18O ABUNDANCE AND DISSOLVED SILICATE IN THE LENA DELTA AND LAPTEV SEA (RUSSIA)

Water samples from the Lena River and stratified Laptev Sea (northeastern Siberia) have been analyzed to determine their stable oxygen isotope composition (O-18/O-16). Measurements at the Lena River reference station give a deltaO-18 value of -18.9 parts per thousand in both surface and bottom waters. In the brackish water surface plume, a nearly perfect correlation is found between deltaO-18 and chlorinity deltaO-18 = -18.9 + 0.787Cl- (n = 15; r = 0.999) A few values lie distinctly below this correlation; they all correspond to surface samples collected in the semi-enclosed Buorkhaya Gulf, and they most likely reveal the occurrence of 'old' water masses. Some of the deltaO-18 values in the deep waters collected in the same zone also fall below the surface-plume correlation line. Dissolved silicate concentrations exhibit a large variability. However, when they are related to the different water masses identified using oxygen isotope data, a more coherent picture is obtained. Concentrations in the surface plume decrease more or less regularly from 50 to 72 mumol in the Lena River, to 7 mumol at the 'marine' end-member (Cl- = 14 g l-1). Dissolved silicate results in the Buorkhaya Gulf are quite distinct, with a clear deficiency in the surface waters, and an excess in the deep waters. These deltaO-18 and dissolved silicate variations are discussed in relation to the hydrology and the biological productivity of the investigated area.