Characterization of the solar light field within the ocean mesopelagic zone based on radiative transfer simulations

The solar light field within the ocean from the sea surface to the bottom of the mesopelagic zone was simulated with a radiative transfer model that accounts for the presence of inelastic radiative processes associated with Raman scattering by water molecules, fluorescence of colored dissolved organic matter (CDOM), and fluorescence of chlorophyll-a contained in phytoplankton. The simulation results provide a comprehensive characterization of the ambient light field and apparent optical properties (AOPs) across the entire visible spectral range within the depth range 200–1000 m of the entire mesopelagic zone for varying chlorophyll-a concentration and seawater optical properties in the mixed surface layer of the ocean. With increasing depth in the mesopelagic zone, the solar irradiance is reduced by ~9–10 orders of magnitude and exhibits a major spectral maximum in the blue, typically centered around a light wavelength of 475 nm. In the green and red spectral regions, the light levels are significantly lower but still important owing to local generation of photons via inelastic processes, mostly Raman scattering and to a lesser extent CDOM fluorescence. The Raman scattering produces a distinct secondary maximum in irradiance spectra centered around 565 nm. Comparisons of our results with light produced by the radioactive decay of the unstable potassium isotope contained in sea salt (⁴⁰K) indicates that the solar irradiance dominates over the ⁴⁰K-produced irradiance within the majority of the mesopelagic zone for most scenarios considered in our simulations. The angular distribution of radiance indicates the dominance of downward propagation of light in the blue and approach to uniform distribution in the red throughout the mesopelagic zone. Below the approximate depth range 400–500 m, the shape of the angular distribution is nearly invariant with increasing depth in the green and red and varies weakly in the blue. The AOPs at any light wavelength also assume nearly constant values within the deeper portion of the mesopelagic zone. These results indicate that the mesopelagic light field reaches a nearly-asymptotic regime at depths exceeding ~400–500 m.     

Major contribution from mesopelagic plankton to heterotrophic metabolism in the upper ocean

Measurements of plankton respiration and heterotrophic bacterial abundance and production were made at seven deep water stations within the upper 500 m of the Gulf of Mexico during the summer of 1995. Bacterial abundance [(1.1–4.6)×108 1⁻¹] and rates of bacterial production (2–19 nM C h⁻¹) and plankton respiration (50–245 nM O₂ h⁻¹) decreased with depth by four- to nine-fold, and were similar to those reported for oligotrophic waters. Bacterial turnover times increased with depth from approximately 1 to 5 days. Bacterial growth efficiencies decreased from 15% at the surface to 8% at 500 m. Depth-integrated plankton respiration exceeded known estimates of primary production for the region, suggesting that heterotrophic utilization of previously and concurrently produced organic matter (e.g. spring phytoplankton growth, and summer blooms of Trichodesmium sp.) was occurring during the summer. Estimates for the upper 500 m showed that roughly half of the bacterial biomass (56%), bacterial production (49%), and plankton respiration (60%) occurred below the euphotic zone. Routine oceanographic studies have focused exclusively on the metabolic activity occurring within the euphotic zone. Our measurements, however, indicate that mesopelagic plankton also contribute substantially to heterotrophic metabolism and nutrient cycling in the ocean.     

两层简单斜压海洋模式及冬、夏季平均海流模拟

设计了一个两层简单斜压海洋模式(H₁=501,H₁+H₂=250m),利用1月和7月多年平均的海平面气温场、海平面气压场和由该气压场诊断出的海面风场,对此模式的性能进行了检验.结果表明,模拟得出的冬、夏季海洋表层及次表层流与实况较为一致.     

