Evaluating carbonate saturation effects on magnesium calcium core top calibration in benthic foraminifera

• Main Author: Jordan, Katherine A.
• Format: Text
• Language: unknown
• Published: No Publisher Supplied 2008
• Subjects: Norwegian Sea
• Online Access: https://dx.doi.org/10.7282/t39k4bk1; https://rucore.libraries.rutgers.edu/rutgers-lib/24406/

Magnesium calcium paleothermometry in benthic foraminifera is intended to provide a salinity-independent reconstruction of bottom water temperature. The temperature calibration is however, rather uncertain, due to evidence of additional carbonate saturation effects on Mg incorporation into benthic tests. The overarching goal of this thesis is to directly determine the magnitude of the carbonate saturation effect and the threshold at which this effect significantly alters temperature estimates based on the Mg/Ca content of benthic foraminiferal tests. My research has focused on using homothermal homohaline depth transects from the Norwegian Sea and Gulf of Mexico in the context of a global core top calibration. This thesis consists of 45 core tops from four cruises. In these transects, the relationship between carbonate saturation and Mg/Ca is not strongly pronounced, possibly due to the large variability in the data. As is, it does support the hypothesis that in saturated waters, temperature exerts the primary control on benthic foraminiferal Mg/Ca. A two-part global calibration equation has been constructed for under-(∆[CO3-2] [less than] 3µmol/kg) and over-saturated (∆[CO3-2] [greater than] 3µmol/kg) waters. However, these equations produce unrealistic bottom water temperature reconstruction when applied to a downcore record in the eastern basin of the tropical Atlantic Ocean (1°21'S 11°58'W; 3912 m). These results suggest that further work is needed to improve the reliability of the Mg/Ca-T-∆[CO3-2] equation and to accurately determine the threshold at which ∆[CO3-2] effects become pronounced. Likewise, future studies should look into using multi proxy approaches for correcting Mg/Ca-T estimates for saturation effects, such as the use of B/Ca measurements as a proxy for ∆[CO3-2].