Biogeochemical Cycle Study Of Aluminum In The Changjiang Drianage Basin, The Yellow Sea And The East China Sea

Aluminum is the trace metal in the ocean, mainly comes from the dissolution of weathering products and can be used as the tracer of terrestrial input, atmospheric deposition and mixing of different water masses. The study of marine biogeochemical cycles of aluminum in natural waters gets more and more attentions these years. The Yellow Sea (YS) and East China Sea (ECS) is the most extensive continental shelf in the world. It receives huge amounts of terrestrial materials from large rivers (e.g., Changjiang, Huanghe etc.) and from East Asia dust storms. The current systems in the YS and ECS are regulated by the seasonal variability of monsoons and incursion of Kuroshio and Taiwan Warm Current. Therefore, the YS and ECS is an ideal place to study the source, sink, internal cycles of aluminum in the continental shelf region. The knowledge of biogeochemical cycles of aluminum in the shelf region can help scientist to deeply understand its behavior in the ocean. This thesis presents the results of biogeochemical cycles of aluminum in the Changjiang drainage basin, the YS and the ECS shelf. The distributions, seasonal variations and its effect factors are mainly discussed. The main results are listed below.Water samples were collected from river mouth upstream over a distance of 3500-4000 km in Changjiang and its 15 northern and southern tributaries during September-October in 2009. The concentrations of dissolved aluminum in the main stream of Changjiang range from 146 to 1263 nmol/L, with an average of 548±354 nmol/L. The concentrations of dissolved aluminum are higher in the upstream than the downstream, and are also higher in the northern tributaries than the southern tributaries. Compared with the results obtained at April-May 1997 before the construction of Three Gorges Dam (TGD), the concentrations of dissolved aluminum in the upstream of Changjiang and major northern tributaries show little annual variations. However, the concentrations of dissolved aluminum in the downstream of Changjiang and southern tributaries decrease dramatically after the construction of TGD. Source rocks in the drainage basin and weathering characteristics are the first control factor on the concentrations of dissolved aluminum in the northern and southern tributaries. Several tributaries are contaminated by human activity (e.g. Nulanjiang). Construction of TGD in the main stream at the Yichang brings several influences, for example, decreasing the water flow and increasing the water residence time before the dam, decreasing the suspended particle contents after the dam etc. All of these influences will change the concentrations and transportations of dissolved aluminum in the Changjiang. Compared with the other Chinese and world rivers, the concentrations of dissolved aluminum remain at the lower level. The concentrations of dissolved aluminum in different drainage basins in China are mainly controlled by the weather characteristics.Three cruises were carried out aboard R/V Beidou in March-April 2007, February 2009 and March-April 2009, respectively, to understand the biogeochemical behaviors of aluminum in the YS. The distributions of dissolved aluminum in the YS are influenced by the mixing of Yellow Sea Coastal Current, Korean Coastal Current and the Yellow Sea Warm Current. The concentrations of dissolved aluminum are high in the coastal area of Shandong peninsula and relative low in the central YS. The concentrations of dissolved aluminum in winter are higher than the average concentrations in spring. Strong East Asia dust storms in spring have important impacts on the distributions of dissolved aluminum in the YS. The concentrations of dissolved aluminum show surface maximum profiles after the dust deposition, which are quite different with the normal profiles in the shelf. The impact of one dust deposition on the increment of dissolved aluminum in the top mix layer of YS is estimated. The estimation matches with the real measured results very well. The concentrations of dissolved aluminum in the top mixed layer of the central YS decrease sharply during the spring bloom. The vertical profiles of dissolved Al are similar with that of silicate during the bloom, which shed light on the biological mediation of dissolved Al in the YS. The HAc-Al fractions in the suspended particles increase during the bloom. Input fluxes of dissolved Al from atmospheric deposition, riverine input and exchanges with the East China Sea and Bohai are estimated, in which the atmospheric deposition is the major source of dissolved Al in the SYS. Combined the dissolved Al inventory with the total input flux, a 87±9 days'residence time of dissolved Al in the SYS is inferred.Two cruises were carried out aboard R/V Beidou in November-December 2006 and February 2007, respectively, to understand the biogeochemical behaviors of aluminum in the ECS shelf. The distributions of aluminum in the YS and ECS show the effects of land-source inputs from the Changjiang and the adjacent rivers in the Zhejiang and Fujian Province and also from water masses mixing from Kuroshio and Taiwan Warm Current, with obvious seasonal variations. The concentrations of dissolved aluminum are high in the coastal area of the ECS shelf and decrease with the distance from the coast. The distributions and seasonal variations of dissolved aluminum in a southeast transect from the Changjiang Estuary to the Ryukyu Islands (i.e. PN section) in the ECS shelf are mainly discussed in the thesis. Combining the different inputs from the Changjiang, atmospheric deposition, Kuroshio waters and Taiwan Warm Current with the total amount of Al, a simple budget was established for the ECS Shelf. The incursion of Kuroshio Subsurface Water and Taiwan Warm Current has significant impact on the distribution of dissolved aluminum in the ECS Shelf. The input flux from atmospheric deposition is similar with that of river. The results reveal an average residence time of 337±154 days for dissolved Al. The impact of Changjiang terrestrial materials over the continental shelf (PN section) is discussed. Using three distinct aluminum-salinity end-members, it is determined that the contribution of the Changjiang in the autumn is significant than the spring. The impact of Changjiang is highest at the station nearest the Changjiang Estuary, and decreased seaward along the PN section within a distance of 150 km. At a distance of 300 km from the Changjiang mouth, the freshwater input was hardly seen and the incursion of Kuroshio waters became dominant. Results of aluminum enrichment incubation experiments both in situ during the cruise in the YS and in lab show that phytoplankton can scavenge the dissolved aluminum from the water column effectively. The enrichment of aluminum will decrease the specific growth rate of phytoplankton in the initial stage, and this effect become serious with the increasing aluminum enrichments. However, the enrichment of Al in the incubation system doesn't affect the total phytoplankton biomass compared with the control group. Several phytoplankton samples were collected during 2007 and 2009 in the YS and ECS by the 20μm phytoplankton sieves. The phytoplankton samples were dealt with the trace metal clean reagent to differentiate the intra-cellular Al and extra-cellular Al contents. The total contents of aluminum in phytoplankton collected in the YS and ECS shelf range in 4.4-48.6 mg/g, with the intra-cellular Al contents of 2.2-48.0 mg/g. Total aluminum contents in phytoplankton are high in the coastal area and decrease with the increasing of distance from the coast. However, the ratio of intra-cellular to total Al contents in phytoplankton is high in the central YS and low in the Changjiang Estuary. Aluminum exists in the intra-cellular pool for the diatom, while in the extra-cellular pool for the dinoflagellates. During the diatom bloom in the YS, Al exists in the interior pool at the early stage of bloom and change to surface associate phase during the decay of bloom.Gravity core sediments were collected in the central YS (E2), Changjiang Estuary (A13), coastal area of Zhejiang and Fujian (E5), mid ECS shelf (E6) and ECS edge (P4) aboard the R/V Dong Fang Hong 2 in September 2002. The average ratios of Al to Ti in the gravity core sediments collected in stations E2, E5 and P4 are higher than the source terrestrial materials from Huanghe, Changjiang, loess and soil, which indicate the existence of excess aluminum in the continental shelf. The average contents of excess aluminum at stations E2, E5 and P4 are 11.8%,5.4% and 5.1%, respectively, which shed light on the biogenic deposition of aluminum in the YS and ECS Shelf.