Interlaboratory comparison study of Mg/Ca and Sr/Ca measurements in planktonic foraminifera for paleoceanographic research

Abstract

International audience[1] Thirteen laboratories from the USA and Europe participated in an intercomparison study of Mg/Ca and Sr/Ca measurements in foraminifera. The study included five planktonic species from surface sediments from different geographical regions and water depths. Each of the laboratories followed their own cleaning and analytical procedures and had no specific information about the samples. Analysis of solutions of known Mg/Ca and Sr/Ca ratios showed that the intralaboratory instrumental precision is better than 0.5% for both Mg/Ca and Sr/Ca measurements, regardless whether ICP-OES or ICP-MS is used. The interlaboratory precision on the analysis of standard solutions was about 1.5% and 0.9% for Mg/Ca and Sr/Ca measurements, respectively. These are equivalent to Mg/Ca-based temperature repeatability and reproducibility on the analysis of solutions of +/- 0.2 degreesC and +/- 0.5 degreesC, respectively. The analysis of foraminifera suggests an interlaboratory variance of about +/-8% (%RSD) for Mg/Ca measurements, which translates to reproducibility of about +/- 2 - 3 degreesC. The relatively large range in the reproducibility of foraminiferal analysis is primarily due to relatively poor intralaboratory repeatability (about +/- 1 - 2 degreesC) and a bias (about 1 degreesC) due to the application of different cleaning methods by different laboratories. Improving the consistency of cleaning methods among laboratories will, therefore, likely lead to better reproducibility. Even more importantly, the results of this study highlight the need for standards calibration among laboratories as a first step toward improving interlaboratory compatibility

Planktonic foraminiferal Mg/Ca as a proxy for past oceanic temperatures: a methodological overview and data compilation for the Last Glacial Maximum

“…The effect of reductive cleaning is to lower Mg/Ca. This is highlighted by a systematic difference (of about 8%) between laboratories using the ''Cd'' and ''Mg'' cleaning protocols (Rosenthal et al, 2004) (Fig. 3).…”

“…de Villiers et al, 2002). Both of these techniques provide high precision measurements of Mg/ Ca ratios of the order 70.5% (Rosenthal et al, 2004). An alternative approach to Mg/Ca analysis is by use of electron microprobe (e.g.…”

“…Brown and Elderfield, 1996;Haley and Klinkhammer, 2002;Barker et al, 2003). This has resulted in the evolution of essentially two variants of the Mgcleaning protocol; one with a reductive step for removal of an oxide coating, the other without (the ''Cd'' and ''Mg'' versions as described by Rosenthal et al, 2004). The effect of reductive cleaning is to lower Mg/Ca.…”

“…Differences in methodology and analysis could lead to discrepancies between laboratories and as such it is important to know what differences exist and how these might affect the determination of Mg/Ca ratios. These questions have recently been addressed by an interlaboratory comparison study by Rosenthal et al (2004) in which several foraminiferal samples and standard solutions were run by a number of laboratories throughout Europe and the USA in order to assess the relative accuracy, precision and reproducibility of their methodologies. Results from this work suggest that analytical techniques used by a selection of laboratories produced generally good precision but poor inter-lab consistency.…”

Holocene and Eemian sea surface temperature trends as revealed by alkenone and Mg/Ca paleothermometry

“…The Mg/Ca ratio measured on surface-dwelling planktonic foraminifera is a well-established SST proxy that has been internationally calibrated by 25 laboratories (Rosenthal et al, 2004;Greaves et al, 2008). Systematic biases owing to, e.g., ecological behaviour of planktonic foraminifera (Regenberg et al, 2009), local hydrologic characteristics such as calcite saturation levels at depth (Dekens et al, 2002), and different cleaning protocols used to remove the coating of Mg-rich contaminant phases (Rosenthal et al, 2004) have led to a wide range of calibration equations to convert Mg/Ca into temperature. As for the alkenones results, we report the Mg/Ca-based SST data as they were estimated in the original publications.…”