Toward better paleocarbonate ion reconstructions: New insights regarding the CaCO3 size index



[1] The reconstruction of paleocarbonate ion concentrations provides an important constraint on the contribution of the CaCO3 cycle to the decrease in atmospheric CO2 content during glacial time. Such reconstructions have been challenging because each of the existing paleo-[CO32−] indices has serious limitations. In this study, we reexamine the Broecker-Clark CaCO3 size index by analyzing the <20 μm, 20 to 38 μm, and 38 to 63 μm fractions in sediments from the Ontong-Java Plateau and the Ceara Rise. Scanning electron microscope analyses demonstrate that the less than 20 μm CaCO3 is dominated by coccoliths and the greater than 20 μm CaCO3 is dominated by foraminifera. Our results clearly indicate that the coccoliths are far more resistant to dissolution than the foraminifera. Referenced to a core top sample from 2.31 km depth in a core top sample from 4.04 km depth on the Ontong-Java Plateau, ∼70% of the foraminifera CaCO3 was dissolved as opposed to only ∼7% of the coccolith CaCO3. We found that the dissolution of foraminifera shells did not produce a significant amount of fragments smaller than 63 μm in size, and thus the Broecker-Clark size index is not a measure of the extent of fragmentation. Rather, it is a measure of the extent of differential dissolution of foraminifera relative to coccoliths. On the basis of these results, we propose a new dissolution index which involves the ratio of dissolution-susceptible foraminifera CaCO3 to total CaCO3.