BIOGEOCHEMICAL CYCLES IN THE EASTERN MEDITERRANEAN; ACTUO (SEASONAL/ANNUAL) VARIATIONS <B>BIOGEOCHEMICAL CYCLES IN THE EASTERN MEDITERRANEAN; ACTUO (SEASONAL/ANNUAL) VARIATIONS</B> <HR><P> Ziveri</B><SUP>1</SUP><B>,P., De Lange</B><SUP>2</SUP><B>, G. J., Rossignol-Strick</B><SUP>3</SUP><B>, M., Schrader</B><SUP>4</SUP><B>, H. & Thomson</B><SUP>5</SUP><B>, J.</B><I><P><P> 1Dept. of Earth Sciences, University of Milan, Milan, Italy; present address: Geomarine Center, Free University, Amsterdam, the Netherlands<BR> 2Inst. of Earth Sciences, Dept. of Geochemistry, Utrecht, The Netherlands<BR> 3Lab. of Palaeontology and Stratigraphy, University of P. and M. Curie, Paris, France<BR> 4Geological Institute, University of Bergen, Bergen, Norway<BR> 5Southampton Oceanography Centre, United Kingdom</I><P><P> The eastern Mediterranean is a perfect natural laboratorium for biogeochemical processes. In only few places in the world, the deep pelagic sediment can be compared between oxic and anoxic conditions. In addition, the eastern Mediterranean is recognised to be a perfect recorder of global climatic variations. The occurrence of deep anoxic brines in the eastern Mediterranean offers the unique opportunity to study quantity and composition of particle fluxes from same site, under oxic and anoxic conditions. A study of biogenic and non-biogenic fluxes is necessary so as to determine the boundary conditions for the origin of climate related variations in sediment compositions / preservation.<P><P> The direct measurement of ocean fluxes has become possible through the development of sediment traps capable of collecting particle fluxes at deep stations, continuously, in time fractionated sequences, over a long period. Deployment of sediment traps in deep water layers has thus provided qualitative and quantitative information on the time and spatial variability of ocean particle rain rates. <P><P> Two time-series sediment traps (Technicap PPS5/2) have been deployed in the south western Bannock Basin (34(o)17.8'N-20(o)01'E, Libeccio sub-basin), a central eastern Mediterranean anoxic basin (Fig. 1), to allow comparison of fluxes and their preservation under oxic and anoxic conditions. The trap sampling programme at Bannock Basin was started in November 1991 and ended in August 1994, collecting 4 sets of 24 samples for each trap, the total been 192 samples. The opening-closing sampling schedule was synchronised for the two traps in approximately 10-day intervals. The traps have been moored at 2900 dbar and 3700 dbar of water depth, the deepest trap was at 60 dbar above the bottom of the basin, the seawater/brine interface is located at 3331 dbar (Fig. 2). To our knowledge, this is the first deployment of sediment traps in the deep eastern Mediterranean, in such oxic/anoxic environment. This study will therefore, form the basis for the missing knowledge of particle fluxes in the eastern Mediterranean. The trap samples were processed immediately upon retrieval: the material of each sample was divided and filtered immediately on board on glass microfiber membranes (0.8micrometer(s) pore size; 47 mm diameter). Those samples from the trap below the seawater/brine interface were processed in a nitrogen-filled (anoxic) glove box, because of potential oxidative alteration of the lower trap samples from anoxic conditions, those from above the interface at ambient atmosphere. The relatively low present day productivity and related particle flux have been clearly shown in the very small amount of material collected by the trap samples, particularly those from the trap in the oxic water. A special methodology and equipment have to be developed in order to analyse those extremely small samples.<P><P> The principal objective is to understand the nature and the magnitude of inorganic, organic and biogenic fluxes throughout the 3-year period. The particles collected in the shallower trap, in oxic condition, are usually originated in the upper layers and clearly recorded the seasonal pattern; the fluxes in the deeper trap are strictly related to preservation and resedimentation. In particular, our approach include the study of delivery fluxes of radio nuclides in the light of seasonal productivity and the export production of the major calcareous and siliceous phytoplankton, and zooplankton produced in an oligotrophic environment and how these patterns are imprinted in the deeper trap dominated by anoxic conditions. The suspended matter in the brines largely contribute to the particle fluxes. The biogenic fluxes are 2 - 3 orders of magnitude higher in the anoxic trap samples and the seasonal signal, clearly recorded in the shallower trap, is largely hidden. During the 3-year study, the biogenic particle fluxes are always higher in the trap positioned in the brines. Coccolithophores are the major contributor to the biogenic carbonate flux in the basin and are also the dominant phytoplankton group. In particular seasonal peaks in coccosphere fluxes (a coccosphere represents an individual organism whereas the coccoliths covering the coccosphere are calcite platelets different in number per species and life cycle) are recorded with delay in the anoxic trap (Figs. 3 and 4). There is a major difference in the opal flux between the upper and lower traps. The opal flux is always much greater in the lower traps.<P><P> Clear spring/early summer peaks are observed in the delivery fluxes of all radio nuclides to the oxic trap in all 3-year study, which contrast of low early winter fluxes recorded in the same trap. The flux record is more complex in the trap in the brine. In the oxic water trap, the greater part of the <SUP>2</SUP><SUP>1</SUP><SUP>0</SUP>Po (Fig. 5) and <SUP>2</SUP><SUP>3</SUP><SUP>0</SUP>Th measured has been sorbed from solution by settling material, whereas the bulk <SUP>2</SUP><SUP>3</SUP><SUP>0</SUP>Th and <SUP>2</SUP><SUP>3</SUP><SUP>8</SUP>U are trace components in detrital material. As documented in the biogenic fluxes, the seasonal pulse is recorded by all four radio nuclides only in the oxic trap.<BR> This is a contribution of MAST2 Palaeoflux Programme (MAS2-CT93-0051)<P><P>