Mid- and long-chain leaf wax δ2H values in modern plants and lake sediments from mid-latitude North America

Abstract

Compound-specific δ2H composition of leaf-wax n-alkanes are increasingly being used to infer past hydroclimates. However, differences in n-alkane production and apparent fractionation factors (εapp) among different plant groups complicate the relationships between n-alkane and environmental water δ2H. Mid- and long-chain n-alkanes in sedimentary archives (i.e., C23 and C29) are thought to derive from aquatic and terrestrial plants, respectively, and track the isotopic composition of either lake water or precipitation. Yet, the relationship between the δ2H composition of alkane C23 and that of lake water is not well constrained. Moreover, recent studies show that n-alkane production is greater in terrestrial plants than in aquatic plants, which has the potential to obscure n-alkane aquatic inputs to sedimentary archives. Here, we investigated n-alkane contributions to sedimentary archives from both aquatic and terrestrial plants by analyzing their distributions and δ2H signatures in plants and lake sediments at 29 sites across mid-latitude North America. We find that both aquatic and terrestrial plants synthesize alkane C23 and that sedimentary C23 δ2H values parallel those of terrestrial plants and differ from those of aquatic plants. Our results indicate that across mid-latitude North America, and globally, both mid- and long-chain n-alkanes in lake sediments commonly derive from terrestrial higher plants challenging the assumption that submerged aquatic plants produce the C23 alkane preserved in lake sediments. Moreover, angiosperms and gymnosperms exhibit similar εapp values between the δ2H of alkane C29 and mean annual precipitation. Therefore, vegetation shifts between angiosperms and gymnosperms do not strongly affect the εapp between alkane C29 and MAP. Our results show that both mid- and long-chain alkanes track the isotopic composition of mean annual precipitation in the temperate region of North America.