EPISODE · Deciphering the tEmperature history of troPIcal oceanS: a cOccolith clumpeD isotopE approach

Deciphering the tEmperature history of troPIcal oceanS: a cOccolith clumpeD isotopE approach

Accurate predictions of climate’s response to increasing CO2, and a timely implementation of mitigation strategies, depends on our ability to reconstruct past Earth’s Climate Sensitivity (ECS) during conditions similar to those expected in the future, for which reliable absolute estimates of temperature and CO2 are required. Given the uncertainties of widely-used temperature proxies, this project evaluated the viability of clumped isotope (Δ47) thermometry applied to coccolith calcite, a mineral produced by photosynthetic organisms called coccolithophores, to reliably reconstruct absolute temperatures of surface oceans (SST). High uncertainties are associated to temperatures of warm, high CO2 worlds, as perhaps the currently best-trusted SST proxy (alkenone unsaturation index UK’37) becomes insensitive > 28 °C. Since warm waters covered larger parts of high CO2 worlds compared to today, a significant part of Earth’s temperature history is unexplored. A further paleoclimate conundrum is the extreme polar amplification shown by widely-used temperature proxies for warm, high CO2 worlds. Climate modelers struggle to reproduce this small latitudinal temperature gradient, posing the question of whether it is models or the interpretation of proxies that should be improved. To ensure a new proxy resolves a targeted parameter, in this case, temperature, further controls on the proxy must be ruled out or corrected for. The first goal was to determine if CO2 changes affect coccolith Δ47. For this, we proposed a mixed culture and core top-based study, considering a range of temperatures and CO2. The second main goal was to validate the coccolith Δ47 performance by comparing temperatures with those obtained by UK’37, which is based on lipids produced by coccolithophores. Finally, we proposed to apply the Δ47 proxy to tropical, well conserved, coccoliths throughout the Cenozoic, so as to produce the first absolute long-term tropical temperature record, and compare with high latitude temperatures to evaluate the latitudinal gradient in high CO2 worlds. EPISODE project proved the invaluable potential of coccolith Δ47 to reconstruct mixed layer depth temperatures, where most coccolithophores’ productivity occurs. We have found no evidence for vital effects. We also show that coccolith Δ47 may provide more reliable absolute temperatures than UK’37, especially in high latitudes. Coccolith Δ47 suggest a more modest polar amplification during the high mid-Miocene CO2 world compared to other temperature proxies.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз