Discovery of native aluminum and its possible origin from prospective gas hydrate areas in the South China Sea

Aluminum is an active amphoteric metal element, but native aluminum (Al) can be formed and preserved in special conditions so that its occurrence has unique geological significance. In this paper, the coarse fractions (> 63 mu m) of 269 samples of surface sediments and of 165 sediment samples in four cores from the South China Sea (SCS) were picked out and analyzed. Al particles were found in only five surface samples and three samples in two sedimentary cores. The particles are grayish or silver white in color with a strongly metallic luster and high plasticity, and they occur as irregular plats, elongated shapes and spherules with botryoidal structures. The identified Al degrees particles belong to cubical system, Fm3m space group with a varying from 4.028x10(-1) nm to 4.059x10(-1) nm. Their chemical compositions consist mainly of Al (95.07%-99.84%) and minor amounts of Si, Fe, Ti, S, Zn, Mg, Ca and trace amounts of K, Na, Cu, Co, P and Mn. Our results show that the identified Al is authigenic, formed in situ in the prospective areas of gas hydrates, and that it is an unreported type of Al occurrence. A possible mechanism of Al formation is proposed: under the strongly reducing micro environments at cold seeps, dissolved Al3+ in sediments is reduced to its metallic state by the strongly reducing agent of H-2 through microbial processes and enzyme action. As a result, the Al particles from the prospective gas hydrate areas in the SCS are the special products, representing the first report on Al from the sediments at cold seeps. The proposed mechanism provides a theoretical basis for further studies on the special biogeochemical processes at cold seeps.