Alkenone Records of Ocean Warming Since the Last Glacial Maximum and Prospects for Their Use in Lacustrine Settings

Simon C Brassell (Biogeochemical Laboratories, Department of Geological Sciences, Indiana University, Bloomington, IN 47401-1403; ph. 812-855-3786; fax: 812-855-7961; internet: (AGU Sponsor: Simon C Brassell)

The unsaturation of alkenones is dependent on the growth temperature of their biological source, principally the coccolithophorid Emiliania huxleyi. Thus, the preservation of these molecular signals in marine sediments provides a proxy for sea surface temperatures (SSTs).

Comparison of the alkenone unsaturation profiles for glacial/interglacial sequences from various oceanic settings enables assessment of the timing, magnitude and progression of ocean warming following the last glacial maximum (LGM). These molecular stratigraphic records exhibit SST increases since the LGM of <1 to > 5 degrees C in the tropical and equatorial regions, and of <5 to >8 degrees C at higher latitudes. The SST change is rapid at some sites, while it occurs gradually at other locations. These temperature records are constrained by the accuracy of sediment dating and, in some cases, by sampling intervals. In addition, they depend on various assumptions, including the presumption that a single calibration relating alkenone unsaturation to SSTs is globally valid. Moreover, a critical need in the assessment of these alkenone records is the capacity to deconvolute temperature changes attributable to global climate change from local factors, including the effect on SSTs of fluctuations in upwelling intensity, of shifts in the seasonal timing of peak plankton production, and of changes in the speciation of alkenone-contributing coccolithophorids and in their growth habit. Efforts to separately recognize and assess the potential impact of each of these factors suggest that they can exert an influence on the timing and magnitude of warming observed in the alkenone records, but also offer the possibility of adjustments to account for these effects.

The recognition of alkenones in an increasing number of lacustrine sediments offers the possibility to extend this molecular approach to continental climate records. A 30ka record from Zabuye Salt Lake in Tibet exhibits temporal variations in alkenones unsaturation that appear to correspond with global changes in climate. However, the data do not conform to the temperature calibrations established for the marine environment, prompting the need for independent temperature measures to calibrate alkenone responses in these settings.