Oxygen concentration at time series station DYNAPROC

Continuous measurements between 0 and 200 m depth were performed every 2 h over two separate periods of four days at a station in the open northwestern Mediterranean Sea (Dyfamed Station) during the Dynaproc cruise in May 1995. Estimates of the daily variations in profiles of temperature, partial pressure of CO2, oxygen, chlorophyll a and nutrients were obtained. The distributions of the various physical and chemical properties were clearly different during the two time series, which were separated by a period of 11 days during which a wind event occurred. The mean daily utilization or production due to biological processes of dissolved inorganic carbon (DIC), nitrate+nitrite and oxygen were calculated along isopycnals using a vertical diffusion model. Between the surface and about 20 m depth, DIC was consumed and O2 released during the two time series while the nitrate+nitrite concentrations as well as supplies were zero. After the wind event, the O2 : C : N ratios of consumption (or production) were, on average, near the Redfield ratios, but during the first time series, the C : N utilization ratio between 20 and 35 m was two to three times that of Redfield stoichiometry and the oxygen release was low. The integrated net community production (NCP) in terms of carbon was equivalent during the two time series, whereas the chlorophyll a biomass was twice as high, on average, during the first time series but did decrease. These results imply that the production systems were different during the two periods. The first time series corresponds to a period at the end of production, due to the nutrient depletion in the euphotic layer. The formation of degradation products of the living material in dissolved organic form is probably important as indicated by the high C : N utilization ratios. The second time series corresponds to a reactivation of the primary production due to the upward shift of nutrients after the wind event.

Observations on the propagation of internal tidal waves on the continental slope off Sierra Leone

During R/V Meteor-cruise no. 30 4 moorings with 17 current meters were placed on the continental slope of Sierra Leone at depths between 81 and 1058 meters. The observation period started on March 8, 1973, 16.55 hours GMT and lasted 19 days for moorings M30_068MOOR, M30_069MOOR, M30_070MOOR on the slope and 9 days for M30_067MOOR on the shelf. One current meter recorded at location M30_067MOOR for 22 days. Hydrographic data were collected at 32 stations by means of the "Kieler Multi-Meeressonde". Harmonic analysis is applied to the first 15 days of the time series to determine the M2 and S2 tides. By vertically averaging of the Fourier coefficients the field of motion is separated into its barotropic and its baroclinic component. The expected error generated by white Gaussian noise is estimated. To estimate the influence of the particular vertical distribution of the current meters, the barotropic M2 tide is calculated by ommitting and interchanging time series of different moorings. It is shown that only the data of moorings M30_069MOOR, M30_070MOOR and M30_067MOOR can be used. The results for the barotropic M2 tide agree well with the previous publications of other authors. On the slope at a depth of 1000 m there is a free barotropic wave under the influence of the Coriolis-force propagating along the slope with an amplitude of 3.4 cm S**-1. On the shelf, the maximum current is substantially greater (5.8 cm s**-1) and the direction of propagation is perpendicular to the slope. As for the continental slope a separation into different baroclinic modes using vertical eigenmodes is not reasonable, an interpretation of the total baroclinic wave field is tried by means of the method of characteristis. Assuming the continental slope to generate several linear waves, which superpose, baroclinic tidal ellipses are calculated. The scattering of the direction of the major axes M30_069MOOR is in contrast to M30_070MOOR, where they are bundled within an angle of 60°. This is presumably caused by the different character of the bottom topography in the vicinity of the two moorings. A detailed discussion of M30_069MOOR is renounced since the accuracy of the bathymetric chart is not sufficient to prove any relation between waves and topography. The bundeling of the major axes at M30_070MOOR can be explained by the longslope changes of the slope, which cause an energy transfer from the longslope barotropic component to the downslope baroclinic component. The maximum amplitude is found at a depth of 245 m where it is expected from the characteristics originating at the shelf edge. Because of the dominating barotropic tide high coherence is found between most of the current meters. To show the influence of the baroclinic tidal waves, the effect of the mean current is considered. There are two periods nearly opposite longshore mean current. For 128 hours during each of these periods, starting on March 11, 05.00, and March 21, 08.30, the coherences and energy spectra are calculated. The changes in the slope of the characteristics are found in agreement with the changes of energy and coherence. Because of the short periods of nearly constant mean current, some of the calculated differences of energy and coherence are not statistically significant. For the M2 tide a calculation of the ratios of vertically integrated total baroclinic energy and vertically integrated barotropic kinetic energy is carried out. Taking into account both components (along and perpendicular to the slope) the obtained values are 0.75 and 0.98 at the slope and 0.38 at the shelf. If each component is considered separately, the ratios are 0.39 and 1.16 parallel to the slope and 5.1 and 15.85 for the component perpendicular to it. Taking the energy transfer from the longslope component to the doenslope component into account, a simple model yields an energy-ratio of 2.6. Considering the limited application of the theory to the real conditions, the obtained are in agreement with the values calculated by Sandstroem.

