Does phylogeny control U37K-temperature sensitivity? Implications for lacustrine alkenone paleothermometry

Alkenone paleothermometry (via the U37K and U37K′ indices) has long been used to reconstruct sea surface temperature and has more recently been proven effective in lacustrine settings. Genetic analyses indicate that there is a diversity of different alkenone-producing lacustrine haptophytes, and differences among U37K-temperature calibrations suggest that unique calibrations might be required to quantify past temperature variation from individual lakes. The only term necessary to quantify U37K-inferred temperature relative to a reference period (e.g., modern temperature 20th Century mean) is the slope of the calibration regression, the U37K-temperature sensitivity (i.e., the change in U37K per °C temperature change). Here, we bring together all of the existing U37K-temperature calibrations in order to compare the variability among U37K-temperature sensitivities. We also report a new in situ U37K-temperature calibration along with environmental genomic analysis based on the 18S rRNA gene of an alkenone producing haptophyte from lake Vikvatnet in Norway. We propose and test the hypothesis that U37K-temperature sensitivity is controlled by phylogeny and that this term can be used to quantify past temperature variation from lake sediments if the genetic identity of the lake's alkenone-producer is known. Using the existing calibration data sets, we determine four phylotype-specific U37K-temperature sensitivities for use in cases where in situ calibration is unavailable but the phylogeny of the alkenone producers is known.