Jean-Luc Fuda

Effect of natural iron fertilization on carbon sequestration in the Southern Ocean

The availability of iron limits primary productivity and the associated uptake of carbon over large areas of the ocean. Iron thus plays an important role in the carbon cycle, and changes in its supply to the surface ocean may have had a significant effect on atmospheric carbon dioxide concentrations over glacial–interglacial cycles. To date, the role of iron in carbon cycling has largely been assessed using short-term iron-addition experiments. It is difficult, however, to reliably assess the magnitude of carbon export to the ocean interior using such methods, and the short observational periods preclude extrapolation of the results to longer timescales. Here we report observations of a phytoplankton bloom induced by natural iron fertilization—an approach that offers the opportunity to overcome some of the limitations of short-term experiments. We found that a large phytoplankton bloom over the Kerguelen plateau in the Southern Ocean was sustained by the supply of iron and major nutrients to surface waters from iron-rich deep water below. The efficiency of fertilization, defined as the ratio of the carbon export to the amount of iron supplied, was at least ten times higher than previous estimates from short-term blooms induced by iron-addition experiments. This result sheds new light on the effect of long-term fertilization by iron and macronutrients on carbon sequestration, suggesting that changes in iron supply from below—as invoked in some palaeoclimatic and future climate change scenarios—may have a more significant effect on atmospheric carbon dioxide concentrations than previously thought.

XBT monitoring of a meridian section across the western Mediterranean Sea

During the Thetis-2/MAST-2 tomography experiment, T7-XBT calibrated (accuracy ∼0.05°C) probes were launched ∼28 km apart between France and Algeria, twice a month from Feb. to Sep. 1994. Combined with infrared images, altimetric data and ship drifts, they provide definite information on the structure, drift and role of the eddy-like mesoscale phenomena generated by the Algerian Current instability. When embedded in this alongslope current, these phenomena generally propagate downstream at a few km/day and are markedly asymmetrical. Because of the topography in the eastern part of the Algerian Basin, they separate from the current, become more symmetrical and follow an anticlockwise circuit in the open basin. These phenomena are deeper than ∼750 m and entrain seaward pieces of the Levantine Intermediate Water (LIW) vein flowing along the Sardinian slope, thus being responsible of the large spatial and temporal variability of the LIW distribution in the open basin. The non-existence of a LIW vein flowing westward across the Algerian Basin is definitely demonstrated. In the Gulf of Lions, new insights are provided into the formation and spreading of the Winter Intermediate Water (WIW), which is the WesternMediterranean counterpart of LIW. Considering the large amount of WIW formed during this mild winter, it is clear that this water has not received enough attention yet, and is certainly a major component of the Mediterranean outflow at Gibraltar. Finally, the XBT data account for the eastward flow of the WesternMediterranean Deep Water (WMDW) off Algeria.

Warming, salting and origin of the Tyrrhenian Deep Water

[1]xa0Data collected from 1996 to 2001 down to 3,500 m in the Tyrrhenian sub-basin with ship-handled and moored instruments show 5-year T and S trends (0.016 °C/yr, 0.008/yr) that are the largest ever evidenced in Mediterranean deep waters. This is not consistent with the usual hypothesis that Tyrrhenian Deep Water (TDW) is a mixture of eastern water flowing from the Sicily Channel and western water flowing from the Sardinia Channel partly since both are reported to encounter lower trends. We argue that TDW might result from a dense water formation process occurring within the Tyrrhenian itself, in a region never reported up to now, east of the Bonifacio Strait. Whatever the validity of our hypothesis, climatic changes are occurring in the whole sea and are efficiently specified with long time series.

Mission results from the first GEOSTAR observatory (Adriatic Sea, 1998)

We assess the first mission of the GEOSTAR (GEophysical and Oceanographic STation for Abyssal Research) deep-sea multidisciplinary observatory for its technical capacity, performance and quality of recorded data. The functioning of the system was verified by analyzing oceanographic, seismological and geomagnetic measurements. Despite the mission’s short duration (21 days), its data demonstrated the observatory’s technological reliability and scientific value. After analyzing the oceanographic data, we found two different regimes of seawater circulation and a sharp and deepening pycnocline, linked to a down-welling phenomenon. The reliability of the magnetic and seismological measurements was evaluated by comparison with those made using on-land sensors. Such comparison of magnetic signals recorded by permanent land geomagnetic stations and GEOSTAR during a “quiet” day and one with a magnetic storm confirmed the correct functioning of the sensor and allowed us to estimate the seafloor observatory’s orientation. The magnitudes of regional seismic events recorded by our GEOSTAR seismometer agreed with those computed from land stations. GEOSTAR has thus proven itself reliable for integrating other deep-sea observation systems, such as modular observatories, arrays, and instrumented submarine cables.

European Seafloor Observatory Offers New Possibilities For Deep Sea Study

The Geophysical and Oceanographic Station for Abyssal Research (GEOSTAR), an autonomous seafloor observatory that collects measurements benefiting a number of disciplines during missions up to 1 year long, will begin the second phase of its first mission in 2000. The 6–8 month investigation will take place at a depth of 3400 m in the southern Tyrrhenian basin of the central Mediterranean. n nGEOSTAR was funded by the European Community (EC) for

Comparison of XCTD/CTD data

2.4 million (U.S. dollars) in 1995 as part of the Marine Science and Technology programme (MAST). The innovative deployment and recovery procedure GEOSTAR uses was derived from the “two-module” concept successfully applied by NASA in the Apollo and space shuttle missions, where one module performs tasks for the other, including deployment, switching on and off, performing checks, and recovery. The observatory communication system, which takes advantage of satellite telemetry, and the simultaneous acquisition of a set of various measurements with a unique time reference make GEOSTAR the first fundamental element of a multiparameter ocean network.

