The Effect of Varying CO2 Concentrations on the Alkenone Paleoclimate Temperature Proxy

Overview

The phytoplankton Emiliania huxleyi is known to produce long chain methyl and ethyl unsaturated ketones, also known as alkenones. Because of the global distribution of Emiliania huxleyi and how resistant alkenones are to degradation, alkenones can commonly be found in sediments all over the world. The UK’37 index is calculated by comparing the unsaturation of alkenones, and varies linearly with past sea surface temperature, making it a useful paleoclimate proxy. However, because CO2 has been a driving force of changing temperature since the earth became biogeochemically modern, it is possible that there may be a systemic difference where UK’37 based temperature records may be affected by varying CO2 levels and do not only reflect past sea surface temperature. In order to test this, isothermal culture experiments simulating different CO2 conditions representing pre-industrial/historic, current day, end of century and future levels were simulated by continuously bubbling gasses with a specific concentration of CO2 through the cultures. Temperatures calculated from UK’37 were always lower than actual temperature, but this is a common occurrence in studies involving laboratory cell cultures. There is no statistical difference in UK’37 or total amounts of alkenones produced caused by the simulated CO2 in samples at any one sampled time. There were differences between day 7 and day 14 samples, with day 7 cells having lower calculated temperatures from UK’37, suggesting the possible impacts of growth phase on UK’37 values. Scanning electron microscope (SEM) imaging demonstrated that CO2 levels did not affect cell morphology. Coloumetric data and SEM imaging revealed that cells did not calcify, but the full effects of this are still unknown. Based on the data from this study, CO2 levels do not appear to have a direct effect on UK’37, ensuring the integrity of UK’37 based temperature records from the effects of varying CO2 concentrations.