Abstract. Two haptophyte algae, Emiliania huxleyi and Gephyrocapsa oceanica, were cultured at different temperatures and salinities to investigate the impact of these factors on the hydrogen isotopic composition of long chain alkenones synthesized by these algae. Results showed that alkenones synthesized by G. oceanica were on average depleted in D by 30‰ compared to those of E. huxleyi when grown under similar temperature and salinity conditions. The fractionation factor, α alkenones−H 2 O , ranged from 0.760 to 0.815 for E. huxleyi and from 0.741 to 0.788 for G. oceanica. There was no significant correlation of α alkenones−H 2 O with temperature but a positive linear correlation was observed between α alkenones−H 2 O and salinity with ∼3‰ change in fractionation per salinity unit and a negative correlation between α alkenones−H 2 O and growth rate. This suggests that both salinity and growth rate can have a substantial impact on the stable hydrogen isotopic composition of long chain alkenones in natural environments.

494Engel et al. AbstractWe studied the direct effects of CO 2 and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (ϳ11 m 3 each), the partial pressure of CO 2 (pCO 2 ) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (ϳ190 parts per million by volume (ppmV) CO 2 ), present (ϳ410 ppmV CO 2 ), and high (ϳ710 ppmV CO 2 ) pCO 2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO 2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO 2 -related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO 2 . This broad influence of CO 2 on the E. huxleyi bloom suggests that changes in CO 2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.

[1] We report a detailed programmed-temperature pyrolysis/combustion methodology for radiocarbon ( 14 C) dating of Antarctic sub-ice shelf sediments. The method targets the autochthonous organic component in sediments that contain a distribution of acid-insoluble organic components from several sources of different ages. The approach has improved sediment chronology in organic-rich sediments proximal to Antarctic ice shelves by yielding maximum age constraints significantly younger than bulk radiocarbon dates from the same sediment horizons. The method proves adequate in determining isotope ratios of the pre-aged carbon end-member; however, the isotopic compositions of the low-temperature measurements indicate that no samples completely avoided mixing with some proportion of pre-aged organic material. Dating the unresolved but desired young end-member must rely on indirect methods, but a simple mixing model cannot be developed without knowledge of the sedimentation rate or comparable constraints. A mathematical approach allowing for multiple mixing components yields a maximum likelihood age, a first-order approximation of the relative proportion of the autochthonous component, and the temperature at which allochthonous carbon begins to volatilize and mix with the autochthonous component. It is likely that our estimation of the cutoff temperature will be improved with knowledge of the pyrolysis kinetics of the major components. Chronology is improved relative to bulk acid-insoluble organic material ages from nine temperature interval dates down to two, but incorporation of inherently more pre-aged carbon in the first division becomes more apparent with fewer and larger temperature intervals.

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