A proxy modelling approach to assess the potential of extracting ENSO signal from tropical Pacific planktonic foraminifera

Abstract

Abstract. A complete understanding of past El Niño–Southern Oscillation (ENSO) fluctuations is important for the future predictions of regional climate using climate models. One approach to reconstructing past ENSO dynamics uses planktonic foraminifera as recorders of past climate to assess past spatio-temporal changes in upper ocean conditions. In this paper, we utilise a model of planktonic foraminifera populations, Foraminifera as Modelled Entities (FAME), to forward model the potential monthly average δ18Oc and temperature signal proxy values for Globigerinoides ruber, Globigerinoides sacculifer, and Neogloboquadrina dutertrei from input variables covering the period of the instrumental record. We test whether the modelled foraminifera population δ18Oc and Tc associated with El Niño events statistically differ from the values associated with other climate states. Provided the assumptions of the model are correct, our results indicate that the values of El Niño events can be differentiated from other climate states using these species. Our model computes the proxy values of foraminifera in the water, suggesting that, in theory, water locations for a large portion of the tropical Pacific should be suitable for differentiating El Niño events from other climate states. However, in practice it may not be possible to differentiate climate states in the sediment record. Specifically, comparison of our model results with the sedimentological features of the Pacific Ocean shows that a large portion of the hydrographically/ecologically suitable water regions coincide with low sediment accumulation rate at the sea floor and/or of sea floor that lie below threshold water depths for calcite preservation.

Modeling the impact of bioturbation and species abundance upon discrete-depth individual foraminifera analysis used in ENSO-type climate reconstructions.

“…We furthermore have shown in our best-case study that a species' abundance response to SST can inherently bias IFA-derived reconstructions of past SST variance. We propose that the coupling of a single foraminifera sediment model approach to foraminiferal ecological models (Lombard et al, 2011;Metcalfe et al, 2020;Roche et al, 2018) could further help to constrain the total uncertainty associated with IFA-derived SST reconstructions.…”

“…4). The equivalent figures for the 5 cm ka -1 and 10 cm ka -1 scenarios, which may more be representative best-case values for the open ocean areas of the Pacific (Metcalfe et al, 2020), can be found in the supplement.…”

“…The ability of discrete-depth IFA to accurately reproduce a time period's true SST distribution can be clouded by a number of environmental, biological and logistical issues, which occur in the water domain (pre-deposition), sediment archive domain (post-deposition) and laboratory domain (post-retrieval). Regarding issues in the water domain, it is possible that a foraminifera species may not exist continually at a single surface water location, thus giving a non-continuous record of SST, which can have consequences for ENSO reconstructions (Metcalfe et al, 2020). Secondly, a species' foraminiferal abundance through time is not constant and can be influenced by SST itself, which may bias IFA-derived SST distribution reconstructions, which is especially relevant in the case of ENSO, which itself influences SST.…”

Using sedimentological priors to improve 14C calibration of bioturbated sediment archives.

“…the downcore, discrete-depth record) is itself modified by bioturbation (Lougheed 2020), and therefore does not reflect the original species abundance signal in the time domain. Species abundance in the time domain, which is called for in the calibration protocol outlined here, could be based on an estimates from, e.g., a transient palaeoclimate model run linked to an ecological model (Lombard et al 2011;Morard et al 2013;Roche et al 2018;Metcalfe et al 2020), although estimating relative temporal abundance of a…”

Testing the effect of bioturbation and species abundance upon discrete-depth individual foraminifera analysis

“…Regarding issues in the water domain, it is possible that a foraminifera species may not continually inhabit a single surface water location or water depth, thus giving a non-continuous record of SST, which can have consequences for, e.g., ENSO reconstructions (Metcalfe et al, 2020). Secondly, a species' foraminiferal abundance through time is not constant and can be influenced by SST itself, which may bias IFA-derived SST distribution reconstructions, which is especially relevant in the case of ENSO, which itself influences SST.…”