Constraining ecological and biological bias in planktonic foraminiferal Mg/Ca and δ18Occ: A multispecies approach to proxy calibration testing

Abstract

Parallel stable isotope (δ18Occ) and Mg/Ca measurements performed in three planktonic foraminifer species provide multiple perspectives on past temperature change and thus allow published calibrations for these geochemical proxies to be tested against each other. The comparison of “simultaneous” temperature records, as recorded by both the δ18Occ and Mg/Ca signature of each species, reveals very clearly the nonpassive character of foraminifera as proxy carriers and allows the systematic temperature response of δ18Occ and Mg/Ca in different foraminifer species to be explored. On the basis of their Mg/Ca signatures, Neogloboquadrina pachyderma (right), Globigerina bulloides, and Globigerinoides ruber (white) are each found to exhibit a specific and narrow range of temperature variability; however, the interspecies offsets in “optimum” temperature habitat that are suggested in this way are not expressed by similarly scaled δ18Occ offsets. Hence it is demonstrated that the temperature modes/variations that are recorded by each species cannot be calibrated to Mg/Ca and δ18Occ variations via a single pair of calibrations for all three species: species‐specific δ18Occ and/or Mg/Ca temperature calibrations are required. On the basis of the average δ18Occ and Mg/Ca offsets observed between the three species, it appears that many of the temperature calibrations that are currently proposed for both of these proxies are inconsistent. The data presented here cannot provide species‐specific Mg/Ca and δ18Occ temperature calibrations; however, they do provide a simple method for testing our calibrations in application and clearly indicate that further calibration work using paired Mg/Ca and δ18Occ is required before the full potential of these proxies may be realized.