The alkenone temperature signal in western North Atlantic surface waters

Abstract

Haptophyte algae-derived long-chain C37-C39 alkenones and alkyl alkenoates were analyzed in euphotic zone particulate matter collected over a 7 yr period at the Oceanic Flux Program/Bermuda Atlantic Time Series (OFP/BATS) site in the western Sargasso Sea. Surface water temperatures at the site range annually from 19 to 29°C. Alkenone concentrations ranged from < 0.1 ng L−1 in summer to > 100 ng L−1 following the passage of storms. Highest seasonal concentrations occurred during the late winter and spring. Under stratified conditions, alkenone concentrations in the surface mixed layer (0–20 m) were generally 2 to 4 times higher than in the deep fluorescent maximum (75–110 m), consistent with Emiliania huxleyi concentration profiles (Haidar and Thierstein, 2001) and indicated that alkenone production primarily occurs within the upper euphotic zone in this region.Alkenone compound distributions and the temperature calibrations of C37 and C38 methyl and ethyl alkenone unsaturation (U37K′, U38MeK, and U38EtK, respectively) were remarkably similar to that observed in an E. huxleyi strain previously isolated from the same area (Conte et al., 1998), providing strong evidence that E. huxleyi is the predominant alkenone synthesizer and that characteristics exhibited by randomly isolated clones in culture are, in many cases, consistent with those of populations in the region of origin.The Bermuda calibration of U37K′ vs. water temperature (U37K′ = −1.9835 + 0.2004T − 0.0034T2, r2 = 0.95, n = 91) is nonlinear and falls along the same trendline as euphotic zone particulates from warm (> 15°C) waters of the eastern North Atlantic (Conte and Eglinton, 1993) and Mediterranean (Ternois et al., 1997). The combined North Atlantic temperature calibration (U37K′ = − 1.1365 + 0.1257T − 0.0018T2, r2 = 0.963, n = 134) differs significantly from published coretop sediment calibrations (Rosell-Melé et al., 1995; Müller et al., 1998) based on sea surface temperature maps (Levitus and Boyer, 1994), having a steeper slope at temperatures < 22°C but a shallower slope at temperatures > 24°C. The reasons are not clearly apparent and manifest a need for detailed studies linking production patterns in surface waters with the signal arriving at the seafloor to fully understand the generation of the sedimentary signal.