On the fidelity of shell-derived δ¹⁸O_seawater estimates

详细信息    查看全文

• 作者：Jennifer Arbuszewski ; Peter deMenocal ; Alexey Kaplan ; E. Christa Farmer
• 关键词：Atlantic ; stable isotopes ; foraminifera ; paleoceanography ; sea surface temperature ; salinity
• 刊名：Earth and Planetary Science Letters
• 出版年：2010
• 出版时间：1 December 2010
• 年：2010
• 卷：300
• 期：3-4
• 页码：185-196
• 全文大小：1749 K

文摘

Paired foraminiferal Magnesium/Calcium ratio and oxygen isotope analyses are widely used to estimate surface ocean δ¹⁸O_seawater, a robust proxy for surface salinity. We assess the fidelity of shell-derived δ¹⁸O_seawater estimates for the surface-dwelling foraminifer Globigerinoides ruber (white) using an Atlantic meridional coretop transect spanning basin-scale temperature and salinity gradients. Shell-derived and observed δ¹⁸O_seawater values are well correlated (r² = 0.77), but a large systematic bias is observed. Shell Mg/Ca ratios are significantly elevated above values expected from observed and isotopic calcification temperatures in the saline subtropical gyres of both hemispheres. This temperature-independent shell Mg/Ca ratio variability, termed “excess Mg/Ca”, is highly correlated with surface salinity (r² = 0.77), and the observed salinity dependence (27 ± 4 % ) is much higher than indicated by culture studies (6 ± 2 % ). Our coretop data are used to develop new Atlantic Basin temperature and salinity calibration equations that are accurate (± 1.1 °C and ± 0.20, respectively), precise (r² = 0.82 and 0.81, respectively), and verifiable using previously published data. These results are valid for the relatively high salinities of the subtropical Atlantic (35.5–37.3). We discover that inclusion of other published data from lower salinity regions (< 35) indicates little or no excess Mg/Ca. Taken together, these results point to a strongly non-linear, positive salinity effect on shell Mg/Ca ratios that significantly affects the accuracy of SST and δ¹⁸O_seawater estimates in high salinity settings (> 35).