Assessing the saponification effect on the quantification of long chain alkenones and the U₃₇^Kʹ paleothermometer

Abstract

In this study, we evaluated the saponification effect on alkenone quantification and the U₃₇^Kʹ paleothermometer using two internal standards (2-nonadecanone and squalane) by analyzing in-house reference sediment and eleven surface sediments taken from the Yellow Sea. The C_37:3 and C_37:2 alkenone concentrations of the in-house reference sediment showed significant differences between before and after the saponification (ΔC_37:3 and ΔC_37:2) depending on the internal standards used. However, the differences in the U₃₇^Kʹ and the estimated sea surface temperatures were insignificant. The ΔC_37:3 and ΔC_37:2 concentrations were larger when calculating based on 2-nonadecanone (46 ± 26 and 136 ± 67 ng/g, respectively) than those based on squalane (–7 ± 4 and –12 ± 10 ng/g, respectively) in the surface sediments. Interestingly, the ΔC_37:3 and ΔC_37:2 concentrations based on 2-nonadecanone showed a strong correlation with the loss of 2-nonadecanone that occurred during saponification. Accordingly, our study indicated that preferential loss of internal standards (e.g., 2-nonadecanone) can occur during saponification, which affects alkenone quantification and, thus, attention should be paid when choosing an internal standard.