Evidence for a reduction in the carbonate ion content of the deep sea during the course of the Holocene

The paleo carbonate ion proxy proposed by Broecker et al. [1999] is applied in a search for trends in the Holocene acidity of waters in the transition zone between North Atlantic Deep Water and Antarctic Bottom Water (AABW). A clear signal emerges that the carbonate ion content of waters in this zone declined during the past 8000 years. In order to determine whether this decline represents a strengthening of the northward penetrating tongue of low CO 3 = content AABW or a global reduction of CO 3 = ion, measurements were made on a core from the Ontong Java Plateau in the western equatorial Pacific. Evidence for a similar decline in CO 3 = ion over the course of the Holocene was obtained lending support of the latter explanation. Such a drop is consistent with the recent finding by Indermühle et al. [1999] that the CO 2 content of the atmosphere (as recorded in the Taylor Dome Antarctica ice core) rose by 20–25 ppm during the past 8000 years.