Review of alkenone calibrations (culture, water column, and sediments)



[1] I review the status of alkenone unsaturation parameters as proxies for growth temperature and the potential for pCO2 estimates using the carbon isotopic composition of alkenones. Culture studies allow investigators to manipulate parameters that could influence the relative proportions of C37 and C38 ketones in producing organisms and to assess the possible influence of genetic variability on the calibration of unsaturation to growth temperature. Culture studies suggest large variability in the relation of unsaturation indices to growth temperature and hint that growth rate may play a role along with temperature in controlling unsaturation. Water column studies match the unsaturation index measured in particulate matter to ambient temperature, providing a snapshot of unsaturation-temperature relationship. In comparison to culture work, assemblages of regional water column data sets give more consistent relations of the unsaturation index to temperature, although the argument can be made for regional rather than basin-wide or global calibrations. A much simpler picture emerges from analyzing a global array of core top sediments. Regressions show that the sedimentary unsaturation indices follow a linear trend that is best correlated to mean annual temperature in the upper 10 m of the overlying water column. The regression is indistinguishable from the original Prahl et al. [1988] culture calibration of the U37k′ index to growth temperature. Exceptions occur at high latitudes, where the effects of highly seasonal production and the possible influence of salinity may affect the unsaturation index in sediments. Core top analyses also fail to reveal the variability of other alkenone parameters (for example, the ratio of C37 to C38 ketones) which have been found in culture studies. Much of the variance that appears in culture and water column calibrations is thus apparently removed in sediments, perhaps by temporal averaging. The promising paleo-pCO2 approach based on the isotopic composition of alkenones awaits the arrival of a significant number of core top and stratigraphic studies that will determine the extent to which physiological factors compromise the alkenone paleobarometer.