Yu
Zhigang
Yu
Zhigang
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ArticleClosing the global marine Ra-226 budget reveals the biological pump as a dominant removal flux in the upper ocean(American Geophysical Union, 2022-06-10) Xu, Bochao ; Cardenas, M. Bayani ; Santos, Isaac R. ; Burnett, William C. ; Charette, Matthew A. ; Rodellas, Valenti ; Li, Sanzhong ; Lian, Ergang ; Yu, ZhigangRadium isotopes are powerful proxies in oceanography and hydrology. Radium mass balance models, including assessments of submarine groundwater discharge (SGD), often overlook particle scavenging (PS) as a pathway for dissolved radium removal from the world ocean. Here, we build a global ocean 226Ra mass balance model and reevaluate the potential importance of PS. We find that PS is the major 226Ra sink for the upper ocean, removing about 96% of the total input from various sources. Aside from vertical exchange with the lower ocean, SGD is the largest 226Ra source into the upper ocean. The biological pump transfers particles to the deep ocean, resulting in a major but often overlooked impact on the global 226Ra marine budget. Our findings suggest that radium mass balance models should consider PS in systems with high siliceous algae production and export fluxes and long water residence times to prevent underestimation of large-scale SGD fluxes.
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ArticleA new perspective for assessing water transport and associated retention effects in a large reservoir(John Wiley & Sons, 2018-09-23) Xu, Bochao ; Yang, Disong ; Yao, Peng ; Burnett, William C. ; Ran, Xiangbin ; Charette, Matthew A. ; Huang, Xinying ; Liu, Sumei ; Yu, ZhigangRadioactive tracer techniques may be useful for assessing water transport and the overall effects of concurrent biogeochemical processes in river‐reservoir systems. In this study, we show that radium isotopes can assess the hydrodynamics and sediment/nutrient retention in the Xiaolangdi Reservoir, the largest impoundment along the Yellow River, China. Activity ratios of 224Ra/226Ra and 223Ra/226Ra were used for water mass age calculations in the riverine, transition, and lentic reaches of the reservoir. Water ages were combined with the length scale of three river‐reservoir zones to determine water transport rates of 3.6 ± 1.2, 1.3 ± 0.3, and 0.16 ± 0.14 km/day, respectively. Radium ages were also used to quantify the net retention of sediment and nutrients in different parts of the river‐reservoir system. Suspended sediment was removed at a rate of 1.4 ± 0.6 g/m3/day, mainly in the riverine zone. Nutrient dynamics were more complicated, with addition or removal at different rates within the three zones.
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ArticleA benthic monitor for coastal water dissolved oxygen variation: Mn/Ca ratios in tests of an epifaunal foraminifer(American Geophysical Union, 2021-12-17) Guo, Xiaoyi ; Wei, Qinsheng ; Xu, Bochao ; Burnett, William C. ; Bernhard, Joan M. ; Nan, Haiming ; Lian, Ergang ; Yang, Shouye ; Yu, ZhigangAn appropriate proxy could help to better understand dissolved oxygen variations in the past, helping to predict potential outcomes of future environmental changes. In the Changjiang Estuary (China), the foraminifer Cribrononion subincertum (C. subincertum) shows a distinct population maximum in the topmost sediment, an indication of an epifaunal species. Therefore, the geochemical composition of C. subincertum tests could record changes in the region’s bottom water chemistry. Our results showed that Mn/Ca ratios in tests of living (Rose-Bengal stained) C. subincertum analyzed by LA-ICP-MS were responsive to variations of bottom water dissolved oxygen concentrations, with average foraminiferal Mn/Ca ratios three times higher during low-oxygen period than in winter. In the uppermost centimeters of sediment, wider ranges of foraminiferal Mn/Ca occurred in summer compared to winter ranges. Our results imply that this epifaunal benthic foraminiferal species could serve as a useful benthic monitor with the Mn/Ca ratios representing a reliable proxy of hypoxia in the past.