The effects of temperature, salinity, and the carbonate system on Mg/Ca in Globigerinoides ruber (white): A global sediment trap calibration

Abstract

The Mg/Ca of planktic foraminifera Globeriginoides ruber (white) is a widely applied proxy for tropical and subtropical sea-surface temperature. The accuracy with which temperature can be reconstructed depends on how accurately relationships between Mg/Ca and temperature and the multiple secondary controls on Mg/Ca are known; however, these relationships remain poorly quantified under oceanic conditions. Here, we present new calibrations based on 440 sediment trap/plankton tow samples from the Atlantic, Pacific and Indian Oceans, including 130 new samples from the Bay of Bengal/Arabian Sea and the tropical Atlantic Ocean. Our results indicate temperature, salinity and the carbonate system all significantly influence Mg/Ca in G. ruber (white). We propose two calibration models: The first model assumes pH is the controlling carbonate system parameter. In this model, Mg/Ca has a temperature sensitivity of 6.0±0.8 %/°C (2s), a salinity sensitivity of 3.3±2.2 %/PSU and a pH sensitivity of-8.3±7.7 %/0.1 pH units; The second model assumes carbonate ion concentration ([CO 3 2-]) is the controlling carbonate system parameter. In this model, Mg/Ca has a temperature sensitivity of 6.7±0.8 %/°C, a salinity sensitivity of 5.0±3.0 %/PSU and a [CO 3 2-] sensitivity of-0.24±0.11 %/µmol kg-1. In both models, the temperature sensitivity is significantly lower than the widely-applied sensitivity of 9.0±0.6 %/°C. Application of our new calibrations to down-core data from the Last Glacial Maximum, considering whole ocean changes in salinity and carbonate chemistry, indicate a cooling of 2.4±1.6 °C in the tropical oceans if pH is the controlling parameter and 1.5±1.4 °C if [CO 3 2-] is the controlling parameter.

Considerations for Globigerinoides ruber (White and Pink) Paleoceanography: Comprehensive Insights From a Long‐Running Sediment Trap

“…The δ 18 O sw was calculated by converting monthly climatological salinity at 20‐m water depth (Levitus, ) using the regional surface water 18 O sw ‐salinity equation presented in section . Gray symbols show the Mg/Ca‐T global sediment trap data set for G. ruber compiled in Gray et al (). Black dashed line indicates the Mg/Ca‐T relationship determined by Gray et al () [Eq.S1: Mg/Ca = 1.26 exp(0.53T)], and the gray dashed line indicates the Mg/Ca‐T relationship determined by Anand et al () [ G. ruber (white) equation with fixed exponential constant, Mg/Ca = 0.449*exp(0.09*T)].…”

“…Gray symbols show the Mg/Ca‐T global sediment trap data set for G. ruber compiled in Gray et al (). Black dashed line indicates the Mg/Ca‐T relationship determined by Gray et al () [Eq.S1: Mg/Ca = 1.26 exp(0.53T)], and the gray dashed line indicates the Mg/Ca‐T relationship determined by Anand et al () [ G. ruber (white) equation with fixed exponential constant, Mg/Ca = 0.449*exp(0.09*T)]. The solid black curve in each figure is the exponential Mg/Ca‐T relationship determined from the combined pink and white G. ruber data set from this study, with the 95% confidence intervals shown in gray shaded intervals.…”

“…We test the impact of accounting for both salinity and pH change in our multicore from the nGoM, which is able to resolve the twentieth century trend due to rapid sediment accumulation rates. In order to look at the impact of twentieth century pH changes on Mg/Ca using the Gray et al () equation, we estimated surface pH change over the twentieth century in the nGoM using the atmospheric CO 2 data from Mauna Loa Observatory (Keeling et al, ) (http://scrippsco2.ucsd.edu/assets/data/atmospheric/merged_ice_core_mlo_spo/merged_ice_core_yearly.csv). We used the climatic mean annual SSS, SST and A T in the nGoM, along with the pCO 2 data to estimate surface ocean pH for the years sampled in according to our age model (Tables S3 and S4, supporting information).…”

“…TEX 86 and U ‐SST were published in Richey and Tierney (). Globigerinoides ruber pink and white Mg/Ca was converted to SST using the multivariate equation of Gray et al () that accounts for temperature, salinity, and pH influence on foraminiferal Mg/Ca.…”

Calibration and Validation of Environmental Controls on Planktic Foraminifera Mg/Ca Using Global Core‐Top Data

“…This novel approach has the advantage of potentially accounting for the variations in depth habitat and seasonality of foraminifera calcification between sites, assuming the δ 18 O of seawater is invariant. The approach is not without its own uncertainties however, and comparison of δ 18 O‐based and in situ temperatures can show offsets of up to 3 °C (Anand, Elderfield, et al, ; Gray et al, ). δ 18 O‐based temperatures are also poorly suited to multivariate calibrations considering that independent proxies for variables other than T either do not exist or have large uncertainties (Allen et al, ).…”