Impact of the ocean carbonate chemistry on living foraminiferal shell weight: Comment on ‘‘Carbonate ion concentration in glacial-age deep waters of the Caribbean Sea’’ by W. S. Broecker and E. Clark

[1] 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. On the basis of 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 mmol kg 1 km 1 [Broecker and Clark, 2001], (3) the weight loss slope is universal and ca. 0.3 mg (mmol kg ) 1 [Broecker and Clark, 2001], and (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 mmol 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.