TY - JOUR
T1 - Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution
T2 - Evidence of shallow Mg-dependent dissolution
AU - Brown, Sarah J
AU - Elderfield, Henry
PY - 1996/10
Y1 - 1996/10
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0030499085&partnerID=8YFLogxK
M3 - Article
VL - 11
SP - 543
EP - 551
JO - Paleoceanography
JF - Paleoceanography
IS - 5
ER -