Gallium behavior and isotopic compositions in marine siliceous sediments from the southern Mariana Trench

Abstract

To further constrain the contribution of marine siliceous sediments in the subduction zones to the oceanic gallium (Ga) cycle, we investigated Ga geochemical behavior by examining the mineral, elemental and Ga isotopic compositions of siliceous sediments from the southern Mariana Trench (SMT). The results show that Ga contents vary from 15.6 to 17.6 μg/g (average = 16.7 μg/g) in the lower part of the sediment core but the lower Ga contents (2.7 to 10.9 μg/g, average = 5.9 μg/g) in the upper part. The systematic variation in geochemistry and mineralogical compositions indicates that abundant Ga in the lower part originates from volcanogenic/basaltic rocks, while the addition of biogenic SiO2 to the upper part dilutes the concentration of these source-rock components, resulting in the concomitant decreases in contents of Ga and other elements. Particularly, reverse weathering leads to an increase of Ga in the top section of the upper part by favoring an uptake of Ga in the seawater into aluminosilicates, as evidenced by the range of pH, Al/Nb, Ga/Nb, and Ga/Al variations. In contrast, δ71Ga values (relative to the Ga-IPGP standard) show a monotonous range from −0.06 to 0.05 ‰ (average = −0.01 ‰, n = 17), which is consistent with those for basalts (0.00 ± 0.05 ‰). We find that the addition of biogenic SiO2 and reverse weathering do not significantly change the Ga isotopic compositions in the SMT marine siliceous sediments. Collectively, our study highlights that marine siliceous sediments in the SMT may act as an isotopically light sink of Ga in the modern ocean via reverse weathering, and could shed new light for understanding the oceanic Ga cycles.