The East Caledonian Current: A Case Example for the Intercomparison between AltiKa and In Situ Measurements in a Boundary Current

XCTD (eXpendable Conductivity Temperature Depth) probes, developed recently by SIPPICAN Inc., have been used simultaneously with a CTD sonde in order to test, in the field, their performance and accuracy (interpreted as ±2 standard deviations of the XCTD-CTD differences). We have taken advantage, during the THETIS-I experiment in March 1992, of both the homogeneous and the stratified areas encountered in winter in the northern part of the western Mediterranean Sea to differentiate the errors due to the experimental conditions from those effectively due to the sensors. Although some intrinsic problems are evident, so that only seven out of the nine probes considered for comparison are usable, the accuracy specified by the manufacturer for the temperature (AT = ± 0.03°C) is reached after standard processing, while the accuracies in conductivity, salinity and potential density are AC ≈ ± 0.06 mS/cm (the specified value is AC = ± 0.03 mS/cm), AS ≈ ± 0.04 and Aσθ ≈ ±3 kg/m3. However, when the experimental errors (in situ natural variability, relatively rough estimation of the XCTD depth) are considered, it appears that the effective accuracies of the XCTD sensors are better than ± 0.02°C and ± 0.04 mS/cm, that is to say better than and close to the specified values of ± 0.03°C and ± 0.03 mS/cm. Occasional offsets in conductivity can further be well corrected for by using a temperature-salinity relation in some limited depth range and area where this relation is known to hold well; the conductivity-sensor accuracy then significantly improves to AC≈ ± 0.02 mS/cm resulting, for our study area, in corresponding salinity and potential density accuracies of AS≈ ± 0.03 and Aσθ ≈ ± 0.02 kg/m3. Thus, such instruments promise to be useful tools for many experimental studies. Complementary comparisons, performed with new versions of the XCTD probes under less convenient experimental conditions, are also presented

Deep submarine gas vents in the aeolian offshore

ABSTRACT This paper presents an assessment of SARAL/AltiKa satellite altimeter for the monitoring of a tropical western boundary current in the south-western Pacific Ocean: the East Caledonian Current. We compare surface geostrophic current estimates obtained from two versions of AltiKa along-track sea level height (AVISO 1 Hz and PEACHI 40 Hz) with two kinds of dedicated in situ datasets harvested along the satellite ground tracks: one deep-ocean current-meter mooring deployed in the core of the boundary current and five glider transects. It is concluded that the AltiKa-derived current successfully captures the velocity of the boundary current, with a standard error of 11 cm/s with respect to the in situ data. It also appears important to reference AltiKa sea level anomaly to the latest mean dynamic topography available in our area. Doing so, Ka-band altimetry provides a satisfactory representation of the western boundary current. Thereby, it usefully contributes to observing its variability in such a remote and under-observed ocean region. However, the rather long repeat period of SARAL (35 days) in comparison to the high frequency variability seen in the flow velocity of the boundary current calls for a combined use of SARAL with the other satellite altimetry missions.

GEOSTAR-scientific goals of the project and results of the first test phase

Abstract Consistent results, concerning the detection of deep submarine hydrothermal vents, were obtained during two cruises (1991 and 1996) offshore from the Aeolian Islands, by CTD profiling from sea surface down to seafloor, and water-sampling casts. In 1991 an echo sounder showed a wide plume at a depth of about 800 m, within which water samples displayed anomalies in He and NH a content, suggesting also the presence of a water-vapour phase. The latter, in 1996, was remarkably observed as a horizontally diffusing plume at about 350 m. Near-plume casts were characterised by high CO2 and CH4 and low O2 concentrations in seawater, disturbed light transmission profiles, and false bottom outputs appearing at ∼300–350 m down to the seafloor from the rosette-mounted altimeter. No significant temperature/salinity anomalies were noted during either events. These preliminary results show the presence of deep hydrothermal activity, over an area where, one century ago, the occurrence of submarine eruptions was detected.

Large warming and salinification of the Mediterranean outflow due to changes in its composition

GEOSTAR is a project funded by the European Union (EU) in the framework of the Marine Science and Technology Programme (MAST-III, CT95-0007). Its aim is to realise the first prototype of an abyssal benthic laboratory, following the scientific recommendation of the EU for deep sea research and in continuity with previous studies promoted by the EU itself in the framework of the MAST-II Programme. The prototype of GEOSTAR is presently equipped with sensors for geophysical, geochemical and physico-oceanographic measurements. The whole set of sensors includes a three-component seismometer, two magnetometers (scaler and bi-axial), CTD, a transmissometer, an electrochemical package for the measurement of pH, Eh, H/sub 2/, H/sub 2/S, and a short range ADCP. The biaxial magnetometer and the electrochemical sensor package have been properly designed and realised for GEOSTAR; the other sensors have been modified to be integrated in GEOSTAR and adapted to the mission scientific goal. The project, started in November 1995, is to be completed at the end of October 1998, after the first shallow water test mission.