Determining the Effect of Growth Rate on Hydrogen Isotope Fractionation of Algal Lipids in Two North Pacific Sites

Tropical hydrologic changes have a large effect on global climate, but there does not yet exist a good indicator of rainfall variation in the tropics. Understanding past natural variability of such features as the Intertropical Convergence Zone and El Niño Southern Oscillation provides information about the extent of anthropogenic climate change today. The hydrogen isotopic composition (D/H ratio) of algal lipids has been shown to track the isotopic composition of source water in which the organism grew, providing information about precipitation variability over time. However, culture work has revealed that environmental factors such as salinity, temperature, growth rate, and irradiance also influence algal lipid D/H ratios. Here I present work determining the effect of growth rate and irradiance on the hydrogen isotope composition of alkenone-producing algae in the water column in two North Pacific locations, off the coast of Oregon and near the Hawaii Ocean Time Series site. This work corroborates empirical relationships observed in culture studies and indicates that the effects of growth rate and irradiance should be taken into account when applying the D/H isotope ratio rainfall proxy to reconstruct past climates.