Evaluation of the CaCO3 dissolution proxies in sediment cores from above the lysocline

Calcium carbonate analyses were carried out from two size fractions (<63 μm and >63 μm) from the Ocean Drilling Program Sites 752 and 715 collected from above the lysocline depth in the Indian Ocean, to evaluate the calcium carbonate size index as paleocarbonate ion proxy in these cores. Size index values varied from 69 to 81 at Site 752 and showed a similar pattern with that of total carbonate content at this site. High values of size index and the location of this site, far from any landmass, suggest that only changes in productivity of overlying waters are responsible for the observed variations.Low values of size index at Site 715 suggest supralysoclinal dissolution, the inverse relation between size index variations and the total CaCO3 content shows that the total carbonate variations are controlled by the finer fraction composed of juvenile foraminifera, foraminifera fragments and coccolithophores. This further confirms the observed variations of CaCO3 content of the <63 μm fraction, which follows the trend of total CaCO3 content. This study demonstrates that apart from factors such as breakup of shells after burial due to organic matter respiration and dilution due to terrigenous material, changes in surface water productivity and coccolithophores might be a constraint to use size index as a paleocarbonate ion proxy for sediments from above the lysocline.