Bayesian calibration of the Mg/Ca paleothermometer in planktic foraminifera

Abstract

The Mg/Ca ratio of planktic foraminifera is a widely used proxy for sea‐surface temperature but is also sensitive to other environmental factors. Previous work has relied on correcting Mg/Ca for nonthermal influences. Here, we develop a set of Bayesian models for Mg/Ca in four major planktic groups—Globigerinoides ruber (including both pink and white chromotypes), Trilobatus sacculifer, Globigerina bulloides, and Neogloboquadrina pachyderma (including N. incompta)—that account for the multivariate influences on this proxy in an integrated framework. We use a hierarchical model design that leverages information from both laboratory culture studies and globally distributed core top data, allowing us to include environmental sensitivities that are poorly constrained by core top observations alone. For applications over longer geological timescales, we develop a version of the model that incorporates changes in the Mg/Ca ratio of seawater. We test our models—collectively referred to as BAYMAG—on sediment trap data and on representative paleoclimate time series and demonstrate good agreement with observations and independent sea‐surface temperature proxies. BAYMAG provides probabilistic estimates of past temperatures that can accommodate uncertainties in other environmental influences, enhancing our ability to interpret signals encoded in Mg/Ca.

A global database of Holocene paleotemperature records

“…The most frequent marine proxy types are δ 18 O, alkenones (U K ' 37 ), and Mg/Ca, including data from 135 out of 260 sites that were assimilated from the DADT project. For nearly all of these records, plus those based on TEX 86 , we generated paleotemperature time series by using the Bayesian calibration methods of Malevich et al 20 , Tierney and Tingley 21,22 , and Tierney et al 23 respectively, with their published model parameters, as specified in 'Calibration Method' . The original temperature time series are retained among the ancillary records in the larger collection of temperature-sensitive datasets ('In Compilation' = Tverse, see below) and are noted as such in the 'QC Comments' field.…”

“…16 and most of the records based on marine sediments (other than those from microfossil assemblages) are calculated for this data product using the Bayesian procedures of refs. [20][21][22][23] . For other proxy types, paleotemperature values and their uncertainties are based on multiple generations of analytical and calibration methods.…”

Overturning circulation, nutrient limitation, and warming in the Glacial North Pacific

“…For Globigerinoides ruber and Globigerina bulloides , we use the species-specific equations from ( 136 ). For Neogloboquadrina pachyderma , we use the species-specific sensitivities given in ( 137 ), which we incorporate into the MgCaRB protocol (note that N. pachyderma Mg/Ca appears insensitive to pH). While the direct temperature sensitivity of Mg/Ca in planktic foraminifera is ~6%/°C ( 136 ), because of the effect of temperature on pH through the disassociation constant of water ( K w ), the “apparent” Mg/Ca temperature sensitivity is higher in pH-sensitive species ( 138 ).…”

Relationship between individual chamber and whole shell Mg/Ca ratios in Trilobatus sacculifer and implications for individual foraminifera palaeoceanographic reconstructions

“…These foraminifera also contained minimal residual sediment, and showed few visible signs of dissolution upon visual inspection. With these limitations and conditions in mind, the correspondence that we find between solution chemistry results and those from LA-ICPMS suggests that calibration equations generated from solution chemistry methods 4 – 6 are applicable for use with LA-ICPMS data.…”

“…The relationship between calcification temperature and Mg/Ca ratios in the shells (or tests) of foraminifera has long been used in paleoclimate reconstructions 1 – 3 . This relationship has been quantified from core-top, sediment trap, and culture studies, resulting in various single-species and multi-species paleotemperature calibration equations relating temperature to shell Mg/Ca 4 – 6 . Typically, sea surface temperature (SST) reconstructions generated from Mg/Ca ratios are obtained from monospecific aggregates of 5–50 shells.…”