A two-layer simple baroclinic equation ocean model and its simulations of ocean currents in winter and summer

INTRODUCTIONThe world ocean is one of the important components of the human environment. With thedevelopment of marine resource exploit and marine engineering, more and more attention shouldbe paid to the study of ocean current movements by the means of numerical simulation and numerical prediction. It can not only improve our knowledge about the law of ocean circulation but alsocheck the reliability of theoretical research and scientific prediction. On the other hand, it canmake up for t…     

Estuarine classification and response to nitrogen loading: Insights from simple ecological models

Estuaries exhibit a large range in their responses to nitrogen loadings determined in part by characteristics of the driver, such as magnitude and frequency, but also by such intrinsic characteristics as physical/chemical factors (e.g., depth, volume, hypsometry, salinity, turbidity) and biological factors (e.g., nature of ecological communities, trophic interactions). To address the richness of estuarine response to driver variables, the aim ultimately is to establish a simple estuarine classification scheme, beginning with a river-dominated subset of estuarine systems and focusing on the role of water residence time in the estuary. Residence time (or flushing time) is related to other drivers (streamflow, nutrient, and sediment loads) and drives much of the biological response of estuaries because of flushing effects on plankton, temperature, nutrients, and light. Toward this goal, nutrient–phytoplankton–zooplankton (NPZ) models have been used to examine a range of subjects including effects of nutrient limitation and zooplankton predation on phytoplankton dynamics and fish predation. This class of model can admit a wide range of behavior, including multiple steady-states and oscillatory behavior. The NPZ equations include terms for nutrient recycling, phytoplankton settling, benthic regeneration, and zooplankton mortality. Analysis of the equations suggests that both the nature of nitrogen loading (i.e., whether it is correlated with discharge or independent of it) and residence time are critical in determining the steady-state response of the system.     

Generation of long internal waves in the ocean by a moving pressure zone

The generation of plane internal waves in a continuously stratified ocean by a steadily moving anomaly of the atmospheric pressure is studied in a linear statement using a quasi-static approximation. Quantitative estimates for the parameters of internal waves in the wake are obtained for the average distribution of the Väisälä-Brunt frequency in the Caribbean Sea.Translated by Mikhail M. Trufanov.     

Source and magmatic evolution of ocean island basalts from the Pohnpei Island,Northwest Pacific Ocean: Insights from olivine geochemistry

The compositional variability of ocean island basalts (OIBs) is thought to reflect partial melting of a lithologically-heterogeneous mantle source dominated by ...     

Sensitivity of a global ocean model to increased run-off from Greenland

We study the reaction of a global ocean–sea ice model to an increase of fresh water input into the northern North Atlantic under different surface boundary conditions, ranging from simple restoring of surface salinity to the use of an energy balance model (EBM) for the atmosphere. The anomalous fresh water flux is distributed around Greenland, reflecting increased melting of the Greenland ice sheet and increasing fresh water export from the Arctic Ocean. Depending on the type of surface boundary condition, the large circulation reacts with a slow-down of overturning and gyre circulations. Restoring of the total or mean surface salinity prevents a large scale redistribution of the salinity field that is apparent under mixed boundary conditions and with the EBM. The control run under mixed boundary conditions exhibits large and unrealistic oscillations of the meridional overturning. Although the reaction to the fresh water flux anomaly is similar to the response with the EBM, mixed boundary conditions must thus be considered unreliable. With the EBM, the waters in the deep western boundary current initially become saltier and a new fresh water mass forms in the north-eastern North Atlantic in response to the fresh water flux anomaly around Greenland. After an accumulation period of several decades duration, this new North East Atlantic Intermediate Water spreads towards the western boundary and opens a new southward pathway at intermediate depths along the western boundary for the fresh waters of high northern latitudes.     

Carbon cycling within the upper methanogenic zone of continental rise sediments; An example from the methane-rich sediments overlying the Blake Ridge gas hydrate deposits

