Bulk carbonate content in the upper part of Subunit 1A of Megasequence D in Hole 1198A is generally very high (Table T1, Fig. F2). More than 95% of the 153 intervals sampled between 0 and 23.69 mbsf have bulk carbonate concentrations >80 wt%, with over 76% of these intervals being >85 wt% carbonate. Maximum bulk carbonate content of 94.1 wt% occurs at 13.63 mbsf and minimum bulk carbonate content of 72.7 wt% occurs at 14.54 mbsf. The interval between 14.54 and 14.35 mbsf shows the highest short-term range in bulk carbonate content with a variation of ~20 wt% occurring in samples separated by <20 cm.
Variations in bulk carbonate content occur with a general cyclicity that has a period of between 3 and 7 m and an amplitude of between 10 and 20 wt%. Overall, five cyclic trends are observed. Peaks occur at 3.35, 6.85, 13.63, 17.00, and 22.04 mbsf, and troughs occur at 0.35, 5.04, 7.50, 14.54, and 20.69 mbsf. The five cyclic variations in bulk carbonate content in Hole 1198A are potentially related to the five major glacial events that have occurred in the last 500 k.y. of the Quaternary. Based on shipboard biostratigraphic datums, including the fact that it is likely that the Holocene sediment section is missing from Hole 1198A (Shipboard Scientific Party, 2002b), peaks in bulk carbonate content may loosely correlate with peaks in fifth-order sea level and marine isotope stages 3, 5, 7, 9, and 11.