Fractionation of germanium and silicon during scavenging from seawater by marine Fe (oxy)hydroxides: Evidence from hydrogenetic ferromanganese crusts and nodules

Abstract

Germanium (Ge) and silicon (Si) are an element pair which shows rather coherent behaviour in many geochemical systems. In aqueous systems and in the critical zone, however, Ge and Si may fractionate from each other, leading to slight differences between the Ge/Si ratios of continental crust, river water and seawater. We here show that even stronger fractionation of the GeSi couple occurs within seawater during the formation of hydrogenetic ferromanganese (FeMn) crusts and nodules. We determined Ge/Si ratios of hydrogenetic FeMn crusts from the Pacific Ocean following a new analytical approach using laser-ablation ICP-MS on nano-particulate tablets prepared from the FeMn crust material. Data quality was monitored and demonstrated by repetitive measurements of FeMn nodule reference material (RM) NOD-P1 from the Pacific Ocean (Ge/Siwt: 16.8 × 10−6 g/g; Ge/Simol: 6.47 × 10−6 mol/mol), which is the only RM for which high-quality GeSi reference data are available. We also report Ge/Si ratios for FeMn nodule RMs NOD-A1 from the Atlantic Ocean (Ge/Siwt: 24.9 × 10−6; Ge/Simol: 9.6 × 10−6) and for JMn-1 from the Pacific Ocean (Ge/Siwt: 17.9 × 10−6; Ge/Simol: 6.9 × 10−6). The Ge/Siwt ratios of the hydrogenetic FeMn crusts from the Pacific Ocean range from 13.2 × 10−6 to 33.5 × 10−6 (Ge/Simol: 5.1 × 10−6 to 13 × 10−6). The Ge/Si ratios of both FeMn crusts and nodules are significantly higher than those of ambient seawater (ca. Ge/Siwt: 1.7 × 10−6; Ge/Simol: 0.7 × 10−6). Their Ge/Si ratios are also well-above those of potential (alumino)silicate detritus. Hence, these high Ge/Si ratios reveal fractionation of Ge and Si during scavenging from seawater by the iron (oxy)hydroxide component in FeMn crusts, and do not result from the presence of detrital material in the chemical precipitate. Their high Ge/Si ratios relative to seawater also suggest that despite higher complexation constants with organic ligands for Ge than Si, dissolved organic ligands do not control marine GeSi fractionation via preferential retention of Ge in seawater.