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

[摘要] The stable hydrogen isotope composition of lipid biomarkers, such asalkenones, is a promising new tool for the improvement of palaeosalinityreconstructions. Laboratory studies confirmed the correlation between lipidbiomarker δD composition (δD_Lipid), water δDcomposition (δD_H2O) and salinity; yet there is limited insightinto the applicability of this proxy in oceanic environments. To fill thisgap, we test the use of the δD composition of alkenones (δD_C37) and palmitic acid (δD_PA) as salinity proxies usingsamples of surface suspended material along the distinct salinity gradientinduced by the Amazon Plume. Our results indicate a positive correlationbetween salinity and δD_H2O, while the relationship betweenδD_H2O and δD_Lipid is more complex: δD_PAM correlates strongly with δD_H2O (r² = 0.81) andshows a salinity-dependent isotopic fractionation factor. δD_C37 only correlates with δD_H2O in a small number (n = 8) ofsamples with alkenone concentrations > 10 ng L⁻¹, whilethere is no correlation if all samples are taken into account. Thesefindings are mirrored by alkenone-based temperature reconstructions, whichare inaccurate for samples with low alkenone concentrations. Deviations inδD_C37 and temperature are likely to be caused by limitedhaptophyte algae growth due to low salinity and light limitation imposed bythe Amazon Plume. Our study confirms the applicability of δD_Lipid as a salinity proxy in oceanic environments. But it raises anote of caution concerning regions where low alkenone production can beexpected due to low salinity and light limitation, for instance, understrong riverine discharge.

[学科分类] 地球化学与岩石