The impact of alkenone degradation on U-37(K)' 37 paleothermometry: a model-derived assessment
Freitas, F.S.; Pancost, R.D.; Arndt, S. (2017). The impact of alkenone degradation on U-37(K)' 37 paleothermometry: a model-derived assessment. Paleoceanography 32(6): 648-672. https://dx.doi.org/10.1002/2016PA003043 In: Paleoceanography. American Geophysical Union: Washington, DC. ISSN 0883-8305; e-ISSN 1944-9186, more |  |
| Authors | | Top | - Freitas, F.S., more
- Pancost, R.D.
- Arndt, S., more
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| Abstract | The U37K proxy for past sea surface temperature (SST) is based on the unsaturation ratio of C-37 alkenones. It is considered a diagenetically robust proxy, but biases have been invoked because the index can be altered by preferential degradation of the C-37:3 alkenone, resulting in higher reconstructed SST. However, alkenone degradation rate constants are poorly constrained, making it difficult to evaluate the plausibility of such a bias. Therefore, we quantitatively assessed the effect of (1) different alkenone degradation rate constants; (2) differential degradation factors between diunsaturated and triunsaturated C-37 alkenones; (3) and initial U37K values on the U37K paleothermometer for two depositional environments (shelf and upper slope), by means of a reaction-transport model (RTM). RTM results reveal that preferential degradation of C-37:3 can potentially alter the original signal of the U37K paleothermometer, but SST biases (SST) are largely within U37K calibration error (SST <1.5 degrees C) assuming realistic model parameters. The magnitude of SST is largely determined by the degradation rate constant, but it also increases with higher differential degradation factors. Additionally, initial U37K values exert a nonlinear influence on the extent of potential SST bias, with midrange values (0.4 < U37K < 0.6) being most sensitive. The most significant changes occur in the shallowest sediment layers and are attenuated with burial time/depth. Scenarios where SST >1.5 degrees C are associated with marked downcore decreases in alkenone concentration. Consequently, we caution against the interpretation of U37K indices when extensive degradation results in very low alkenone concentrations (<5ng g(-1)). |
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