Nonthermal Influences on Mg/Ca in Planktonic Foraminifera: A Review of Culture Studies and Application to the Last Glacial Maximum

Abstract

Planktonic foraminiferal Mg/Ca is one of the most widely applied sea surface temperature proxies. While the influence of salinity on Mg/Ca has led the accuracy of Mg/Ca‐temperatures to be questioned, the effect of seawater carbonate chemistry (pH) is seldom accounted for down‐core. Using published data sets, we review controls on Mg/Ca in laboratory cultures of planktonic foraminifera Globigerinoides ruber (white), Trilobatus sacculifer, Globigerina bulloides, and Orbulina universa. All are characterized by a temperature sensitivity of ~6% per °C and a salinity sensitivity of ~4% per salinity unit; all except T. sacculifer are sensitive to carbonate chemistry (−5 to −9% per 0.1 pH units). We demonstrate the down‐core manifestation of these sensitivities using data spanning the last deglaciation and an Earth System Model forced with last glacial maximum conditions. While the effects of salinity are relatively minor, seawater carbonate chemistry exerts a large bias on Mg/Ca temperature if unaccounted for; however, as local pH changes beyond the effect of atmospheric CO2 are relatively small across most of the ocean (less than ±0.05 pH units, 2σ), atmospheric CO2 can be used to accurately correct Mg/Ca temperatures. We present protocols to correct Mg/Ca for pH down‐core using either atmospheric CO2 or (preferably) boron isotopes in a new software package “MgCaRB.”

Bayesian Calibration of the Mg/Ca Paleothermometer in Planktic Foraminifera

“…This could indicate that temperature sensitivity for G. ruber is systematically underestimated; however, the trend is only slightly ameliorated after running a version of the group‐specific model with variable SST coefficients for each species ( r =0.61, p <0.0001), and the derived ca. 6% sensitivity of the seasonal group‐specific model is very similar to values calculated from G. ruber culture and sediment trap data (Gray & Evans, ; Gray et al, ). Alternatively, this residual trend could suggest that our relatively simple inference of seasonal SST (based on sediment trap abundances) does not apply well to G. ruber .…”

“…As described below, we also use Mg/Ca data from cultured foraminifera to constrain sensitivities to environmental parameters. We use the compilation of Gray and Evans (), with the addition of the G. ruber pink data from Allen et al () and N. incompta data from Von Langen et al () and Davis et al (). This updated culture data set includes 30 G. ruber observations, 20 T. sacculifer observations, 12 G. bulloides observations, 29 O. universa observations, and 12 N. incompta observations for a total of 103 data points.…”

“…However, pH does not seem to impact Mg/Ca in cultures of N. pachyderma , N. incompta (Davis et al, ), and T. sacculifer (Allen et al, ). Laboratory experiments indicate a moderate sensitivity of planktic Mg/Ca to salinity (3–5% per psu) (Allen et al, ; Gray & Evans, ; Hönisch et al, ; Kisakürek et al, ; Lea et al, ). Previous core top studies suggested a much larger sensitivity (15–59%, Arbuszewski et al, ; Ferguson et al, ; Mathien‐Blard & Bassinot, ), but reanalyses indicated that these high estimates are due either to environmental covariates (Hertzberg & Schmidt, ; Hönisch et al, ; Khider et al, ) or to analytical issues (Dai et al, ).…”

“…We then interpolate these scaled curves to the ages at which there are Mg/Ca observations and input them into BAYMAG. We do not explicitly account for the temperature effect on pH (e.g., Gray & Evans, ) because while it scales with the magnitude of local cooling, it is a small source of error for the LGM (0.65 ∘ C, Gray & Evans, ). Since the salinity and pH sensitivities are of opposite sign, the glacial‐interglacial changes partly cancel each other out; however, LGM cooling is still 0.8 ° C warmer than estimates made with constant salinity and pH (not shown) due to the pH effect.…”

“…Previous calibrations for Mg/Ca have been based on laboratory culturing experiments (Gray & Evans, ; Lea et al, ; Nürnberg et al, ), sediment trap data (Anand et al, ; Gray et al, ), or modern core tops (Dekens et al, ; Elderfield & Ganssen, ; Khider et al, ; Saenger & Evans, ). Culture experiments provide precise constraints on environmental sensitivities but are limited in that laboratory conditions are not perfect analogs for the natural environment.…”