Geochemistry of marine sediments of the Brazilian Northeastern continental shelf

Abstract

The marine sediment samples collected from the northeastern Brazilian continental shelf, at water depths between 20 and 80 m, consisted mainly of sands with an almost total absence of gravel and granules. Medium, coarse and very coarse sand grains are mostly composed of halimeda, lithothamnium, rodoliths and bioclastic sands with a carbonate content varying between 77 and 96 %. The chemistry in general shows a decreasing content of Ca (86.1 %) >Si (6 %) > Cl (3.6 %) > Sr (0.8 %) > K (0.66 %) > S (0.62 %) > Al (0.6 %) > Na (0.55%) > Mg (0.43 %) > Fe (0.4 %) > P (0.2 %) > Br (0.04 %) in the samples. There was no correlation between CaCO3 and chemical contents and grain size with depth and biocomponents. With the exception of Sr of marine origin, all other elements (P, S, Br, Cl, Fe) are of continental origin. The lithothamnium of some offshore samples shows higher CaCO3 content, while Mg and Na are present only in halimedas. Bioclastic sands contain no Br, and silt and clay fractions are rare and characterize samples closer to the coast. These marine bioclastic granulates are of very pure biogenic calcium carbonates and are thus highly to be recommended for economic purposes.