Data from piston cores collected from Carolina Rise and Blake Ridge, and from many DSDP/ODP sites indicate that extreme ¹³C-depletion of methane and ΣCO₂ occurs within the uppermost methanogenic zone of continental rise sediments. We infer that ¹³C-depleted methane is generated near the top of the methanogenic zone when carbon of ¹³C-depleted ΣCO₂, produced by microbially-mediated anaerobic methane oxidation, is recycled back to methane through CO₂ reduction. Interstitial water and gas samples were collected in 27 piston cores, 16 of which penetrated through the sulfate reduction zone into methane-bearing sediments of the Carolina Rise and Blake Ridge. Isotopic measurements (δ¹³C_CH4, δ¹³C_CO2, δD_CH4, and δD_H2O) indicate that this methane is microbial in origin, produced by microbially-mediated CO₂ reduction. Methane samples form two distinct isotopic pools. (1) Methane from a seafloor seep site shows a mean δ¹³C_CH4 value of − 69 ± 2%., mirroring values found at ≥ 160 mbsf from a nearby DSDP site. (2) Twenty, areally-separated sites (sample depth, 10 to 25 mbsf) have δ¹³C_CH4 values ranging from −85 to −103%., and δ¹³C_CO2 as negative as −48%.. The very low δ¹³C values from the methane and CO₂ pools highlight the importance of carbon cycling within continental rise sediments at and near the sulfate-methane boundary.     

大洋真光层中的新生产力和N的来源

新生产力是理解海洋生物生产和碳循环的重要概念。20世纪80年代初,用地球化学方法得出的大洋的新生产力大大高于用生物方法估计的数值,这一结果在海洋学界引起不小的轰动,并成为海洋学研究的主线之一。在随后的时间里,许多海洋学家致力于解释这一差异形成的原因,直到今天,这一努力还在继续。作者对该研究的发展历程和研究现状做一综述。     

POC export from ocean surface waters by means of Th/U and Po/Pb disequilibria: A review of the use of two radiotracer pairs

²³⁴Th (T_1/2=24.1 d) and ²¹⁰Po (T_1/2=138.4 d) are particle reactive radioisotopes that are used as tracers for particle cycling in the upper ocean. Particulate organic carbon (POC) export has frequently been estimated using ²³⁴Th/²³⁸U disequilibrium. Recent evidence suggests that ²¹⁰Po/²¹⁰Pb disequilibrium may be used as an additional tool to examine particle export, given the direct biological uptake of ²¹⁰Po into cellular material. Differences in these two radioisotope pairs with regard to their half-lives, particle reactivity and scavenging affinity in seawater should provide complementary information to be obtained on the processes occurring in the water column. Here, we review eight different studies that have simultaneously used both approaches to estimate POC export fluxes from the surface ocean. Our aim is to provide a complete “dataset” of all the existing POC flux data derived from the coupled use of both ²³⁴Th and ²¹⁰Po and to evaluate the advantages and limitations of each tracer pair. Our analysis suggests that the simultaneous use of both radiotracers provides more useful comparative data than can be derived from the use of a single tracer alone. The difference in half-lives of ²³⁴Th and ²¹⁰Po enables the study of export production rates over different time scales. In addition, their different biogeochemical behaviour and preferred affinity for specific types of particles leads to the conclusion that ²³⁴Th is a better tracer of total mass flux, whereas ²¹⁰Po tracks POC export more specifically. The synthesis presented here is also intended to provide a basis for planning future sampling strategies and promoting further work in this field to help reveal the more specific application of each tracer under specific water column biogeochemistries.     

Local remineralization patterns in the mesopelagic zone of the Eastern North Atlantic, off the NW Iberian Peninsula

The short-time-scale variability of the remineralization patterns in the domain of Eastern North Atlantic Central Waters (ENACW) off the NW Iberian Peninsula is studied based on biogeochemical data (oxygen, nutrient salts, total alkalinity, pH, dissolved organic matter and fluorescence of dissolved humic substances) collected weekly between May 2001 and April 2002. The temporal variability of inorganic variables points to an intensification of remineralization during the summer and autumn, with an increase of nutrients, total inorganic carbon and fluorescence and a decrease of oxygen. During the subsequent winter mixing, there is a biogeochemical reset of the system, with lower nutrients, total inorganic carbon and fluorescence and higher oxygen. In contrast to inorganic variables, the levels of dissolved organic matter in the ENACW seem to respond to short-term events probably associated with fast sinking particles, where solubilisation of organic matter prevails over remineralization. Applying a previously published stoichiometric model, we observed a vertical fractionation of organic-matter remineralization. Although there is a preferential remineralization of proteins and P compounds in the entire domain of ENACW, the percentage was higher in the upper ENACW (σ<27.10 kg/m³) than in the lower; the percentage of N and P compounds in the oxidised organic matter was >80% for the upper ENACW and 63% for the lower. Likewise, the redissolution of calcareous structures contributes about 6% and 13% to the carbon regenerated in the upper and lower layers of ENACW, respectively.     

