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

Abstract

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 μm) 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° particles belong to cubical system, Fm3m space group with a varying from 4.028×10 −1 nm to 4.059×10 −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 Al 3+ 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. native aluminum, cold seep, gas hydrate, reducing environment, South China Sea Citation:Chen Z, Huang C Y, WU B H, et al. Discovery of native aluminum and its possible origin from prospective gas hydrate areas in the South China Sea.

Geochemical characteristics of sediment and pore water affected by cold seeps in southern South China Sea

“…The trough areas such as Zengmu, Wan' an, Nanwei, Nansha and Beikang in southern SCS are reported to store rich hydrocarbon gas and gas hydrate resources, and southern SCS has tectonic environment conducive to the migration of hydrocarbon-rich fluids and extensive methane outlets (Zhu et al, 2001;Wang et al, 2006;Yao, 2007;Zhang et al, 2010;Liu et al, 2011;Trung, 2012;Wei et al, 2012). In the Beikang Basin, some evidences that may indicate existence of cold seeps have been found, including some geophysical features (such as BSR) and authigenic minerals (such as authigenic pyrite) (Figure 1) (Wang et al, 2003;Su et al, 2005;Chen et al, 2009;Chen Z. et al, 2010;Liu et al, 2011). Nonetheless, till now there are only two studies focusing on the activities of cold seeps in the Beikang Basin (Yao, 2007;Li et al, 2018;, their studies showed that the sulfatemethane transition zones (SMTZ) of the four sites are between 5.3 and 8.8 mbsf and the dissolved SO 2− 4 was predominantly consumed by the anaerobic oxidation of methane (AOM) at rates between 27 and 44 mmol m −2 yr −1 only by geochemical characteristics and model of porewater near our study area in the Beikang Basin .…”