Using Paired Mg/Ca and Oxygen Isotopic Measurements of Planktonic Foraminifera to Estimate Tropical Atlantic Thermocline Shape
Abstract
Dynamic feedbacks between atmospheric circulation and upper ocean water masses determine sea surface temperature patterns in the tropics. Easterly tropical trade winds shoal the thermocline and drive vigorous upwelling in eastern basins, cooling upper ocean temperatures there. Warmer SSTs and a deeper thermocline characterize western tropical warm pools. Geochemical compositions of depth-stratified planktonic foraminifera allow paleoceanographic reconstructions of tropical wind-driven upper ocean temperatures. Initial results using only oxygen isotopic ratios indicated that the tropical thermocline shape (between 0 and 40m) could be reconstructed with an average standard error of +/-1.6degC for a set of 24 Atlantic coretops. This study uses paired Mg/Ca and isotopic data and new statistical techniques to enhance the utility of this approach. Mg/Ca values were measured by ICP-OES in the same samples used in the oxygen isotope calibration, as well as for many additional new coretop samples. Foraminiferal Mg/Ca has been shown to be exponentially dependent on the temperature at which the organisms grew. While this relationship is subject to complication by dissolution and ontogenetic effects, it is unaffected by variations in continental ice volume. In general, Mg/Ca of G. sacculifer (without final chamber), G. ruber (white), N. tumida, and G. menardii species show a higher correspondence with upper ocean temperatures than was apparent for the oxygen isotopic dataset, promising increased accuracy of thermocline reconstruction estimates. Additional calibration improvements result from reconstructing depth and temperature of inflection points in the upper ocean temperature profile rather than simply temperature at discrete depths. So instead of regressing Mg/Ca or isotopic data of any one species on temperature at 50m, for example, higher correlations and better predictive skill are obtained by relating Mg/Ca of each species to the temperature and depth of the mixed layer, the steepest point in the thermocline, and the base of the thermocline.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2003
- Bibcode:
- 2003AGUFMPP21D..04F
- Keywords:
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- 1000 GEOCHEMISTRY (New field;
- replaces Rock Chemistry);
- 3030 Micropaleontology;
- 3344 Paleoclimatology;
- 4267 Paleoceanography