Testing the D / H ratio of alkenones and palmitic acid as salinity proxies in the Amazon Plume

Abstract. 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 δD composition (δD_Lipid), water δD composition (δD_H2O) 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 δD composition of alkenones (δD_C37) and palmitic acid (δD_PA) 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 δD_H2O, while the relationship between δD_H2O and δD_Lipid is more complex: δD_PAM correlates strongly with δD_H2O (r² = 0.81) and shows a salinity-dependent isotopic fractionation factor. δD_C37 only correlates with δD_H2O 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 δD_C37 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 δD_Lipid 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.