CaCO3, organic carbon and mass accumulation rates at DSDP Site 75-532

The average periodicity for carbonate cycles is about 36 k.y. Darker parts of color cycles usually contain higher concentrations of organic carbon, but the organic- carbon record does not follow the cyclicity of the color cycles in detail, at least for the last 2.5 m.y. Organic-carbon cycles have an average periodicity of about 34 k.y. for the Quaternary and upper Pliocene.

The cycles of CaCO3 and color have periodicities similar to those reported from carbonate stratigraphies from the northeast Atlantic, Caribbean, and eastern equatorial Pacific. The carbonate cycles at Site 532 are the result of external forcing, probably related to global climate, that affected fluctuations in both sediment supply from the African continental margin and productivity of siliceous organisms. The organic-carbon cycles have similar periodicities and similar changes in periodicities to those of the CaCO3 cycles.

Semiquantitative estimates of diatom abundance from smear slides and concentrations of biogenic SiO2 calculated from chemical analyses suggest that upwelling at Site 532 was minor until about 3 m.y. ago. The Benguela-Current upwelling system either began at that time or, more likely, migrated into the area of Site 532, where it prevailed until some time between about 1.2 and 0.5 m.y. ago. The increase and decline of upwelling in the area of Site 532, however, did not disturb the trend of cyclicities of carbonate and organic carbon.

The latest change in conditions at Site 532 was an increase in intensity of bottom currents during the past 0.5 m.y. that winnowed nannofossils, diatoms, and clay and left a lag deposit represented by a foraminifer-rich fades.