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

Water samples from the Lena River and stratified Laptev Sea (northeast ern Siberia) have been analyzed to determine their stable oxygen isoto pe composition (O-18/O-16). Measurements at the Lena River reference s tation give a deltaO-18 value of -18.9 parts per thousand in both surf ace and bottom waters. In the brackish water surface plume, a nearly p erfect correlation is found between deltaO-18 and chlorinity deltaO-18 = -18.9 + 0.787Cl- (n = 15; r = 0.999) A few values lie distinctly be low 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-plu me correlation line. Dissolved silicate concentrations exhibit a large variability. However, when they are related to the different water ma sses identified using oxygen isotope data, a more coherent picture is obtained. Concentrations in the surface plume decrease more or less re gularly from 50 to 72 mumol in the Lena River, to 7 mumol at the 'mari ne' end-member (Cl- = 14 g l-1). Dissolved silicate results in the Buo rkhaya Gulf are quite distinct, with a clear deficiency in the surface waters, and an excess in the deep waters. These deltaO-18 and dissolv ed silicate variations are discussed in relation to the hydrology and the biological productivity of the investigated area.