LSU Digital Commons LSU Digital Commons Coastal Eutrophication near the Mississippi River Delta Coastal Eutrophication near the Mississippi River Delta-猫眼课题宝

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由于土地利用的变化和人为排放，河流携带的营养物质（如溶解的磷酸盐、硝酸盐和硅酸盐）的输送发生变化，已知会导致富营养化1-浮游植物大量繁殖-以及许多封闭海湾和海洋中更严重的缺氧事件2 - 4。虽然类似的生态效应可能会在大陆架上，发生这种富营养化仍然没有得到解决5。在这里，我们提出的证据，富营养化的大陆架附近的流出密西西比河，通过量化生物结合硅（BSi）的硅藻残留物内的过时的沉积物芯。BSi在河口100 km范围内20~50 m水深处的累积速率最大，世纪以来增加了100%。到1980年，尽管同期硅负荷下降了50%，但河流氮负荷相对于世纪初增加了一倍。因此，河流营养负荷的变化可以改变沿海食物网，并影响大陆架尺度的底层沃茨的氧气数量和分布。密西西比河是北美大陆上最大的河流，流经美国本土的40%，在长度、产沙量和流量方面分别居世界第三、第六和第八位。本世纪的土地利用变化导致溶解硝酸盐浓度增加了一倍，溶解硅酸盐浓度减少了一半。这些变化以及由此导致的年平均溶解硅：氮（硅酸盐和硝酸盐）原子比的下降来自地球上地幔的岩石样品通常显示出化学改性的间接证据。这种变质作用或"交代作用"可以通过外来矿物如金云母、角闪石或磷灰石的沉淀来识别。地幔岩石的整体成分被一种化学特征所覆盖，这种化学特征涉及钾和其他"不相容"元素的富集。在这里，我们研究的交代剂的组成更直接地通过检查熔体和流体包裹体被困在地幔矿物。这些包裹体是第二类，它们沿沿着愈合的断裂面形成痕迹。对来自大陆和大洋板内区的14个超镁铁橄榄岩包裹体的化学成分、包裹温度和压力的系统研究表明，整个岩石圈都存在富挥发分和富硅的交代熔体。它们的组成，

CHANGES in delivery of river-borne nutrients such as dissolved phosphate, nitrate and silicate, owing to land-use changes and anthropogenic emissions, are known to result in eutrophication 1- enhanced phytoplankton blooms-and more severe hypoxic events2-4 in many enclosed bays and seas. Although similar ecolog ical effects might be expected on continental shelves, the occur rence of such eutrophication has remained unresolved 5• Here we present evidence of eutrophication of the continental shelf near the outflow of the Mississippi river, obtained by quantifying biologi cally bound silica (BSi) in diatom remnants within dated sediment cores. BSi accumulation rates are greatest in water depths of 20 to 50 m within 100 km of the river mouth, and have increased by as much as 100% this century. The increases were substantial by 1980, by which time riverine nitrogen loading had doubled relative to the beginning of the century, even though the silica loading had declined by 50% over the same period. Thus changes in river-borne nutrient loadings can modify coastal food webs and affect the amount and distribution of oxygen in bottom waters on the scale of continental shelves. The Mississippi river, the largest river of the North American continent, drains 40% of the conterminous United States, and is third, sixth and eighth largest in the world in terms of length, sediment yield and discharge, respectivell. Land-use changes this century have resulted in a doubling of dissolved nitrate con centration and a halving of dissolved silicate concentration 5. These changes and the resulting decline in the annual average dissolved Si:N (as silicate and nitrate) atomic ratio RocK samples derived from the Earth's upper mantle commonly show indirect evidence for chemical modification. Such modifi cation, or 'metasomatism', can be recognized by the precipitation of exotic minerals such as phlogopite, amphibole or apatite 1, and by the overprinting of the bulk compositions of the mantle rocks by a chemical signature involving the enrichment of potassium and other 'incompatible' elements 2• Here we study the composition of the metasomatic agents more directly by examining melt and fluid inclusions trapped in mantle minerals. These inclusions are second ary, forming trails along healed fracture planes_ A systematic study of the chemical compositions and entrapment temperatures and pressures of inclusions from 14 ultramafic peridotites from both continental and oceanic intraplate regions shows that volatile- and silica-rich metasomatic melts are present throughout the litho sphere. Their compositions,

LSU Digital Commons LSU Digital Commons Coastal Eutrophication near the Mississippi River Delta Coastal Eutrophication near the Mississippi River Delta

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