Successful carbonate Δ47 analyses requires low organic matter contents. Therefore, we developed a protocol for organic matter removal, and proved that this method successfully eliminates organic contaminants, and does not affect stable isotope or Δ47 estimates. The advantage of using coccolith calcite is that it ensures a surface signal. However, this requires a careful separation from other calcite sources. Further separation of discrete size fractions may be needed, if, for instance, a specific coccolith fraction is more affected by diagenesis, or if there are important changes in coccolith species through time. Therefore, a key success of this project was the development of a coccolith size separation method, which combines centrifugation, settling and semi-automated microfiltering techniques, and produces enough calcite for Δ47 analysis. The centrifugation part was submitted to Biogesciences, and the implementation of the whole method produced 90-98% pure coccolith fractions.
We applied Δ47 thermometry to 85 coccolith fractions, i.e. to 21 additional ones than those planned. One of the key results is the Δ47 temperature record from pure and well-preserved North Atlantic coccoliths of the last 15 Ma, which show similar cooling trends but significantly colder absolute temperatures compared to UK’37-derived SSTs from the same samples. We showed that at least for high latitudes, coccolith Δ47 may provide more reliable absolute temperatures compared to UK’37. These results are in review in Nature Geosciences. We have had complications with coccolithophore culturing until a couple of months ago. Therefore, since carbon limitation is expected to affect more large cells due to their lower capacity of CO2 diffusion, we decided to alternatively test if carbon limitation influences coccolith Δ47 by measuring Δ47 in three discrete, well-separated coccolith size fractions from the same North Atlantic sediments. These results, combined with vital effects of coccolith δ13C and δ18O, and carbon isotopic fractionation of alkenones (εp), are currently prepared to be submitted to EPSL.
We obtained Δ47 temperatures from coretop coccolith separations from diverse oceanographic settings with varying SSTs, CO2, seasonalities, and depths of production. UK’37 SSTs measured in the same samples show differences compared to Δ47 temperatures. We also obtained εp estimates, which, combined with ratios of alkenones C37:C38, can provide information on factors affecting the alkenone CO2 proxy. This study is being prepared for submission to EPSL. Finally, we have measured Δ47 from coccolith separations of low latitudes in the Equatorial Pacific over the last 60 Ma. This tropical absolute temperature record is expected to provide the first long-term mixed layer depth absolute temperature estimates of tropical oceans, in latitudes where the alkenone proxy is insensitive. Results are being prepared for submission, likely to a high impact journal like Nature.
Results have been presented in international conferences, including Goldschmidt 2018, 2019, EGU 2020, ICP 2019 and SGM 2018. Outreach activities include a talk and a video directed to the public and politicians in Colombia, the organization and participation in a Doctoral Geological excursion to Colombia, and an interview on my experience as a Marie Curie fellow to EURAXESS LAC. Manuscripts are in review (Nature Geosciences, Biogeoscienes) or in preparation for submission and a field guide to the excursion was published.
