Abstract
Abstract. Individual foraminifera analysis (IFA) holds promise to reconstruct seasonal to interannual oceanographic variability. Even though planktonic foraminifera are reliable recorders of environmental conditions on a population level, whether they also are on the level of individuals is unknown. Yet, one of the main assumptions underlying IFA is that each specimen records ocean conditions with negligible noise. Here we test this assumption using stable isotope data measured on groups of four shells of Neogloboquadrina pachyderma from a 16–19 d resolution sediment trap time series from the subpolar North Atlantic. We find a within-sample variability of 0.11 ‰ and 0.10 ‰ for δ18O and δ13C respectively that shows no seasonal pattern and exceeds water column variability in spring when conditions are homogeneous down to hundreds of metres. We assess the possible effect of life cycle characteristics and delay due to settling on foraminifera δ18O variability with simulations using temperature and δ18Oseawater as input. These simulations indicate that the observed δ18O variability can only partially be explained by environmental variability. Individual N. pachyderma are thus imperfect recorders of temperature and δ18Oseawater. Based on these simulations, we estimate the excess noise on δ18O to be 0.11±0.06 ‰. The origin and nature of the recording imprecision require further work, but our analyses highlight the need to take such excess noise into account when interpreting the geochemical variability among individual foraminifera.
Highlights
Planktonic foraminifera hold the promise to provide palaeoenvironmental information at high temporal resolution, owing to their short life cycle, which is on the order of weeks to months, and rapid calcification that takes place over hours to days
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 environmental conditions with less precision than average populations
Stable isotope measurements on groups of four shells of N. pachyderma from a 16–19 d resolution sediment trap time series in the subpolar North Atlantic show large within sample variability
Summary
Planktonic foraminifera hold the promise to provide palaeoenvironmental information at high temporal resolution, owing to their short life cycle, which is on the order of weeks to months, and rapid calcification that takes place over hours to days. This potential is exploited in individual foraminifera analysis (IFA), when instead of measuring groups of shells, shells are measured individually, and the variability among the individual shells is used to reconstruct environmental variability during deposition of the sample. Analytical issues aside (Fehrenbacher et al, 2020), the uncertainty associated with IFA is often viewed from the perspective of whether the population is well enough characterised, how habitat tracking may affect the results or how variability at different timescales (seasonality, El Niño–Southern Oscillation – ENSO) can be distinguished (Glaubke et al, 2021; Leduc et al, 2009; Metcalfe et al, 2020; Thirumalai et al, 2013), and only a few studies consider calibration issues as-
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