitude. The range of KwHe variation over the globe is over an order of magnitude, varying from 5 to 75 cm hr−1. Equatorial values are generally 10‐20 cm hr−1 and high latitude regions experience values in excess of 60 cm hr−1. The annual variability of the transfer velocity in the high‐latitude northern hemisphere is larger than in the high‐latitude southern hemisphere. This work shows that the spatial and temporal variation in the global transfer velocity field of He is significant. This result should be included in future high‐resolution 3‐dimensional coupled ocean‐atmosphere tracer models when attempting to quantitatively deconvolute vertical transport and air‐sea exchange in surface ocean He budgets. This approach may allow high‐resolution global estimates of nitrate flux into the euphotic zone, an important component in oceanic biogeochemical modeling efforts.

Correlation of Co concentration and growth rate — A method for age determination of ferromanganese crusts

Based on the analysis and comparison of metal content and growth rate determination by the 10Be method an empirical law has been developed to calculate the growth rate. Evaluating after 520 chemical analyses we have shown that the flux of Co into ferromanganese seamount crusts has been constant over the entire growth period of all crusts investigated. The amount of Co supplied per unit of time and seabottom area is nearly constant in the oceanic water column. The layered texture of ferromanganese crusts reflects the change of phases and hydrogenetically precipitated amounts as well as their different growth rates. The most important indicator revealing the growth rate is the Co enrichment of each crust layer: The higher the growth rate the lower the Co concentration. Therefore, it is possible to recalculate the age and growth rate of each ferromanganese crust layer by: G(mm Ma −1)=1.28/[Co (%)-0.24] Based on the intense phosphorite impregnation of the lower part of the ferromanganese crust which has grown within the time period 18-12 Ma (so-called older crust) two additional effects have to be taken into account: (a) the intense phosphorite impregnation causing dilution; and (b) the chemical dissolution of the ferromanganese crusts caused by the phosphate-rich pore water. Both reduce the Co concentration and the relating Co contents, Co ( x) ′ and Co ( x) ″, have thus to be compensated: (a) Co (x)′= Co (x)m Mn/Co (x) Mn/Co (b) (b) Co ( x)″ =Co ( x)′ (1-0.05 ΔP) −1 where Co ( x) ″ = double-corrected Co concentration of the layer ( x) of the older crust generation; Co ( x) ′ = corrected Co concentration of the layer ( x) of the older crust generation concerning the phosphate dilution; Co ( x)m = measured Co concentration of the layer ( x) of the older crust generation; Mn Co (x) = Mn Co ratio measured of the layer ( x) of the older crust generation; Mn Co ( b) = Mn Co ratio measured of the boundary layer between younger and older crust generation; and ΔP = difference between the apatite fraction of the older crust of the layer ( x) and the average value of the younger crust generation. This simple cobalt calculation model makes it possible to recalculate the growth rate and age of each simple layer of ferromanganese crusts.

Ocean primary production and available light: further algorithms for remote sensing

In the context of remote sensing of the ocean, the general problem of estimating water column production from surface irradiance and column chlorophyll concentration is examined, and some refinements are made to the linear theory presented by Platt (1986, Deep-Sea Research, 33, 149–163). Further empirical evidence is presented to show the stability of the relationship between surface light and biomass-normalized primary production of the ocean water column. A theoretical explanation is given for the non-zero intercept often obtained when these two variables are regressed. The systematic errors in the estimation of primary production by remote sensing, due to non-uniformity in the biomass profile, are examined through sensitivity analyses on a generalized biomass profile. The errors are shown to be functions of the parameters of the biomass profile, of the photosynthetic parameters and of the optical properties of the water. The probable random errors in the estimation of water column primary production using remotely sensed data are evaluated. Some general issues related to the collection and assimilation of data on ocean primary production are addressed.

