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Lysocline reconstructions play an important role in scenarios purporting to explain the lowered atmospheric CO 2 content of glacial time. These reconstructions are based on indicators such as the CaCO 3 content, the percent of coarse fraction, the ratio of fragments to whole foraminifera shells, the ratio of solution‐susceptible to solution‐resistant species, and the ratio of coarse to fine CaCO 3 . All assume that changes with time in the composition of the input material do not bias the result. However, as the composition of the input material does depend on climate, none of these indicators provides an absolute measure of the extent of dissolution. In this paper we evaluate the reliability of the ratio of >63 µm CaCO 3 to total CaCO 3 as a dissolution indicator. We present here results that suggest that in today's tropics this ratio appears to be determined solely by CO 3 = ion concentration and water depth (i.e., the saturation state of bottom waters). This finding offers the possibility that the size fraction index can be used to reconstruct CO 3 = ion concentrations for the late Quaternary ocean to an accuracy of ±5 µmol kg −1 .
Broecker et al. (Fri,) studied this question.