High‐Resolution Mg/Ca and δ¹⁸O Patterns in Modern Neogloboquadrina pachyderma From the Fram Strait and Irminger Sea

Abstract

Neogloboquadrina pachyderma is the dominant species of planktonic foraminifera found in polar waters and is therefore invaluable for paleoceanographic studies of the high latitudes. However, the geochemistry of this species is complicated due to the development of a thick calcite crust in its final growth stage and at greater depths within the water column. We analyzed the in situ Mg/Ca and δ 18 O in discrete calcite zones using laser ablation-inductively coupled plasma-mass spectrometry, electron probe microanalysis, and secondary ion mass spectrometry within modern N. pachyderma shells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. Here we compare shell geochemistry to the measured temperature, salinity, and δ 18 O sw in which the shells calcified to better understand the controls on N. pachyderma geochemical heterogeneity. We present a relationship between Mg/Ca and temperature in N. pachyderma lamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. We also document highly variable secondary ion mass spectrometry δ 18 O results (up to a 3.3‰ range in single shells) on plankton tow samples which we hypothesize is due to the granular texture of shell walls. Finally, we document that the δ 18 O of the crust and lamellar calcite of N. pachyderma from an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts in N. pachyderma δ 18 O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions.

Variability in Neogloboquadrina pachyderma stable isotope ratios from isothermal conditions: implications for individual foraminifera analysis

“…Notwithstanding the fact that the exact cause of the excess variability in N. pachyderma stable isotope ratios needs to be constrained in future studies, our analysis shows that individual planktonic foraminifera record 345 environmental conditions with less precision than average populations. Our study thus confirms earlier indications (Groeneveld et al, 2019;Livsey et al, 2020), but we have attempted a first quantification of this noise for δ 18 O, which has up to now been ignored in the interpretation of individual foraminifera data.…”

“…The excess variability could also arise from differences in the proportion of crust to lamellar calcite. Crust and lamellar calcite have different δ 18 O even in N. pachyderma shells formed in isothermal conditions (Livsey et al, 2020). Variable crust to lamellar calcite ratios among foraminifera could therefore add temperature-independent 335 noise, similar to what has been suggested for Mg/Ca (Jonkers et al, 2021(Jonkers et al, , 2016.…”

Tracking Southern Ocean Sea Ice Extent With Winter Water: A New Method Based on the Oxygen Isotopic Signature of Foraminifera

“…Similarly, Livsey et al. (2020) found that N. pachyderma δ 18 O c in the hydrographically complex Fram Strait have an average offset from equilibrium values of 0.3 ± 0.8‰ (using the Kim and O'Neil (1997) equation). Calcification temperature is another key consideration when calculating δ 18 O c .…”

“…For example, when the predicted δ 18 O c for the Bauch et al (1997) data are determined using the Kim and O'Neil (1997) calibration, the average offset with N. pachyderma δ 18 O c decreases to ∼0.1‰. Similarly, Livsey et al (2020) found that N. pachyderma δ 18 O c in the hydrographically complex Fram Strait have an average offset from equilibrium values of 0.3 ± 0.8‰ (using the Kim and O'Neil (1997) equation). Calcification temperature is another key consideration when calculating δ 18 O c .…”

Comment on cp-2021-87

“…Laboratory-based calibrations of δ 18 O-temperature relationships hint at a similar non-temperature related noise (Bemis et al, 1998;Erez and Luz, 1982). Observations from plankton nets and sediment traps also demonstrate marked variability (Livsey et al, 2020;Davis et al, 2020b;Haarmann et al, 2011). These observations are not 65 conclusive in their own right, but together they suggest that there are reasonable grounds to assess if the composition of individual foraminifera can be used as a precise environmental indicator.…”