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

Abstract

Compound-specific δ2H values 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 δ2H values and those of environmental water. Mid- and long-chain n-alkanes in sedimentary archives (i.e., n-C23 and n-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 n-C23 δ2H values and lake water δ2H values 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 n-alkane δ2H values in plants and lake sediments at 29 sites across mid-latitude North America. We find that both aquatic and terrestrial plants synthesize n-C23 and that sedimentary n-C23 δ2H values parallel those of terrestrial plants and differ from those of aquatic plants. Our results indicate that across mid-latitude North America, 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 n-C23-alkane preserved in lake sediments. Moreover, angiosperm and gymnosperm plants exhibit similar εapp values between n-C29 and mean annual precipitation (MAP) δ2H values across North America. Therefore, vegetation shifts between angiosperm and gymnosperm plants do not strongly affect εapp values between n-C29 and MAP. Our results show that both mid- and long-chain n-alkanes track the isotopic composition of MAP in temperate North America.