Relationship of the tetra‐unsaturated C 37 alkenone to salinity and temperature: Implications for paleoproxy applications

Abstract

This study assesses the relationship to salinity and temperature of the levels of the tetra‐unsaturated 37‐carbon methyl alkenone (C 37:4 ) in the surface ocean. U K ′ 37 , a measure of the relative abundances of the C 37:2 and the C 37:3 alkenones, has a well constrained linear relationship to temperature in the open ocean [ Prahl and Wakeham , 1987 ] and is a well‐established technique for estimating past sea surface temperatures in the sediments (e.g. [ Müller et al. , 1998 ]). Unlike the di‐ and tri‐unsaturated C 37 alkenones, the temperature response of the tetra‐unsaturated C 37 alkenone is less certain [ Sikes et al. , 1997 ], and recent work has suggested a relationship to salinity instead [ Rosell‐Melé , 1998 ; Schulz et al. , 2000 ]. Our study examined 106 surface water and sediment trap samples from the Atlantic, Pacific, and Southern Oceans to assess the relationship of the relative abundance of C 37:4 to temperature and salinity. We also examined the relative unsaturation of C 37:2 and C 37:3 (the parameter U K ′ 37 ) to the same parameters to place the C 37:4 results in context. U K ′ 37 has a strong correlation to salinity in the Atlantic, but the relationship does not hold worldwide, whereas U K ′ 37 shows a strong linear relationship to temperatures in all ocean basins as shown in previous calibrations. The salinity response in the Atlantic does not confirm cause and effect and interpretation of the broader data set suggests any correlation is an artifact of the strong correlation of salinity to temperature in this basin implying salinity has no effect on the unsaturation of the C 37:2 or C 37:3 alkenones. The C 37:4 alkenone shows no discernable relationship to temperature or salinity across the several basins, even when correlations are restricted to cooler temperatures where the tetra‐unsaturated alkenone would be expected to be present. These results indicate that C 37:4 alkenone levels in the open ocean do not reflect either salinity levels or temperature but respond most strongly to some other environmental variable, perhaps changes in growth rate, light, or nutrient supply as suggested by culture studies.