Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg-dependent dissolution
Article first published online: 4 MAY 2010
Copyright 1996 by the American Geophysical Union.
Volume 11, Issue 5, pages 543–551, October 1996
How to Cite
1996), Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg-dependent dissolution, Paleoceanography, 11(5), 543–551, doi:10.1029/96PA01491., and (
- Issue published online: 4 MAY 2010
- Article first published online: 4 MAY 2010
- Manuscript Accepted: 3 MAY 1996
- Manuscript Received: 18 DEC 1995
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.