Tracers of chemical scavenging in the ocean: boundary effects and large-scale chemical fractionation

Several nuclides in the natural radioactive decay series display a strong non-conservative behaviour in the oceanic water column because of their removal to the sediments by uptake at the solid—solution interface. This removal process is commonly referred to as scavenging, and it is believed to be important in governing the behaviour of many heavy metals and other trace substances in the ocean. Measurements of radioactive disequilibrium in seawater yield estimates of the rate at which scavenging occurs. Results show that removal times by scavenging are comparable to within-basin lateral mixing times. It follows that the distribution of the scavenged tracers should be sensitive to the spatial distribution of their sinks, and this has been demonstrated by observation, most notably in the case of210Pb. The distribution of210Pb suggests a strong control by intensified uptake at boundaries, especially at the ocean margins. This conclusion has been confirmed by sediment traps and by measurements of210Pb accumulations in the sediment column. A similar phenomenon of intensified scavenging at ocean margins has been observed for231Pa. Studies with230Th, on the other hand, show that its delivery to the sediments is mainly caused by the local flux of sinking particles. Thus two transport pathways for removal by scavenging are envisioned, one with a strong horizontal component due to the intensified uptake at the margins and the other with a strong vertical component due to the particle flux. Important large-scale chemical fractionations, best illustrated by comparative studies of230Th and231Pa, are produced by differential partitioning of elements between these two pathways. Model calculations suggest that horizontal mixing rate and particle flux are important variables controlling the partitioning of reactive chemical substances between pelagic sediments of the interior ocean and hemipelagic sediments of the ocean margins. Changes in the extent of this partitioning through time may be recorded as changes in the230Th/231Pa ratio through the sediment column.

Selected examples of Scholte wave propagation in deep and shallow water

The Naval Ocean Research and Development Activity recently completed a series of three experiments to measure very low‐frequency (VLF) seismo‐acoustic propagation in both shallow and deep water marine environments. A vertical hydrophone array and ocean bottom instruments were deployed in all three experiments. The ocean bottom instruments contained both hydrophone and three‐component geophone sensors. In the single shallow water experiment, measurements were also made with buried three‐component geophones. We compare the VLF propagation characteristics in deep and shallow water seen with ocean bottom and water column sensors. The importance of Scholte wave propagation in accounting for both signal and ambient noise energy at near bottom receivers is shown for selected frequencies using both measured and modeled results. Necessary environmental conditions for Scholte wave excitation are also examined.

Paleoceanography of the mid-Pleistocene

The fossil record of the Pleistocene calcareous nannoplankton indicates that during the mid-Pleistocene (0.93–1.25 my) occurred an episode of overwhelming dominance of small Gephyrocapsa. During this episode normally abundant, large size specimens of this genus (mainly Gephyrocapsa oceanica) were virtually excluded from the phytoplankton of tropical and subtropical oceans. The best modern analog of this dominantly small Gephyrocapsa assemblage is the subpolar Emiliania huxleyi assemblage, which implies that nutrient content was significantly greater and water temperature was lower in the photic water column of the tropical oceans than they are today. Increased equatorial upwelling in the oceans, on a scale much greater than today, may explain the above pattern. To achieve such broad equatorial upwelling there must be a source and a drive for cold, dense water. The Arctic Ocean, which was probably seasonally free of ice during this interval of the mid-Pleistocene, is capable of providing the requisite source as well as a drive for the inferred equatorial upwelling. The energy balance of a predominantly ice-free Arctic Ocean requires an approximately three to seven fold increase of hydrospheric heat transport from the North Atlantic to the Arctic Ocean, which dictates a corresponding or even greater increase in the volume of warm water entering the Arctic Ocean at the surface and cold dense water exiting at depth to the North Atlantic. Such enhanced dense water formation in the Arctic Ocean could drive the intensified equatorial upwelling implied by the small Gephyrocapsa dominance interval. If the above scenario is correct then the climate of the earth's northern hemisphere during the mid-Pleistocene may have been very different from the younger Pleistocene climate. One manifestation of this difference may be the mid-Pleistocene shift in climatic cycle periodicity from 40 ky to 100 ky. Another important aspect is that the enhanced greenhouse effect expected during the next century because of an increase of atmospheric CO 2 is thought to lead directly to melting of the Arctic Ocean ice cover and of the Greenland ice sheet. Thus, the “greenhouse” Arctic Ocean and its attendant ocean circulation would resemble the inferred mid-Pleistocene conditions.

