Diagenetic Degradation of Long-Chain Alkenones: Implications for Uk37 Paleo-Thermometry

Arnim Flügge SFB 313, University of Kiel, Olshausenstraße 40, D-24118 Kiel, Germany


The alkenone unsaturation index (Uk37) is the most widely used tool to estimate paleo sea-surface temperatures. However, due to high variations of the Uk37 vs. T calibration at the low temperature range, the application of this molecular paleo-thermometer is uncertain for high latitude oceanic regions (T < 5-8 °C).

High resolution Uk37 determinations on sediment cores from the Northern North Atlantic show a significant apparent shift towards warmer temperatures during the last thousand years. This temperature increase is not seen in *18O signals of planktonic foraminifera. Thus, increasing Uk37 values with sediment depth may be an early diagenetic phenomenon. A degradation
experiment conducted with an E. huxley culture of low initial Uk37 (0.23) verified microbially induced Uk37 alterations to higher values during the course of the experiment. The observations indicate that the rate constants of the di- to tetra-unsaturated C37 ketones increase with the degree of unsaturation, leading to systematic variations (increase) of the Uk37 index during early
diagenesis. A simple diffusion-reaction model adequately describes sedimentary alkenone degradation. The model demonstrates that diagenetic Uk37 variations are not linear and preferentially affect low Uk37 values. As a consequence, Uk37 based climatic signals are attenuated, and particulary at high latitudes (low T), the Uk37 based paleo-temperatures are over-estimated. The simple model can help to correct for diagenetic Uk37 shifts.