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

• Main Authors: C. Häggi, C. M. Chiessi, E. Schefuß
• Format: Article
• Language: English
• Published: Copernicus Publications 2015-12-01
• Series: Biogeosciences
• Online Access: http://www.biogeosciences.net/12/7239/2015/bg-12-7239-2015.pdf
• ISSN: 1726-4170; 1726-4189

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 (&delta;D_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: &delta;D_PAM correlates strongly with δD_H₂O (<i>r</i>² = 0.81) and shows a salinity-dependent isotopic fractionation factor. δD_C37</sub></sub> only correlates with δD_H₂O in a small number (<i>n</i> = 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</sub></sub> 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 &delta;D_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.