Depth and seasonality of alkenone production along the California margin inferred from a core top transect

Alkenone unsaturation indices (U-37(k')) of marine sediment could prove particularly useful on organic-rich continental margins where carbonate dissolution hampers the use of other paleoclimatic proxies [McCaffrey et al., 1990; Kennedy and Brassell, 1992]. Forty core top samples of Recent sediment from a latitudinal transect (23 degrees-40 degrees N) along the California margin yield U-37(k') values that correlate linearly with modem mean annual sea surface temperatures (SSTs) in the range of 12 degrees-23 degrees C. Reproducibility of the unsaturation value in closely spaced cores is near analytical error. U-37(k') data define a relationship to temperature nearly identical to the Prahl et al. [1988] laboratory cultures of Emiliania huxleyi. The close agreement is particularly significant in light of the nannofossil composition of the sediments, where the abundance of the coccolith taxon Gephyrocapsa oceanica (known to synthesize alkenones) equals or exceeds that of E. huxleyi. Comparison with seasonal temperature variations at different depths indicates that little if any alkenone production occurs at depths >30 m along the continental margin (water depths <2 km). Sediments in more pelagic locations exhibit small but consistent biases toward winter and/or subsurface production similar to previously reported sediment trap and core top data from the Oregon margin [Prahl et al., 1993, Doose et al., 1997].