Characteristics of alkenones synthesized by a bloom of Emiliania huxleyi in the Bering Sea

Abstract

We investigated the characteristics of the alkenones produced by a bloom of Emiliania huxleyi in the eastern Bering Sea in 2000. Alkenones were detected in surface waters between 57degreesN and 63degreesN, where phosphate concentrations were low and the ammonium/nitrate ratio was high. The total alkenone content (C-37:2, C-37:3, and C-37:4) ranged from 22.0 to 349 mug g(-1) in suspended particles and from 0.109 to 1.42 mug 9 1 in surface sediments. This suggests that a large proportion of the particulate alkenones synthesized in the surface water rapidly degraded within the water column and/or at the water-sediment interface of the Bering Shelf. The change in the stable carbon isotopic composition (delta(13)C) of C-37:3 alkenone could not be explained only by variation in [CO2(aq)] in the surface water but also depended on the growth rate of E. huxleyi. The alkenone unsaturation index (U-37(K')) was converted into an alkenone "temperature" with three equations (Prahl et al., 1988; Sikes et al., 1997; Miiller et al., 1998); Sikes et al.'s (1997) equation gave the best correlation with the observed sea surface temperature (SST) in the eastern Bering Sea. However, some temperatures estimated by Sikes et al.'s (1997) equation from the U-37(K') varied from the observed SST, possibly because of the rapidly changing rate of alkenone synthesis in the logarithmic growth stage or the low rate of alkenone synthesis when nutrients were limiting. Temperatures estimated from U-37(K') in the surface sediments (6.8 - 8.2degreesC) matched the observed SST in September (7-8degreesC) but differed from the annual average SST of 4 to 5degreesC, suggesting that most of the alkenone in the eastern Bering Sea was synthesized during limited periods, for instance, in September. The relative amounts of C-37:3 alkenone as proportions of the total alkenones (referred to as C-37:4%) were high, ranging from 18.3 to 41.4%. Low-salinity water (<32 psu) within the study area would have contributed to the high C-37:4% because a negative linear relationship between C-37:4% and salinity was found in this study. Copyright (C) 2003 Elsevier Science Ltd.