VARIATIONS IN MG CA AND SR/CA RATIOS OF PLANKTONIC-FORAMINIFERA CAUSED BY POSTDEPOSITIONAL DISSOLUTION - EVIDENCE OF SHALLOW MG-DEPENDENT DISSOLUTION/

A comparative study has been made of two species of planktonic foramin ifera, G. tumida and G. sacculifer, in a depth transect on the Ontong Java Plateau, western equatorial Pacific. G. tumida tests from core-to p sediments showed decreasing Mg/Ca (2.65-1.25 mmol/mol) and Sr/Ca (1. 50-1.16 mmol/mol) ratios with increasing water depth (1600-4400 m), wh ile no such variation was found in G. sacculifer tests in excess of 35 5 mu m (average Mg/Ca, 3.6 mmol/mol, Sr/Ca, 1.4 mmol/mol). Artificial dissolution of G. tumida tests led to a decrease in both Mg/Ca and Sr/ Ca, but in G. sacculifer, there was no significant change in the ratio s. Analyses by electron microprobe revealed that Mg/Ca of the inner (c hamber) calcite in G. tumida tests was much higher than that of the ca lcite crust (keel), whereas Sr/Ca was only slightly elevated. There we re no consistent spatial differences in either Mg/Ca or Sr/Ca for G. s acculifer tests. Dissolution in the oceans gives rise to the removal o f the chamber calcite in G. tumida (about 30% of the total calcite), l eaving the keel calcite. Although G. tumida is thought of as a dissolu tion-resistant form, the majority of Mg/Ca and Sr/Ca depletion (and ch amber loss) occurs above the lysocline. It is also possible that much of the Mg-enriched gametogenic calcite has been lost by dissolution fr om G. sacculifer above 1600 m. Mg content has a significant effect on the dissolution susceptibility of planktonic foraminifera. Calculation s show that the saturation horizon for Mg-rich parts of tests may be e levated by several hundred meters compared with normal calcite.