Tracing continental weathering and estuarine redox conditions with U/Th isotopes in Pearl River estuarine sediments

  • 蔡 孟涵

Student thesis: Master's Thesis

Abstract

Estuary is an important region where the fluxes of dissolved and particulate weathering materials in rivers transport from continent into the ocean and lots of biogeochemical reactions occurred To understand the weathering processes and deposited environment recorded in sediments This study employed a new method to investigate the U/Th isotope disequilibrium respectively in authigenic and detrital phases of estuarine sediments and showed that U-series disequilibrium in authigenic and/or detrital phases of estuarine sediments could provide useful information on the changes of continental weathering and/or land-sea interactions at the estuary in the present and past A sediment core (P11) was collected from the Pearl River estuary and sediments sampled from various depths of the core were further separated into the authigenic and detrital phases using 6N HCl leaching technique which has been confirmed that do not affect the U/Th activity ratio with a set of leaching experiment U-series radioisotopes 210Pb (via 210Po) 234U 238U 228Th 230Th 232Th and 228Ra in authigenic and detrital phases were measured respectively with alpha-counting techniques following radiochemical purification procedures Results show that excess 210Pb activity exhibits frequent fluctuation with a general decreasing trend suggesting periodic changes in 210Pb flux driven by climate change From this decreasing trend the average sedimentation rate of P11 core is dated to be 3 56 ± 0 89 cm/yr In the authigenic phases both 238U and 234U activities decrease with the depth implying that the estuarine water and the underlying sediments become more and more reducing toward the present which supported by the (228Th/228Ra) activity ratio (228Ra/232Th) ratios show that there is an enrichment of 228Ra and seem to correlate with the BaSO4 in detrital phase The (234U/238U) ratio in the detrital phase is measured to be mostly near unity reflecting that the Pearl River sediments have experienced a relatively rapid chemical weathering The relatively lower ratios at some depths are mostly due to alpha recoil effect in association with the physical weathering of rocks This conclusion is further supported by (230Th/234U) in the detrital phases with (230Th/234U) > 1 indicating the preferential release of 234U due to chemical weathering The correlation between chemical weathering and 210Pbex can also be found and is coincident with the significant climate circulation This study shows that by separating authigenic from detrital phases uranium isotope disequilibrium can provide more valuable information than that based on total sediments It will be a useful tool to understand a variety of continental/estuarine processes such as the origin of estuarine sediments the chemical/physical weathering rate of continental rocks and changes in estuarine biogeochemical process driven by human activity and/or natural climate change
Date of Award2014 Jun 18
Original languageEnglish
SupervisorShangde Luo (Supervisor)

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