Increasing hypoxia in the Changjiang Estuary during the last three decades deciphered from sedimentary redox-sensitive elements

Abstract

Ranked as one of the world's largest seasonal hypoxic water bodies (Dissolved oxygen, DO≤2 mg l−1), the Changjiang hypoxia has been reported to grow rapidly worse in recent decades according to cruise observations, but it has been seldom studied by sedimentary records. In this paper, four gravity cores (E1-E4), retrieved from the Changjiang Estuary, were dated by excess 210Pb and analyzed with grain-size compositions, total organic carbon (TOC) contents, and RSEs (redox sensitive elements) compositions. We aim to decipher RSEs enrichment characteristics and controlling mechanisms in the Changjiang Estuary. The results show that Mo and V enrichment is highly promoted by particle absorption and FeMn redox cycling at the shallow estuarine environment (E1 and E2) with occasional hypoxic disturbances. Scavenging of Mo and V by organic complexation becomes significant at the hypoxic center (E3), together with great influence by FeMn redox cycling, but they do not work effectively for U enrichment because of its easy remobilization and abundant riverine input. Moreover, upcore increasing trends of Mo, V, and U in E3 match well with a general lowering trend of bottom water DO minima since the mid-1980s. There are two progressive hypoxic development stages intercalated with a less DO-depleted period 1991–1997 as shown by both cruise observations and RSEs/Al records. These findings are vital to better understanding coastal hypoxic development and RSEs enrichment mechanisms in the seasonal hypoxic settings, because hypoxia is predicted to increase in the near future due to intensifying human disturbances.