Equilibrium adsorption of thorium by metal oxides in marine electrolytes

The equilibrium adsorption of Th by the hydrous oxides goethite (α-FeOOH) and δ-MnO 2 in marine electrolytes is not affected by the major cations Ca 2+ and Mg 2, relative to NaCl electrolyte, while SO 2− 4 decreased adsorption through competitive ion pairing with Th in solution. A triple layer model of surface speciation in NaCl electrolyte suggested that adsorption of Th mainly involves the hydrolysed forms Th(OH) 2+ 2, Th(OH) + 3 and Th(OH) 4. Intrinsic surface binding constants for these species were determined by fitting the model calculations to experimental adsorption data. In sulphate-containing electrolytes, including seawater, the model correctly predicted the slope of the adsorption curves for both oxides, with the pH of the adsorption edge depending on values used for Th-sulphate ion-pairing equilibrium constants. Competition experiments using synthetic organic ligands to buffer the Th concentrations with δ-MnO 2 lowered the concentration free Th available for adsorption to within the range of oceanic dissolved Th concentrations. This gave rise to shifts in the adsorption edge to higher pH close to that of seawater, which were consistently predicted by the model. Application of the model to the deep ocean water column suggested that δ-MnO 2 would not be a powerful scavenger for Th because of the development of a strong negative surface charge at pH 8, while adsorption on goethite should be significant.

Submersible observations along the southern Juan de Fuca Ridge: 1984 Alvin Program

In September 1984, the research submersible Alvin provided direct observations of three major hydrothermal vent areas along the southernmost segment of the Juan de Fuca Ridge (JFR). The submersible operations focused on specific volcanologie, structural, and hydrothermal problems that had been identified during the preceding 4 years of photographic, dredging, acoustic imaging, and geophysical studies along a 12‐km‐long section of the ridge. A continuously maintained (from 1981 to the present) net of seafloor‐anchored acoustic transponders allowed the observations from Alvin to be directly tied to all previous U.S. Geological Survey data sets and National Oceanic and Atmospheric Administration water column surveys from 1984 to the present. The three vent areas studied are the largest of at least six areas identified by previous deep‐towed camera surveys that lie within a deep cleft, which marks the axis of symmetry of the JFR in this region. The cleft appears to be the locus of eruption for this segment of the JFR. The vent areas, at least in part, are localized near what appear to be previous volcanic eruptive centers marked by extensive lava lake collapse features adjacent to the cleft at these sites. Each hydrothermal area has several active discharge sites, and sulfide deposits occur as clusters (15–100 m2) of small chimneys, individual large chimneys, or clusters of large branched chimneys. We review the dive program and present a brief synthesis of the geology of the vent sites together with sample and track line compilations.

Simulation of marine ecosystem effects due to PCB waste incineration in the Gulf of Mexico

A four-level atmosphere—ocean-sediments transport model has been coupled to an ecosystem model to estimate the magnitude of effects of ocean-based incineration of PCB wastes. The atmospheric (top) layer of the model has a seasonally variable thickness, a reflecting boundary at the top, and an absorbing boundary at the air—water interface. Both wet and dry deposition are included. Rainfall occurs stochastically. Upper and lower layers of the oceanic water column constitute the second and third layers of the transport model, while the sediments on the seafloor are the fourth layer. Adsorption of pollutants to particulate matter in the water column, and resuspension of pollutants in shallow coastal regions are included. The ecosystem model focuses on linkages among major trophic compartments, and includes predation, metabolic, and pollutant assimilation/depuration components. Simulation results suggest that, for destruction and removal efficiencies (DRE) on the order of 0.9999 (‘four nines’), measurable bioaccumulation effects may occur in the benthos over a 50-year policy horizon, whereas if a DRE of six nines can be established and maintained, effects will be negligible. Efficiency monitoring and control during the incineration process is therefore of primary importance if a policy of ocean incineration of hazardous wastes is to be adopted on a large scale.

