Global temperature calibration of the alkenone unsaturation index (UK′37) in surface waters and comparison with surface sediments

Abstract

In this paper, we compile the current surface seawater C 37 alkenone unsaturation (U K′37) measurements (n = 629, −1 to 30°C temperature range) to derive a global, field‐based calibration of U K′37 with alkenone production temperature. A single nonlinear “global” surface water calibration of U K′37 accurately predicts alkenone production temperatures over the diversity of modern‐day oceanic environments and alkenone‐synthesizing populations (T = −0.957 + 54.293(U K′37) − 52.894(U K′37)2 + 28.321(U K′37)3, r 2 = 0.97, n = 567). The mean standard error of estimation is 1.2°C with insignificant bias in estimated production temperature among the different ocean regions sampled. An exception to these trends is regions characterized by strong lateral advection and extreme productivity and temperature gradients (e.g., the Brazil‐Malvinas Confluence). In contrast to the surface water data, the calibration of U K′37 in surface sediments with overlying annual mean sea surface temperature (AnnO) is best fit by a linear model (AnnO = 29.876(U K′37) − 1.334, r 2 = 0.97, n = 592). The standard error of estimation (1.1°C) is similar to that of the surface water production calibration, but a higher degree of bias is observed among the regional data sets. The sediment calibration differs significantly from the surface water calibration. U K′37 in surface sediments is consistently higher than that predicted from AnnO and the surface water production temperature calibration, and the magnitude of the offset increases as the surface water AnnO decreases. We apply the global production temperature calibration to the coretop U K′37 data to estimate the coretop alkenone integrated production temperature (coretop IPT) and compare this with the overlying annual mean sea surface temperature (AnnO). We use simple models to explore the possible causes of the deviation observed between the coretop temperature signal, as estimated by U K′37, and AnnO. Our results indicate that the deviation can best be explained if seasonality in production and/or thermocline production as well as differential degradation of 37:3 and 37:2 alkenones both affect the sedimentary alkenone signal.