Sedimentary Geochemical Records Of Environmental Changes In The Changjiang Estuary And Its Adjacent Waters For The Past Century

This study investigated the geochemical characterics and environmental significances of biogenic elements and trace elements in sediments of the Changjiang Estuary and its adjacent waters, then established the geochemical proxies of environmental changes and further revealed the correlations between sedimentary records and environmental changes in the Changjiang Estuary and its adjacent waters. The following tasks have been completed:1) The geochemical behavior including sources and variations of particulate inorganic and organic P and N in the Changjiang Estuary and adjacent waters was investigated, and the budgets of particulate P and N were calculated;2) The sources, governing factors and budgets of biogenic elements (TOC, TN, BSi, TP), potential harmful elements (PHE:V, Cr, Co, Ni, Cu, Zn, Mo, Cd, Pb, Bi) and chemical forms of P and PHEs in surface sediments of the Changjiang Estuary and adjacent waters were investigated, and the environmental indicative significances of these elements were revealed.3) The correlations of sedimentary biogenic elements,13C, P forms, trace elements and their forms in two sediment cores from the Zhejiang coast and the offshore cold-eddy with environmental changes in waters were discussed, and the histories of natural and human-induced environmental changes in the Changjiang Estuary and adjacent waters for the past hundred years were reconstructed. Major results and conclusions are as the following:1. Constitution, sources, variations and budgets of particulate P and N in the Changjiang Estuary and adjacent waters were revealed. Particulate inorganic P (PIP) and organic N (PON) were the major constitutions of particulate P and N, respectively. PIP was mainly input from the Changjiang. POP was from both marine biological input and terrestrial input with the former contributed more than79%of total input. Higher than89%of particulate N was from phytoplankton. PIP was stable, but most of the POP and TPN were recycled.PIP and PON were the major components of particulate P and N, accounting for57.2±11.4%of TPP and78.2±16.8%of TPN, respectively. Distributions and variations of particulate P and N illustrated that PIP was mainly from the Changjiang River and the Jiangsu coastal current (JCC). POP was from both terrestrial and marine biological input. PIN and PON were primarily of marine source. Impacted by the seasonal variations of primary productivity and Changjiang sediment load, the concentrations of particulate P and N were higher in spring than those in autumn, while the percentage of organic component were lower in spring. The budgets of particulate P and N showed that Changjiang sediment was the major source of SPM in the Changjiang Estuary and adjacent area, contributing88%of SPM influx in spring and summer. Fifty-seven percent of SPM was finally buried in seabed. Phytoplankton input contributed87%and79%of total POP input in spring-summer and autumn-winter, respectively.95%in spring-summer and89%in autumn-winter of particulate N were from phytoplankton input. The influx and outflow of PIP were largely equal. The missing flux of POP and TPN accounted for92%and95%of their total influx, respectively, implying that most of the particulate organic matter likely was dissolved or mineralized.2. In-depth knowledge of the sources, governing factors and environment indicative significances of biogenic elements and P species in sediments were gained. The Changjiang estuary, the JCC and marine biological input were three major sources of biogenic elements in sediments. In the coastal core sediment, the variations of Fe-P, Al-P, Det-P, BSi and δ13C were used to reflect the dry/wet variations of the Changjiang runoff for the past hundred years; Increases in grain size-normalized elements in the upper core recorded the aggravated eutrophication and elevation of productivity in inshore waters after the1990s. In the offshore core, the variations of TOC, TN,δ13C, BSi, Ex-P, Fe-P and lea-OP reflected the periodic fluctuations of the strength of the East Asia Winter Monsoon.The distributions of biogenic elements and P species showed that TOC and Lea-OP were from both river and marine biological inut. According to δ13C, about34-70%of organic matter was of terrestrial origin in coastal sediments while66-79%was of marine source in offshore sediments. TN was governed by marine primary productivity