The stable hydrogen isotope composition of lipid biomarkers, such as alkenones, is a promising new tool for the improvement of palaeosalinity reconstructions. Laboratory studies confirmed the correlation between lipid biomarker dD composition (dDLipid), water dD composition (dDH2O) and salinity; yet there is limited insight into the applicability of this proxy in oceanic environments. To fill this gap, we test the use of the dD composition of alkenones (dDC37) and palmitic acid (dDPA) as salinity proxies using samples of surface suspended material along the distinct salinity gradient induced by the Amazon Plume. Our results indicate a positive correlation between salinity and dDH2O, while the relationship between dDH2O and dDLipid is more complex: dDPAM correlates strongly with dDH2O (r2 = 0.81) and shows a salinity-dependent isotopic fractionation factor. dDC37 only correlates with dDH2O in a small number (n = 8) of samples with alkenone concentrations > 10 ng L**-1, while there is no correlation if all samples are taken into account. These findings are mirrored by alkenone-based temperature reconstructions, which are inaccurate for samples with low alkenone concentrations. Deviations in dDC37 and temperature are likely to be caused by limited haptophyte algae growth due to low salinity and light limitation imposed by the Amazon Plume. Our study confirms the applicability of dDLipid as a salinity proxy in oceanic environments. But it raises a note of caution concerning regions where low alkenone production can be expected due to low salinity and light limitation, for instance, under strong riverine discharge.

Values for salinity and temperature are the average of on-board measurements taken in 1 s intervals during each filtering period. Errors represent the standard deviation of these measurements. The dD values of water represent the mean of two samples taken at the beginning and the end of each filtering period; each sample represents the mean of 10 replicate injections. Errors represent the propagated standard deviation of these measurements. The dD values of C37 and palmitic acid are the means of duplicate measurements. Errors represent the range between the duplicate measurements.a: C37 yield was not high enough for isotope analysis; b: no clear peak distinction for C38.

Supplement to: Häggi, Christoph; Chiessi, Cristiano Mazur; Schefuß, Enno (2015): Testing the D / H ratio of alkenones and palmitic acid as salinity proxies in the Amazon Plume. Biogeosciences, 12, 7239-7249