21. UPPER QUATERNARY DIATOMS IN THE AMAZON FAN OF THE WESTERN ATLANTIC1Naja Mikkelsen2 |
Diatoms are rare in the upper Pleistocene/Holocene sediments recovered during Ocean Drilling Program (ODP) Leg 155 from the Amazon Fan. The diatom assemblages are generally characterized by a few solution-resistant species of marine origin and are only found in hemipelagic sediments younger than 12 ka. Most assemblages have a minor contribution of freshwater diatoms (average 10%-30%), reflecting the influence of discharge from the Amazon River. The highest freshwater diatom abundance is off the flank of the fan (75%), and it is likely a result of the northward sweep of the Amazon freshwater plume. The high production of diatom frustules reported from the estuarine area of the Amazon River is not reflected in Amazon Fan sediments due to rapid dissolution and the recycling of opaline diatom frustules. Diatoms are absent from glacial-aged Amazon Fan deposits. Glacial sediments are barren of diatoms and characterized by terrigenous material funneled directly into the deep sea through the Amazon Submarine Canyon during low sea-level stands. Diatoms are similarly rare in upper Quaternary sequences of the open ocean environment of the Ceara Rise region. In contrast to the western equatorial Atlantic, diatoms are common in the eastern equatorial Atlantic. The general low abundance of diatoms in the Amazon Fan region is therefore controlled by post-depositional dissolution superimposed on a cross-basinal productivity gradient. |
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