The evolution of hypoxia off the Changjiang Estuary in the last 3000 years: Evidence from benthic foraminifera and elemental geochemistry

Abstract

Estuarine hypoxia has exerted great influence on the ecological system and fishery production in the past decades, consequently attracting intensive concern. In this study, the upper 18.0 m of Core YD0902, retrieved from the hypoxic area off the Changjiang Estuary, was dated by AMS 14C method and analyzed with lithology, grain size, benthic foraminifera, and geochemical elements to decipher the paleoenvironmental change and hypoxic history in the last 3000 years. The core location was in the neritic to prodelta transitional environment during 3000–2160 cal yr BP and the delta-front environment after 2160 cal yr BP. No hypoxia occurred during 3000–2600 cal yr BP, potentially related to the contemporary occurrence of a rapid cold climate event in the northern hemisphere. Hypoxia has likely existed persistently since 2600 cal yr BP. The period 2600–600 cal yr BP characterizes persistently severe hypoxia mainly linked with the warm climate, strong East Asian Summer Monsoon (EASM) and rainfall, intensified Taiwan Warm Current intrusion and upwelling, and high primary production. Lessened hypoxia followed in the period 600–260 cal yr BP because of the cold climate, weak EASM, and minor precipitation. The recent period (post 260 cal yr BP) was not disclosed by the core due to lost sediments in the top 4.5 m, and could only be surmised to have severe hypoxia because of climate factors similar to the period 2600–600 cal yr BP. Our findings shed new light on long-term hypoxic evolution and controlling factors in mega-river estuaries.