Chapter

Use of Proxies in Paleoceanography

pp 315-344

Organic Carbon and Carbonate as Paleoproductivity Proxies: Examples from High and Low Productivity Areas of the Tropical Atlantic

  • C. RühlemannAffiliated withFachbereich Geowissenschaften, Universität Bremen Email author 
  • , P. J. MüllerAffiliated withFachbereich Geowissenschaften, Universität Bremen
  • , R. R. SchneiderAffiliated withFachbereich Geowissenschaften, Universität Bremen

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Abstract

We reviewed the factors influencing organic carbon and carbonate preservation in marine sediments. The application and problems of these two biogenic components as paleoproductivity proxies are exemplified by comparison of two sediment cores: one from the high productivity upwelling region off Angola, and the other from the low productivity area off North Brazil. Unlike the upwelling site, sedimentary organic carbon in the low productivity area is problematic as a paleoproductivity indicator. There, calcium carbonate, the predominant biogenic material of the pelagic ocean, serves as an alternative estimator for paleoproductivity. During the last 300,000 years, high and variable paleoproductivity in the eastern Atlantic contrasts with low and relatively constant values in the West. Beyond this, a countercyclicity of paleoproductivity variations between the eastern and the western tropical Atlantic is observed. During cold climatic substages, paleoproductivity was at maximum in the East, whereas minimum values were recorded in the West. These inverse relationships are attributed to a lowered nutrient concentration of glacial intermediate waters which are the nutrient source of open ocean new production. At the same time, the supply of nutrients was enhanced in the eastern Atlantic due to intensified wind-driven upwelling. Moreover, the increased zonal wind stress may have caused a deepening of the nutricline in the West coupled with a synchronous shallowing in the East. The sharp Glacial-to-Holocene decrease in paleoproductivity in the upwelling areas (dominated by opal producers), occurred contemporaneously with an increase in the oligotrophic open ocean (dominated by carbonate producers). This should have resulted in a decrease of the carbon rain ratio (Corg /CCaCO3), possibly contributing to the observed Glacial-to-Holocene increase in atmospheric pCO2