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>Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg‐dependent dissolution
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Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg‐dependent dissolution
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机译：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 foraminifera,G. tumidaandG. sacculifer, in a depth transect on the Ontong Java Plateau, western equatorial Pacific.G. tumidatests 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 inG. sacculifertests in excess of 355 µm (average Mg/Ca, 3.6 mmol/mol, Sr/Ca, 1.4 mmol/mol). Artificial dissolution ofG. tumidatests led to a decrease in both Mg/Ca and Sr/Ca, but inG. sacculifer, there was no significant change in the ratios. Analyses by electron microprobe revealed that Mg/Ca of the inner (chamber) calcite inG. tumidatests 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 forG. sacculifertests. Dissolution in the oceans gives rise to the removal of the chamber calcite inG. tumida(about 30 of the total calcite), leaving the keel calcite. AlthoughG. tumidais 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 fromG. sacculiferabove 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
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