Suspended matter and bottom deposits of the Orinoco delta: Turbidity, mineralogy and elementary composition
Eisma, D.; Van der Gaast, S.J.; Martin, J.-M.; Thomas, A.J. (1978). Suspended matter and bottom deposits of the Orinoco delta: Turbidity, mineralogy and elementary composition. Neth. J. Sea Res. 12(2): 224-251
In: Netherlands Journal of Sea Research. Netherlands Institute for Sea Research (NIOZ): Groningen; Den Burg. ISSN 0077-7579; e-ISSN 1873-1406, meer
A three-layer stratification was found in the Boca Grande estuary: a layer of fluid mud near the bottom, a layer of low salinity and low turbidity at the surface, and a salt wedge of high salinity and low turbidity in between. The position of the fluid mud layer is related to the nodal point where the movement of the bottom water changes from a resultant landward to a resultant seaward direction. It is not clear to what extent the formation of the fluid mud is influenced by the dredging of the navigation channel and to spoiling practices. The data on sediment supply from the Orinoco river and from the Amazon along the north coast of South America indicate that about twice as much sediment is supplied by the Amazon to the Orinoco delta as by the Orinoco river. The clay mineral and elementary composition of the Orinoco delta sediments supports this. In the Orinoco delta iron- and aluminiumhydroxydes are adsorbed onto clay minerals and other particles as coatings and form chlorite-like structures with swelling minerals. This prevents swelling and makes that the swelling minerals are not recognized as such with normal X- ray diffraction procedures when the hydroxydes are not removed. It is possible that through adsorption of hydroxydes on degraded illites also some swelling minerals are formed, resulting in relatively low illite contents and high kaolinite-illite ratios. pyrophyllite is present in the Orinoco river sediments in large quantities, the supply being in the order of 106 tons y-1. It is considered to be of natural origin. The elemental composition of the sediments of the Orinoco delta, the Amazon estuary and the Gulf of Paria is very homogeneous. The distribution of most elements, when normalized to Al2O3, does not show any significant variation in relation to salinity, going from the river to the outer delta, which indicates that mobilization processes are insignificant or masked by strong mixing of Orinoco and Amazon sediment. Only the contents of Ca, Mg, Sr and B increase in the outer delta due to the inflow of marine carbonate and uptake of B from sea water. Removal of silica, iron, aluminium and boron from solution, if this occurs, also does not lead to a zonal accumulation of these elements in the bottom deposits of the estuary.
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