Laurent Memery

Simulation of primary production and export è uxes in the Northwestern Mediterranean Sea

A biogeochemical model, BIOMELL (BIOgeochemical Model of the Euphotic Layer of Lodyc), has been developed to simulate the temporal evolution of the main nitrogen stocks and e uxes at the DyFAMed station (Dynamique des FluxAtmospheriques en Mediterranee), located in the Northwest- ern Mediterranean Sea. This mainly oligotrophic region is characterized by a strong seasonal cycle, and a signie cant export of dissolved organic matter compared to the particulate export measured by sediment traps. Validation of the model is made using temperature, nitrate and chlorophyll proe les acquired at DyFAMed approximately every month in 1991. Extended datasets from specie c years are also used to validate seasonal variations of other variables, for which the coverage in 1991 was rather poor (new and total production, particulate export, dissolved organic matter export, bacteria, zooplankton). Sensitivity studies on selected parameters are carried out in order to give an idea of the margin of error in the model predictions. The model is then used to analyze the behavior of the biogeochemical system during various production regimes (winter, spring bloom, oligotrophy, autumn bloom). It is shown that several processes, which are often neglected in biogeochemical models, must be taken into account: phytoplankton growth limitation by deep mixing, C:Chl ratio dependence on light, nitrie cation, and semi-refractory dissolved organic matter accumulation in the surface layer.

The coupled physical‐new production system in the equatorial Pacific during the 1992–1995 El Niño

We investigate the coupling between the physics and new production variability during the period April 1992 to June 1995 in the equatorial Pacific via two cruises and simulations. The simulations are provided by a high-resolution Ocean General Circulation Model forced with satellite-derived weekly winds and coupled to a nitrate transport model in which biology acts as a nitrate sink. The cruises took place in September-October 1994 and sampled the western Pacific warm pool and the upwelling region further east. The coupled model reproduces these contrasted regimes. In the oligotrophic warm pool the upper layer is fresh, and nitrate-depleted, and the new production is low. In contrast, the upwelling waters are colder, and saltier with higher nitrate concentrations, and the new production is higher. Along the equator the eastern edge of the warm pool marked by a sharp salinity front, also coincides with a “new production front”. Consistent with the persistent eastward surface currents during the second half of 1994, these fronts undergo huge eastward displacement at the time of the cruises. The warm/fresh pool and oligotrophic region has an average new production of 0.9 mmol NO3 m−2 d−1, which is almost balanced by horizontal advection from the central Pacific and by vertical advection of richer water from the nitrate reservoir below. In contrast, the upwelling mesotrophic region shows average new production of 2.1 mmol NO3 m−2 d−1 and the strong vertical nitrate input by the equatorial upwelling is balanced by the losses, through westward advection and meridional divergence of nitrate rich waters, and by the biological sink.

The distribution of helium 3 in the deep western and southern Indian Ocean

Almost a decade after the Geochemical Ocean Sections Study Indian Expedition, the new deep 3He data from the INDIGO program give a further insight into the distribution of this tracer in the Indian Ocean. This distribution exhibits some major features related on one hand to a hydrothermal 3He input in the Gulf of Aden and on the Mid-Indian Ocean Ridge, and on the other to the origin of the water masses and to the characteristics of the deep circulation. The main pattern is a significant north-south 3He gradient, with deep waters of the southern ocean showing δ3He values around 8–9% due to the influence of the Atlantic deep waters poor in 3He and relatively high values in the northern and central regions (15% to 18% between 2000 m and 3000 m depth) originating from the hydrothermal activity. In the easternmost part of the basin, the 3He values exhibit a significant increase at shallower depths (around 1000 m) probably due to the Pacific water flow through the Indonesian sills, whereas the data in the Indian sector of the Antarctic ocean show a maximum of the order of 10%, south of the Polar Front, interpreted as showing the presence of the Pacific deep waters in the Antarctic Circumpolar Current. These different aspects are summarized by mapping the horizontal distribution of the δ3He maxima all over the Indian basin. This map points out some characteristics of the deep circulation but also stresses the need for further measurements in order to clarify the description of this tracer in several key areas.

3He and chlorofluorocarbons (CFC) in the Southern Ocean: tracers of water masses

Abstract The distribution of 3 He across the Southern Ocean is depicted on the basis of a meridional section between Antarctica and South Africa measured during the INDIGO-3 survey (1988). A core of δ 3 He values above 10% is observed south of the Polar Front, associated with very low CFC concentrations. This 3 He enriched layer is documented from the GEOSECS and INDIGO 3 He data in the Southern Ocean. It is found at a density level around σ θ =27.8 in all the waters close to Antarctica (i.e. south of 50°S ). Its zonal distribution suggests that it is likely that it originates from the central/eastern Pacific. Hence, it provides an indication of the deep Pacific waters in the Antarctic Circumpolar Current, which are not easily detectable from the standard hydrographie parameters.