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A comparative study has been made of two species of planktonic foraminifera, G. tumida and G. sacculifer , in a depth transect on the Ontong Java Plateau, western equatorial Pacific. G. tumida tests from core‐top 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), while no such variation was found in G. sacculifer tests in excess of 355 µm (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 ratios. Analyses by electron microprobe revealed that Mg/Ca of the inner (chamber) calcite in G. tumida tests was much higher than that of the calcite crust (keel), whereas Sr/Ca was only slightly elevated. There were no consistent spatial differences in either Mg/Ca or Sr/Ca for G. sacculifer tests. Dissolution in the oceans gives rise to the removal of the chamber calcite in G. tumida (about 30% of the total calcite), leaving the keel calcite. Although G. tumida is thought of as a dissolution‐resistant form, the majority of Mg/Ca and Sr/Ca depletion (and chamber loss) occurs above the lysocline. It is also possible that much of the Mg‐enriched gametogenic calcite has been lost by dissolution from G. sacculifer above 1600 m. Mg content has a significant effect on the dissolution susceptibility of planktonic foraminifera. Calculations show that the saturation horizon for Mg‐rich parts of tests may be elevated by several hundred meters compared with normal calcite.
Brown et al. (Tue,) studied this question.