A Method for Quantitative Evaluation of Carbonate Dissolution in Deep-Sea Sediments and its Application to Paleoceanographic Reconstruction

Abstract

A method is proposed by which the degree of attrition of the tests of certain foraminifera species, such asGloborotalia menardiiandGloborotalia tumida, is used to “scale” the amount of CaCO3that has been dissolved from sediment. The scale is calibrated experimentally in the laboratory. The method has been applied to three calcareous cores from the Pacific and the Indian Oceans. It is shown that the original CaCO3contents in these cores were high (82–95%) and relatively uniform compared to the present down-core values. About 65 to 85% of the originally deposited CaCO3has been dissolved, corresponding to dissolution rates on the order of 0.1-0.3 moles/cm2/yr. These results indicate that appreciable solution could have occurred on sea floor rich in calcareous sediments and that the variation in CaCO3content in a core may have resulted largely from dissolution. The difference in the degree of solution between glacial and interglacial sediments in these cores is not so distinct, with ⋍ 10% less intense dissolution during glacial times on the average. However, the dissolution minimum occurring around the late Wisconsin glaciation (10,000–20,000 yr B.P.) previously noted in several cores elsewhere is confirmed. At that time, near the site of core M70 PC-20 in the southwest Pacific, the CO32−concentration of the bottom water is estimated to have been approximately 5% higher than the present value, and the calcite lysocline was about 300 m deeper. To evaluate possible variations in CaCO3deposition rate across the glacial-interglacial transitions requires precise age control, which the present study lacks.