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

Abstract

[1] In this paper, we compile the current surface seawater C 37 alkenone unsaturation (U 37 K 0 ) measurements (n = 629, À1 to 30°C temperature range) to derive a global, field-based calibration of U 37 K 0 with alkenone production temperature. A single nonlinear ''global'' surface water calibration of U 37 K 0 accurately predicts alkenone production temperatures over the diversity of modern-day oceanic environments and alkenonesynthesizing populations (T = À0.957 + 54.293(U 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 37 ) À 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 37 K 0 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 37 K 0 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 37 K 0 , 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.

Seasonal and depth variations in molecular and isotopic alkenone composition of sinking particles from the western North Pacific

“…Nearly identical relationships were found in a comparison of the U K' 37 of core-top and surface sediments with the mean annual temperature at 0 m in the overlying waters (e.g., Müller et al, 1998;Conte et al, 2006). The calibration equations based on these relationships have been used for assessing sea-surface paleotemperature.…”

“…High U K' 37 -based temperature in the southern Indian Ocean during the last glacial maximum has been interpreted as the lateral advection of warm-water detrital alkenones by the Agulhas Current (Sicre et al, 2005). Other research has attributed the low U K' 37 -based temperature of the suspended particulates in the Brazil-Malvinas Confluence to the lateral advection of cold-water detrial alkenones by the Malvinas Current (Rühlemann and Butzin, 2006;Conte et al, 2006).…”

“…The calibration equations based on these relationships have been used for assessing sea-surface paleotemperature. Recently Conte et al (2006) showed that the surface sediment calibration differs significantly from the surface-water calibration and attributed the deviation to the combined effects of seasonality and thermocline production as well as the differential degradation of triand di-unsaturated alkenones.…”

Holocene and Eemian sea surface temperature trends as revealed by alkenone and Mg/Ca paleothermometry

“…In spite of a recognized potential bias in alkenone-based SST reconstructions linked to alkenone production in the thermocline, to the long-distance alkenone advection by oceanic currents and/or to a differential degradation of C37:3 and C37:2 alkenones within the sediment (see discussion in Conte et al, 2006 and references therein), alkenone-based thermometry is widely used as a SST proxy. Since the global core-top calibration equation of Mü ller et al (1998) is remarkably well correlated with annual-mean SST, most of the alkenone-derived SST records are implicitly interpreted in terms of annual-mean SST.…”

The role of North Brazil Current transport in the paleoclimate of the Brazilian Nordeste margin and paleoceanography of the western tropical Atlantic during the late Quaternary

“…Previous studies have shown that in regions of strong surface currents, such as the NBC, alkenone temperature reconstructions may be temporally offset from the nominal age of sedimentation given by planktonic foraminifers as a result of fine particle advection (e.g., Benthien andMüller, 2000 andConte et al 2006). Jaeschke et al (2007) address the discrepancy between Mg/Ca-derived SST from Weldeab et al (2006) and their own alkenone-derived SST in the case of the H1 event and suggest that postdepositional resuspension and transport of the alkenones could be a cause, but they did not address the potential for the same discrepancy for the entirety of their record.…”