Modulated orbital cycles in planktonic foraminifera δ18O

Measurements of δ¹⁸O on planktonic foraminifera represent an important geological archive. In the study of climate change on orbital time-scales, δ¹⁸O is used either as a proxy for temperature change, to construct age models through pattern matching, to deduce the drivers of climate change by looking at frequency power spectra or all of the above. The validity of these approaches hinges on a two-step assumption, namely that the signal of the orbital cycles is transferred unaltered from 1) solar radiation that reaches the top of the atmosphere, insolation, to the sea surface, and 2) from the sea surface translated into the δ¹⁸O composition of the foraminiferal shell. The complexity of these two steps make it difficult to validate the assumptions behind these major paleoclimatological approaches. In this research we aim to disentangle the problem, here focussing only on the latter part: how do species-specific living habitats of foraminifera in the water column and throughout the year shape the δ¹⁸O response were the Earth’s surfce climate to perfectly reflect the orbital forcing(s). To this end we combine an isotope-enabled climate model (iLOVECLIM) with a foraminifera growth model and investigate the response of δ¹⁸O from three species of foraminifera (Globigerinoides ruber, Neogloboquadrina pachyderma, and Globigerina bulloides) to obliquity and precessional cycles.

Our results show that the planktonic foraminifera δ¹⁸O response to astronomical forcings is dominated by annual mean changes in sea-surface temperature, thus corroborating a key assumption underlying many geological climate reconstructions. However, in various places changes in the depth habitat, temperature-dependent growth rates, seasonality and δ¹⁸O of the sea water dominate the foraminifera δ¹⁸O signal. Because of these modulations, planktonic foraminifera δ¹⁸O time series can have very different characteristics compared to the orbital forcings, including limited spatial coherence as well as limited inter-species coherence at a single location.