Cyclic variations in calcium carbonate and organic carbon in Miocene to Holocene sediments, Walvis Ridge, South Atlantic Ocean

Abstract

The entire upper Miocene to Holocene sedimentary sequence recovered in a hydraulic piston core at DSDP Site 532 on Walvis Ridge shows distinct cycles in amount of CaCO sub(3) that correlate with dark and light cycles of sediment color. The average periodicities of the carbonate cycles for the Quaternary, upper Pliocene, and lower Pliocene are about 35, 46, and 28 ky, respectively, with an overall average of about 36 ky for the last 5 my. Most minima in carbonate abundance correspond to dark parts of the color cycles, and most maxmia in carbonate abundance correspond to dark parts of color cycles, The darker parts of the color cycle usually contain higher concentrations of organic carbon, but organic carbon does not follow the color cycles in detail. Organic carbon cycles were analyzed only for the last 2.5 my, and for this interval they have an average periodicity of about 34 ky. The carbonate and color cycles persist through more than 5my during which major changes in relative proportions of siliceous-biogenic, calcareous- biogenic and terrigenous- clastic components occurent in response to climate change and to the waxing and waning of the Benguela Current upwelling systems off southwest Africa. The cyclic nature of these sediments probably is the result of dilution by terrigrnous clastic material and not dissolution of carbonate. We believe that the forcing mechanisms that produced the cycliclity was external to the area of site 532. Because of the similarity among the periodicities of the Walvis Ridge cycles and those of carbonate cycles in the northeastern Atlantic, Caribbean and eastern equatorial Pacific, these cycles probably are responses to global events. We conclude that fluctuations om global sea level with an average period of about 36 ky during the last 5 my caused variations in influx of terrigenous clastic materials from the Africans continental margin