Biogeochemical Characteristics Of Elements And The Selection Of Environmental Indicator In The Bohai Bay Sediments

The Bohai Bay is a semi-enclosed epicontinental sea in the southwestern Bohai Sea. With experienced rapid industrial developments, marine environmental quality of the Bohai Bay has seen a major decline due to the increase of populations and pollutants. Sediment samples were collected at 27 stations and two cores of Bohai Bay, North China in 2008. Sequential extractions have been carried out. Combining with heavy metals in sea water and ecological environmental information, a series of results and viewpoints with regard to biogeochemical characteristics of major and trace elements were presented, going with ecological risk and controlling factors:1. Geochemical characteristics and NASC patterns of REE in sediment were discussed in detail to reveal the source and their main regulating factors, and a large flood event can be reflected by the burial fluxes of REEThe total REE fluctuate slightly in Bohai Bay sediment. Four labile phases were leached in Exchangeable (L1), Bound to carbonates (L2), Bound to Fe-Mn oxides (L3), Bound to organic matter (L4), and the remainder was Residual (R5). Percentages of fractions followed the order: R5> L3> L2> L4> L1. Sediments in Bohai Bay were mainly composed of clay and silt. Leached REE would rather enrich in the fine-grained particles. Normalized to North American Shale Composite (NASC), REE generally indicated significantly negtive Eu anomaly, showing no significant fractionations of REE. According to REE patterns and Y/Ho ratios of samples, REE are not anthropogenic or oceanic sources but riverine input, whereas suitable environment varieties can slightly affect the patterns and fractionations of REE, espacially for L2.Based on the distributions and burial fluxes of REE, it reflected that there was a large flood event in 1939, followed by the decreasing discharge of sediments from then on.2. Geochemical characteristics of heavy metals in sea water and sediment were discussed to reveal their sources, main regulating factors and ecological risk. Based on total contents of heavy metals, the quality of Bohai Bay sediment was good, while there were different results from fraction view, that was, Zn, Co, Cu, Ni, Pb all were in potential ecological risk. It suggested that the quality of sediment should be estimated not only by total contents but also by concentrations in fraction. The contents of heavy metals (except Pb) was low in central area while high inshore especailly near Haihe estuary. Heavy metals mainly came from river input, while the source of Pb were composed of river input and atmospheric deposition. Heavy metals in sediments were mainly concentrated in the middle region with fine particle. According to sedimentary data and vertical distribution of heavy metal, there were several break which could be corresponding as follow: The lowest level occurred in 1939 resulting from the flood of Haihe basin; there was another flood in 1963 resulting in the decreasing of heavy metals by the dilution of flood; the contents of heavy metals kept increasing from 1963. Percentages of fractions followed the order: R5 >L3 >L2 >L4 >L1. Labile heavy metals were enriched near the estuary of Haihe River.The order of enrichment factors (EF) of heavy metals was Pb> Cd> Zn> Cr> V> Co> Cu> Ni. And the EF of Pb, Cd and Zn were above 1, suggesting that they could be polluted by exterior input. The ratios of labile and residual fraction showed that V and Cr were clean, Zn was mostly clean or in slightly polluted, Co was mostly slightly polluted, Cu and Ni were polluted in different levels at different stations, Pb was polluted heaviest in all heavy metals. In conclusion, the ecological risk of Bohai Bay was low and indicated that the sediment quality was relatively good in general. What have been studied on ecological risk of heavy metals in sediment suggested that the level of polluted sediment should be related to labile fractions of heavy metals, not simply employing the total content of heavy metals.3. Distributions of alkali metals and alkaline earth metals in Bohai Bay sediment were initially elucidated to reflect the influence of riverine input and filtrate the main factor indicating environmental changes. Li/Ba and Rb/Sr are good indicators reflecting the environmental change.Based on Principal Component Analysis (PCA), it was terrestrial input that held the key factor controlling the distribution of alkali metals and alkaline earth metals, suitable environmental varieties and sedimentary conditions can slightly affect the distributions. In different fractions, the R5 percentage of Rb, Cs, Ba were much higher than the sum of other fractions; while the L2 percentage of Sr was higher than R5, suggesting Sr was relatively sensitive to the environmental conditions.The significant enrichment of Sr and Ca can be explained that carbonate played an important role in Bohai Bay, the Sr/Ba ratio showed that core A6 was much more influenced by the decomposition of biology than core A2. The vertical distributions of Rb/Sr and Li/Ba showed that there were two floods in 1939 and 1963, resulting in the increasing input of Sr and Ba. The decreasing ratios of Rb/Sr and Li/Ba suggested that Sr and Ba were transferring into Bohai Bay more and more, and the ratios of Rb/Sr and Li/Ba in A6 were lower than those in A2, indicating that Rb/Sr and Li/Ba were also transferring from coastal area to the middle of Bohai Bay.4. Labile fractions of Nb, Ta, Zr, Hf were discussed to reveal that he specific ratios of Nb, Ta, Zr, Hf could account for the environmental change in Bohai Bay, such as the floods and the sandiness input. The environmental change could promote the fractionation of Nb and Ta, but the effect was much less on Zr and Hf.Nb, Ta, Zr, Hf in Bohai Bay sediments were mainly concentrated in the middle region with fine particle. Percentages of fractions followed the order Nb: R5>> L4> L3> L1> L2; Ta:R5>> L1> L2> L3> L4; Zr: R5>> L4> L3 > L1> L2; Hf: R5>> L4> L3> L2> L1. In different fractions, the R5 percentage almost control the total contents of Nb, Ta, Zr, Hf, while labile fractions can be used to indicate the environmental change. Ta preferred to exist in inorganic fractions, such as L1 and L2, and Ta can be easily combined in organic fractions, such as L3 and L4. The combinative ability of Zr in labile fraction was similar with that of Hf.Based on the vertical distribution of Ti/Nb, the environmental evens that affect core A2 were as follow: Yellow River changing its route to the south in 1938, the flood of northern Haihe drainage basin in 1939 and the flood of southern Haihe drainage basin in 1963. The environmental evens that have impacted A6 were Yellow River changing its route to the north in 1947 and the flood in heavy sandiness from 1984 to 1990. Recently the environment of Bohai Bay changed relatively, which could lead to the fractionations of Nb and Ta in labile fraction were more and more, especially for L4. But the change of environment played less impact on the fractionation of Zr and Hf in labile fractions.