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

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 (δDLipid), water δD composition (δDH2O) 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 (δDC37) and palmitic acid (δDPA) 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 δDH2O, while the relationship between δDH2O and δDLipid is more complex: δDPAM correlates strongly with δDH2O (r2 = 0.81) and shows a salinity-dependent isotopic fractionation factor. δDC37 only correlates with δDH2O 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 δDC37 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 δDLipid 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.

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

The stable hydrogen isotope composition of lipid biomarkers, such as alkenones, is a promising new tool for the improvement of paleosalinity reconstructions. Laboratory studies confirmed the correlation between lipid biomarker δD composition (δDLipid), water δD composition (δDH2O) 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 (δDC37) and palmitic acid (δDPA) 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 δDH2O, while the relationship between δDH2O and δDLipid is more complex: δDPA correlates strongly with δDH2O (r2 = 0.81) and shows a salinity dependent isotopic fractionation factor. δDC37 only correlates with δDH2O in samples with alkenone concentrations > 10 ng L−1 (r2 = 0.51). These findings are mirrored by alkenone based temperature reconstructions, which are inaccurate for samples with alkenone concentrations < 10 ng L−1. Deviations in δDC37 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 δDLipid 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 very low salinity conditions. To circumvent these limitations, we suggest the complementary use of δDC37 and δDPA.

Variation in the Stable-Hydrogen Isotope Composition of Northern Goshawk Feathers: Relevance to the Study of Migratory Origins

The analysis of stable-hydrogen isotope ratios in feathers (δDf) allows researchers to investigate avian movements and distributions to an extent never before possible. Nonetheless, natural variation in δDf is poorly understood and, in particular, its implications for predictive models based on stable-hydrogen isotopes remain unclear. We employed hierarchical linear modeling to explore multiple levels of variation in the stable-hydrogen isotope composition of Northern Goshawk (Accipiter gentilis) feathers. We examined (1) inter-individual variation among goshawks from the same nest, and (2) intra-individual variation between multiple feathers from the same individual. Additionally, we assessed the importance of several factors (e.g., geographic location, climate, age, and sex characteristics) in explaining variation in δDf. Variation among individuals was nearly eight times the magnitude of variation within an individual, although age differences explained most of this inter-individual variation. In contrast, most variation in δD values between multiple feathers from an individual remained unexplained. Additionally, we suggest temporal patterns of δD in precipitation (δDp) as a potential explanation for the geographic variability in age-related differences that has precluded the description of movement patterns of adult raptors using δDf. Furthermore, intra-individual variability necessitates consistency in feather selection and careful interpretation of δDf-based models incorporating multiple feather types. Finally, although useful for describing the movements of groups of individuals, we suggest that variability inherent to environmental and intra-individual patterns of δDp and δDf, respectively, precludes the use of stable-hydrogen isotopes to describe movements of individual birds. Variación en la Composición de Isótopos Estables de Hidrógeno de las Plumas de Accipiter gentilis: Relevancia para los Estudios sobre el Origen de la Migración Resumen. El análisis de los cocientes de isótopos estables de hidrógeno presentes en las plumas (δDf) permite a los investigadores estudiar los movimientos y distribuciones de las aves en un grado nunca antes posible. Sin embargo, la variación natural en δDf es poco entendida, y particularmente sus implicaciones sobre modelos que hacen predicciones con base en isótopos estables de hidrógeno aún permanecen poco claras. Empleamos un modelo lineal jerárquico para explorar múltiples niveles de variación en la composición de isótopos estables de hidrógeno en las plumas de Accipiter gentiles. Examinamos (1) la variación entre individuos de un mismo nido y (2) la variación entre varias plumas de un mismo individuo. Además, determinamos la importancia de varios factores (e.g., aislamiento geográfico, clima, edad y características sexuales) para explicar las variaciones en δDf. La variación entre individuos fue casi ocho veces mayor que la variación en un mismo individuo, aunque diferencias en la edad explicaron la mayoría de esta variación entre individuos. De manera contrastante, la mayor parte de la variación en los valores de δD entre varias plumas de un mismo individuo permaneció inexplicada. Además, sugerimos patrones temporales de δD en la precipitación (δDp) como una posible explicación para la variabilidad geográfica en las diferencias relacionadas con la edad que han imposibilitado la descripción de los patrones de movimiento de aves rapaces adultas utilizando δDf. Asimismo, la variabilidad intra-individual requiere que exista coherencia en la selección de plumas y una interpretación cuidadosa de los modelos basados en δDf que incorporen múltiples tipos de plumas. Finalmente, a pesar de ser útiles para describir los movimientos de grupos de individuos, sugerimos que la variabilidad inherente al ambiente y los patrones intra-individuos de δDp y δDf, respectivamente, impiden el uso de isótopos estables de hidrógeno para describir los movimientos de aves individuales.