Response of a simple two-layer ocean to divergent and rotational wind stress

Response of an unbounded two-layer ocean to traveling atmospheric disturbances is analytically investigated. Under the assumption that (1) lower layer is motionless, (2) each layer is homogeneous and in hydrostatic balance, (3) the Coriolis parameter is constant, and (4) the density difference between the two layers is small compared to the density itself, the one-dimensional Klein-Gordon's equation is solved analytically for both the divergent and rotational wind stress. Numerical examples of the wake pattern of the upwelling behind the regional divergent and rotational wind fields are also represented.The results may be summarized as follows: (1) the effect of the divergent wind stress is dominant when the forced Rossby numberR ₀ (the ratio of the frequency of the wind stress to the Coriolis parameter) is larger than unity unless the Mach numberM (the ratio of the traveling velocity of the wind stress to the internal gravitational velocity) is far less than unity, (2) the effect of the rotational wind stress is important, when the forced Rossby numberR ₀ is less than unity.     

An assessment of particulate organic carbon to thorium-234 ratios in the ocean and their impact on the application of Th as a POC flux proxy

Thorium-234 is increasingly used as a tracer of ocean particle flux, primarily as a means to estimate particulate organic carbon export from the surface ocean. This requires determination of both the ²³⁴Th activity distribution (in order to calculate ²³⁴Th fluxes) and an estimate of the C / ²³⁴Th ratio on sinking particles, to empirically derive C fluxes. In reviewing C / ²³⁴Th variability, results obtained using a single sampling method show the most predictable behavior. For example, in most studies that employ in situ pumps to collect size fractionated particles, C / ²³⁴Th either increases or is relatively invariant with increasing particle size (size classes > 1 to 100s μm). Observations also suggest that C / ²³⁴Th decreases with depth and can vary significantly between regions (highest in blooms of large diatoms and highly productive coastal settings). Comparisons of C fluxes derived from ²³⁴Th show good agreement with independent estimates of C flux, including mass balances of C and nutrients over appropriate space and time scales (within factors of 2–3). We recommend sampling for C / ²³⁴Th from a standard depth of 100 m, or at least one depth below the mixed layer using either large volume size fractionated filtration to capture the rarer large particles, or a sediment trap or other device to collect sinking particles. We also recommend collection of multiple ²³⁴Th profiles and C / ²³⁴Th samples during the course of longer observation periods to better sample temporal variations in both ²³⁴Th flux and the characteristic of sinking particles. We are encouraged by new technologies which are optimized to more reliably sample truly settling particles, and expect the utility of this tracer to increase, not just for upper ocean C fluxes but for other elements and processes deeper in the water column.     

Phytoplankton and iron: validation of a global three-dimensional ocean biogeochemical model