and BSi mainly originated from diatom. Fe-P, Al-P and Det-P were primarily of terrestrial sources from the Changjiang and the JCC. CFA-P was impacted by material input from the JCC and calcium organisms. TOC, TN, TP and non-apatite P were associated with fine sediment while Det-P was largely enriched in coarse particles.In the coastal core, the distributions of Fe-P and Al-P exhibited opposite trends to that of Det-P. Their variations were in response to the changed sediment grain size and Changjiang runoff, resulting from the interactions between runoff, sediment grain size and P contents. Marked low concentrations of BSi recorded the decreased diatom production in the late1980s due to low Changjiang runoff and the minimum flux of dissolved silicon. In the upper section of the core, grain-size normalized TOC, TN, TP, Ex-P, Fe-P and lea-OP showed marked increases. This revealed the eutrophication and elevated primary productivity in the coastal waters after the1990s. In the offshore core, the variations of TOC, TN,δ13C, BSi, Ex-P, Fe-P and lea-OP recorded the decadal oscillations of the winter monsoon strength. Strong winter monsoon strengthened the JCC and increased material delivery to the offshore water, and thus led to elevated primary productivity and higher biogenic element contents in sediments.3. The governing factors and environmental indicative significances of PHE and their chemical forms in sediments of the Changjiang Estuary and adjacent waters were investigated. Residual fraction was the major form of V, Cr, Co, Ni, Cu, Zn, Mo, Cd and Bi in sediments. These elements mainly came from terrestrial input, and the Changjiang River contributed82-90%of their total input. Enrichment of Cu, Zn, Cd and Bi was observed in the estuary and Zhejiang coast, and Pb enrichmenten in the northern middle shelf. In the coastal core, the variations of PHE, lithogenic elements and their residual fractions reflected the dry/wet fluctuations of the Changjiang runoff after the1850s. Increases of the enrichment factors (EFs) and labile fraction contents of Zn, Pb, Bi and Cu revealed pollution of these elements in the coastal sediments. In the offshore core, the changes in trace element contents recorded the periodic oscillations of the winter monsoon strength for the past century. The results of sedimentary PHE in reflecting the environmental changes were in agreement with those of sedimentary biogenic elements.Residual fraction was the major form of V, Cr, Co, Ni, Cu, Zn, Mo, Cd and Bi in sediments of the Changjiang Estuary and adjacent waters. Contents and distributions showed that these elements and their residual fractions came primarily from the sediment input of the Changjiang and the JCC. Sedimentary Cu, Zn, Cd, Bi and Pb had higher proportions of labile fractions which account for22-51%of their total contents. According to the distributions of EFs and labile fractions, coastal sediments might be polluted with Cu, Zn, Cd and Pb and the northern middle shelf sediments were enriched with Pb. The pollution sources of these elements likely included the Changjiang, coastal pollution discharge and atmospheric input for Pb. The budges revealed that the Changjiang was the major source of PHEs, contributing82-90%of total PHE input. Thirty-eight to77percent of PHEs were finally buried in sediments, primarily in the inshore area. In the coastal core, the vertical distributions of PHEs, lithogenic elements (Li, Sc, Rb, Cs and Th) and residual fractions were identical. Their sediment records reflected the dry/wet fluctuations of the Changjiang runoff after the1850s. The mechanisms of this reflection included direct river input, grain-size effect and organic matter association. In the upper section of the core, the EFs of Zn, Pb, Bi and Cu increased by13%,19%,20%and4%, respectively, and the contents of the labile fractions of Zn, Pb, Bi and Cu were also elevated. This implied the anthropogenic input of these elements after the1980s. In the upper16cm, oxidizable Co, Ni, Cu, Zn, Pb and Bi increased upward, which resulted from the elevated productivity in water after the1990s. In the offshore core, influenced by the JCC, the variations of PHEs and lithogenic elements were in agreement with the decadal fluctuations of winter monsoon. Strengthened winter monsoon increased the material input from the JCC and atmosphere, and then promoted the phytoplankton growth in offshore water, which jointly led to increased contents of trace elements in sediments.