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Sedimentary Records Of Hypoxia In The Changjiang Estuary Over Last 100 Years

Posted on:2010-10-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:X W FengFull Text:PDF
GTID:1100360302479560Subject:Geology
Abstract/Summary:PDF Full Text Request
Seasonal hypoxia areas (dead zones) in the estuaries and coastal oceans have spread exponentially since the 1960s and have serious consequences for ecosystem functioning especialy changed significantly the benthic ecosystem. As far no special monitoring or observating programmes focused on the seasonal hypoxia in the Changjiang Estuary in China. The purpose of this study was to reconstruct the hypoxia history by the core sediments in the Changjiang Estuary. So two cores were selected for comparison study, one within the region of seasonal hypoxia and the other nearby but outside the hypoxia region. The geological and geochemistrical analyzing methods were used to (a) reveal the distribution characteristic of the biogenic elements, stable organic carbon and nitrogen isotopic ratios, redox sensitive elements (RSE) and the mineral phase; (b) select several hypoxia marked indicators combined with historic and in-situ data on watershed environment; (c) deduce the onset of eutrophic and hypoxia in the Changjiang Estuary as well as the developing trend; (d) discuss the relations between the hypoxia anthropogenic activities especially as like the fertilizer application.The grain size and elements, including ten kinds of major elements and thirteen kinds of trace elements were determined on the 210Pb-dated sediment cores. The two core sediments had different resources and represented different element distribution characteristics. Major sources of the core sediment outside the hypoxia region were from the old Huanghe estuary, and the distributions of most elements were controlled by "grain size effects". The sources of the core sediment within the hypoxia region were mainly from the riverine input of the Changjiang River in summer and the burial fluxes of the dead marine biology. The distribution of some redox sensitive elements (RSE) and biogenic elements did not conform to "grain size effects". The Mo, Cd, As et al. have been enriched significantly, which concentrations increased about 83%, 73% and 50% respectively since the late 1960s to 1970s. Conversely the Mn has been depleted since 1970s. The results indicate the redox environment of the bottom water-sediment interface has been changed which might be caused by the eutrophication and hypoxia in the Changjiang Estuary. The concentrations of biogenic elements as well as Ca, Sr, P have also increased about 129%, 65% and 38% respectively since the late 1960s, which reflected the increasing of productivity and biomass influenced by the anthropogenic activities during the last forty years. With the X-ray diffraction (XRD) method the mineral phase characteristics were studied and implied the two cores sediments had the stable but different sources. The contents of calcite increased in some degree over the last 100 years in the two cores, which distribution consisted with the Ca's. The results demonstrated the increasing of calcinosis organism productivity in the Changjiang Estuary and its adjacent area.The total organic carbon (TOC), total nitrogen (TN), biogenic silica (BSi), stable carbon and nitrogen isotopic ratios (δ13 Corg andδ15 N) were determined on the 210Pb-dated sediment core. The results showed the concentrations of BSi were almost the constant, the concentrations of TOC and TN increased before 1940s~1950s andδ13 Corg increased before 1970s in the core sediments outside the hypoxia region, which reflected the biomass and the productivity hardly changed in the past several decades. For the core sediments within the hypoxia region, the concentrations of TOC, TN, BSi as well as their sedimentation fluxes have increased in some degree since 1970s. TOC, TN fluxes increased about 45%, 36% respectively. The averageδ13Corg value of the core was -23.67‰which remained nearly constant before the twentieth century. Theδ13 Corg values increased after 1900s, two marked increases were observed from 1950s and 1970s. A simpleδ13Corg model was used to estimate the contribution of terrestrial and marine organic matter inputs for the sediment, which indicated the increase in accumulation since 1970s was almost exclusively marine. The increasing of marine organic matter accumulation (TOC, TN and BSi) was corresponding with the increasing of fertilizer consumption and the NO3-N budgets from the Changjiang River. The riverine runoff of fertilizers and nutrients stimulated the algae blooming. Enhanced primary production resulted in an enrichment of organic matter in the sediment. These data support the hypothesis that anthropogenic nutrient loading has been a significant factor on the eutrdphication in Changjiang Estuary.The results showed that the contents of carbon and nitrogen stable isotope, fluxes of TOC, biogenic elements as well as Ca, Sr, P and some RSEs just like Mo,Cd, As in sediments could be used to trace or reconstruct history of the coastal eutrophication and hypoxia in the high productivity zone in the Changjiang Estuary over last 100 years. These parameters gave the same conclusions consistently: the estuary eutrophication in the Changjiang Estuary and its adjacent region began in 1950s and accelerated in 1970s, then the enhanced eutrophication has caused and developed hypoxia since 1970s which has been fueled and showed the increasing trend.
Keywords/Search Tags:Changjiang Estuary, eutrophication and hypoxia, total organic carbon (TOC), total nitrogen (TN), biogenic silica (BSi), stable carbon and nitrogen isotopes (δ13 Corg andδ15N), redox sensitive element (RSE), sedimentary environment effects
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