The JGOFS program and NASA ocean-color satellites have provided a wealth of data that can be used to test and validate models of ocean biogeochemistry. A coupled three-dimensional general circulation, biogeochemical, and radiative model of the global oceans was validated using these in situ data sources and satellite data sets. Biogeochemical processes in the model were determined from the influences of circulation and turbulence dynamics, irradiance availability, and the interactions among four phytoplankton functional groups (diatoms, chlorophytes, cyanobacteria, and coccolithophores) and four nutrients (nitrate, ammonium, silica, and dissolved iron).Annual mean log-transformed dissolved iron concentrations in the model were statistically positively correlated on basin scale with observations (P<0.05) over the eight (out of 12) major oceanographic basins where data were available. The model tended to overestimate in situ observations, except in the Antarctic where a large underestimate occurred. Inadequate scavenging and excessive remineralization and/or regeneration were possible reasons for the overestimation.Basin scale model chlorophyll seasonal distributions were positively correlated with SeaWiFS chlorophyll in each of the 12 oceanographic basins (P<0.05). The global mean difference was 3.9% (model higher than SeaWiFS).The four phytoplankton groups were initialized as homogeneous and equal distributions throughout the model domain. After 26 years of simulation, they arrived at reasonable distributions throughout the global oceans: diatoms predominated high latitudes, coastal, and equatorial upwelling areas, cyanobacteria predominated the mid-ocean gyres, and chlorophytes and coccolithophores represented transitional assemblages. Seasonal patterns exhibited a range of relative responses: from a seasonal succession in the North Atlantic with coccolithophores replacing diatoms as the dominant group in mid-summer, to successional patterns with cyanobacteria replacing diatoms in mid-summer in the central North Pacific. Diatoms were associated with regions where nutrient availability was high. Cyanobacteria predominated in quiescent regions with low nutrients.While the overall patterns of phytoplankton functional group distributions exhibited broad qualitative agreement with in situ data, quantitative comparisons were mixed. Three of the four phytoplankton groups exhibited statistically significant correspondence across basins. Diatoms did not. Some basins exhibited excellent correspondence, while most showed moderate agreement, with two functional groups in agreement with data and the other two in disagreement. The results are encouraging for a first attempt at simulating functional groups in a global coupled three-dimensional model but many issues remain.     

Diurnal cycling of carbon monoxide (CO) in the upper ocean near Bermuda

A coupled photochemical–physical model for the upper ocean carbon monoxide concentration is presented. The coupled model uses a spectral optical model and a number of different mixing parameterizations. It is run using the meteorological forcing conditions measured during a 9-day investigation near Bermuda in the Sargasso Sea in March 1993 in the declining phase of the spring bloom. The baseline runs of the model are successful in simulating measurements made during the campaign, and form the basis of an optimization routine to find better estimates of the production and destruction constants of the photochemical tracer. The optimized values derived in this inverse procedure depend on the mixing parameterization and the assumed dark production rate of carbon monoxide.     

Stable carbon isotope distribution of particulate organic matter in the ocean: a model study

The stable carbon isotopic composition of particulate organic matter in the ocean, δ¹³C_POC, shows characteristic spatial variations with high values in low latitudes and low values in high latitudes. The lowest δ¹³C_POC values (−32‰ to −35‰) have been reported in the Southern Ocean, whereas in arctic and subarctic regions δ¹³C_POC values do not drop below −27‰. This interhemispheric asymmetry is still unexplained. Global gradients in δ¹³C_POC are much greater than in δ¹³C_DIC, suggesting that variations in isotopic fractionation during organic matter production are primarily responsible for the observed range in δ¹³C_POC. Understanding the factors that control isotope variability is a prerequisite when applying δ¹³C_POC to the study of marine carbon biogeochemistry. The present model study attempts to reproduce the δ¹³C_POC distribution pattern in the ocean. The three-dimensional (3D) Hamburg Model of the Oceanic Carbon Cycle version 3.1 (HAMOCC3.1) was combined with two different parametrizations of the biological fractionation of stable carbon isotopes. In the first parametrization, it is assumed that the isotopic fractionation between CO₂ in seawater and the organic material produced by algae, _P, is a function of the ambient CO₂ concentration. The two parameters of this function are derived from observations and are not based on an assumption of any specific mechanism. Thus, this parametrization is purely empirical. The second parametrization is based on fractionation models for microalgae. It is supported by several laboratory experiments. Here the fractionation, _P, depends on the CO₂ concentration in seawater and on the (instantaneous) growth rates, μ_i, of the phytoplankton. In the Atlantic Ocean, where most field data are available, both parametrizations reproduce the latitudinal variability of the mean δ¹³C_POC distribution. The interhemispheric asymmetry of δ¹³C_POC can mostly be attributed to the interhemispheric asymmetry of CO₂ concentration in the water. However, the strong seasonal variations of δ¹³C_POC as reported by several authors, can only be explained by a growth rate-dependent fractionation, which reflects variations in the cellular carbon demand.