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

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 (ε app) 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 -C 23 and n -C 29) 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 -C 23 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 -C 23 and that sedimentary n -C 23 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 -C 23 -alkane preserved in lake sediments. Moreover, angiosperm and gymnosperm plants exhibit similar ε app values between n -C 29 and mean annual precipitation (MAP) δ2H values across North America. Therefore, vegetation shifts between angiosperm and gymnosperm plants do not strongly affect ε app values between n -C 29 and MAP. Our results show that both mid- and long-chain n -alkanes track the isotopic composition of MAP in temperate North America. [ABSTRACT FROM AUTHOR]