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Sediment Dynamics and Morphodynamics of River-Sea Sediment Dispersal Systems through Space and Time: A Source-to-Sink Perspective
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The evolution of hypoxia off the Changjiang Estuary in the last ~3,000 years: evidence from the benthic foraminiferal assemblages
P-G1-05-S
Fahui Ren* , State Key Laboratory of Marine Geology, Tongji University
Daidu Fan, State Key Laboratory of Marine Geology, Tongji University
Quanhong Zhao, State Key Laboratory of Marine Geology, Tongji University
Yijing Wu, State Key Laboratory of Marine Geology, Tongji University
Jianfeng Su, State Key Laboratory of Marine Geology, Tongji University
Presenter Email: fahui1229@126.com |
| Coastal hypoxia exerts increasing impacts on the ecological system and fishery production, consequently attracting increased concern in the past decades. A lot of efforts have been made to better understand hypoxia trigger mechanism and future development tendency under the global change and increasing human disturbances. The hypoxia off the Changjiang Estuary occurs seasonally with a relatively stable core area. A continuous long core of YD0902 has been retrieved from the hypoxia core area in 2009. The core strata have been studied in detail and the stratal chronology has been established by the AMS14C dating result. In this study, hypoxia conditions will be reconstructed using benthic foraminifera assemblages, including the A-P index (A% + P%) and LOFA index (Bolivina spp.% + Bulimina marginata% +Epistominella naraensis%), together with sedimentary elemental ratios Mn/Ti and Cd/Al.
The results show that, the benthic dissolved oxygen (DO) should be high during 3.00~2.56 cal. ka BP, and then it decreased rapidly to produce hypoxia. The former was inferred to link with the rapid cold climate event in the northern hemisphere during 3.50~2.50 cal. ka BP (Mayewski, 2004). Alternations of mild and severe hypoxia were identified over centennial scales with severe stages: 1.98~1.84, 0.80~0.76 and 0.48~0.42 cal. ka BP, versus mild stages: 2.36~2.26, 2.20~2.14, 1.84~1.60, 1.42~1.28 and 0.30~0.16 cal. ka BP. The sea level was considered to be relatively stable in the last ~3000 years, so we assume that the alternations of severe and mild hypoxia events were mainly linked to the enhanced East Asia Summer Monsoon (EASM) and cold climate events, respectively. The enhanced EASM could increase precipitation in the Changjiang drainage basin and river discharge into the sea, resulting in water stratification and eutrophication off the estuary to enhance the hypoxia. The cold climate events in the late Holocene, such as the Little Ice Age (LIA) of Ming and Qing dynasty (AD 1550~1851), could make the EASM weaken and precipitation decrease, so hypoxia would become weaker. Recently, the impact of anthropogenic activities on hypoxia has been observed to grow significantly. |
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