Chamber formation leads to Mg/Ca banding in the planktonic foraminifer Neogloboquadrina pachyderma

Lukas Jonkers, Benjamin Buse, Geert-Jan Brummer, Ian Hall

Research output: Contribution to journalArticle (Academic Journal)peer-review

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Abstract

Many species of planktonic foraminifera show distinct banding in the intratest distribution of Mg/Ca. This heterogeneity appears biologically controlled and thus poses a challenge to Mg/Ca paleothermometry. The cause of this banding and its relation with chamber formation are poorly constrained and most of what we know about intratest Mg/Ca variability stems from culture studies of tropical, symbiont-bearing foraminifera. Here we present data on the non-spinose, symbiont-barren Neogloboquadrina pachyderma from the subpolar North Atlantic where wintertime mixing removes vertical gradients in temperature and salinity. This allows investigation of biologically controlled Mg/Ca intratest variability under natural conditions. We find that intratest Mg/Ca varies between <0.1 and 7 mmol/mol, even in winter specimens. High Mg/Ca bands occur at the outer edge of the laminae, indicating reduced Mg removal at the end of chamber formation. Our data thus provide new constraints on the timing of the formation of such bands and indicate that their presence is intrinsic to the chamber formation process.

Additionally, all specimens are covered with an outer crust consisting of large euhedral crystals. The composition of the crust is similar to the low Mg/Ca bands in the laminar calcite in winter and summer specimens, indicating a tight biological control on crust formation and composition. Nevertheless, despite high intratest variability, the median Mg/Ca of summertime tests is higher than that of wintertime tests. This provides support for Mg/Ca paleothermometry, but to improve the accuracy of paleotemperature estimates biological effects on Mg incorporation need to be better accounted for
Original languageEnglish
Pages (from-to)177-184
Number of pages8
JournalEarth and Planetary Science Letters
Volume451
Early online date29 Jul 2016
DOIs
Publication statusPublished - 1 Oct 2016

Keywords

  • Mg/Ca paleothermometry
  • Neogloboquadrina pachyderma
  • planktonic foraminifera
  • intratest Mg/Ca variability
  • sediment trap

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