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

• Published in: Climate of the past Vol. 18; no. 1; pp. 89 - 101
• Main Authors: Jonkers, Lukas, Brummer, Geert-Jan A, Meilland, Julie, Groeneveld, Jeroen, Kucera, Michal
• Format: Journal Article
• Language: English
• Published: Katlenburg-Lindau Copernicus GmbH 20-01-2022; Copernicus Publications
• Subjects: Calcification; Carbon; Chemical analysis; Environmental conditions; Foraminifera; Isotope ratios; Isotopes; Laboratories; Life cycle; Life cycles; Neogloboquadrina pachyderma; Noise; Oceanographic variability; Population; Recorders; Seasonal variability; Seasonal variations; Seawater; Sediment traps; Sediments; Sediments (Geology); Simulation; Stable isotopes; Temperature; Time series; Variability; Water analysis; Water circulation; Water column; Irminger Sea
• ISSN: 1814-9332; 1814-9324; 1814-9332
• DOI: 10.5194/cp-18-89-2022

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.