Constraints on redox conditions in the Japan Sea in the last 47,000 years based on Mo and W as palaeoceanographic proxies

Abstract

Both molybdenum (Mo) and tungsten (W) form soluble oxyanions in oxic seawater, whereas Mo forms insoluble thiomolybdate and W forms soluble thiotungstate in sulfidic seawater. Thus, concentrations and stable isotope ratios of Mo and W in sediments may fluctuate due to changes in redox conditions and can be used to estimate paleoenvironmental changes. The modern Japan Sea is oxic from the surface to the bottom, whereas deep water became anoxic several times from the late Pleistocene to the Holocene. Detailed information on redox conditions is still lacking. In this study, we analyzed a sediment core that was collected from offshore Iwanai, Hokkaido (43°22ʹ36ʺ N, 140°04ʹ10ʺ E, water depth 900 m). To the best of our knowledge, our study is the first to report stable isotope data of Mo and W in sediments of the Japan Sea. We observed maxima in the Mo concentration of up to 29 ppm in the sediment layers of 11–10 ka, 17–14 ka (the last glacial maximum), 31 ka, and 45 ka in accordance with the maxima of total sulfur, thereby indicating the deposition of thiomolybdate MoO_xS_4−x²⁻ (0 ≤ x ≤ 3). δ⁹⁸Mo, however, was between –0.19 and 0.69‰ at these ages, suggesting that the H₂S concentration in bottom water never exceeded 11 μmol kg⁻¹. The concentration and isotopic ratio of W were relatively constant throughout the core; W = 1.2 ± 0.2 ppm and δ¹⁸⁶W = 0.03 ± 0.03‰ (ave ± sd). The authigenic Mo and W ratio, Mo_auth/W_auth (mol/mol), was 10.5 ± 7.3 except for the above four ages, supporting the control of Mn and Fe (oxyhydr)oxides on Mo_auth and W_auth under oxic conditions.