Changes in the extent of shell fragmentation, the wall thicknesses of foraminifera shells, and the bulk CaCO3 content of deep Pacific sediments demonstrate that the extent of dissolution has steadily increased during the past 8 kyr. On the basis of measurements on sediment core top material from a range of water depths on the Ontong-Java Plateau, these dissolution proxies have been calibrated, allowing the extent of the decrease in carbonate ion concentration to be quantified. The results suggest that the decrease was larger than that of 6 mu mol/km expected to accompany the 20 mu atm rise in atmospheric CO2 content that occurred during this time interval. However, the inconsistency between the magnitude of the change based on the decrease in shell weight on one hand and that based on the increase in fragmentation on the other hand is problematic. If indeed the drop in carbonate ion concentration has been larger than expected, then 8 kyr ago either a change in the pattern of thermohaline circulation or in the strength of the biological pump must have kicked in and become ever stronger as the millennia passed. If thermohaline circulation is the villain, then a possible explanation is that the strength of deepwater formation in the northern Atlantic has weakened relative to that in the Southern Ocean.
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