Mauritanian Upwelling Research Articles

Interpretation of the nitrogen isotopic signal variations in the Mauritanian upwelling with a 2D physical‐biogeochemical model

A physical‐biogeochemical model is used to simulate the evolution of the δ15N signal during the last glacial‐interglacial transition in sedimentary cores offshore from the Mauritanian upwelling. The biological model is a classical nitrogen‐based trophic chain model, which also computes the nitrogen isotope fractionation. The 2D physical primitive equation model simulates the coastal upwelling circulation and is applied for different sea level scenarios. The effect of the sea level rise, inducing the shelf immersion, seems to be a main factor explaining the organic nitrogen flux and isotopic signal variations along the last deglaciation. This effect is modulated by an upwelling seasonality that may have been much longer at the Last Glacial Maximum, around 10–11 months instead of 4–5 months at present. Between 15 and 5.5 kyr ago, 60% of the sedimentary δ15N variations could be explained by this local shelf immersion effect. This reconstruction also reproduces the strong isotopic fall occurring between 5.5 kyr and the present.

Ship-borne measurements of ocean colour: Development of a CCD-based reflectance radiometer and trials on a longitudinal transect of the Atlantic Ocean

A ship-borne reflectance radiometer was constructed capable of making sequential downward irradiance (Ed) and upward radiance (Lu) measurements from a fixed position over a wavelength range from 350 to 800 nm with 4 nm spectral resolution. The radiometer used a cooled CCD array as the sensing element and a bifurcated fibre optic bundle for the optical inputs. It was mounted on a ship's superstructure so that the radiance collector viewed the sea surface at the Brewster angle (53° to the vertical) through a linear polarizing filter. Comparative measurements were made with the polarizer oriented horizontally and vertically in order to estimate the spectral effect of skylight reflected directly from the sea surface on reflectance spectra (Lu/Ed) measured in air. Regression algorithms based on waveband ratios were devised to relate the reflectance spectra to surface chlorophyll concentrations and to the diffuse attenuation coefficient for downwards irradiance at 490nm. The results obtained were well correlated with in situ optical measurements and with chemical analyses of water samples. Ship-borne reflectance data obtained during an Atlantic Meridianal Transect cruise from the UK to the Falklands in September 1997 showed high particle concentrations in the English Channel and high chlorophyll concentrations in the Mauritanian upwelling and the south Atlantic.

Modeling δ<SUP>15</SUP>N evolution: First palaeoceanographic applications in a coastal upwelling system

The delta(15)N Signal in marine sediments appears to be a good palaeoceanographic tracer. It records biological processes in the water column and is transferred to and preserved in the sediments. Changes in forcing factors in upwelling systems may be recorded by delta(15)N. These forcing conditions can be of a biogeochemical nature, such as the initial isotopic signal of the nutrients or the trophic structure, or of a physical nature, such as wind stress, insolation, temperature or dynamic recycling. A simple nitrogen-based trophic chain model was used to follow the development of the nitrogen isotopic signal in nutrients, phytoplankton, zooplankton and detritus. Detrital delta(15)N, influenced by the isotopic signature of the upwelled nutrients and isotopic fractionation along the trophic chain (photosynthesis and zooplankton excretion), was then compared to the sedimentary signal measured off Mauritania. In our model, the biological variables are transported at shallow depths by a simple circulation scheme perpendicular to the coast depicting a continental shelf recirculation cell. Because cell length depends on the extension of the continental shelf, modifications of the cell length mimic sea level changes. Long cell length (high sea lever) scenarios produce higher delta(15)N values whereas short cell length scenarios result in lower values as in the glacial low sea level periods. Despite changes in many climatic parameters throughout this period, our results show that changing the sea level is sufficient to reconstruct the main pattern of the sedimentary delta(15)N variations offshore of the Mauritanian upwelling, i.e. an increase from about 3 parts per thousand to 7 parts per thousand during the deglaciation, without invoking any change in nitrogen fixation or denitrification.

Continuous pCO<sub>2</sub> measurements in surface water of the Northeastern tropical Atlantic

The partial pressure of CO 2 in the area of the northeastern tropical Atlantic influenced by the Mauritanian upwelling is highly variable in time and space. Partial pressures between 300 and 450 µatm were observed in autumn 1991 and spring 1992. In the oceanic region off the Mauritanian coast, the range of spatial variations in pCO 2 was low (between 365 and 395 µ atm, in autumn, and between 332 and 353 µ atm in spring). In this area, the seasonal pCO 2 variations were mainly due to temperature effects. Even in the upwelling region, where the biomass was high, there was not a single relationship valid for the whole area between chlorophyll fluorescence and pCO 2 . On the other hand, pCO 2 at a constant temperature was, on an average, decreasing with temperature. A comparison with older data shows that pCO 2 in the surface seawater of the open ocean has evolved as the atmospheric pCO 2 the last twenty years. DOI: 10.1034/j.1600-0889.47.issue1.9.x