Anundance of phytoplankton at time series station DYNAPROC

A general study of biogeochemical processes (DYNAPROC cruise) was conducted in May 1995 at a time-series station in the open northwestern Mediterranean Sea where horizontal advection was weak. Short-term variations of the vertical distributions of pico- and nanophytoplankton were investigated over four 36-h cycles, along with parallel determinations of metabolic CO2 production rates and amino acid-containing colloid (AACC) concentrations at the chlorophyll maximum depth. The vertical (0-1000-m depth) distributions of (i) AACC, (ii) suspended particles and (iii) metabolic CO2 production rate were documented during the initial and final stages of these 36-h cycles. This study was concerned with diel vertical migration (DVM) of zooplankton, which provided periodic perturbations. Accordingly, the time scale of the experimental work varied from a few hours to a few days. Although all distributions exhibited a periodic behaviour, AACC distributions were generally not linked to diel vertical migrations. In the subsurface layer, Synechococcus made the most abundant population and large variations in concentration were observed both at day and at night. The corresponding integrated (over the upper 90 m) losses of Synechococcus during one night pointed to a potential source of exported organic matter amounting to 534 mg C/m**2. This study stresses the potential importance of organic matter export from the euphotic zone through the daily grazing activity of vertically migrating organisms, which would not be accounted for by measurements at longer time scales. The metabolic CO2 production exhibited a peak of activity below 500 m that was shifted downward, apparently in a recurrent way and independently of the vertical distributions of AACC or of suspended particulate material. To account for this phenomenon, a 'sustained wave train» hypothesis is proposed that combines the effect of the diel superficial faecal pellet production by swarming migrators and the repackaging activity of the nonmigrating midwater populations. Our results confirm the recent finding that the particulate compartment is not the major source of the observed instantaneous remineralisation rate and shed a new light on the fate of organic matter in the aphotic zone.

The photo-physiological response of a model cnidarian-dinoflagellate symbiosis to CO2-induced acidification at the cellular level

We measured the relationship between CO2-induced seawater acidification, photo-physiological performance and intracellular pH (pHi) in a model cnidarian-dinoflagellate symbiosis - the sea anemone Aiptasia sp. -under ambient (289.94 ± 12.54 µatm), intermediate (687.40 ± 25.10 µatm) and high (1459.92 ± 65.51 µatm) CO2 conditions. These treatments represented current CO2 levels, in addition to CO2 stabilisation scenarios IV and VI provided by the Intergovernmental Panel on Climate Change (IPCC). Anemones were exposed to each treatment for two months and sampled at regular intervals. At each time-point we measured a series of physiological responses: maximum dark-adapted fluorescent yield of PSII (Fv/Fm), gross photosynthetic rate, respiration rate, symbiont population density, and light-adapted pHi of both the dinoflagellate symbiont and isolated host anemone cell. We observed increases in all but one photo-physiological parameter (Pgross:R ratio). At the cellular level, increases in light-adapted symbiont pHi were observed under both intermediate and high CO2 treatments, relative to control conditions (pHi 7.35 and 7.46 versus pHi 7.25, respectively). The response of light-adapted host pHi was more complex, however, with no change observed under the intermediate CO2 treatment, but a 0.3 pH-unit increase under the high CO2 treatment (pHi 7.19 and 7.48, respectively). This difference is likely a result of a disproportionate increase in photosynthesis relative to respiration at the higher CO2 concentration. Our results suggest that, rather than causing cellular acidosis, the addition of CO2 will enhance photosynthetic performance, enabling both the symbiont and host cell to withstand predicted ocean acidification scenarios.

