A suite of 15 piston cores up to 23 m long taken in an eastern equatorial Atlantic fracture zone at 8°N. and in the nearby Sierra Leone basin document climatic variations over the last 1.8m.y. Foraminiferal and paleomagnetic stratigraphies were used to correlate the cores and select the most representative pelagic record.
"Total fauna" analysis of foraminiferal variations in a suite of shorter cores spanning the last 200,000 yrs substantiates in detail the oxygen-isotope trends over that interval. When applied to two cores containing 1.8 m.y. of equatorial sedimentary history, this analysis pinpoints two prominent, large-scale climatic shifts: (l) at 1.3 m.y. B.P., the mean climatic situation deteriorated, and short but severe cold pulses began to punctuate the previous moderate warmth of the late Matuyama; (2) following 900,000 yrs B.P., the duration of cold intervals increased. Prior to the Jaramillo, no cold pulse exceeded 30,000 yrs; three post-Jaramillo cold intervals ranged in duration from about 50,000 to 150,000 yrs. The shortest and most recent of these correlates with the Wisconsin glaciation.
In addition to pelagic carbonates, continental sediment is introduced into these cores by turbidity currents flowing down the axis of the fracture zone and by wind blowing off Saharan and equatorial Africa. Although the absolute input rate of pelagic carbonate to these sediments increases during cold intervals, the glacial carbonate percentages tend to decrease due to even greater influxes of continental detritus. Beginning in the Jaramillo event at roughly 900,000 yrs B.P., this terrigenous dilution depresses carbonate percentages in these cores, often to very low values. Pre-Jaramillo sections are generally calcareous oozes.