2. MAJOR AND TRACE ELEMENT GEOCHEMISTRY OF SEDIMENTS FROM EAST GREENLAND CONTINENTAL RISE: AN IMPLICATION FOR SEDIMENT PROVENANCE AND SOURCE AREA WEATHERING1Saneatsu Saito2 |
ABSTRACTThe bulk geochemistry of the nonbiogenic fraction of 70 samples from Ocean Drilling Program Site 918 in the Irminger Basin was determined by X-ray fluorescence. These data enabled the reconstruction of several aspects of Cenozoic paleoenvironmental evolution. By means of chemical stratigraphic analysis of the bulk sediments, seven major geochemical stages were identified and correlated with the significant lithologic units. The chemical compositions of the geochemical stages were basically controlled by differences in source area (volcanic/plutonic) and the degree of source area weathering. Important elements for characterizing source rock lithology are iron and sodium, whereas those for intensity of weathering are sodium, calcium, and potassium. The sodium-ferric iron ratio and potassium-calcium ratio provide useful indexes for the degree of the plutonic input and source area weathering, respectively. Major chemical shifts occur at 550 meters below seafloor, which coincides with the beginning of ice-rafted debris (IRD) deposition in the upper Miocene. Mobile elements from weathering, such as calcium, manganese, and sodium, decrease downsection while immobile potassium increases. IRD is clearly distinguished from turbidites by its high content of the mobile elements. The chemical difference is explained by the difference between mechanical erosion by ice sheets and chemical weathering on land. Mechanical erosion by ice sheets does not involve source area chemical weathering and selective transportation by rivers. Turbidite materials were derived, however, from chemically weathered plutonic, metamorphic, and basaltic rocks on Greenland. Potassium in turbidites was recycled from basement rocks and paleosols, while sodium was removed during weathering and transportation. The major Cenozoic paleoenvironmental changes in the East Greenland Margin are reconstructed from sediment compositions and geochemistry. Eocene sediments are derived from a weathered basaltic region. The upper Oligocene to lowermost Miocene is characterized by a weathered continental input. Fresh volcanic input began during the early Miocene. The supply of continental materials became dominant again during the middle Miocene, which coincides with the development of modern North Atlantic Deep Water. Unweathered IRD has been supplied since the late Miocene, which coincides with the first occurrence of dropstones. |
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