Nutrient regeneration and denitrification in low oxygen fjords

A plot of total combined nitrogen (NO 3 −, NO 2 −, NH 4 +) versus phosphate for the Jervis Inlet system (British Columbia, Canada) reveals ΔN: ΔP = 11.1 – 11.7: 1 instead of 16:1, for the upper 30 m of water of the entire system and for the deep water in the main body of Jervis Inlet. In Narrows Inlet and Princess Louisa Inlet, shallow-silled fjords attached to the main body, the slope in stagnant deep water is even smaller and sometimes negative. This is caused by denitrification in the sediments. These conditions are reflected in the estimated ration at which phytoplankton take up nitrogen and phosphorus, 12.5:1. Rates of nutrient regeneration and microbial respiratory processes for the deep regions of Narrows Inlet and Princess Louisa Inlet were calculated from observed changes in oxygen and nutrient concentrations during summer months when the deep water was stagnant. The mean oxygen consumption rate (some of which occured in the water), normalized to area of sediment, was 0.45–0.55 μg-at. m −2 s −1 for Narrows Inlet and 0.71 μg-at. m −2 s −1 for Princess Louisa Inlet. The mean phosphate regeneration rate was 2.3 × 10 −3 μg-at. m −2 s −1 for Narrows Inelt and 4.9 × 10 −3 μg-at. m −2 s −1 for Princess Louisa Inlet. Rates of denitrification ranged from 0.016 to 0.073 μg-at. N m −2 s −1 in Narows Inlet and 0.075 to 0.101 μg-at. N m −2 s −1 for Princess Louisa Inlet. The range in sulfate reduction rates was 0.018–0.082 μg-at. m −2 s −1 for Narrows Inlet and 0.089–0.102 μg-at. m −2 s −1 for Princess Louisa Inlet. The denitrification rates are high compared to rates determined for other coastal and inshore environments. This appears to be caused by a combination of very low oxygen water overlying the sediments which allows the denitrification zone to be at or very close to the sediment-water interface, and a strong coupling between nitrification and denitrification, in which ammonium released by sulfate reduction in the sediments, diffuses into the aerobic zone and is oxidized to nitrogen species that are consumed by denitrificaon. The nitrification-denitrification couple probably occurs widely in coastal and continental shelf sediments, and sedimentary denitrification may exceed denitrification that occurs in the oxygen deficient portions of the oceanic water column.

Synthetic Pn and Sn phases and the frequency dependence of Q of oceanic lithosphere

The oceanic lithosphere is an extremely efficient waveguide for high‐frequency seismic energy. In particular, the propagation of the regional to teleseismic oceanic Pn and Sn phases is largely controlled by properties of the oceanic plates. The shallow velocity gradient in the sub‐Moho lithosphere results in a nearly linear travel time curve for these oceanic phases and an onset velocity near the material velocity of the uppermost mantle. The confinement of Pn/Sn to the lithosphere imposes a constraint on the maximum range that a normally refracted wave can be observed. The rapid disappearance of Sn and the discontinuous drop in Pn/Sn group velocity beyond a critical distance, dependent upon the local thickness of the lithosphere, are interpreted as a shadowing effect of the low Q asthenosphere. Wave number integration was used to compute complete synthetic seismograms for a model of oceanic lithosphere. The results were compared to data collected during the 1983 Ngendei Seismic Experiment in the southwest Pacific. The Pn/Sn coda is successfully modeled as a sum of leaky organ‐pipe modes in the sediment layer and oceanic water column. While scattering is present to some degree, it is not required to explain the long duration and complicated nature of the Pn/Sn wave trains. The presence of extremely high frequencies in Pn/Sn phases and the greater efficiency of Sn than Pn propagation are interpreted in terms of an absorption band rheology. A shorter high‐frequency relaxation time for P waves than for S waves results in a rheology with the property that Qα &gt; Qβ at low frequency while Qβ &gt; Qα at high frequency, consistent with the teleseismic Pn/Sn observations. The absorption band model is to viewed as only an approximation to the true frequency dependence of Q in the oceanic lithosphere for which analytic expressions for the material dispersion have been developed.

Primary production of the ocean water column as a function of surface light intensity

Primary production of the ocean water column as a function of surface light intensity

Primary production of the ocean water column as a function of surface light intensity: algorithms for remote sensing

Empirically, the phytoplankton production per unit area of sea surface, normalised to total pigment biomass in the water column, is a linear function of the surface insolation. The slope ψ varies by only a factor of two in samples from a variety of environments. A theory is developed that describes the empirical results. A simplifying assumption of the theory, that photosynthesis is a linear function of available light, leads to a bias (overestimate) that can be calculated as a function of a measureable dimensionless parameter. With the bias corrected, ψ is shown to be equal, to within a dimensionless constant, to the initial slope α of the photosynthesis-light curve. The theory can be applied to the calculation of primary production from satellite surveys of ocean colour or from survey data collected by ship.