Simulated Methyl iodide emission and concentration using the ocean biogeochemistry model MPIOM/HAMOCC forced by OMIP data

Production pathways of the prominent volatile organic halogen compound methyl iodide (CH3I) are not fully understood. Based on observations, production of CH3I via photochemical degradation of organic material or via phytoplankton production has been proposed. Additional insights could not be gained from correlations between observed biological and environmental variables or from biogeochemical modeling to identify unambiguously the source of methyl iodide. In this study, we aim to address this question of source mechanisms with a three-dimensional global ocean general circulation model including biogeochemistry (MPIOM-HAMOCC (MPIOM - Max Planck Institute Ocean Model HAMOCC - HAMburg Ocean Carbon Cycle model)) by carrying out a series of sensitivity experiments. The simulated fields are compared with a newly available global data set. Simulated distribution patterns and emissions of CH3I differ largely for the two different production pathways. The evaluation of our model results with observations shows that, on the global scale, observed surface concentrations of CH3I can be best explained by the photochemical production pathway. Our results further emphasize that correlations between CH3I and abiotic or biotic factors do not necessarily provide meaningful insights concerning the source of origin. Overall, we find a net global annual CH3I air-sea flux that ranges between 70 and 260 Gg/yr. On the global scale, the ocean acts as a net source of methyl iodide for the atmosphere, though in some regions in boreal winter, fluxes are of the opposite direction (from the atmosphere to the ocean).

Geochemistry and isotopic ratios of boninites and related rocks of the Izu-Bonin Forearc

Twenty-six samples representing the wide range of lithologies (low- and intermediate-Ca boninites and bronzite andesites, high-Ca boninites, basaltic andesites-rhyolites) drilled during Leg 125 at Sites 782 and 786 on the Izu-Bonin outer-arc high have been analyzed for Sr, Nd, and Pb isotopes. Nd-Sr isotope covariations show that most samples follow a trend parallel to a line from Pacific MORB mantle (PMM) to Pacific Volcanogenic sediment (PVS) but displaced slightly toward more radiogenic Sr. Pb isotope covariations show that all the Eocene-Oligocene samples plot along the Northern Hemisphere Reference Line, indicating little or no Pb derived from subducted pelagic sediment in their source. Two young basaltic andesite clasts within sediment do have a pelagic sediment signature but this may have been gained by alteration rather than subduction. In all isotopic projections, the samples form consistent groupings: the tholeiites from Site 782 and Hole 786A plot closest to PMM, the boninites and related rocks from Sites 786B plot closest to PVS, and the boninite lavas from Hole 786A and late boninitic dikes from Hole 786B occupy an intermediate position. Isotope-trace element covariations indicate that these isotopic variations can be explained by a three-component mixing model. One component (A) has the isotopic signature of PMM but is depleted in the more incompatible elements. It is interpreted as representing suboceanic mantle lithosphere. A second component (B) is relatively radiogenic (epsilon-Nd = ca 4-6; 206Pb/204Pb = ca 19.0-19.3; epsilon-Sr = ca -10 to -6)). Its trace element pattern has, among other characteristics, a high Zr/Sm ratio, which distinguishes it from the ìnormalî fluid components associated with subduction and hotspot activity. There are insufficient data at present to tie down its origin: probably it was either derived from subducted lithosphere or volcanogenic sediment fused in amphibolite facies; or it represents an asthenospheric melt component that has been fractionated by interaction with amphibole-bearing mantle. The third component (C) is characterized by high contents of Sr and high epsilon-Sr values and is interpreted as a subducted fluid component. The mixing line on a diagram of Zr/Sr against epsilon-Sr suggests that component C may have enriched the lithosphere (component A) before component B. These components may also be present on a regional basis but, if so, may not have had uniform compositions. Only the boninitic series from nearby Chichijima would require an additional, pelagic sediment component. In general, these results are consistent with models of subduction of ridges and young lithosphere during the change from a ridge-transform to subduction geometry at the initiation of subduction in the Western Pacific.