Broecker and Clark (2002) use the "size normalized weight" of planktic foraminifera to estimate the carbonate ion concentration ([CO3=]) of Atlantic glacial upper deep water. This method was introduced by Lohmann (1995) and is based on the fact that, within a defined size fraction, dissolution decreases shell weight in proportion to the degree of undersaturation with respect to CaCO3. Based on the following assumptions,1) the thickness of the foraminiferal shell wall does not depend on growth conditions,2) the saturation [CO3=] for calcite, [CO3=]sat., increases by 20 µmol kg-1 km-1 (Broecker and Clark, 2001),3) the weight loss slope is universal and ca. 0.3 µg (µmol kg-1)-1 (Broecker and Clark, 2001),4) the offset between the bottom water and pore water [CO3=] was the same during glacial time as during the Holocene,Broecker and Clark (2002) calculate that the [CO3=] of Atlantic glacial upper deep water was 14 µmol kg-1 higher than during the Holocene. Although they recognize that some of their assumptions are not strictly valid, they do not assess the impact of those assumptions on their [CO3=] estimate. Here we comment on several of those assumptions and attempt to quantify their impact on Broecker and Clark's (2002) calculations.
EPIC Alfred Wegener Institut